Observation of 3–6-Day Meridional Wind Oscillations over the Tropical Pacific, 1973–1992: Horizontal Structure and Propagation

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  • 1 Northwest Research Associates, Bellevue, Washington
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Abstract

Twenty years of rawinsonde data (1973–1992) were examined in conjunction with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and outgoing longwave radiation (OLR) in 1980–1989 to determine the horizontal structure, propagation, and convective coupling of 3–6-day meridional wind oscillations over the tropical Pacific. Wave properties from ECMWF data, determined by lag correlation with respect to rawinsonde or ECMWF principal components, were consistent with what could be determined from the sparse rawinsonde network alone. Gridded analyses allowed a clearer distinction between equatorially trapped Rossby-gravity waves (RGW) and off-equatorial “tropical-depression” (TD) disturbances, so that the contrasting properties of these waves, including their seasonal and interannual variation, could be studied in better detail. Significant correlations with OLR were found, increasing in magnitude from eastern to western Pacific.

The apparent group propagation of disturbances was equatorward in the western Pacific, eastward across the central and eastern Pacific, and upward-downward out of the 150–300-mb layer. Vertical propagation was evident primarily at higher frequencies, implying that only a fraction of the kinetic energy associated with Rossby-gravity waves in the upper troposphere was involved either in convective coupling to the lower troposphere or vertical momentum transport to the lower stratosphere. It is suggested that in addition to convective and lateral forcings, Rossby-gravity waves are sometimes excited by energetic TD disturbances in the western Pacific.

Abstract

Twenty years of rawinsonde data (1973–1992) were examined in conjunction with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and outgoing longwave radiation (OLR) in 1980–1989 to determine the horizontal structure, propagation, and convective coupling of 3–6-day meridional wind oscillations over the tropical Pacific. Wave properties from ECMWF data, determined by lag correlation with respect to rawinsonde or ECMWF principal components, were consistent with what could be determined from the sparse rawinsonde network alone. Gridded analyses allowed a clearer distinction between equatorially trapped Rossby-gravity waves (RGW) and off-equatorial “tropical-depression” (TD) disturbances, so that the contrasting properties of these waves, including their seasonal and interannual variation, could be studied in better detail. Significant correlations with OLR were found, increasing in magnitude from eastern to western Pacific.

The apparent group propagation of disturbances was equatorward in the western Pacific, eastward across the central and eastern Pacific, and upward-downward out of the 150–300-mb layer. Vertical propagation was evident primarily at higher frequencies, implying that only a fraction of the kinetic energy associated with Rossby-gravity waves in the upper troposphere was involved either in convective coupling to the lower troposphere or vertical momentum transport to the lower stratosphere. It is suggested that in addition to convective and lateral forcings, Rossby-gravity waves are sometimes excited by energetic TD disturbances in the western Pacific.

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