Spatial Characteristics of Interannual Variability in Wind Stress over the Western North Pacific

Abstract

About 4.5 million wind data observations reported from ships during 1961?83 are used to elucidate the spatial structure of interannual variations of the wind stress over the western North Pacific (0°?0°N, 120°E?170°W). Spatially coherent features are described in terms of time series and cross-spectral analyses of monthly data of the zonal and meridional components in 2° lat-5° long quadrangles. In a large portion of the study area, the interannual variation of the zonal stress dominates that of the meridional stress. In the region of the westerlies north of 28°N, the strength of the wintertime maximum in the eastward stress changes from year to year, and the interannual variation of the zonal stress is not coherent over the whole region. The interannual energy in the region is highest in the western North Pacific and is not concentrated in any particular period bands. In the doldrums, located in the western portion of the region between the equator and 10°N, where the stress magnitude is smaller than 0.03 N m?2 in the mean, the zonal stress changes coherently over the whole region in the period band around 2 yr, in which the interannual energy is concentrated. In the trades, south of 28°N, where westward stress is dominant in the mean, the interannual energy of the zonal stress is concentrated in the period band around 3.6 yr, and the variations are not coherent over the whole region. The relationship between the zonal stress and Southern Oscillation index (SOI) is examined for periods in which the interannual energy of the SOI is concentrated. In the 3.6-yr period in which the major spectral peak is found, the SOI is coherent with the zonal stress not only in the tropical area between the equator and 20°N but also in the subtropical area between 30° and 40°N; while in the 2.1-yr period in which a secondary spectral peak is found, the SOI is coherent with the zonal stress in the doldrums region. It is suggested that the variations in these period bands are related to the large eastward wind anomalies in the western equatorial Pacific occurring in the years of El Niño events.