The Atlantic's Meridional Overturning Circulation (AMOC) collapsed during the last glacial termination, and the collapse led to warmings in both hemispheres. It is not clear, however, how the warmings occurred. The warmings are attributed here to a component of the AMOC that has received relatively little attention—the upshifts of the AMOC's return flow. The upshifts occur off Namibia and Peru, where the upshifted water is very cool. The cool water is warmed up as it spreads across the ocean basins, and the warmed-up water is then drawn away to the North Atlantic. The net result is a cooling of the tropics. The cooling would be eliminated when the AMOC collapses, and the demise of the cooling would allow the tropics and both hemispheres to warm together. The AMOC is also shown to keep the tropics and both hemispheres relatively cool when the summer insolation over the Northern Hemisphere is at a minimum in the 23-kyr precessional band. The cooling is then weakened as the summer insolation increases. Thus, cooling gives way to warming in much the same way that it did during the last glacial termination. These findings lead to a new description of how warming and cooling occurred during the ice ages.
Toggweiler, J R., E R M Druffel, Robert M Key, and Eric D Galbraith, April 2019: Upwelling in the Ocean Basins north of the ACC Part 1: On the Upwelling Exposed by the Surface Distribution of Δ14C. Journal of Geophysical Research: Oceans, 124(4), DOI:10.1029/2018JC014794. Abstract
The upwelling associated with the ocean's overturning circulation is hard to observe directly. Here, a large data set of surface Δ14C measurements is compiled in order to show where deep water is brought back up to the surface in the ocean basins north of the Antarctic Circum‐polar Current (ACC). Maps constructed from the data set show that low‐Δ14C deep water from the ACC is drawn up to the surface in or near the upwelling zones off Northwest Africa and Namibia in the Atlantic, off Costa Rica and Peru in the Pacific, and in the northern Arabian Sea in the Indian Ocean. Deep water also seems to be reaching the surface in the subarctic Pacific gyre near the Kamchatka Peninsula. The low‐Δ14C water drawn up to the surface in the upwelling zones is also shown to spread across the ocean basins. It is easily seen, for example, in the western Atlantic off Florida and in the western Pacific off New Guinea and Palau. The spreading allows one to estimate the volumes of upwelling, which, it turns out, are similar to the volumes of large‐scale upwelling derived from inverse box models. This means that very large volumes of cool subsurface water are reaching the surface in and near the upwelling zones − much larger volumes than would be expected from the local winds.
Toggweiler, J R., E R M Druffel, Robert M Key, and Eric D Galbraith, April 2019: Upwelling in the Ocean Basins north of the ACC Part 2: How Cool Subantarctic Water Reaches the Surface in the Tropics. Journal of Geophysical Research: Oceans, 124(4), DOI:10.1029/2018JC014795. Abstract
Large volumes of cool water are drawn up to the surface in the tropical oceans. A com‐panion paper shows that the cool water reaches the surface in or near the upwelling zones off northern and southern Africa and Peru. The cool water has a subantarctic origin and spreads extensively across the Atlantic and Pacific basins after it reaches the surface. Here, we look at the spreading in two low‐resolution ocean general circulation models and find that the spreading in the models is much less extensive than observed. The problem seems to be the way the upwelling and the spreading are connected (or not connected) to the ocean's large‐scale over‐turning. As proposed here, the cool upwelling develops when warm buoyant water in the western tropics is drawn away to become deep water in the North Atlantic. The “drawing away” shoals the tropical thermocline in a way that allows cool subantarctic water to be drawn up to the surface along the eastern margins. The amounts of upwelling produced this way exceed the amounts generated by the winds in the upwelling zones by as much as four times. Flow restrictions make it difficult for the warm buoyant water in our models to be drawn away.
To explore the mechanisms involved in the global ocean circulation response to the shoaling and closure of the Central American Seaway (CAS), we performed a suite of sensitivity experiments using the Geophysical Fluid Dynamics Laboratory Earth System Model (ESM), GFDL‐ESM 2G, varying only the seaway widths and sill depths. Changes in large‐scale transport, global ocean mean state, and deep‐ocean circulation in all simulations are driven by the direct impacts of the seaway on global mass, heat and salt transports. Net mass transport through the seaway into the Caribbean is 20.5‐23.1 Sv with a deep CAS, but only 14.1 Sv for the wide, shallow CAS. Seaway transport originates from the Antarctic Circumpolar Current in the Pacific and rejoins it in the South Atlantic, reducing the Indonesian Throughflow and transporting heat and salt southward into the South Atlantic, in contrast to present‐day and previous CAS simulations. The increased southward salt transport increases the large‐scale upper ocean density, and the freshening and warming from the changing ocean transports decreases the intermediate and deep‐water density. The new ocean circulation pathway traps heat in the Southern Hemisphere oceans and reduces the northern extent of Antarctic Bottom Water penetration in the Atlantic, strengthening and deepening Atlantic meridional overturning, in contrast to previous studies. In all simulations, the seaway has a profound effect on the global ocean mean state and alters deep‐water mass properties and circulation in the Atlantic, Indian and Pacific basins, with implications for changing deep‐water circulation as a possible driver for changes in long‐term climate.
Sosdian, S M., Y Rosenthal, and J R Toggweiler, June 2018: Deep Atlantic carbonate ion and CaCO3 Compensation during the Ice Ages. Paleoceanography and Paleoclimatology, 33(6), DOI:10.1029/2017PA003312. Abstract
Higher alkalinity is compensation for reduced CaCO3 burial in the deep ocean in response to increased carbon sequestration in the deep ocean. This process accounts for about half of the reduction in glacial atmospheric CO2. To date our understanding of this process comes from benthic carbon isotope and CaCO3 burial records. Here we present a 1.5 My orbitally resolved deep ocean calcite saturation record (∆CO32‐) derived from benthic foraminiferal B/Ca ratios in the North Atlantic. Glacial ∆CO32‐ declines across the mid‐Pleistocene transition (MPT) suggesting increased sequestration of carbon in the deep Atlantic. The magnitude, timing, and structure of deep Atlantic Ocean ∆CO32‐ parallels changes in %CaCO3 and contrasts the small amplitude, anti‐phased swings in IndoPacific ∆CO32‐ and %CaCO3 during the mid‐to‐late Pleistocene questioning the classic view of CaCO3 compensatory mechanism. We propose that the increasing corrosivity of the deep Atlantic causes the locus of CaCO3 burial to shift into the equatorial Pacific where the flux of CaCO3 to the seafloor was sufficiently high to overcome low saturation and establish a new burial “hot spot”. Based on this mechanism, we propose that the persistently low∆CO32‐ levels at Marine Isotope Stages (MIS) 12, set the stage for the high pCO2 levels at MIS 11 and subsequent interglacials via large swings in ocean alkalinity caused by shifts in CaCO3 burial. Similarly, the development of classic (‘anti‐correlated’) CaCO3 patterns was driven by enhanced ocean stratification and an increase in deep ocean corrosivity in response to MPT cooling.
We introduce a composite tracer, Alk*, that has a global distribution primarily determined by CaCO3 precipitation and dissolution. Alk* also highlights riverine alkalinity plumes that are due to dissolved calcium carbonate from land. We estimate the Arctic receives approximately twice the riverine alkalinity per unit area as the Atlantic, and 8 times that of the other oceans. Riverine inputs broadly elevate Alk* in the Arctic surface and particularly near river mouths. Strong net carbonate precipitation lowers basin mean Indian and Atlantic Alk*, while upwelling of dissolved CaCO3 rich deep waters elevates Northern Pacific and Southern Ocean Alk*. We use the Alk* distribution to estimate the carbonate saturation variability resulting from CaCO3 cycling and other processes. We show regional variations in surface carbonate saturation are due to temperature changes driving CO2 fluxes and, to a lesser extent, freshwater cycling. Calcium carbonate cycling plays a tertiary role. Monitoring the Alk* distribution would allow us to isolate the impact of acidification on biological calcification and remineralization.
Cheon, W G., Y-G Park, J R Toggweiler, and Sang-Ki Lee, February 2014: The relationship of Weddell polynya and open-ocean deep convection to the Southern Hemisphere westerlies. Journal of Physical Oceanography, 44(2), DOI:10.1175/JPO-D-13-0112.1. Abstract
The Weddell polynya of the mid 1970s is simulated in an Energy Balance Model (EBM) sea-ice/ocean coupled General Circulation Model (GCM) with an abrupt 20% increase in intensity of Southern Hemisphere (SH) westerlies. This small up-shift of applied wind stress is viewed as a stand-in for the stronger zonal winds that developed in the mid 1970s following a long interval of relatively weak zonal winds between 1954 and 1972. Following the strengthening of the westerlies in our model, the cyclonic Weddell gyre intensifies, raising relatively warm Weddell Sea Deep Water to the surface. The raised warm water then melts sea ice or prevents it from forming to produce the Weddell polynya. Within the polynya, large heat loss to the air causes surface water to become cold and sink to the bottom via open-ocean deep convection. Thus, the underlying layers cool down, the warm-water supply to the surface eventually stops, and the polynya can not be maintained anymore. During our 100-year-long model simulation we observe two Weddell polynya events. The second one occurs a few years after the first one disappears; it is much weaker and persists for less time than the first one because the underlying layer is cooler. Based on our model simulations, we hypothesize that the Weddell polynya and open-ocean deep convection were responses to the stronger SH westerlies that followed a prolonged weak phase of the Southern Annular Mode.
