CR 22:141-146 (2002)  -  doi:10.3354/cr022141

Interannual variability of the bimodal distribution of summertime rainfall over Central America and tropical storm activity in the far-eastern Pacific

Scott Curtis*

Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, Department of Geography, NASA/Goddard Space Flight Center, Code 912, Greenbelt, Maryland 20771, USA

ABSTRACT: The summer climate of southern Mexico and Central America is characterized by a mid-summer drought (MSD), where rainfall is reduced by 40% in July as compared to June and September. A mid-summer reduction in the climatological number of eastern Pacific tropical cyclones has also been noted. Little is understood about the climatology and interannual variability of these minima. The present study uses a novel approach to quantify the bimodal distribution of summertime rainfall for the globe and finds that this feature of the annual cycle is most extreme over Pan America and adjacent oceans. One dominant interannual signal in this region occurs the summer before a strong winter El Niño/Southern Oscillation (ENSO). Before El Niño events the region is dry, the MSD is strong and centered over the ocean, and the mid-summer minimum in tropical cyclone frequency is most pronounced. This is significantly different from Neutral cases (non-El Niño and non-La Niña), when the MSD is weak and positioned over the land bridge. The MSD is highly variable for La Niña years, and there is not an obvious mid-summer minimum in the number of tropical cyclones.

KEY WORDS: Rainfall · Tropical storms · Summer · Central America · ENSO

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