The varied impacts of El Nino-Southern Oscillation on Pacific Island climates

The varied impacts of El Nino-Southern Oscillation on Pacific Island climates

El Nino-Southern Oscillation (ENSO) drives interannual climate variability in many tropical Pacific island countries, but different El Nino events might be expected to produce varying rainfall impacts. To investigate these possible variations, El Nino events were divided into three categories based on where the largest September-February sea surface temperature (SST) anomalies occur: warm pool El Nino (WPE), cold tongue El Nino (CTE), and mixed El Nino (ME), between the other two. Large-scale SST and wind patterns for each type of El Niño show distinct and significant differences, as well as shifts in rainfall patterns in the main convergence zones. As a result, November to April rainfall in many Pacific island countries is significantly different among the El Nino types. In western equatorial Pacific islands, CTE events are associated with drier than normal conditions whereas ME and WPE events are associated with significantly wetter than normal conditions. This is due to the South Pacific convergence zone and intertropical convergence zone moving equatorward and merging in CTE events. Rainfall in the convergence zones is enhanced during ME and WPE and the displacement is smaller. La Nina events also show robust impacts that most closely mirror those of ME events. In the northwest and southwest Pacific strong CTE events have much larger impacts on rainfall than ME and WPE, as SST anomalies and correspondingly large-scale surface wind and rainfall changes are largest in CTE. While variations in rainfall exist between different types of El Nino and the significant impacts on Pacific countries of each event are different, the two extreme CTE events have produced the most atypical impacts.

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