The Regulation of Northern Overturning by Southern Hemisphere Winds

Abstract

Coarse-resolution models are used to study the interaction of zonal wind stress at the latitude of Drake Passage and the production of northern deep water. A nearly linear response to wind stress at the tip of South America is seen in northern deep-water production in both one- and two-basin configurations. A comparison of steady-state circulations forced by different wind stress strengths over the Southern Hemisphere westerlies shows the changes in circulation away from the Southern Ocean largely confined to western boundary currents. The change in circulation accompanying stronger winds is similar to that predicted by the southern upwelling/northern sinking couplet discussed by Stommel. Transient studies are conducted in which wind stress is increased over the region of the Southern Hemisphere westerlies. In an ocean initially stratified and at rest, forced only by the Southern Hemisphere westerlies, the development of the flow is similar to that observed by Kawase in a two-layer model: baroclinic signals propagate from the wind-forced region to the Northern Hemisphere. In an ocean with surface buoyancy forcing that creates deep water in the north, the baroclinic modes “leak on” to the existing sinking region. The effect is to enhance the deep-water formation and associated western boundary currents. The process connecting the wind stress in the latitude of Drake Passage with the deep-water production in the north also readjusts the ocean's thermal structure. As stronger winds are applied in the south, more northern source deep water upwells in the latitude of Drake Passage and less into the thermocline, allowing it to deepen. This increases the density across the Antarctic Circumpolar Current (ACC) and. through the JEBAR term in the vorticity equation, produces a stronger ACC.