Druffel, E R., B M Griffin, D Glynn, R B Dunbar, D Muciarone, and J R Toggweiler, July 2014: Seasonal radiocarbon and oxygen isotopes in a Galapagos coral: Calibration with climate indices. Geophysical Research Letters, 41(14), DOI:10.1002/2014GL060504. Abstract
We present seasonal ∆14C and δ18O measurements from a Galapagos coral sequence that grew during the early 20th century. Our results show that both ∆14C and δ18O values are correlated with sea surface temperature in the Niño 3.4 region and are indicators of El Niño/Southern Oscillation. There is a significant inverse correlation between ∆14C and δ18O values when ∆14C is lagged by ~2 months, indicating that sea surface temperature changes precede upwelling changes at this eastern equatorial location. We find that cold season low ∆14C values were higher after the Pacific Decadal Oscillation (PDO) changed from a positive to a negative phase. Cold season high δ18O values were significantly higher after the PDO shift, as well. These findings suggest that there are two sources of low ∆14C waters that upwell at the Galapagos, Subantarctic Mode Water and shallow overturning water from the subpolar North Pacific.
Straub, M, M M Tremblay, Daniel M Sigman, A S Studer, Hong-Li Ren, J R Toggweiler, and Gerald H Haug, March 2013: Nutrient conditions in the subpolar North Atlantic during the last glacial period reconstructed from foraminifera-bound nitrogen isotopes. Paleoceanography, 28, DOI:10.1002/palo.20013. Abstract
Surface nitrate concentration is a potentially useful diagnostic in reconstructing the past circulation of high latitude North Atlantic waters. Moreover, nutrient consumption in the North Atlantic surface impacts the atmospheric concentration of carbon dioxide. To reconstruct nutrient conditions in the subpolar North Atlantic region during the last ice age, a record of foraminifera-bound δ15N was measured in N. pachyderma (sin.) from core V28-73 south of Iceland (57.2°N, 20.9°W). Foraminifera-bound δ15N is up to 2‰ lower during the last ice age than during the Holocene, suggesting as much as ~25% less complete nitrate consumption during the former. This is consistent with stronger light limitation associated with a deeper summer surface mixed layer, perhaps related to the formation of Glacial North Atlantic Intermediate Water previously suggested to have occurred near the core site. However, three single-point maxima in δ15N in the glacial section and the sharp deglacial δ15N rise coincide with Heinrich event layers. This suggests that increased water column stratification during Heinrich events, presumably due to surface freshening, reduced the nutrient supply from below and led to nearly complete nitrate consumption in the summertime mixed layer. The Heinrich layers in V28-73 are not accompanied by δ18O minima in either N. pachyderma (sin.) or G. bulloides, which we tentatively attribute to extreme mixed layer shoaling. The reconstructed subpolar North Atlantic upper water column changes – both glacial/interglacial and millennial – are inverse to those inferred for the Antarctic.
We assess the global balance of calcite export through the water column and burial in sediments as it varies regionally. We first drive a comprehensive 1-D model for sediment calcite preservation with globally gridded field observations and satellite-based syntheses. We then reformulate this model into a simpler five-parameter box model, and combine it with algorithms for surface calcite export and water column dissolution for a single expression for the vertical calcite balance. The resulting metamodel is optimized to fit the observed distributions of calcite burial flux. We quantify the degree to which calcite export, saturation state, organic carbon respiration, and lithogenic sedimentation modulate the burial of calcite. We find that 46% of burial and 88% of dissolution occurs in sediments overlain by undersaturated bottom water with sediment calcite burial strongly modulated by surface export. Relative to organic carbon export, we find surface calcite export skewed geographically toward relatively warm, oligotrophic areas dominated by small, prokaryotic phytoplankton. We assess century-scale projected impacts of warming and acidification on calcite export, finding high sensitive to inferred saturation state controls. With respect to long term glacial cycling, our analysis supports the hypothesis that strong glacial abyssal stratification drives the lysocline towards much closer correspondence with the saturation horizon. Our analysis suggests that, over the transition from interglacial to glacial ocean, a resulting ~0.029 PgC a-1 decrease in deep Atlantic, Indian and Southern Ocean calcite burial leads to slow increase in ocean alkalinity until Pacific mid-depth calcite burial increases to compensate.
Kwon, Eun Young, M P Hain, Daniel M Sigman, Eric D Galbraith, Jorge L Sarmiento, and J R Toggweiler, May 2012: North Atlantic ventilation of "southern-sourced" deep water in the glacial ocean. Paleoceanography, 27, PA2208, DOI:10.1029/2011PA002211. Abstract
One potential mechanism for lowering atmospheric CO2 during glacial times is an increase in the fraction of the global ocean ventilated by the North Atlantic, which produces deep water with a low concentration of unused nutrients and thus drives the ocean's biological pump to a high efficiency. However, the data indicate that during glacial times, a water mass low in 13C/12C and 14C/C occupied the deep Atlantic, apparently at the expense of North Atlantic Deep Water (NADW). This water is commonly referred to as "southern-sourced", because of its apparent entry into the Atlantic basin from the south, prompting the inference that it was ventilated at the Southern Ocean surface. Here, we propose that this deep Atlantic water mass actually included a large fraction of North Atlantic-venitlated water, the chemical characteristics of which were altered by recirculation in the deep Southern and Indo-Pacific Oceans. In an ocean model sensitivity experiment that reduces Antarctic Bottom Water formation and weakens its overturning circulation, we find that a much greater fraction of NADW is transported into the Southern Ocean without contacting the surface and is entrained and mixed into the southern-sourced deep water that spreads into the global abyssal ocean. Thus, North Atlantic ventilation takes over more of the ocean interior, lowering atmospheric CO2, and yet the abyssal Atlantic is filled from the south with old water low in 13C/12C and 14C/C, consistent with glacial data.
Katz, M E., B S Cramer, and J R Toggweiler, et al., May 2011: Impact of Antarctic Circumpolar Current development on late Paleogene ocean structure. Science, 332(6033), DOI:10.1126/science.1202122. Abstract
Global cooling and the development of continental-scale Antarctic glaciation occurred in the late middle Eocene to early Oligocene (~38 to 28 million years ago), accompanied by deep-ocean reorganization attributed to gradual Antarctic Circumpolar Current (ACC) development. Our benthic foraminiferal stable isotope comparisons show that a large δ13C offset developed between mid-depth (~600 meters) and deep (>1000 meters) western North Atlantic waters in the early Oligocene, indicating the development of intermediate-depth δ13C and O2 minima closely linked in the modern ocean to northward incursion of Antarctic Intermediate Water. At the same time, the ocean’s coldest waters became restricted to south of the ACC, probably forming a bottom-ocean layer, as in the modern ocean. We show that the modern four-layer ocean structure (surface, intermediate, deep, and bottom waters) developed during the early Oligocene as a consequence of the ACC.
Kwon, Eun Young, Jorge L Sarmiento, J R Toggweiler, and T DeVries, September 2011: The control of atmospheric pCO2 by ocean ventilation change: The effect of the oceanic storage of biogenic carbon. Global Biogeochemical Cycles, 25, GB3026, DOI:10.1029/2011GB004059. Abstract
A simple analytical framework is developed relating the atmospheric partial pressure of CO2 to the globally-averaged concentrations of respired carbon ( ) and dissolved carbonate ( ) in the ocean. Assuming that the inventory of carbon is conserved in the ocean-atmosphere system (i.e. no seawater-sediment interactions), the resulting formula of = −0.0053Δ + 0.0034Δ suggests that atmospheric pCO2 would decrease by 5.3% and increase by 3.4% when and increase by 10 μmol kg−1, respectively. Using this analytical framework along with a 3-D global ocean biogeochemistry model, we show that the response of atmospheric pCO2 to changes in ocean circulation is rather modest because ∼30% of the change in atmospheric pCO2 caused by the accumulation of respired carbon is countered by a concomitant accumulation of dissolved carbonate in deep waters. Among the suite of circulation models examined here, the largest reduction in atmospheric pCO2 of 44–88 ppm occurs in a model where reduced overturning rates of both southern and northern sourced deep waters result in a four-fold increase in the Southern Ocean deep water ventilation age. On the other hand, when the ventilation rate of the southern-sourced water decreases, but the overturning rate of North Atlantic Deep Water increases, the resulting decrease in atmospheric pCO2 is only 14–34 ppm. The large uncertainty ranges in atmospheric pCO2 arise from uncertainty in how surface productivity responds to circulation change. Although the uncertainty is large, this study suggests that a synchronously reduced rate for the deep water formation in both hemispheres could lead to the large glacial reduction in atmospheric pCO2 of 80–100 ppm.
Denton, G H., R F Anderson, J R Toggweiler, R L Edwards, J M Schaefer, and A E Putnam, June 2010: The Last Glacial Termination. Science, 328(5986), DOI:10.1126/science.1184119. Abstract
A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age. A prerequisite was the growth of very large Northern Hemisphere ice sheets, whose subsequent collapse created stadial conditions that disrupted global patterns of ocean and atmospheric circulation. The Southern Hemisphere westerlies shifted poleward during each northern stadial, producing pulses of ocean upwelling and warming that together accounted for much of the termination in the Southern Ocean and Antarctica. Rising atmospheric CO2 during southern upwelling pulses augmented warming during the last termination in both polar hemispheres.
We overview problems and prospects in ocean circulation models, with emphasis on certain developments aiming to
enhance the physical integrity and flexibility of large-scale models used to study global climate. We also consider elements
of observational measures rendering information to help evaluate simulations and to guide development priorities.
http://www.oceanobs09.net/blog/?p=88
Toggweiler, J R., and D W Lea, June 2010: Temperature differences between the hemispheres and ice age climate variability. Paleoceanography, 25, PA2212, DOI:10.1029/2009PA001758. Abstract
The Earth became warmer and cooler during the ice ages along with changes in the Earth's orbit, but the orbital changes themselves are not nearly large enough to explain the magnitude of the warming and cooling. Atmospheric CO2 also rose and fell, but again, the CO2 changes are rather small in relation to the warming and cooling. So, how did the Earth manage to warm and cool by so much? Here we argue that, for the big transitions at least, the Earth did not warm and cool as a single entity. Rather, the south warmed instead at the expense of a cooler north through massive redistributions of heat that were set off by the orbital forcing. Oceanic CO2 was vented up to the atmosphere by the same redistributions. The north then warmed later in response to higher CO2 and a reduced albedo from smaller ice sheets. This form of north-south displacement is actually very familiar, as it is readily observed during the Younger Dryas interval 13,000 years ago and in the various millennial-scale events over the last 90,000 years.
