Global warming can intensify tropical rainfall



Posted on 19 September 2012  | 
With every 1 degree Celsius rise in temperature, tropical regions will see 10 percent heavier rainfall extremes, with possible impacts for flooding in populous regions.This is an estimate given by a MIT study based on model simulations and observations.Extreme precipitation in the tropics comes in many forms: thunderstorm complexes, flood-inducing monsoons and wide-sweeping cyclones like the recent Hurricane Isaac. Global warming is expected to intensify extreme precipitation, but the rate at which it does so in the tropics has remained unclear - until now."The study includes some populous countries that are vulnerable to climate change," said Paul O'Gorman, the Victor P. Starr Career Development Assistant Professor of Atmospheric Science at MIT, "and impacts of changes in rainfall could be important there."O'Gorman found that, compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change. "It seems rainfall extremes in tropical regions are more sensitive to global warming. We have yet to understand the mechanism for this higher sensitivity," O'Gorman said. 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 spanned the last 20 years, the extent of the satellite record. He then compared the observations 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. Some years are warmer than others, and it's known to rain more overall in those years," O'Gorman said.This year-to-year variability is mostly due to El Nino - a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean. El Nino causes localized warming and changes in rainfall patterns and occurs independent of global warming. Looking through the climate models, which can simulate the effects of both El Nino and global warming, O'Gorman found a pattern. Models that showed a strong response in rainfall to El Nino also responded strongly to global warming, and vice versa. The results, he said, 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 Nino 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 nontropical 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.
"Results from the study are published online this week in the journal Nature Geoscience. (ANI)
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