ENSO Seasonality: 1950–78 versus 1979–92

Todd P. Mitchell Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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John M. Wallace Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington

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Abstract

ENSO-related seasonal-to-interannual variability in the Pacific basin is documented, based on marine surface observations of monthly mean sea surface temperature, sea level pressure, and wind, together with satellite-based estimates of rainfall and mean tropospheric temperature. Anomalies in these fields are linearly regressed onto simultaneous values of an index of equatorial Pacific SST anomalies. The analysis is performed separately on the data for earlier (1950–78) and later (1979–92) epochs of the record. The analyses are further stratified in terms of the climatological-mean warm and cold seasons in the equatorial Pacific, which correspond to January–May and July–November, respectively. Composite SST, wind, and rainfall fields for he warm and cold seasons that fall within typical warm and cold ENSO episodes are also presented.

Despite the dramatic differences in the sequencing of ENSO warm episodes with respect to the annual march in the two epochs, the anomaly patterns are found to be remarkably similar and generally consistent with the Rasmusson and Carpenter composite. SST and zonal wind anomalies were quite comparable in strength in the warm and cold seasons, although the distributions were somewhat different. Rainfall anomalies and the associated anomalies in surface wind convergence and mean tropospheric temperature were much stronger during the warm season (and particularly during January–February) than during the cold season. Some aspects of the observed rainfall anomaly seasonality can be explained on the basis of simple thermodynamical considerations.

Abstract

ENSO-related seasonal-to-interannual variability in the Pacific basin is documented, based on marine surface observations of monthly mean sea surface temperature, sea level pressure, and wind, together with satellite-based estimates of rainfall and mean tropospheric temperature. Anomalies in these fields are linearly regressed onto simultaneous values of an index of equatorial Pacific SST anomalies. The analysis is performed separately on the data for earlier (1950–78) and later (1979–92) epochs of the record. The analyses are further stratified in terms of the climatological-mean warm and cold seasons in the equatorial Pacific, which correspond to January–May and July–November, respectively. Composite SST, wind, and rainfall fields for he warm and cold seasons that fall within typical warm and cold ENSO episodes are also presented.

Despite the dramatic differences in the sequencing of ENSO warm episodes with respect to the annual march in the two epochs, the anomaly patterns are found to be remarkably similar and generally consistent with the Rasmusson and Carpenter composite. SST and zonal wind anomalies were quite comparable in strength in the warm and cold seasons, although the distributions were somewhat different. Rainfall anomalies and the associated anomalies in surface wind convergence and mean tropospheric temperature were much stronger during the warm season (and particularly during January–February) than during the cold season. Some aspects of the observed rainfall anomaly seasonality can be explained on the basis of simple thermodynamical considerations.

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