Cramer, B S., J R Toggweiler, J D Wright, M E Katz, and K G Miller, November 2009: Ocean overturning since the Late Cretaceous: Inferences from a new benthic foraminiferal isotope compilation. Paleoceanography, 24, PA4216, DOI:10.1029/2008PA001683. Abstract
Benthic foraminiferal oxygen isotopic (δ18O) and carbon isotopic (δ13C) trends, constructed from compilations of data series from multiple ocean sites, provide one of the primary means of reconstructing changes in the ocean interior. These records are also widely used as a general climate indicator for comparison with local and more specific marine and terrestrial climate proxy records. We present new benthic foraminiferal δ18O and δ13C compilations for individual ocean basins that provide a robust estimate of benthic foraminiferal stable isotopic variations to ∼80 Ma and tentatively to ∼110 Ma. First-order variations in interbasinal isotopic gradients delineate transitions from interior ocean heterogeneity during the Late Cretaceous (>∼65 Ma) to early Paleogene (35–65 Ma) homogeneity and a return to heterogeneity in the late Paleogene–early Neogene (35–0 Ma). We propose that these transitions reflect alterations in a first-order characteristic of ocean circulation: the ability of winds to make water in the deep ocean circulate. We document the initiation of large interbasinal δ18O gradients in the early Oligocene and link the variations in interbasinal δ18O gradients from the middle Eocene to Oligocene with the increasing influence of wind-driven mixing due to the gradual tectonic opening of Southern Ocean passages and initiation and strengthening of the Antarctic Circumpolar Current. The role of wind-driven upwelling, possibly associated with a Tethyan Circumequatorial Current, in controlling Late Cretaceous interior ocean heterogeneity should be the subject of further research.
Sijp, W P., Matthew H England, and J R Toggweiler, December 2009: Effect of Ocean Gateway Changes under Greenhouse Warmth. Journal of Climate, 22(24), DOI:10.1175/2009JCLI3003.1. Abstract
The role of tectonic Southern Ocean gateway changes in driving Antarctic climate change at the Eocene–Oligocene boundary remains a topic of debate. One approach taken in previous idealized modeling studies of gateway effects has been to alter modern boundary conditions, whereby the Drake Passage becomes closed. Here, the authors follow this approach but vary atmospheric pCO2 over a range of values when comparing gateway configurations. They find a significantly greater sensitivity of Antarctic temperatures to Southern Ocean gateway changes when atmospheric pCO2 is high than when concentrations are low and the ambient climate is cool. In particular, the closure of the Drake Passage (DP) gap is a necessary condition for the existence of ice-free Antarctic conditions at high CO2 concentrations in this coupled climate model. The absence of the Antarctic Circumpolar Current (ACC) is particularly conducive to warm Antarctic conditions at higher CO2 concentrations, which is markedly different from previous simulations conducted under present-day CO2 conditions. The reason for this is the reduction of sea ice associated with higher CO2. Antarctic sea surface temperature and surface air temperature warming due to a closed DP gap reach values around 5° and 7°C, respectively, for high concentrations of CO2 (above 1250 ppm). In other words, the authors find a significantly greater sensitivity of Antarctic temperatures to atmospheric CO2 concentration when the DP is closed compared to when it is open. The presence of a DP gap inhibits a return to warmer and more Eocene-like Antarctic and deep ocean conditions, even under enhanced atmospheric greenhouse gas concentrations.
De Boer, A M., J R Toggweiler, and Daniel M Sigman, February 2008: Atlantic dominance of the meridional overturning circulation. Journal of Physical Oceanography, 38(2), DOI:10.1175/2007JPO3731.1. Abstract
North Atlantic (NA) deep-water formation and the resulting Atlantic meridional overturning cell is generally regarded as the primary feature of the global overturning circulation and is believed to be a result of the geometry of the continents. Here, instead, the overturning is viewed as a global energy–driven system and the robustness of NA dominance is investigated within this framework. Using an idealized geometry ocean general circulation model coupled to an energy moisture balance model, various climatic forcings are tested for their effect on the strength and structure of the overturning circulation. Without winds or a high vertical diffusivity, the ocean does not support deep convection. A supply of mechanical energy through winds or mixing (purposefully included or due to numerical diffusion) starts the deep-water formation. Once deep convection and overturning set in, the distribution of convection centers is determined by the relative strength of the thermal and haline buoyancy forcing. In the most thermally dominant state (i.e., negligible salinity gradients), strong convection is shared among the NA, North Pacific (NP), and Southern Ocean (SO), while near the haline limit, convection is restricted to the NA. The effect of a more vigorous hydrological cycle is to produce stronger salinity gradients, favoring the haline state of NA dominance. In contrast, a higher mean ocean temperature will increase the importance of temperature gradients because the thermal expansion coefficient is higher in a warm ocean, leading to the thermally dominated state. An increase in SO winds or global winds tends to weaken the salinity gradients, also pushing the ocean to the thermal state. Paleoobservations of more distributed sinking in warmer climates in the past suggest that mean ocean temperature and winds play a more important role than the hydrological cycle in the overturning circulation over long time scales.
http://ams.allenpress.com/archive/1520-0485/38/2/pdf/i1520-0485-38-2-435.pdf
Marinov, I, Anand Gnanadesikan, Jorge L Sarmiento, J R Toggweiler, M J Follows, and B K Mignone, July 2008: Impact of oceanic circulation on biological carbon storage in the ocean and atmospheric pCO2. Global Biogeochemical Cycles, 22, GB3007, DOI:10.1029/2007GB002958. Abstract
We use both theory and ocean
biogeochemistry models to examine the role of the soft-tissue biological
pump in controlling atmospheric CO2. We demonstrate that
atmospheric CO2 can be simply related to the amount of inorganic
carbon stored in the ocean by the soft-tissue pump, which we term (OCSsoft). OCSsoftis linearly
related to the inventory of remineralized nutrient, which in turn is just
the total nutrient inventory minus the preformed nutrient inventory. In a
system where total nutrient is conserved, atmospheric CO2 can
thus be simply related to the global inventory of preformed nutrient.
Previous model simulations have explored how changes in the surface
concentration of nutrients in deepwater formation regions change the global
preformed nutrient inventory. We show that changes in physical forcing such
as winds, vertical mixing, and lateral mixing can shift the balance of
deepwater formation between the North Atlantic (where preformed nutrients
are low) and the Southern Ocean (where they are high). Such changes in
physical forcing can thus drive large changes in atmospheric CO2,
even with minimal changes in surface nutrient concentration. If Southern
Ocean deepwater formation strengthens, the preformed nutrient inventory and
thus atmospheric CO2 increase. An important consequence of these
new insights is that the relationship between surface nutrient
concentrations, biological export production, and atmospheric CO2
is more complex than previously predicted. Contrary to conventional wisdom,
we show that OCSsoftcan increase and atmospheric
CO2 decrease, while surface nutrients show minimal change and
export production decreases.
A new mechanism is proposed to explain the
100,000-year timescale for variations in Antarctic temperatures and
atmospheric CO2 over the last 650,000 years. It starts with
fluctuations in the oceanic overturning around Antarctica that release CO2
up to the atmosphere or trap it in the deep ocean. Every 50,000 years one of
these fluctuations coincides with a changeover in the burial of CaCO3
in the deep ocean. The changeover alters the atmospheric pCO2
in a way that augments the tendency of the overturning. The augmented
overturning then enhances the tendency of the CaCO3 burial, which
augments the overturning, etc. In this way, an individual random fluctuation
becomes one of the big transitions seen in the Antarctic ice cores.
Alternating transitions toward the warm and cold states every 50,000 years
produce the 100,000-year timescale. The 50,000-year time interval is set by
the turnover time for CO3= ions in the ocean with
respect to the CO2-induced weathering of silicate rocks and the
burial of CaCO3 on the seafloor.
De Boer, A M., Daniel M Sigman, J R Toggweiler, and Joellen L Russell, June 2007: Effect of global ocean temperature change on deep ocean ventilation. Paleoceanography, 22, PA2210, DOI:10.1029/2005PA001242. Abstract
A growing number of paleoceanographic observations suggest that the ocean's deep ventilation is stronger in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is, at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection. In order to isolate this factor from other climate-related feedbacks we change the model ocean temperature only where it is used to calculate the density (to which we refer below as “dynamic” temperature change). We find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With dynamic cooling, convection decreases markedly in regions that have strong haloclines (i.e., the Southern Ocean and the North Pacific), while overturning increases in the North Atlantic, where the positive salinity buoyancy is smallest among the polar regions. We propose that this opposite behavior of the North Atlantic to the Southern Ocean and North Pacific is the result of an energy-constrained overturning.
