Global and Regional Scale Precipitation Patterns Associated with the El Niño/Southern Oscillation

Abstract

We investigate the “typical” global and large-scale regional precipitation patterns that are associated with the El Niño/Southern Oscillation (ENSO). Monthly precipitation time series from over 1700 stations are analyzed using an empirical method designed to identify regions of the globe that have precipitation variations associated with ENSO. Monthly mean ranked precipitation composites are computed over idealized 2-year ENSO episodes for all stations that include data for at least five ENSOs. The amplitude and phase of the Arm harmonic fitted to the 24-month composite values are plotted in the form of a vector for each station. When plotted on a global map, these vectors reveal both the regions of spatially coherent ENSO-related precipitation and the phase of this signal in relation to the evolution of the composite episode. Time cries of precipitation for the coherent regions identified in the harmonic vector map are examined to determine the magnitudes of the ENSO-related precipitation and the percentages of the time the identified relationship actually occurred in conjunction with ENSO episodes. This study expands previous results by placing the regional precipitation relationships into a global framework and by providing a consistent methodology for the definition of the geographical regions and the temporal phase of ENSO-related precipitation. In addition to the Pacific Ocean basin where precipitation patterns could be directly related to the ENSO, several other regions, which showed consistent ENSO-related precipitation, were identified. Specifically, four regions in Australia, two regions each in North America South America, the Indian subcontinent, and Africa and one region in Central America were all found to have coherent ENSO-related precipitation. In most of thew regions, the “season” of ENSO-related precipitation was found to be in phase with the normal annual precipitation cycle. Time series of area-averaged precipitation for the appropriate “seasons” show departures consistent with the composites occurring for at 1east 80% of the ENSO events in almost every region. The analysis further indicates that variations in precipitation related to ENSO occur as early as April of the composite episode through May of the following year.