Some Climatological Aspects of the Circulation in Southern Hemisphere Temperate Latitudes as Determined From 200-Millibar GHOST Balloon Flights

Abstract

The climatology of the 200-mb flow in Southern Hemisphere temperate latitudes is investigated for a 1-yr period using data obtained from nine global horizontal sounding technique (GHOST) balloon flights in 1966?67. In the mean for the year, the west wind is a maximum over the Indian Ocean and a minimum near Cape Horn while a mean equatorward flow of about 1 m/s exists over the South Atlantic and a mean poleward flow of similar magnitude exists near South Africa. The poleward eddy flux of westerly momentum produced by wave numhers 1?4 is a maximum over the South Atlantic. Between 40° and 60°S, semiannual and annual variations in zonal and meridional wind are of similar magnitude, resulting in a west-wind maximum near the vernal equinox and a secondary maximum near the autumnal equinox. The inferred mean meridional velocity attains values of 0.3 m/s, with equatorward flow indicated as occurring in summer and autumn (when the west wind is relatively weak) and poleward flow in winter and spring (when the west wind is relatively strong). It is hypothesized that the mean equatorward flow results statistically from the occasional establishment of regions of inertial instability on the anticyclonic shear sides of strongly diffluent troughs and the mean poleward flow from the occasional excess of pressure gradient force over Coriolis force following air parcels moving into the anticyclonic shear zone of strongly confluent troughs. The time rate of change of zonal wind between 40° and 60°S is well correlated with the GHOST-derived convergence of meridional eddy momentum flux in this belt, but the inferred mean meridional velocity helps balance momentum accounts and helps explain the observed large semi-annual variation in zonal wind.