Lamy, F, J Kaiser, H W Arz, D Hebbeln, U Ninnemann, O Timm, Axel Timmermann, and J R Toggweiler, 2007: Modulation of the bipolar seesaw in the Southeast Pacific during Termination 1. Earth and Planetary Science Letters, 259(3-4), DOI:10.1016/j.epsl.2007.04.040. Abstract
The
termination of the last ice age (Termination 1; T1) is crucial for our
understanding of global climate change and for the validation of climate
models. There are still a number of open questions regarding for example the
exact timing and the mechanisms involved in the initiation of deglaciation
and the subsequent interhemispheric pattern of the warming. Our study is
based on a well-dated and high-resolution alkenone-based sea surface
temperature (SST) record from the SE-Pacific off southern Chile (Ocean
Drilling Project Site 1233) showing that deglacial warming at the northern
margin of the Antarctic Circumpolar Current system (ACC) began shortly after
19,000 years BP (19 kyr BP). The timing is largely consistent with Antarctic
ice-core records but the initial warming in the SE-Pacific is more abrupt
suggesting a direct and immediate response to the slowdown of the Atlantic
thermohaline circulation through the bipolar seesaw mechanism. This response
requires a rapid transfer of the Atlantic signal to the SE-Pacific without
involving the thermal inertia of the Southern Ocean that may contribute to
the substantially more gradual deglacial temperature rise seen in Antarctic
ice-cores. A very plausible mechanism for this rapid transfer is a
seesaw-induced change of the coupled ocean–atmosphere system of the ACC and
the southern westerly wind belt. In addition, modelling results suggest that
insolation changes and the deglacial CO2 rise induced a
substantial SST increase at our site location but with a gradual warming
structure. The similarity of the two-step rise in our proxy SSTs and CO2
over T1 strongly demands for a forcing mechanism influencing both,
temperature and CO2. As SSTs at our coring site are particularly
sensitive to latitudinal shifts of the ACC/southern westerly wind belt
system, we conclude that such latitudinal shifts may substantially affect
the upwelling of deepwater masses in the Southern Ocean and thus the release
of CO2 to the atmosphere as suggested by the conceptual model of
[Toggweiler, J.R., Rusell, J.L., Carson, S.R., 2006. Midlatitude westerlies,
atmospheric CO2, and climate change during ice ages.
Modelling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air–sea balance of CO2 and global biological production1, 2, 3, 4, 5, 6, 7, 8. Box model studies1, 2, 3, 4 first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure (pCO2). This early research led to two important ideas: high latitude regions are more important in determining atmospheric pCO2 than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric pCO2 are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling pre-industrial atmospheric CO2 because it serves as a lid to a larger volume of the deep ocean5, 6. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide7 and in controlling global biological production8. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air–sea CO2 balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO2 and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.
A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.
Russell, Joellen L., Keith W Dixon, Anand Gnanadesikan, J R Toggweiler, and Ronald J Stouffer, August 2006: The once and future battles between Thor and the Midgard Serpent: The Southern Hemisphere Westerlies and the Antarctic Circumpolar Current. Geochimica et Cosmochimica Acta, 70(18 Supp 1), DOI:10.1016/j.gca.2006.06.1010. PDF
Toggweiler, J R., Joellen L Russell, and S Carson, 2006: Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages. Paleoceanography, 21, PA2005, DOI:10.1029/2005PA001154. Abstract
An idealized general circulation model is constructed of the ocean's deep circulation and CO2 system that explains some of the more puzzling features of glacial-interglacial CO2 cycles, including the tight correlation between atmospheric CO2 and Antarctic temperatures, the lead of Antarctic temperatures over CO2 at terminations, and the shift of the ocean's δ13C minimum from the North Pacific to the Atlantic sector of the Southern Ocean. These changes occur in the model during transitions between on and off states of the southern overturning circulation. We hypothesize that these transitions occur in nature through a positive feedback that involves the midlatitude westerly winds, the mean temperature of the atmosphere, and the overturning of southern deep water. Cold glacial climates seem to have equatorward shifted westerlies, which allow more respired CO2 to accumulate in the deep ocean. Warm climates like the present have poleward shifted westerlies that flush respired CO2 out of the deep ocean.
Toggweiler, J R., 2005: Climate change from below. Quaternary Science Reviews, 24(5-6), 511-512. PDF
The presence of low-latitude circumglobal passage from the late Jurassic (~160 Ma) through the Miocene (~14 Ma) provides a possible mechanism for increased poleward ocean heat transport during periods of warm climate and may help explain low meridional temperature gradients of the past. Experiments using an ocean general circulation model (GCM) with an energy-balance atmosphere and idealized bathymetry reveal that, like the modern Drake Passage, a circumglobal Tethyan Passage might have induced high rates of wind-driven upwelling of relatively cold and deep water, but at low latitudes. With no change in radiative forcing, a low-latitude circumglobal passage increases simulated northern high-latitude temperatures by 3°-7°C, while tropical temperatures cool by up to 2°C relative to a scenario with solid meridional boundaries. Combining this mechanism of heat transport with increased radiative forcing allows substantial warming of northern high latitudes by 7°-11°C, while tropical temperatures remain within 3°C of present-day temperatures.
Toggweiler, J R., Anand Gnanadesikan, S Carson, R Murnane, and Jorge L Sarmiento, March 2003: Representation of the carbon cycle in box models and GCMs: 1. Solubility pump. Global Biogeochemical Cycles, 17(1), 1026, DOI:10.1029/2001GB001401. Abstract
Bacastow [1996], Broecker et al. [1999], and Archer et al.[2000] have called attention recently to the fact that box models and general circulation models (GCMs) represent the thermal partitioning of CO2 between the warm surface ocean and cold deep ocean in different ways. They attribute these differences to mixing and circulation effects in GCMs that are not resolved in box models. The message that emerges from these studies is that box models have overstated the importance of the ocean's polar regions in the carbon cycle. A reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial - interglacial changes in atmospheric CO2. In parts 1 and 2 of this paper, a new analysis of the ocean's carbon pumps is carried out to examine these findings. This paper, part 1, shows that unresolved mixing and circulation effects in box models are not the main reason for box model-GCM differences. The main factor is very different kinds of restrictions on gas exchange in polar areas. Polar outcrops in GCMs are much smaller than in box models, and they are assumed to be ice covered in an unrealistic way. This finding does not support a reduced role for the ocean's polar regions in the cycling of organic carbon, the subject taken up in part 2.
Box models of the ocean/atmosphere CO2 system rely on mechanisms at polar outcrops to alter the strength of the ocean's organic carbon pump. GCM-based carbon system models are reportedly less sensitive to the same processes. Here we separate the carbon pumps in a three-box model and the GCM-based Princeton Ocean Biogeochemistry Model to show how the organic pumps operate in the two kinds of models. The organic pumps are found to be quite different in two respects. Deep water in the three-box model is relatively well equilibrated with respect to the pCO2 of the atmosphere while deep water in the GCM tends to be poorly equilibrated. This makes the organic pump inherently stronger in the GCM than in the three-box model. The second difference has to do with the role of polar nutrient utilization. The organic pump in the GCM is shown to have natural upper and lower limits that are set by the initial PO4 concentrations in the deep water formed in the North Atlantic and Southern Ocean. The strength of the organic pump can swing between these limits in response to changes in deep-water formation that alter the mix of northern and southern deep water. Thus, unlike the situation in the three-box model, the organic pump in the GCM can become weaker or stronger without changes in polar nutrient utilization.
Toggweiler, J R., and Robert M Key, 2003: Ocean Circulation / Thermohaline Circulation In Encyclopedia of Atmospheric Sciences, Vol. 4, San Diego, CA, Academic Press, 1549-1555.
Gildor, H, E Tziperman, and J R Toggweiler, 2002: Sea ice switch mechanism and glacial-interglacial CO2 variations. Global Biogeochemical Cycles, 16(3), DOI:10.1029/2001GB001446. Abstract PDF
A physical mechanism is proposed for the glacial-interglacial variations in the rate of vertical mixing and in sea ice cover in the Southern Ocean. Such variations were postulated without an explanation by Toggweiler [1999] and Stephens and Keeling [2000], who used them to explain the glacial-interglacial CO2 variations. In the physical mechanism explored here, initially given by Gildor and Tziperman [2001b], changes in the stratification of the Southern Ocean due to the cooling of North Atlantic Deep Water (NADW) during glacial maxima reduce the rate of vertical mixing of the surface water with the deep water. The changed temperature of the NADW arriving to the Southern Ocean and the reduced vertical mixing there also increase the Southern Ocean sea ice cover during glacial maxima. These vertical mixing and sea ice cover changes are shown to be a natural result of the sea ice switch mechanism of the glacial cycles. A box model of the climate system is used to demonstrate the above physical mechanism and its effect on the atmospheric CO2. Because of the uncertainties in the exact dependence of the vertical mixing on vertical stratification, it is impossible to quantify the exact separate contribution of reduced vertical mixing and larger sea ice cover to the glacial CO2 variations. The CO2 variations are not essential to the existence of the glacial cycles in the sea ice switch mechanism, yet they are shown to amplify the glacial-interglacial variability, consistent with the view of the role of CO2 deduced from proxy observations.
Bjornsson, H, and J R Toggweiler, 2001: The climatic influence of Drake Passage In The Oceans and Rapid Climate Change: Past, Present, and Future, Geophysical Monograph 126, Washington, DC, American Geophysical Union, 243-259. Abstract
The influence of Drake Passage on the earth's temperature distribution is explored in an idealized coupled model. In a version of the model without Drake Passage the temperature distribution is symmetric about the equator, due in large part to the fact that the meridional overturning in the ocean is symmetric about the equator with deep water formation in both hemispheres. With Drake Passage open, the overturning takes on the form of an interhemispheric conveyor with deep water formation primarily in the opposite (northern) hemisphere. Surface temperatures rise in the northern hemisphere and fall in the southern hemisphere as the ocean transports a large amount of heat northward across the equator. The magnitude of the thermal asymmetry between the hemispheres depends on the depth of the circumpolar channel and the strength of the winds over the channel. The opening of Drake Passage also leads to the formation of a low-salinity intermediate water mass reminiscent of Antarctic Intermediate Water. The intermediate water mass is associated with a warming and thickening of the thermocline that extends from the circumpolar channel into the highest latitudes of the northern hemisphere. Paleoclimatic implications of this work are discussed.
Orr, James C., E Maier-Reimer, U Mikolajewicz, Patrick Monfray, Jorge L Sarmiento, J R Toggweiler, N K Taylor, J Palmer, Nicolas Gruber, C L Sabine, C Le Quéré, Robert M Key, and J Boutin, 2001: Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochemical Cycles, 15(1), 43-60. Abstract PDF
We have compared simulations of anthropogenic CO2 in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ± 19%, giving a range of 1.85±0.35 Pg C yr -1 for the 1980-1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real ocean's present uptake, based on comparison of regional data-based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the date-based estimates could be provided regionally.
