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The Mean Upper-Air Flow in Southern Hemisphere Temperate Latitudes Determined from Several Years of GHOST Balloon Flights at 200 and 100 mb

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  • 1 National Center for Atmospheric Research, Boulder, Colo. 80302
  • | 2 Air Resources Laboratories, NOAA, Silver Spring, Md. 20910
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Abstract

Five years of constant-level balloon (GHOST) flights at 200 mb and three years of flights at 100 mb in Southern Hemisphere temperate latitudes furnish information on the mean variation with time of year and longitude of zonal and meridional winds. As found previously from one year of data at 200 mb, there is evidence of a large annual variation in mean meridional wind at these surfaces, with the GHOST balloons moving toward the South Pole at a mean speed exceeding 20 cm sec−1 in the Southern Hemisphere winter and toward the equator at a similar speed in summer. The poleward flow in winter is associated with relatively strong west winds. The semiannual variations in meridional wind are indicated to be out of phase at 200 and 100 mb, with the maximum poleward flow occurring near the equinoxes at 200 mb and near the solstices at 100 mb, implying the existence of meridional circulation cells with flows of opposite sense in the upper troposphere and lower stratosphere. The west wind is strongest over the Indian Ocean and weakest near Cape Horn, and west and south winds are almost out of phase, i.e., standing wavenumber 1 efficiently transports momentum toward the South pole. The poleward eddy flux of momentum is a maximum in winter and occurs with greatest vigor over the South Atlantic. The meridional convergence of the eddy momentum flux obtained from the GHOST balloons does not come close to counterbalancing the large meridional flux of earth angular momentum associated with the GHOST-derived mean meridional velocities, suggesting either that the derived mean meridional velocities are unrepresentative or that other terms in the momentum equation are of surprisingly large magnitude. This dilemma is discussed, but is not satisfactorily resolved at this time.

Abstract

Five years of constant-level balloon (GHOST) flights at 200 mb and three years of flights at 100 mb in Southern Hemisphere temperate latitudes furnish information on the mean variation with time of year and longitude of zonal and meridional winds. As found previously from one year of data at 200 mb, there is evidence of a large annual variation in mean meridional wind at these surfaces, with the GHOST balloons moving toward the South Pole at a mean speed exceeding 20 cm sec−1 in the Southern Hemisphere winter and toward the equator at a similar speed in summer. The poleward flow in winter is associated with relatively strong west winds. The semiannual variations in meridional wind are indicated to be out of phase at 200 and 100 mb, with the maximum poleward flow occurring near the equinoxes at 200 mb and near the solstices at 100 mb, implying the existence of meridional circulation cells with flows of opposite sense in the upper troposphere and lower stratosphere. The west wind is strongest over the Indian Ocean and weakest near Cape Horn, and west and south winds are almost out of phase, i.e., standing wavenumber 1 efficiently transports momentum toward the South pole. The poleward eddy flux of momentum is a maximum in winter and occurs with greatest vigor over the South Atlantic. The meridional convergence of the eddy momentum flux obtained from the GHOST balloons does not come close to counterbalancing the large meridional flux of earth angular momentum associated with the GHOST-derived mean meridional velocities, suggesting either that the derived mean meridional velocities are unrepresentative or that other terms in the momentum equation are of surprisingly large magnitude. This dilemma is discussed, but is not satisfactorily resolved at this time.

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