Global Rainfall and the Classification of ENSO

Peter Baines



From an analysis of global observations of sea surface temperature, rainfall and atmospheric circulation (surface winds and vertical velocity at 550 hPa) from 1979 to the present, it is shown that El Niño-Southern Oscillation (ENSO) events may be classified into two types with different structure.  The first type consists of the two large events of 1982/3 and 1997/8, and the second consists of the remaining ENSO events over this period.  The analysis is based on a breakdown of the data into empirical orthogonal functions taken over “ENSO years”, which run from June to May.  For each variable, the leading EOF describes the familiar ENSO pattern, with a subsequent EOF describing a similar but different “Modoki mode”, identified in SST by Ashok et al. (2007).  When these EOFs are correlated with monthly data, they describe the structure of the various ENSO events.   The Modoki mode is prominent in the large ENSO events and embodies their different structure, but has modest amplitude at most other times.  Its presence is attributed to the initial large zonal wind anomalies in the western equatorial Pacific that initiate the large events, and result in anomalously warm water reaching the extreme eastern equatorial Pacific, a prominent feature of the Modoki mode in SST.  This forces the Modoki mode in rainfall and the associated circulation, aided by the nonlinear response in evaporation to warm SST anomalies.