Chai, Fei, S T Lindley, J R Toggweiler, and R T Barber, 2000: Testing the importance of iron and grazing in the maintenance of the high nitrate condition in the equatorial Pacific Ocean: A physical-biological model study In The Changing Ocean Carbon Cycle, Cambridge University Press, 155-186.
Suntharalingam, P, Jorge L Sarmiento, and J R Toggweiler, 2000: Global significance of nitrous-oxide production and transport from oceanic low-oxygen zones: A modeling study. Global Biogeochemical Cycles, 14(4), 1353-1370. Abstract PDF
Recent studies of marine nitrous oxide have focused attention on the suboxic and low-oxygen zones associated with ocean basin eastern boundaries. It has been suggested that complex N2O cycling mechanisms in these regions may provide a net source to the oceanic interior and a significant portion of the ocean-atmosphere flux. In this study we evaluate the global significance of N2O formation in these regions. N2O is treated as a nonconserved tracer in an ocean general circulation model: a simple source function is developed which models N2O production as a function of organic matter remineralization and local oxygen concentration. Model results are evaluated against both surface and deep observational data sets. The oceanic oxygen minimum zones are predominantly found in the upperwater column of tropical latitudes and overlain by regions of strong upwelling in the surface ocean. Simulations of increased N2O production under low-oxygen conditions indicate that the majority of the N2O thus formed escapes directly to the atmosphere and is not subject to significant meridional transport. Results indicate that while enhanced N2O production in these regions cannot be held accountable for the majority of the sea-air flux and interior distribution, it may, however, have significance for the local distribution and provide as much as 25-50% of the global oceanic source.
Toggweiler, J R., and H Bjornsson, 2000: Drake Passage and palaeoclimate. Journal of Quaternary Science, 15(4), 319-328. Abstract PDF
The effect of Drake Passage on the Earth's climate is examined using an idealised coupled model. It is found that the opening of Drake Passage cools the high latitudes of the southern hemisphere by about 3°C and warms the high latitudes of the northern hemisphere by nearly the same amount. This study also attempts to determine whether the width and depth of the Drake Passage channel is likely to be an important factor in the thermal response. A deeper channel is shown to produce more southern cooling but the magnitude of the effect is not large. Channel geometry is relatively unimportant in the model because of a haline response that develops when the channel is first opened up.
Gnanadesikan, Anand, and J R Toggweiler, 1999: Constraints placed by silicon cycling on vertical exchange in general circulation models. Geophysical Research Letters, 26(13), 1865-1868. Abstract PDF
The flux of biogenic silica out of the oceanic mixed layer (the export flux) must balance the import of high-silicate deep waters associated with mass exchange between the surface and deep ocean. Recent regional estimates of the export flux limit it to 50-80 Tmol yr-1. In order to reproduce such low export fluxes, coarse-resolution general circulation models must have low pycnocline diffusivity and a lateral exchange scheme which involves advection of tracers by unresolved mesoscale eddies. Failure to capture low rates of vertical exchange will result in climate models which overpredict the uptake of anthropogenic carbon dioxide and fail to capture the proper locations and rates of density transformation.
Toggweiler, J R., 1999: Oceanography: An ultimate limiting nutrient. Nature, 400(6744), 511-512. PDF
Toggweiler, J R., 1999: Variation of atmospheric CO2 by ventilation of the ocean's deepest water. Paleoceanography, 14(5), 571-588. Abstract PDF
A new box model for glacial-interglacial changes in atmospheric CO2 produces lower levels of atmospheric CO2 without changes in biological production or nutrient chemistry. The model treats the boundary between middepth water and deep water as a chemical divide that separates low-CO2 water above from high-CO2 water below. Atmospheric CO2 is reduced 21 ppm by reduced ventilation of the deep water below the divide. A further reduction of 36 ppm is due to CaCO3 compensation in response to lower CO3= below the divide. Colder surface temperatures account for an additional 23 ppm of CO2 reduction. The new mechanism leaves the glacial atmosphere lighter in delta 13 C than in preindustrial time, as seen in ice cores and fossil plant material. Bottom water below the divide becomes strongly depleted in delta 13 C without a change in nutrient concentrations.
Schlosser, P, W M Smethie, Jr, and J R Toggweiler, 1998: Introduction to special section: Maurice Ewing Symposium on applications of trace substance measurements to Oceanographic problems. Journal of Geophysical Research, 103(C8), 15,815. PDF
Toggweiler, J R., and Bonita L Samuels, 1998: On the ocean's large-scale circulation near the limit of no vertical mixing. Journal of Physical Oceanography, 28(9), 1832-1852. Abstract PDF
By convention, the ocean's large-scale circulation is assumed to be a thermohaline overturning driven by the addition and extraction of buoyancy at the surface and vertical mixing in the interior. Previous work suggests that the overturning should die out as vertical mixing rates are reduced to zero. In this paper, a formal energy analysis is applied to a series of ocean general circulation models to evaluate changes in the large-scale circulation over a range of vertical mixing rates. Two different model configurations are used. One has an open zonal channel and an Antarctic Circumpolar Current (ACC). The other configuration does not. The authors find that a vigorous large-scale circulation persists at the limit of no mixing in the model with a wind-driven ACC. A wind-powered overturning circulation linked to the ACC can exist without vertical mixing and without much energy input from surface buoyancy forces.
The comment by Rahmstorf suggests that a numerical problem in Tziperman et al. (1994, TTFB) leads to a noisy E - P field that invalidates TTFB's conclusions. The authors eliminate the noise, caused by the Fourier filtering used in the model, and show that TTFB's conclusions are still valid. Rahmstorf questions whether a critical value in the freshwater forcing separates TTFB's stable and unstable runs. By TTFB's original definition, the unstable runs in both TTFB and in Rahmstorf's comment have most definitely crossed a stability transition point upon switching to mixed boundary conditions. Rahmstorf finally suggests that the instability mechanism active in TTFB is a fast convective mechanism, not the slow advective mechanism proposed in TTFB. The authors show that the timescale of the instability is, in fact, consistent with the advective mechanism
Chavez, F P., and J R Toggweiler, 1995: Physical estimates of global new production: The upwelling contribution In Dahlem Workshop on Upwelling in the Ocean: Modern Processes and Ancient Records, Chichester, UK, John Wiley & Sons, 313-320. Abstract
We have estimated the amount of nitrate available to support new production for component regions of the global ocean according to its physical supply. We differentiate new production between two components: one in which nitrate input is directly attributed to wind-driven upwelling and another in which nitrate input is supported by vertical mixing or other processes. Global new production according to our estimates and literature sources amounts to about 7.2 gigatons of carbon per year (GtCy-1). This is towards the lower end of the range or previous global estimates. The fraction supported directly by upwelling amounts to 4.8 GtCy-1, or 67% of the total.
Lampitt, R, and J R Toggweiler, et al., 1995: Group report: Does upwelling have a significant influence on the global carbon cycle? In Upwelling in the Ocean: Modern Processes and Ancient Records, Chichester, UK, John Wiley & Sons, 383-404. Abstract
Current interest and concern about the dynamics of the global carbon cycle led our group to an assessment of the influence of upwelling on this cycle. We suggested a number of relevant questions from which the following report is derived. In the first section, we define our terms of reference and establish what oceanographic processes should be considered as upwelling. Next we go on to outline the mechanisms that are likely to be significant in terms of the global carbon cycle and discuss the possible effects of changing the rates of upwelling, both on current atmospheric CO2 levels and on the levels predicted over the next 1000 years as a result of anthropogenic activity. Common themes throughout our discussion were the temporal and spatial scales of any particular process or characteristic, an aspect that is also discussed in a supporting paper (Watson, this volume). Within this is a discussion of the importance of the Redfield ratios of elements intimately connected to the carbon cycle. Finally, we outline areas of research that are required to increase confidence in our understanding of the influence of upwelling on the global carbon cycle.
Toggweiler, J R., 1995: Anthropogenic CO2: The natural carbon cycle reclaims center stage. Reviews of Geophysics, 33(Supplement), 1249-1252.
Toggweiler, J R., and S Carson, 1995: What are upwelling systems contributing to the ocean's carbon and nutrient budgets? In Upwelling in the Ocean: Modern Processes and Ancient Records, Chichester, UK, John Wiley & Sons, 337-360. Abstract PDF
Understanding the effect of upwelling systems on the carbon cycle requires detailed knowledge of how nutrients and carbon enter and leave these systems. In this article we use recent findings of the JGOFS equatorial Pacific process study and a detailed three-dimensional model to look specifically at the nitrate budget in the equatorial Pacific. Nitrate enters the equatorial upwelling system in the far-western Pacific via the Equatorial Undercurrent. Because the equatorial biota tend to recycle nitrogen much more effectively than they export nitrogen in sinking particles, nitrate stocks build up in the eastern Pacific. A significant fraction of the nitrate entering the upwelling system seems to be lost to denitrification in the anoxic zones off Peru and Central America. Through denitrification, the equatorial upwelling system may function as a regulator of global nitrate stocks and air-sea partitioning of CO2.
The Ekman divergence around Antarctica raises a large amount of deep water to the ocean's surface. The regional Ekman transport moves the upwelled deep water northward out of the circumpolar zone. The divergence and northward surface drift combine, in effect, to remove deep water from the interior of the ocean. This wind-driven removal process is facilitated by a unique dynamic constraint operating in the latitude band containing Drake Passage. Through a simple model sensitivity experiment we show that the upwelling and removal of deep water in the circumpolar belt may be quantitatively related to the formation of new deep water in the northern North Atlantic. These results show that stronger winds in the south can induce more deep water formation in the north and more deep outflow through the South Atlantic. The fact that winds in the southern hemisphere might influence the formation of deep water in the North Atlantic brings into question long-standing notions about the forces that drive the ocean's thermohaline circulation.
