The temporal variability of the spatial extent of extremes in interannual rainfall (drought and excessive wetness) is an important characteristic of tropical climate that has received relatively little attention from researchers. Here an observational study is presented which, utilizing a 12-month standardized precipitation index computed for the period 1950-2003, examines such variations from a tropics wide perspective (land areas only, 30šS-30šN) and investigates their association with both the warm and cold phases of individual ENSO events.

Major peaks in the spatial extent of both drought and excessively wet conditions are generally found to be associated with warm and cold ENSO events, respectively. A remarkably robust, linear relationship was identified between ENSO strength (based on Niņo 3.4 SST anomalies) and the spatial extent of drought, with a comparatively weaker relationship for La Niņa and excessive wetness. In addition, it is shown that the likelihood of drought or excessively wet conditions is significantly enhanced in many regions during associated phases of ENSO events, and that this likelihood is greater for severe versus moderate levels of these respective conditions. The results reveal a consistent relationship between the behavior of ENSO and the spatial extent of interannual precipitation extremes over the past half-century, while also indicating asymmetries in the precipitation response between extreme phases of ENSO.