Ocean Color Can Steer Pacific Tropical Cyclones

Anand Gnanadesikan, NOAA GFDL

For all we talk and sing about the “ocean blue”, the crystal-clear waters one sees in pictures of tropical resorts are actually rare.  The real ocean contains all kinds of materials that absorb and scatter light, often giving it a greenish hue. Recent work done in our lab has shown that this fact has a major impact on the distribution of hurricanes, typhoons and tropical storms, known collectively as tropical cyclones.

The oceanic subtropical gyres are vast high pressure circulations found in all ocean basins. In the North Atlantic the subtropical gyre flows in a clockwise direction and is bounded by the fast Gulf Stream on its western edge. In the Pacific a similar circulation is bounded by the Kuroshio current along the coast of Taiwan and Japan.  Because the subtropical gyres are regions where surface waters are pushed downwards by the prevailing winds, they tend not have a lot of the nutrients needed to allow for the growth  of marine plants. As a result, the water in the gyres (the blue regions in the picture at left) is relatively clear.  Mixing from winds and horizontal currents  do allow for some plant growth in these waters, however. The result is that sunlight falls off with depth about twice as quickly in the gyres as it would be in pure water.

Along with Whit Anderson, I’ve been examining the implications of this additional absorption for various phenomena, including ocean circulation and El Nino. We noticed that a number of the changes we were finding had been associated with hurricane formation by Gabe Vecchi and Kerry Emanuel among others.  In a paper that is currently in press in Geophysical Research Letters, a journal of the American Geophysical Union, we team up to show that the additional absorption in the gyres would have a big impact of the paths, numbers, and intensity of typhoons in the North Pacific.

How does this work? In perfectly blue ocean water, sunlight would penetrate deeper into the ocean. In the gyres, this heat would be taken away by subsurface currents, and so the surface would become cooler. Cooler surface waters would tend to produce less intense hurricanes.

Additionally, the atmosphere would then respond to these cooler surface temperatures, with air sinking over such regions and flowing towards the warmer equator. This circulation would have two main impacts- the sinking air over the subtropics is dry, which inhibits hurricane formation, and the air drawn in to replace it from the equator will move to east, as it carries with it faster motion of the equator. Both of these significantly damp the formation of tropical cyclones. Nearer to the equator, however, rising air would increase the humidity of the atmosphere and make the region more favorable to these storms.

We find that without this additional absorption in the North Pacific gyre, the region would serve as a barrier to typhoon formation and propagation. For regions like  South China, Taiwan, and Japan, the result would be a 70% decrease in typhoon activity. However, the Philippines and Vietnam would see about 20% more tropical cyclones. Such huge shifts are much larger than what recent studies have predicted would occur as a result of global warming.

Is this result relevant to changes in hurricane activity over the past 50 years? It’s hard to say. Some investigators have argued that the concentration of chlorophyll over the North Pacific has doubled in the past fifty years, while others have argued that the water in this regions is actually getting clearer.

What is clear from this work is that changes in ocean color can’t be ignored if we want to understand how hurricanes and typhoons will change as the climate changes.

Click here to access the manuscript that is in press.

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