More Wet and Dry Months in the Tropics in Response to Global Warming
Quantifying how global warming impacts the spatiotemporal distribution of precipitation represents a key scientific challenge with profound implications for human systems. Using predictions of monthly precipitation from Coupled Model Intercomparison Project (CMIP3) climate change simulations, the authors show that the occurrence of very dry (<0.5 mm/day) and very wet (>10 mm/day) months comprises a straightforward, robust metric of anthropogenic warming on tropical land region rainfall. In particular, tropics-wide precipitation frequencies for 25-year periods over the late 21st and 20th centuries show increased late, 21st-century occurrence of both extremes in the model ensemble and across individual models. Similar diagnostics are calculated for two 15-year sub-periods from 1979-2008 to assess whether the signature of late, 21st-century warming has already emerged in response to recent warming. While both the observations and CMIP3 ensemble mean hint at similar amplification in the warmer (1994-2008) subinterval, the changes are not robust, as substantial differences are evident among the observational products and the intra-ensemble spread is large. Comparing results from the warmest and coolest years of the observational period further demonstrates effects of internal variability, notably the El Niño/Southern Oscillation, which appear to oppose the impact of quasi-uniform anthropogenic warming on the wet tail of the monthly precipitation distribution. These results identify the increase of very dry and wet occurrences in monthly precipitation as a potential signature of anthropogenic global warming, but also highlight the continuing dominance of internal climate variability on even bulk measures of tropical rainfall.
Lintner, B. R., M. Biasutti, N. S Diffenbaugh, J.-E. Lee, M. J. Niznik, and K. L. Findell. 2012. “Amplification of Wet and Dry Month Occurrence over Tropical Land Regions in Response to Global Warming,” Journal of Geophysical Research Atmospheres, DOI: 10.1029/2012JD01749, in press.