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Climate change and tropical rainfall

Written by Matthew R. Igel, SoGES 2013-2014 Sustainability Leadership Fellow, and PhD Candidate in the Department of Atmospheric Science

One of the most incredible aspects of life in the tropics is the shear force with which raindrops seem to fall. As if shot out of a cannon with the specific aim of teaching a painful lesson that won’t soon be forgotten to silly humans attempting to feebly take shelter under a palm tree, raindrops from tropical thunderstorms are quite impressive already. But what will happen to these raindrops, or more precisely their number, with a warming climate?

This question is one that has been on the minds of atmospheric scientists in the last decade. I am just now sitting down to write this blog entry after attending a colloquium during which the idea that tropical rainfall might be responsible for events occurring in the Arctic was discussed, and, as so often occurs, the subsequent discussion became one on trends in tropical rainfall. And the reason this question is so common a refrain is that understanding how and why precipitation might change under climate change scenarios is crucial to the lives and livelihoods of populations living in the tropics. Local populations in the tropics have developed a dependence on frequent, but not too frequent, heavy, but not too heavy, rainfall. There are two ways in which this balance could be broken. Either the frequency or intensity of rainfall could change.  And, unfortunately, climate change is expected to change both.

As the climate warms, a myriad of changes will probably occur to the tropical atmosphere. The warmer atmosphere will be able to “hold” higher amounts of water vapor than it can today. So, the same storm in a warmer climate would rain more than it would in a cooler one. Also where it rains is expected to change. Today, much of the rain in the tropics is confined to narrow bands around the equator. Due to a variety of influences, these bands are expected to shrink in a warmer climate. Since the atmosphere has to rain a certain amount each day to maintain a constant water cycle, and because this rain will be confined to a smaller area, that effect too will create stronger rain rates.  Combined, these predicted changes portend bad news for many vulnerable populations in the tropics. “Extreme” rainfall will get heavier and “normal” rainfall will become less frequent. The situation is often referred to as the “Rich-get-Richer” response.  This name reflects the nature of regions receiving an abundance of precipitation being predicted to get more.

Traditional global circulation models (GCMs) that climate scientists use to make predictions about climate change have been predicting a “Rich-get-Richer” response to global climate change for some time.  However, these models lack the ability to simulate individual thunderstorms. Recently, work with special, high-resolution models has been conducted to understand how individual thunderstorms will respond to climate change. These simulations have revealed an even grimmer picture of changes to rainfall.  What they have shown is that groups of thunderstorm conspire to exacerbate the problem. It seems that clouds will group together in new ways to pour down rain on rainy areas at the expense of the drier ones. The strongest thunderstorms appear to get even stronger. Together with the results of the large-scale weather well simulated by the GCMs, there is reason for concern.

The “Rich-get-Richer” and thunderstorm difference mechanisms only account for the daily variations in rainfall. Tropical rainfall is also influenced by longer time scale phenomena. Both monthly and yearly time scales see broad changes in rainfall. On monthly scales the so-called Madden-Julian Oscillation (the MJO), a broad region of storminess that slowly moves eastward along the equator, can enhance or deter rainfall. Very recent work has suggested that MJOs may become harder to predict with greater time between rain events.  The events that do occur might result in heavier rainfall. On yearly timescales, hurricanes come and go. But with climate warming, the hurricanes that come will likely be less frequent and more intense than those we see today.

Anecdotally, some of these changes have already been seen. Regions like Australia and the Indian subcontinent have seen changes in the frequency and intensity of their rainfall. The Indian monsoon, a seasonal rain pattern, used to be amazingly consistent year-to-year, and therefore, predictable. Now monsoon onset occurs at a more variable time and monsoon rains are often heavy enough to flood. Australia too has seen drought and flooding with unfamiliar regularity.

Climate warming will likely require adaptation to new weather. While those in the tropics are generally spared the worst effects, they will have to adapt to rising sea levels and, unfortunately, it seems, changes in the rainfall that is so vital to their everyday life.

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