Could Tropical Hurricanes Melt the Polar Ice Caps?

It has long been speculated that increased sea surface temperatures may result in an increased frequency of tropical storms (i.e. climate influences storms). However, according to a recent study published in Nature (Observational evidence for an ocean heat pump induced by tropical cyclones; 447, 577pp), the opposite may also be true: local storms mitigating changes in global climate.

Work by Ryan L. Sriver and Matthew Huber at the Purdue Climate Change Research Center, using a computer model simulation, illustrated that tropical cyclones (hurricanes) can mix up to 15% of the global ocean heat budget below the upper ocean. What significance might this have on global climate?

A more apt question is the following. Why is ocean mixing and circulation important to global climate? The ocean’s are responsible for moving heat the equator to the poles, via thermohaline circulation; in essence, a conveyer belt driven by differential gradients of heat and salt (eg. cold, salty – dense – water sinks, pushes water away; surface water replaces sinking water to maintain a mass balance of water molecules). Although other forms of mixing are proving to be important (tides are more important than you might think), no one seriously considered tropical storms to exert a large enough influence on ocean mixing occurring over depths of thousands of meters.

This might not be the case, and to quote the author’s final paragraph, “Our analysis suggests that changes in global cyclone frequency, duration and/or intensity are closely related to the amount of heat pumped into—and available to be subsequently transported by—the oceans. This relationship may have implications for changes in heat transport associated with past and future climate change. Extrapolation of our results suggests that future increases in tropical temperatures may result in increased dissipation, mixing, heat storage, and eventually heat transport. Moreover, this positive response in transport might feed back on climate by redistributing heat poleward, diminishing the Equator-to-pole temperature gradient, and raising global mean temperature. We have provided some evidence that cyclone-induced mixing is a fundamental physical mechanism that may act to stabilize tropical temperatures, mix the upper ocean, and cause polar amplification of climate change.”

So what are hurricanes doing exactly? They are helping to dissipate energy from the tropics to the poles, assumedly at an increased rate as sea surface temperatures rise in the tropics, to help maintain the regional climate. Although it as equally interesting as it is bizarre to think of the Earth’s climate as “doing something about climate change”, but lets avoid anthropomorphizing it; there is a natural climate state and natural mechanisms to compensate for perturbations to the system.

What I found particularly scary about the author’s final paragraph were the last two sentences. In essence, this system could act as a positive feedback on polar ice cap melting, depending on how important it truly is on ocean mixing (15% is the upper limit, and others argue it might not be important at all). Nevertheless, some more somber, sobering science on our climate.