National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1987

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

Gutzler, D.S., and D.E. Harrison

Mon. Weather Rev., 115(1), 169–192, doi: 10.1175/1520-0493(1987)115<0169:TSAEOS>2 (1987)

The longitude-height-time evolution of seasonally averaged wind anomalies over the near-equatorial eastern Indian and western Pacific Oceans is examined, using multiyear time series derived from a network of eight rawinsonde stations. Data at six pressure levels, between 850 and 150 mb, are considered. The first two 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. This space-time structure is confirmed using a complex empirical orthogonal function analysis, which indicates that over half of the interseasonal zonal wind variance at these stations is associated with an eastward-propagating mode (denoted E1). Wind anomalies described by E1 are negatively correlated in the upper and lower troposphere at each station, and are out of phase between the southern tip of India and the central Pacific, so that E1 can be interpreted as an eastward-propagating pattern of convergence/divergence along the equator. Variations in the phase of this mode are "phase-locked" to the annual cycle, and are highly correlated with a conventional Southern Oscillation Index. The wind anomaly field described by E1 evolves through a characteristic life cycle during El Niño events, which begins before the onset of ocean surface warming in the eastern Pacific; the anomaly pattern then propagates eastward during the course of the event. These results are further confirmed by compositing wind anomalies with respect to the phases of the six most recent El Niño events. During the Northern Hemisphere autumn season prior to the onset of El Niño, anomalous low level convergence and upper level divergence are observed in the vicinity of Indonesia. This pattern subsequently propagates eastward, until the opposite pattern of anomalies is observed during the fully developed phase of El Niño, one year after the initial appearance of the atmospheric anomaly pattern. The eastward 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 six El Niño 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 western Pacific prior to the 1982 event was not significantly different from previous events.

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