The structure and evolution of seasonal wind anomalies over the near- equatorial eastern Indian and western Pacific Oceans.

Abstract

Data at 6 pressure levels, between 850 and 150mb, are considered. The 1st 2 modes of an empirical orthogonal function analysis of zonal wind fluctuations are cross correlated at lag, with spatial structures suggesting that the dominant pattern of variability on seasonal time scales is best described as a propagating oscillation. During the Northern Hemisphere autumn season prior to the onset of El Nino, anomalous low level convergence and upper level divergence are observed in the vicinity of Indonesia. This pattern subsequently propagates E, until the opposite pattern of anomalies is observed during the fully developed phase of El Nino, 1yr after the initial appearance of the atmospheric anomaly pattern. The E phase speed is much slower than an atmospheric Kelvin wave, suggesting that the wind anomalies are part of an air-sea interactive system. The interevent variability for each phase of the 6 El Nino events in the data record is substantial; the significance of the composite anomaly pattern varies considerably from phase to phase. The composite is most robust for the Northern Hemisphere autumn season during the year in which ocean surface warming first occurs. It is particularly noteworthy that the evolution of wind anomalies over the far W Pacific prior to the 1982 event was not significantly different from previous events. -from Authors