Brine rejection during the formation of Antarctic sea ice is known to enhance the salinity of dense shelf waters in the Weddell and Ross Seas. As these shelf waters flow off the shelves and descend to the bottom, they entrain ambient deep water to create new bottom water. It is not uncommon for ocean modelers to modify salinity boundary conditions around Antarctica in an attempt to include a "sea ice effect" in their models. However, the degree to which Antarctic salinities are enhanced is usually not quantified or defended.
In this paper, studies of shelf hydrography and delta18O are reviewed to assess the level of salinity enhancement appropriate for ocean general circulation models. The relevant quantities are 1) the salinity difference between the water masses modified on the shelves and the final offshelf flow and 2) the flux of salt (or freshwater) that gives rise to this salinity difference. Onshelf/offshelf salinity changes in the Weddell and Ross Seas appear to be fairly small, 0.15-0.20 salinity units. The quantity of brine needed to produce this salinification is equivalent to the salt drained from <0.50 m of new sea ice every year.
Salt fluxes and salinity distributions from three GCM simulations are then compared. The first model has its surface salinities simply restored to the Levitus observations. Levitus restoring produces a slight freshening in the area of the Weddell and Ross Sea shelves. The global-mean bottom-water salinity in this model is 34.57 psu, which 0.16 units less than observed. The second model includes a very modest salinity enhancement in the area of the Weddell and Ross Sea shelves. This produces a salt flux equivalent to the formation of ~ 0.50 m yr-1 of new sea ice. Even though this amount of salt input is close to the amount observed, global-average deep salinities in the second model are only 0.02 units greater than the deep salinities in the first model. The third model includes a large salinity enrichment, which is applied throughout the Weddell and Ross embayments without regard to water depth. Its deep salinities are 0.18 units higher than the deep salinities in the first model, but the amount of salt pumped into the model greatly exceeds the salt flux in nature.
The authors conclude that salt from sea ice is probably not a major influence on the salinity of Antarctic bottom waters. Predicted salinities in ocean GCMs are too fresh because of circulation deficiencies, not because of inadequate boundary conditions. Models that employ large salinity modifications near Antarctica run the risk of grossly distorting the processes of deep-water formation.
Each second millions of cubic meters of water circulate between the ocean's surface and its depths. Despite the crucial role this 'overturning' may play in regulating Earth's climate, we still do not know how vulnerable it is to anthropogenic environmental changes.
Tziperman, E, J R Toggweiler, Y Feliks, and Kirk Bryan, 1994: Instability of the thermohaline circulation with respect to mixed boundary conditions: Is it really a problem for realistic models?Journal of Physical Oceanography, 24(2), 217-232. Abstract PDF
A global primitive equations oceanic GCM and a simple four-box model of the meridional circulation are used to examine and analyze the instability of the thermohaline circulation in an ocean model with realistic geometry and forcing conditions under mixed boundary conditions. The purpose is to determine whether this instability should occur in such realistic GCMs.
It is found that the realistic GCM solution is near the stability transition point with respect to mixed boundary conditions. This proximity to the transition point allows the model to make a transition between the unstable and stable regimes induced by a relatively minor change in the surface freshwater flux and in the interior solution. Such a change in the surface flux may be induced, for example, by changing the salinity restoring time used to obtain the steady model solution under restoring conditions. Thus, the steady solution of the global GCM under restoring conditions may be either stable or unstable upon transition to mixed boundary conditions, depending on the magnitude of the salinity restoring time used to obtain this steady solution. The mechanism by which the salinity restoring time affects the model stability is further confirmed by carefully analyzing the stability regimes of a simple four-box model. The proximity of the realistic ocean model solution to the stability transition point is used to deduce that the real ocean may also be near the stability transition point with respect to the strength of the freshwater forcing.
Finally, it is argued that the use of too short restoring times in realistic models is inconsistent with the level of errors in the data and in the model dynamics, and that this inconsistency is a possible reason for the existence of the thermohaline instability in GCMs of realistic geometry and forcing. A consistency criterion for the magnitude of the restoring times in realistic models is formulated, that should result in steady states that are also stable under mixed boundary conditions. The results presented here may be relevant to climate studies that run an ocean model under restoring conditions in order to initialize a coupled ocean-atmosphere model.
Imbrie, J, and J R Toggweiler, et al., 1993: On the structure and origin of major glaciation cycles 2. The 100,000-year cycle. Paleoceanography, 8(6), 699-735. Abstract
Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (~15,000 years) much larger than the other cycles (~5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations.
Whether a glacier or ice sheet influences the climate depends very much on the scale....The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.
Najjar, R G., and J R Toggweiler, 1993: Reply to the comment by Jackson. Limnology and Oceanography, 38(6), 1331-1332. PDF
Sarmiento, Jorge L., Richard D Slater, M J Fasham, H W Ducklow, J R Toggweiler, and G T Evans, 1993: A seasonal three-dimensional ecosystem model of nitrogen cycling in the North Atlantic euphotic zone. Global Biogeochemical Cycles, 7(2), 417-450. Abstract
A seven-component upper ocean ecosystem model of nitrogen cycling calibrated with observations at Bermuda Station "S" has been coupled to a three-dimensional seasonal general circulation model (GCM) of the North Atlantic Ocean. The aim of this project is to improve our understanding of the role of upper ocean biological processes in controlling surface chemical distributions, and to develop approaches for assimilating large data sets relevant to this problem. A comparison of model predicted chlorophyll with satellite coastal zone color scanner observations shows that the ecosystem model is capable of responding realistically to a variety of physical forcing environments. Most of the discrepancies identified are due to problems with the GCM model. The new production predicted by the model is equivalent to 2 to 2.8 mol m-2 yr-1 of carbon uptake, or 8 to 12 GtC/yr on a global scale. The southern half of the subtropical gyre is the only major region of the model with almost complete surface nitrate removal (nitrate<0.1 mmol m-3). Despite this, almost the entire model is nitrate limited in the sense that any addition of nitrate supply would go predominately into photosynthesis. The only exceptions are some coastal upwelling regions and the high latitudes during winter, where nitrate goes as high as ~10 mmol m-3 .
Toggweiler, J R., and James C Orr, 1993: Summary of Workshop on Dissolved Organic Carbon in the Ocean In The Global Carbon Cycle, NATO ASI Series - Vol. 115. Heidelberg, Germany, Springer-Verlag, 583-584.
Toggweiler, J R., and Bonita L Samuels, 1993: New radiocarbon constraints on the upwelling of abyssal water to the ocean's surface In The Global Carbon Cycle, NATO ASI Series - Vol. 115. Heidelberg, Germany, Springer-Verlag, 333-366. Abstract
The output from seven different ocean model simulations is compared on the basis of the Δ 14C difference between North Pacific deep water and Antarctic surface water. This set of models produces a range of North Pacific-Antarctic Δ 14C differences between -173% and -108%, all but the smallest of which are substantially larger than the actual pre-bomb difference, -80 to -110%. Predicted values are highly correlated with the quantity of mid-depth water which flows out of the Pacific to the south. A circulation in which most of the Antarctic bottom water flows back out of the basin at mid-depth produces the smallest North Pacific-Antarctic Δ 14C differences, whereas a circulation in which all the inflow of bottom water upwells through the thermocline produces the largest and least realistic differences. According to the models, upwelled abyssal water becomes entrained into the wind-driven convergence of thermocline water toward the equator. When it reaches the surface it spreads to the north and south, producing a Δ 14C minimum along the equator. A detailed analysis of both pre-bomb and post-bomb Δ 14C data indicates that the oldest water in the tropical Pacific is actually found south of the equator and is associated with the upwelling off Peru, not the upwelling along the equator. Toggweiler et al. (1991) trace the low-Δ 14C signal in the Peru upwelling to deep water raised to the surface around Antarctica which is pushed northward into the thermocline by circumpolar winds. According to the models, even a small amount of abyssal water upwelling through the thermocline (~3x106 m3 s-1) leaves a characteristic signal in the surface Δ 14C distribution which is not observed. One is left with the general conclusion that there is very little upwelling associated with a top-to-bottom thermohaline circulation in the world ocean. Virtually all the upwelling of abyssal water to the ocean's surface occurs around Antarctica where it is mainly wind-forced. The implications of this conclusion for the carbon cycle are discussed.
Toggweiler, J R., and Bonita L Samuels, 1993: Is the magnitude of the deep outflow from the Atlantic Ocean actually governed by Southern Hemisphere winds? In The Global Carbon Cycle, NATO ASI Series - Vol. 115. Heidelberg, Germany, Springer-Verlag, 303-331. Abstract
The large-scale overturning in the Atlantic Ocean and its export of salty water to the other basins of the ocean is usually thought of as a thermohaline process driven by the formation of dense bottom water in the isolated basins of the North Atlantic. In this paper the output from several different runs of a global ocean GCM is used to show that the inflow of upper kilometer water in the South Atlantic and the outflow of deep water varies in direct proportion to the westerly wind stress in the circumpolar region of the southern hemisphere. According to the results presented here, the production of dense bottom water in the North Atlantic makes it possible for an Atlantic overturning to exist, but southern hemisphere winds appear to determine the magnitude of the inflow and outflow. The connection between southern hemisphere winds and the Atlantic overturning is due to a unique dynamic constraint which operates in the latitude band of Drake Passage. This constraint suggests the possibility of a very simple relationship between the magnitude of the northward wind drift at the latitude of the tip of South America and the magnitude of the inflow and outflow from the Atlantic basin.
Imbrie, J, and J R Toggweiler, et al., 1992: On the structure and origin of major glaciation cycles. 1. Linear responses to Milankovitch forcing. Paleoceanography, 7(6), 701-738. Abstract
Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind-driven and density-driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,00 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000- and 41,000-year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end-member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertical constants in a chain of four subsystems.
Murray, J W., M W Leinen, Richard A Feely, J R Toggweiler, and R Wanninkhof, 1992: EqPac: A process study in the central equatorial Pacific. Oceanography, 5(3), 134-142.
