Large Scale, Low Frequency Variability of the 1979 FGGE Surface Buoy Drifts and Winds over the Southern Hemisphere

Abstract

The surface response of the Southern Hemisphere's oceans to the large spatial scale, interseasonal changes in wind forcing during the FGGE year of 1979 is investigated. The primary data are the analyzed daily wind fields, and the trajectories of the FGGE drifting buoy array. The zonal wind forcing is characterized by large spatial patterns of low frequency (annual and semiannual) variability. particular attention is paid to the second harmonic, which has amplitude peaks at 35°?40° S with solstitial maxima, and amplitude peaks at 60°S with equinoctial maxima. The distinct phase change occurs at 50°S. The analysis of the drifting buoy data is guided by the wind patterns, but first the question of the current-following characteristics of the FGGE buoys is addressed. Compared to the wind, the buoy drift has even larger spatial scales, and more low frequency contributions to its intra-annual variance. Like the wind, amplitude peaks in the second harmonic of monthly mean zonal drift are found in each ocean basin sector at 40 ± 5° S and at 55° ?60° S, with a phase change at about 50° S. These wind and drift patterns extend from 30°S to antarctica, and so encompass the entire antarctic Circumpolar Current (ACC) and the poleward halves of the subtropical gyres. The results are discussed in relation to Southern Ocean dynamics and previous studies. A simple barotropic calculation shows that interseasonal changes in buoy drifts are small enough relative to the wind forcing that neither baroclinic surface enhancement nor slip error need be invoked to explain them. Latitudinal shear in zonal drift is shown to have a great deal of temporal variability implying momentum transports across the ACC, to the center or from the center of the ACC, depending on the time of year. The observed buoy drift is consistent with the view of the ACC consisting of multiple narrow cores. Furthermore, it suggests that as the latitude of the peak in zonal wind shifts with the half-year waves, different underlying cores of the ACC are accelerated to be the one with the greatest velocity. The Seasat satellite altimetric results are interpreted as capturing a half-cycle of the second harmonic, and as showing a phase change in zonal geostrophic flow at about 50°S. A second harmonic with equinoctial maxima is found in the 500 m depth pressure difference across the Drake Passage, although we find that this area is not very representative of the ACC as a whole. We propose that the semiannual signals in the winds and surface currents should be important diagnostics in coupled ocean-atmosphere models of the Southern Ocean. This wave is, however, faithfully represented only in products from daily analyzed pressure fields and in their climatological analyses, but not in atmospheric general circulation models nor in wind climatologies based on ship observatons.