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Mean 200-mb Circulation in the Southern Hemisphere Deduced from EOLE Balloon Flights

Pierre MorelLaboratoire de Météorologie Dynamique, C. N. R. S., Paris, France

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Mickel DesboisLaboratoire de Météorologie Dynamique, C. N. R. S., Paris, France

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Abstract

The EOLE experiment has provided a uniquely consistent and uniform set of 85,000 precise wind data, determined from the essentially horizontal displacements of over 480 constant-level balloons released in the Southern Hemisphere at approximately 200 mb. These data are analyzed here for estimating the climatological aspects of the Southern Hemisphere general circulation, i.e., the mean zonal and mean meridional flow and the standing longitudinal wave patterns. Instantaneous velocities in excess of 100 m sec−1 (200 kt) were indeed observed, but the mean zonal velocity, of the order of 10–30 m sec−1, agrees well with previous observational studies. The mean meridional velocity at 200 mb was estimated for various latitudes throughout the period October 1971–March 1972 with hitherto unknown time resolution, uncovering thereby short-term variations or reversals, particularly during the outbreak of the austral summer season, with typical values of the order of 0.2 m sec−1. The largest contribution to the day-to-day variations of the wind is associated with essentially isotropic transient fluctuations with rms amplitudes of 10–15 m sec−1. Significant standing longitudinal wave patterns are also found, however, with dominant wavenumbers 1 and 4 in temperate latitudes and typically 5 m sec−1 rms amplitude.

Abstract

The EOLE experiment has provided a uniquely consistent and uniform set of 85,000 precise wind data, determined from the essentially horizontal displacements of over 480 constant-level balloons released in the Southern Hemisphere at approximately 200 mb. These data are analyzed here for estimating the climatological aspects of the Southern Hemisphere general circulation, i.e., the mean zonal and mean meridional flow and the standing longitudinal wave patterns. Instantaneous velocities in excess of 100 m sec−1 (200 kt) were indeed observed, but the mean zonal velocity, of the order of 10–30 m sec−1, agrees well with previous observational studies. The mean meridional velocity at 200 mb was estimated for various latitudes throughout the period October 1971–March 1972 with hitherto unknown time resolution, uncovering thereby short-term variations or reversals, particularly during the outbreak of the austral summer season, with typical values of the order of 0.2 m sec−1. The largest contribution to the day-to-day variations of the wind is associated with essentially isotropic transient fluctuations with rms amplitudes of 10–15 m sec−1. Significant standing longitudinal wave patterns are also found, however, with dominant wavenumbers 1 and 4 in temperate latitudes and typically 5 m sec−1 rms amplitude.

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