Seasonal variations in the spatial structure of intraseasonal tropical wind fluctuations

Seasonal and geographic variations in tropical intraseasonal wind variance are described by using bandpass filtered 850- and 150-mb-wind time series derived from rawinsonde observations. Three bandpass filters, with central response periods of 31, 47, and 99 days, are applied to the daily time series. The intermediate filter is designed to isolate variance associated with the 40-50-day oscillation. The spatial coherence of the bandpass filtered wind fluctuations is examined by using complex eigenvector analysis. Comparisons are made of u- and upsilon -variance and large-scale structure of filtered wind anomalies for each season and frequency band, with emphasis on the u-component. At stations across the western Pacific, the 47-day filtered u sub(1) sub(5) sub(0) variance is nearly constant with season. The largest seasonal variability in 47-day filtered zonal wind variance is at 150 mb at stations along and to the north of the Equator between Africa and Southeast Asia, and in the central Pacific. Compared to the u sub(1) sub(5) sub(0) variance over the western Pacific, the variance at these stations is much larger in the boreal winter and much smaller in the boreal summer. Large variance at 850 mb is found in each frequency band from the central Indian Ocean eastward to the dateline, with u sub(8) sub(5) sub(0) and u sub(1) sub(5) sub(0) fluctuating out of phase and the largest u sub(8) sub(5) sub(0) variance in the summer hemisphere. Eastward propagation of u sub(1) sub(5) sub(0) anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u sub(1) sub(5) sub(0) anomalies have a wavenumber 2 structure, consistent with the leading pattern of large-scale convection anomalies. From the dateline eastward across Africa, the scale of the u sub(1) sub(5) sub(0) anomalies is broader, closer to a wavenumber 1 scale. The results suggest that the 40-50-day oscillation in the global Tropics has a two-regime character. Across the eastern Indian and western Pacific oceans (the convective regime), the 40-50-day oscillation occurs year-round, and its spatial structure indicates that it is closely coupled to convection. Elsewhere (the dry regime), the oscillation is clearly evident only in the upper troposphere and is subject to strong seasonal modulation.