Mean Wind Forcing And Largescale Oceanic Response

Although detailed knowledge of the wind fields over the southern hemisphere at any particular time is limited by a chronic shortage of observations over large, remote areas of the ocean, the major climatological features of these fields have been fairly well documented (e.g., Taljaard et al., 1969; Jenne et al. ,1971; Han and Lee, 1981). The climatological sea-level pressure distribution is dominated by subtropical highs centred over the oceans at approximately 30°S, low pressure cells around the edges of the Antarctic continent at 65°S to 70°S, and a permanent high over the South Pole. The high gradient region between 40°S and 65°S maintains a strong and prevailing westerly wind. Nearer the Antarctic continent, on the southern flank of the low pressure cells, the mean wind direction (usually between southerly and easterly) is influenced by both the horizontal pressure gradient and the katabatic effects of the massive continental ice shield (see Mullan and Hickman, this volume).

An extensive effort to collect atmospheric data was carried out between December 1978 and December 1979 as part of the First GARP (Global Atmospheric Research Program) Global Experiment (FGGE). The quantity of data obtained and the spatial coverage realized by this effort, especially in the southern hemisphere, are unpredecented. The mean wind stress field over the Pacific Sector of the Southern Ocean as derived from the FGGE data set (Ploshay et al., 1983) is presented in Fig. 3.2. Although these stresses were obtained during only a single year, the distribution is generally consistent with the historical climatology of wind stress presented by Han and Lee (1981).

North of 70°S, the mean wind stress drives the surface waters eastward with some deflection to the left of the wind stress direction due to the influence of the earth's rotation (Ekman, 1905). The northward Ekman transport of surface water sets up a north-south pressure gradient within the Southern Ocean that in turn supports the eastward geostrophic flow. This eastward wind-driven flow has been referred to as the West Wind Drift (Deacon, 1937). As the flow approaches the western coast of the South American continent, it splits into a northward and southward branch at approximately 43°S (Reid and Arthur, 1975). The northern branch, called the Peru or Humboldt Current, forms the eastern limb of the South Pacific subtropical gyre, and the southern branch flows through Drake Passage as the ACC. Most of the eastward-flowing waters south of 40°S in the Pacific are within the ACC. Fig. 3.2 reveals a zonal band of high mean wind stress (greater than 0.15 Nnv2) between 45°S and 65°S. Within this band the stress is highest (greater than 0.25 Nm"2) south of New Zealand between 160°E and 160°W. That

Fig. 3.2. Annual mean wind stress vectors at the sea surface based on data obtained during FGGE. The tail of each vector is indicated by a large dot. Spatial variations in stress magnitude are emphasized with contours. Units are 101 Nm 2.

this band coincides with the region of highest mean ocean current velocities has been confirmed by both indirect (Gordon et al., 1978) and direct (Hofmann, 1985; Patterson, 1985) methods. Within this band, vector-averaged surface velocities inferred from satellite-tracked drifting buoys are typically greater than 20 cm sec."1. In some locations, such as south of New Zealand and South America, they exceed 40 cm sec."1 (Patterson, 1985).

Although the wind variability increases toward the south, the mean wind stress near the Antarctic continent is relatively large (Fig. 3.2) and induces a net flow toward the west. Deacon (1937) referred to this flow as the East Wind Drift and claimed that it was nearly circumpolar, being interrupted only at Drake Passage. More recent evidence (Treshnikov, 1964) indicates that easterly winds help maintain several distinct clockwise gyres south of the ACC, including the Ross Sea Gyre in the Pacific and the Weddell Gyre in the Atlantic. The continuity of the East Wind Drift is therefore probably interrupted in several locations around Antarctica.

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