21 September 2010

Tiny plants and hurricanes: a question

Posted by vivienne

Anne Jefferson (@highlyanne) who blogs at Highly Allochthonous had a question about my recent blogpost “Can tiny marine plants steer some of the world’s biggest storms?“, which reviewed a paper published in the academic journal Geophysical Research Letters.

In my blogpost, I explained how the authors of the paper used a climate model to see whether hurricanes in the North Pacific were affected by removing the plankton – tiny marine plants – from one of the world’s five big ‘gyres’ or circular ocean currents. Removing the plankton, I wrote, prevented hurricanes from travelling as far north into the East China Sea. Instead, hurricanes tended to track along the equator.

My blogpost also described how plankton affected the hurricanes by changing ocean temperatures. Plankton at the ocean surface absorbed lots of the sun’s energy, I explained, warming the surface water and preventing heat reaching deeper waters below. When the plankton were removed, the sun’s heat could penetrate further into and warm the deeper ocean.

Anne felt there was something missing from this explanation. Hurricanes need warm surface and deep water to form. So why weren’t the deep warm waters in the plankton-free areas as good at generating hurricanes as the plankton-rich surface waters?

AGU science writer Colin Schultz (@_ColinS_), who had interviewed the paper’s lead author Anand Gnanadesikan, offered to help answer her question. He explained:

As for your first question, during the interview Gnanadesikan said;

“Gnanadesikan: The reason we focused on the gyres is that we found as part of this research that you can get very different results depending on where you remove the chlorophyll. So it turns out that if you’re in the Gyre…. As we talked about earlier, the basic mechanism is that you’ve got clear water, resulting in cooling at the surface and warming at depth. In the gyres that warmer water gets carried away, and so you’re left with cooling at the surface. In other parts of the ocean that’s not true. That’s really, though we don’t talk about it in this paper, that’s one of the main results of this line of research which is why we focused on the gyres. The surprise then that comes out is, the gyres are already pretty clear, and so were showing that even in these regions that are apparently clear, biologically mediated heating is important.” (emphasis added)

I’m not a specialist in this topic by any means, but from what I gathered from talking to Gnanadesikan, this result is partly due to the unique deep ocean currents which exist in the gyre which transport the deeper water away, effectively leaving the sub-surface cool as well. And, as Gnanadesikan mentioned, this won’t be the case for all parts of the ocean.

A news story in New Scientist about the paper says something similar:

That deeper heat is carried away from the gyre by its associated deep currents, creating a region that is cooler than the rest of the tropics. There are no such currents to carry away the warmer waters of the murky regions.