QUASI-PERIODIC VARIABILITIES OF WIND-STRESS FIELDS OVER THE PACIFIC-OCEAN RELATED TO ENSO EVENTS

Two datasets of monthly-mean wind stress, estimated from Ships-of-Opportunity data covering the North Pacific (0-degrees-50-degrees-N) and the Pacific trade wind region (30-degrees-S-30-degrees-N), are used to describe the spatial character of interannual variability. Two characteristic time scales related to the occurrence of ENSO events are given by period bands around 3.6 years (EN) and 2 years (QB), being associated with spectral peaks found in sea surface temperature (SST) in the eastern equatorial Pacific area. Spatial features of wind stress variations in the two period bands are described by patterns derived from correlations between SST in the area and the zonal and meridional components of the wind stress in the datasets. The variation in the EN-band around 3.6 years is coherent over a large portion of the study areas covering the extratropics as well as the tropics and has a Pacific-basin scale. The wind stress pattern in the maximum phase of the SST corresponding to the peak phase of an El Nino event has not only symmetric features, such as westerly anomalies near the equator, but also asymmetric features such as easterly anomalies covering the western portion north of the equator. These spatial features are similar to those in composite fields for the peak phase of El Nino events, which suggests that the EN-period variation is a principal element in ENSO variability. On the other hand, the variation in the QB-band around 2 years is dominant in the tropical area, especially in the western portion north of the equator, being incoherent over most of the study areas. It is suggested that the QB-period variation is characteristic of the tropical zone covering the Indian Ocean and that its phase relationship with the EN-period variation is important for the occurrence of ENSO events.