Najjar, R G., Jorge L Sarmiento, and J R Toggweiler, 1992: Downward transport and fate of organic matter in the ocean: Simulations with a general circulation model. Global Biogeochemical Cycles, 6(1), 45-76. Abstract
A phosphorous-based model of nutrient cycling has been developed and used in conjunction with a general circulation model to evaluate the roles of the dissolved and sinking particulate phases in the downward transport of organic matter in the ocean. If sinking particles dominate the downward transport and remineralize in accord with observations made primarily with sediment traps, we find in equatorial upwelling regions that particle fluxes and thermocline nutrient concentrations are higher than observed. These enhanced fluxes and concentrations are a result of what we term "nutrient trapping," a positive feedback whereby high upwelling produces high new production that results in remineralization and enhanced nutrient concentrations in the upwelling water, which further increases new production. Nutrient trapping in shallow upwelling zones can be eliminated by increasing the particle flux length scale, which suggests that if sinking particles dominate the downward transport of organic matter then the flux length scale is longer than observed. Even with a longer particle flux length scale, we find that nutrients are trapped in some deep convective regions of the southern ocean, where new production is predicted to be much higher than the observed primary production. In simulations where the downward transport of organic matter takes place primarily in a dissolved phase, nutrient trapping is completely eliminated, in both upwelling and convective regions. The models with dissolved organic matter also agree fairly well with nutrient transports in the north Atlantic Ocean calculated from observed nutrient and hydrographic data (Rintoul and Wunsch, 1991). Our results therefore support the dissolved organic nitrogen and carbon measurements made with the high-temperature combustion technique of Suzuki, et al. (1985) and Sugimura and Suzuki (1988) and suggest that there exists an as-yet undiscovered pool of dissolved organic phosphorous in the ocean. We also use the various models to make an estimate of global new production of 2.9 to 3.6 mol C/m2/yr (12 to 15 Gt C/yr).
Toggweiler, J R., 1992: Catalytic conversions. Nature, 356(6371), 665-666.
Toggweiler, J R., Keith W Dixon, and W Broecker, 1991: The Peru upwelling and the ventilation of the South Pacific thermocline. Journal of Geophysical Research, 96(C11), 20,467-20,497. Abstract PDF
A reconstruction of the prebomb Δ 14C distribution in the tropical Pacific using data from old coral heads shows that surface waters with the lowest Δ 14C content are found distinctly south of the equator. Prebomb, low-Δ 14C surface water appears to owe its origin to the upwelling of ~15°C water off the coast of Peru. The low-Δ 14C water upwelling off Peru is shown to be derived from the "13° Water" thermostad (11° - 14°C) of the Equatorial Undercurrent. Untritiated water in the lower part of the undercurrent had nearly the same Δ 14C content during the Geochemical Ocean Sections Study (GEOSECS) as the prebomb growth bands in Druffel's (1981) Galapagos coral. Similar Δ 14C levels were observed in 9° - 10°C water in the southwest Pacific thermocline in the late 1950s. We suggest that the low-Δ 14C water upwelling off Peru and the thermostad water in the undercurrent both originate as ~8°C water in the subantarctic region of the southwest Pacific. This prescription points to the "lighter variety" of Subantarctic Mode Water (7° - 10°C) as a possible source. Because prebomb Δ 14C is so weakly forced by exchange of carbon isotopes with the atmosphere, thermocline levels of Δ 14C should be particularly unaffected by diapycnal mixing with warmer overlying water types. We argue that successively less dense features of the South Pacific thermocline, like the Subantarctic Mode Water, the equatorial 13°C Water, and the Peru upwelling, may be part of a single process of thermocline ventilation. Each evolves from the other by diapycnal alteration, while prebomb Δ 14C is nearly conserved. Detailed comparisons are made between the coral Δ 14C distribution and a model simulation of radiocarbon in Toggweiler et al. (1989). While the Δ 14C data suggest a southern hemisphere thermocline origin for the equatorial Δ 14C minimum, the model produces its Δ 14C minimum by upwelling abyssal water to the surface via the equatorial divergence. In an appendix to the paper we present a new set of coral Δ 14C measurements produced over the last 10 years at Lamont-Doherty Geological Observatory and compile a post -1950 set of published coral Δ 14C measurements for use in model validation studies.
Toggweiler, J R., 1990: Bombs and ocean carbon cycles. Nature, 347, 122-123. PDF
Sarmiento, Jorge L., M J Fasham, U Siegenthaler, R G Najjar, and J R Toggweiler, 1989: In Models of Chemical Cycling in the Oceans: Progress Report II, Ocean Tracers Laboratory Report #6, Princeton, NJ, Princeton University, 46 pp. Abstract
In a previous progress report (Toggweiler, et al., 1987) we argued that the most difficult obstacle that needed to be overcome in developing predictive, dynamical, 3-D models of geochemical cycling in the oceans was to develop approaches for simulating the role of biological processes. In this report we update our progress on developing an ecosystem-level description of upper ocean fluxes and on simulating the penetration of anthropogenic CO2 into the ocean. if the natural carbon cycle and ocean circulation are in steady state, one needs to know only the pre-anthropogenic surface total carbon and alkalinity to predict the uptake of fossil CO2 by the oceans. As a first simple approximation, we fix the surface alkalinity at a constant value of 2300 ueq kg-1, and fix the pre-anthropogenic surface total carbon to the value that gives the pre-anthropogenic pCO2 of 280 ppm everywhere. This neglects details of the natural cycles of CO2 due to temperature as well as biology that give rise to non-equilibrium pre-anthropogenic pCO2 levels over much of the ocean. Several fossil CO2 uptake experiments have been performed with this approach, both with 3-D ocean circulation models and with a new box model that incorporates features not included in previous box models. Another approach we are working on is a determination of the pre-anthropogenic surface total carbon and alkalinity based on the observed surface nutrient distributions and an assumed Redfield stochiometry. This approach gives us the concentration of pre-anthropogenic total carbon and alkalinity that we need for the steady state simulations of fossil CO2 uptake discussed above. It also provides a simple way of simulating the effects of biology and temperature in the euphotic region of the ocean, allowing us to put major emphasis on processes occurring below the euphotic zone. Our simulations of processes below the euphotic zone suggest an important role for substances not caught in sediment traps, such as dissolved organic matter. We have made considerable progress on the development of ecosystem models of the upper ocean and have performed a first experiment incorporating these models into a 3-D ocean circulation model of the North Atlantic. Such models are necessary for predicting how the atmospheric pCO2 will be affected should the ocean circulation and biology begin to change in response to a greenhouse climate.
Toggweiler, J R., 1989: Are rising and falling particles microbial elevators?Nature, 337, 691-692. PDF
Toggweiler, J R., 1989: Is the downward dissolved organic matter (DOM) flux important in carbon transport? In Productivity of the Ocean: Present and Past, Chichester, UK, John Wiley & Sons, 65-83. Abstract PDF
A new method for measuring the concentration of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in seawater has recently been applied to the study of the material balance in the oceanic water column. These measurements suggest that the downward transport of organic carbon and nitrogen in the dissolved organic phase is every bit as important as the downward transport in sinking particles. It appears that DOC and DON are the most important organic substrates supporting the consumption of oxygen and the remineralization of nitrate below the thermocline. Although still controversial, these findings are supported by a model study which shows that the vertical transport of organic matter cannot be attributed solely to the fast sinking particles caught in sediment traps. A characterization of the vertical flux as such produces a model nutrient distribution which bears little resemblance to observed distributions.
Toggweiler, J R., Keith W Dixon, and Kirk Bryan, 1989: Simulations of radiocarbon in a coarse-resolution world ocean model 1. Steady state prebomb distributions. Journal of Geophysical Research, 94(C6), 8217-8242. Abstract PDF
This paper presents the results of five numerical simulations of the radiocarbon distribution in the ocean using the Geophysical Fluid Dynamics Laboratory primitive equation world ocean general circulation model. The model has a 4.5 degree latitude by 3.75 degree longitude grid, 12 vertical levels, and realistic continental boundaries and bottom topography. The model is forced at the surface by observed, annually averaged temperatures, salinities, and wind stresses. There are no chemical transformations or transport of 14C by biological processes in the model. Each simulation in this paper has been run out the equivalent of several thousand years to simulate the natural, steady state distribution of 14C in the ocean. In a companion paper the final state of these simulations is used as the starting point for simulations of the ocean's transient uptake of bomb-produced 14C. The model reproduces the mid-depth 14C minimum observed in the North Pacific and the strong front near 45 degrees S between old, deep Pacific waters and younger circumpolar waters. In the Atlantic, the model's deep 14C distribution is much too strongly layered with relatively old water from the Antarctic penetrating into the northern reaches of the North Atlantic basin. Two thirds of the decay of 14C between 35 degrees S and 35 degrees N is balanced by local 14C input from the atmosphere and downward transport by vertical mixing (both diffusion and advective stirring). Only one third is balanced by transport of 14C from high latitudes. A moderately small mixing coefficient of 0.3 cm2 s-1 adequately parameterizes vertical diffusion in the upper kilometer. Spatial variation in gas exchange rates is found to have a negligible effect on deepwater radiocarbon values. Ventilation of the circumpolar region is organized in the model as a deep overturning cell which penetrates as much as 3500 m below the surface. While allowing the circumpolar deep water to be relatively well ventilated, the overturning cell restricts the ventilation of the deep Pacific and Indian basins to the north. This study utilizes three different realizations of the ocean circulation. One is generated by a purely prognostic model, in which only surface temperatures and salinities are restored to observed values. Two are generated by a semidiagnostic model, in which interior temperatures and salinities are restored toward observed values with a 1/50 year-1 time constant. The prognostic version is found to produce a clearly superior deep circulation in spite of producing interior temperatures and salinities which deviate very noticeably from observed values. The weak restoring terms in the diagnostic model suppress convection and other vertical motions, causing major disruptions in the diagnostic model's deep sea ventilation.
