19th Conference on Climate Variability and Change


Is Rainfall Increasing in the Tropics?

Robert F. Adler, NASA/GSFC, Greenbelt, MD; and G. Gu and G. J. Huffman

Tropical (25N-25S) rainfall variations and possible long-term changes are examined using the 27-year (1979-2005) monthly dataset from the Global Precipitation Climatology Project (GPCP). Techniques are used to discriminate among the variations due to ENSO, volcanic events and possible long-term changes for rainfall over both land and ocean. The impact of the two major volcanic eruptions over the past 25 years is estimated to be about a 5% maximum reduction in tropical rainfall during each event. Although the global change of precipitation in the data set is near zero, an upward linear change over tropical ocean (0.06 mm/day/10yr) and a slight downward linear change over tropical land (-0.03 mm/day/10yr) are noted. This gives a total tropical (ocean plus land) linear increase of (0.04 mm/day/10yr). These positive changes correspond to about a 5% increase (ocean) and 3% increase (ocean plus land) during this time period. The increases are shown to be statistically significant at the 99% level. Sensitivity tests indicate that the slope of the change line is similar, even with reductions in the length of the record, but the significance level drops below 95% when the record is shortened to about 20 years. Inter-comparisons between the GPCP, SSM/I (1988-2005), and TRMM (1998-2005) satellite rainfall products and alternate gauge analyses over land are made to further assess confidence in the long-term changes seen in the GPCP analysis. The results here indicate that rainfall may very well be increasing in the tropics, especially over oceans, but this needs to be confirmed with additional and improved data sets. .

Session 4A, Changes in hydrometeorological fields
Tuesday, 16 January 2007, 1:30 PM-5:45 PM, 214B

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