18 September 2012
Climatologists estimate extreme rainfall events in tropics
by Will Parker
Thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones will all become more likely in the tropics; suggest MIT projections that show 10 percent heavier rainfall extremes for every 1 degree Celsius rise in temperature. The new estimates, appearing in the journal Nature Geoscience, are based on model simulations and decades of observations.
Most of the possible consequences of a warming climate are still, er, hotly debated, but its effects on rainfall are relatively well-understood. As carbon dioxide and other greenhouse gases enter the atmosphere, they increase the temperature, which in turn leads to increases in the amount of water vapor in the atmosphere. When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.
MIT's Paul O'Gorman said that while most existing precipitation simulations do a reasonable job of simulating rainfall outside the tropics, tropical precipitation models rarely showed any agreement. "It seems rainfall extremes in tropical regions are more sensitive to global warming," O'Gorman says. "We have yet to understand the mechanism for this higher sensitivity."
He believes it may come down to resolution: Climate models simulate weather systems by dividing the globe into a grid, with each square on the grid representing a wide swath of ocean or land. Large weather systems that span multiple squares, such as those that occur in the United States and Europe, are relatively easy to simulate. In contrast, smaller, more isolated storms that occur in the tropics may be trickier to model.
To better understand global warming's effect on tropical precipitation, O'Gorman studied satellite observations of extreme rainfall between the latitudes of 30 degrees north and 30 degrees south - just above and below the Equator. The observations - spanning the last 20 years - were then compared to results from 18 different climate models over a similar 20-year period.
"That's not long enough to get a trend in extreme rainfall, but there are variations from year to year," O'Gorman says. "Some years are warmer than others, and it's known to rain more overall in those years." This year-to-year variability, he explains, is mostly due to El Niño - a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Niño causes localized warming and changes in rainfall patterns and occurs independently of global warming.
Looking through the climate models, O'Gorman found a pattern. Models that showed a strong response in rainfall to El Niño also responded strongly to global warming, and vice versa. The results, he says, suggest a link between the response of tropical extreme rainfall to year-to-year temperature changes and longer-term climate change.
O'Gorman then looked at satellite observations to see what rainfall actually occurred as a result of El Niño in the past 20 years, and found that the observations were consistent with the models in that the most extreme rainfall events occurred in warmer periods.
Using the observations to constrain the model results, he determined that with every 1 degree Celsius rise under global warming, the most extreme tropical rainfall would become 10 percent more intense - a more sensitive response than is expected for non-tropical parts of the world.
"Unfortunately, the results of the study suggest a relatively high sensitivity of tropical extreme rainfall to global warming," O'Gorman says. "But they also provide an estimate of what that sensitivity is, which should be of practical value for planning."
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