Toggweiler, J R., Keith W Dixon, and Kirk Bryan, 1989: Simulations of radiocarbon in a coarse-resolution world ocean model 2. Distributions of bomb-produced carbon 14. Journal of Geophysical Research, 94(C6), 8243-8264. Abstract PDF
Part 1 of this study examined the ability of the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation ocean general circulation model to simulate the steady state distribution of naturally produced 14C in the ocean prior to the nuclear bomb tests of the 1950s and early 1960s. In part 2 we begin with the steady state distributions of part 1 and subject the model to the pulse of elevated atmospheric 14C concentrations observed since the 1950s. This study focuses on the processes and time scales which govern the transient distributions of bomb 14C in the upper kilometer of the ocean. Model projections through 1990 are compared with observations compiled by the Geochemical Ocean Sections Study (GEOSECS) in 1972, 1974, and 1978; the Transient Tracers in the Ocean (TTO) expedition in 1981, and the French INDIGO expeditions in 1985-1987. In their analysis of the GEOSECS 14C observations, Broecker et al. (1985) noted that much of the bomb 14C which entered the ocean's equatorial belts prior to GEOSECS accumulated in the adjacent subtropical zones. Broecker et al. argued that this displacement of bomb 14C inventories was caused by the wind-driven upwelling and surface divergence in the tropics combined with convergent flow and downwelling in the subtropics. Similar displacements were invoked to shift bomb 14C from the Antarctic circumpolar region into the southern temperate zone. The GFDL model successfully reproduces the observed GEOSECS inventories, but then predicts a significantly different pattern of bomb 14C uptake in the decade following GEOSECS. The post-GEOSECS buildup of bomb 14C inventories is largely confined to the subthermocline layers of the North Atlantic, the lower thermocline of the southern hemisphere, and down to 2000 m in the circumpolar region. A great deal of attention is devoted to detailed comparisons between the model and the available radiocarbon data. A number of flaws in the model are highlighted by this analysis. The Subantarctic Mode Waters forming along the northern edge of the circumpolar current are identified as a very important process for carrying bomb 14C into the thermoclines of the southern hemisphere. The model concentrates its mode water formation in a single sector of the circumpolar region and consequently fails to form its mode waters with the correct T-S properties. The model also moves bomb 14C into the deep North Atlantic and deep circumpolar region much too slowly.
We examine the hypothesis that global scale episodes of anoxia such as occurred in the Cretaceous are due to high productivity and/or stagnation of the circulation. Two modes of ocean circulation are considered: a thermohaline overturning cell, essentially vertical, which involves global scale upwelling into the surface followed by sinking in deep water formation regions; and an approximately horizontal cell which connects the abyss directly with deeply convecting waters in deep water formation regions. Modern analogs for these processes are formation of North Atlantic Deep Water and Antarctic Bottom Water, respectively. Over most of the oceans the surface new production is nutrient limited and thus directly proportional to the supply of nutrients by the vertical overturning cell. A reduction in oxygen can only be brought about by increased vertical overturning associated with increased production. In addition, the model shows that as the deep ocean becomes lower in oxygen, the sensitivity of the oxygen levels to the meridional circulation decreases such that it becomes difficult or impossible to achieve.
We examine the causes of anoxia in regions such as the Eastern Mediterranean, which have exchange over sills with adjacent basins. Box models show that the concentration of the limiting nutrient is the major determinant of deep oxygen levels. The most effective way of increasing nutrient concentrations to the point where anoxia occurs is to change the flow pattern across the sills ventilating the basins. With a sill exchange pattern such as that in the present Strait of Sicily, it is difficult to obtain anoxia in the Eastern Mediterranean without also driving the Western Mediterranean to low oxygen and high nutrient levels. Episodes of anoxia in the Eastern Mediterranean are associated with a freshening of surface waters. A reversal in flow directions, presumably resulting from the observed freshening, will inevitably lead to anoxia associated with increased sediment burial rates of the limiting nutrient and will leave the Western Mediterranean largely unaffected, in keeping with the observational evidence.
Sarmiento, Jorge L., J R Toggweiler, and R G Najjar, 1988: Ocean carbon-cycle dynamics and atmospheric pCO2. Philosophical Transactions of the Royal Society of London, A, 325, 3-21. Abstract
Mechanisms are identified whereby processes internal to the oceans can give rise to rapid changes in atmospheric pCO2. One such mechanism involves exchange between the atmosphere and deep ocean through the high-latitude outcrop regions of the deep waters. The effectiveness of communication between the atmosphere and deep ocean is determined by the rate of exchange between the surface and deep ocean against the rate of biological uptake of the excess carbon brought up from the abyss by this exchange. Changes in the relative magnitude of these two processes can lead to atmospheric pCO2 values ranging between 165 p.p.m. (by volume) and 425 p.p.m. compared with a pre-industrial value of 280 p.p.m. Another such mechanism involves the separation between regeneration of alkalinity and total carbon that occurs in the oceans because of the fact that organic carbon is regenerated primarily in the upper ocean whereas CaCO3 is dissolved primarily in the deep ocean. The extent of separation depends on the rate of CaCO3 formation at the surface against the rate of upward mixing of deep waters. This mechanism can lead to atmospheric values in excess of 20000 p.p.m., although values greater than 1100 p.p.m. are unlikely because calcareous organisms would have difficulty surviving in the undersaturated surface waters that develop at this point. A three-dimensional model that is being developed to further study these and other problems provides illustrations of them and also suggests the possibility that there is a long-lived form of non-sinking carbon playing a major role in carbon cycling.
Broecker, W, W C Patzert, J R Toggweiler, and M Stuiver, 1986: Hydrography, chemistry, and radioisotopes in the southeast Asian basins. Journal of Geophysical Research, 91(C12), 14,345-14,354. Abstract PDF
Nutrient constituent, radiocarbon, and radium data are presented for the waters in the Southeast Asian basins. These results suggest very rapid ventilation by waters passing over the sills that separate these basins from one another and from the open Pacific.
Sarmiento, Jorge L., and J R Toggweiler, 1986: A preliminary model of the role of upper ocean chemical dynamics in determining oceanic oxygen and atmospheric carbon dioxide levels In Dynamic Processes in the Chemistry of the Upper Ocean, Plenum Press, 233-240. Abstract
A first version is presented of equations for a three-dimensional model of nutrient and carbon cycling in the oceans. An analytical solution of these equations has been obtained for a one-and-a-half-dimensional "pipe" model. This solution shows that atmospheric CO2 can be varied by changing the level of preformed nutrients. It is suggested that this mechanism may explain the lower pCO values of the last ice age.
Toggweiler, J R., and Jorge L Sarmiento, 1985: Glacial to interglacial changes in atmospheric carbon dioxide: The critical role of ocean surface water in high latitudes In The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present, Geophysical Monograph 32, Washington, DC, American Geophysical Union, 163-184. Abstract PDF
Recent measurements of the CO2 content of air bubbles trapped in glacial ice have shown that the partial pressure of atmospheric COsub>2 during the last ice age was baout 70 ppm lower than during the interglacial. Isotopic measurements on surface- and bottom-dwelling forams living during the ice age have shown that the 13C/12C gradient between the ocean's surface and bottom layers was 25% larger during the last ice age than at present. Broecker (1982) proposed that an increase in the phospate content of the deep sea could explain these observations. We follow up here on a proposal by Sarmiento and Toggweiler (1984) that glacial to interglacial changes in PCO2 are related to changes in the nutrient content of high-latitude surface water. We develop a four-box model of the ocean and atmosphere which includes low- and high-latitude surface boxes, an atmosphere, and a deep ocean. In simplest form the model equations show that the CO2 content of high-latitude surface water is directly connected to the huge reservoir of CO2 in deep water through the nutrient content of high-latitude surface water. The relationship between the CO2 content of low latitude surface water and the deep sea is more indirect and depends to a large extent on transport of CO2 through the atmosphere from high latitudes. We illustrate how the 14C content of the atmosphere and that of high-latitude surface water constrain model solutions for the present ocean and how ice age 13C observations constrain ice age parameters. We propose that the low ice age PCO2 can be produced by a reduction in local exchange between high-latitude surface water and deep water. The model requires that the current exchange rate of about 50 Sv be reduced to about 10 Sv. We review evidence in the geologic record for widespread changes in deep convection around Antarctica about 14,000 years ago which are synchronous with the change in atmospheric PCO2.
Toggweiler, J R., and S Trumbore, 1985: Bomb-test 90Sr in Pacific and Indian Ocean surface water as recorded by banded corals. Earth and Planetary Science Letters, 74, 306-314. Abstract
We report here measurements of bomb-test 90Sr activity in the CaCO3 skeletons of banded head forming corals collected from nine locations in the tropical Pacific and Indian Oceans. Density variations in skeletal carbonate demarcate annual growth bands and allow one to section individual years. Measurements of 90Sr activity in the annual bands reconstruct the activity of the water in which the coral grew. Our oldest records date to the early years of the nuclear era and record not only fallout deposition from the major U.S. and Soviet tests of 1958-1962, but also the huge, and largely unappreciated, localized inputs from the U.S. tests at Eniwetok and Bikini atolls during 1952-1958. In the 1960's, the 90Sr activity in Indian Ocean surface water was twice as high as activity levels in the South Pacific at comparable latitudes. We suggest that substantial amounts of northern hemisphere fallout moved west and south into the Indian Ocean via passages through the Indonesian archipelago. Equatorial Pacific 90Sr levels have remained relatively constant from the mid 1960's through the end of the 1970's in spite of 90Sr decay, reflecting a large-scale transfer of water between the temperate and tropical North Pacific. Activity levels at Fanning Is. (4 degrees N, 160 degrees W) appear to vary in conjunction with the 3-4 year El Niño cycle.
Sarmiento, Jorge L., and J R Toggweiler, 1984: New model for the role of the oceans in determining atmospheric PCO2. Nature, 308(5960), 621-624.
