The South Pacific Current

Abstract

The Southern Hemisphere Subtropical Front (STF) is a narrow zone of transition between upper-level sub-tropical waters to the north and subantarctic waters to the south. It is found near 40°S across the South Atlantic and South Indian Oceans and is associated with an eastward geostrophic current band. The current band in each basin is found at or just north of the surface front except near the eastern boundaries where most of the subtropical waters turn north into the eastern limbs of the subtropical gyres. The bands associated with the STF are thus distinct features separated from the strong zonal flows of the Antarctic Circumpolar Current farther south. The authors have referred to the current bands in the two respective oceans as the South Atlantic Current and the South Indian Ocean Current. In this paper the authors use the historical database from the South Pacific Ocean to investigate the geostrophic flow associated with the STF there. The STF extends across the southern Tasman Sea from south of Tasmania to southern New Zealand, and a weak eastward flow appears to be associated with it. The transport amounts to only about 3 Sv (1Sv &equiv 106 m3 s?1), little of Which passes south of New Zealand. Mixing within the eddy-rich Tasman Sea may account for this weakness, while also setting up another more significant front in the northern Tasman Sea, the Tasman Front. It branches off from the East Australian Current toward the north of New Zealand, along which moves a flow of about 14 Sv. After passing north of New Zealand, a portion of this current flows east to contribute to a current band near 30°S, while another portion turns south as the East Auckland Current and meets with subantarctic waters near Chatham Rise (44°S), thus reestablishing the STF. An enhanced eastward current band is associated with the front there, one that extends across the remainder of the South Pacific and is referred to as the South Pacific Current. In comparison with its counterparts in the other basins, which typically begin by carrying 30 Sv (Atlantic) to 60 Sv (Indian) in the upper 1000 m in their western portions before weakening to 10?15 Sv in the east, the South Pacific Current is weak. Near Chatham Rise, it starts with a transport of approximately 5 Sv, and it remains near this strength as it shifts gradually north across the basin toward South America. The current appears to split into two smaller bands in the region of 115°?85°W, while near 28°S, 83°W it begins to turn more strongly north and becomes shallower and weaker. Potential vorticity distributions indicate that this current acts as an impediment toward the northward spreading of Antarctic Intermediate Water. But why the South Pacific Current east of New Zealand should be so much weaker than its counterparts in the other basins is not particularly clear. It may be due to the presence of New Zealand and other topographic barriers to deep flow east of Australia, to the axis of the subtropical gyre in the South Pacific shifting more rapidly southward with depth than those elsewhere, thus causing greater reductions in the underlying zonal velocities, and to strong poleward eddy heat and salt fluxes in the other two basins leading to smaller cross-STF gradients in the Pacific.