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Seasonal Variations in the Spatial Structure of Intraseasonal Tropical Wind Fluctuations

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  • 1 Atmospheric and Environmental Research, Inc., Cambridge, Massachusetts
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Seasonal and geographical variations in tropical intraseasonal wind variance are described 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 using complex eigenvector analysis.

Comparisons are made of u and v 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 u150 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 u150 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 u850 and u150 fluctuating out-of-phase and the largest u850 variance in the summer hemisphere. Eastward propagation of u150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u150 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 u150 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.

Abstract

Seasonal and geographical variations in tropical intraseasonal wind variance are described 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 using complex eigenvector analysis.

Comparisons are made of u and v 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 u150 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 u150 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 u850 and u150 fluctuating out-of-phase and the largest u850 variance in the summer hemisphere. Eastward propagation of u150 anomalies is found in each season and frequency band. A longitudinally varying wavenumber structure fits the eigenvectors reasonably well. Across the western Pacific, the u150 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 u150 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.

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