JISAO
ENSO seasonality: 1950-78 versus 1979-92

Todd P. Mitchell and John M. Wallace
 Journal of Climate, 9, 3149-3161.

Abstract

Ship observations of monthly-mean sea surface temperature (SST), sea-level pressure (SLP), surface vector winds, and cloudiness for the years 1950-92, and satellite estimates of monthly-mean rainfall, outgoing longwave radiation, and mean-tropospheric temperature for 1979-92 are analyzed to document ENSO variability in the Pacific Ocean basin. Anomalies in these fields are linearly regressed onto simultaneous values of an index of cold tongue SST anomalies (CTI) for early (1950-78) and late (1979-92) epochs of the record. The analyses are further stratified by the seasons December through April and June through October, which correspond to times of large and small amplitude ENSO-related rainfall anomalies in the equatorial Pacific, respectively. Composite SST, surface vector wind, and rainfall fields for the two seasons in typical warm and cold ENSO episodes are also presented.

The spatial relationships between ENSO-associated anomalies of SST, SLP, and surface vector winds are found to be largely reproducible in the early and late epochs of the record. The month-to-month autocorrelation of the atmospheric components of ENSO exhibited a similar seasonality in the two epochs, while comparable statistics for the CTI were found to be less reproducible.

Analyses of Microwave Sounding Unit and GOES Precipitation Index (GPI) rainfall estimates for the later years (1986-92 for GPI) indicate that rainfall anomalies in December through April are located in the core of the equatorial dry zone / cold tongue, while rainfall anomalies during June through October are only about half as large and confined to the edges of the dry zone. It is suggested that these differences are related to the annual march in the strength of the cold tongue. The stronger tropical forcing in December through April may contribute to the stronger influence of ENSO upon the Northern Hemisphere circulation during that season.

Compressed tar file (2 Mbytes) containing PostScript files of the manuscript and figures.

Corresponding author: Todd P. Mitchell (mitchell@atmos.washington.edu)

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JISAO