The Relationship Between Interannual Variability in the 200 mb Tropical Wind Field and the Southern Oscillation

Abstract

The interannual variability in the upper tropospheric tropical wind field is described through empirical orthogonal functions, teleconnections and composite analyses. The data used are 11 years of mean monthly analyzed fields of zonal and meridional components of the 200 mb wind on a 5° Mercator grid extending from 48.1° N to 48.1°S derived from the National Meteorological Center's operational tropical analysis. A substantial portion of the interannual variability in the 200 mb circulation is shown to be related to the Southern Oscillation. The anomalous circulation in the Pacific is characterized by a pair of anticyclonic/cyclonic anomalies straddling the equator during periods of low/high Southern Oscillation Index. Zonal wind differences of 8?11 m s?1 between low- and high-Index phases of the SO were found near 25°N, 25°S and near the equator in the central Pacific. Composites relative to El Niño events during different seasons reveal that anomalous anticyclonic circulations in the Pacific are associated with the presence of a positive sea surface temperature anomaly in the eastern and central equatorial Pacific. The anomalous circulation features outside the tropical and subtropical Pacific vary with season, with the largest anomalies in each hemisphere occurring during the winter.