THE SOUTH-PACIFIC CURRENT

The Southern Hemisphere Subtropical Front (STF) is a narrow zone of tr ansition between upper-level subtropical waters to the north and suban tarctic waters to the south. It is found near 40 degrees S across the South Atlantic and South Indian Oceans and is associated with an eastw ard geostrophic current band, The current band in each basin is found at or just north of the surface front except near the eastern boundari es where most of the subtropical waters turn north into the eastern li mbs of the subtropical gyres. The bands associated with the STF are th us distinct features separated from the strong zonal flows of the Anta rctic Circumpolar Current farther south. The authors have referred to the current bands in the two respective oceans as the South Atlantic C urrent and the South Indian Ocean Current. In this paper the authors u se the historical database from the South Pacific Ocean to investigate the geostrophic flow associated with the STF there. The STF extends a cross the southern Tasman Sea from south of Tasmania to southern New Z ealand, and a weak eastward flow appears to be associated with it. The transport amounts to only about 3 Sv (1Sv = 10(6) m(3) s(-1)), little of which passes south of New Zealand. Mixing within the eddy-rich Tas man Sea may account for this weakness, while also setting up another m ore 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 degrees S, while another portion turns south a s the East Auckland Current and meets with subantarctic waters near Ch atham Rise (44 degrees S), thus reestablishing the STF. An enhanced ea stward current band is associated with the front there, one that exten ds across the remainder of the South Pacific and is referred to as the South Pacific Current. In comparison with its counterparts in the oth er basins, which typically begin by carrying 30 Sv (Atlantic) to 60 Sv (Indian) in the upper 1000 m in their western portions before weakeni ng to 10-15 Sv in the east, the South Pacific Current is weak. Near Ch atham Rise, it starts with a transport of approximately 5 Sv, and it r emains near this strength as it shifts gradually north across the basi n toward South America. The current appears to split into two smaller bands in the region of 115 degrees-85 degrees W, while near 28 degrees 5, 83 degrees W it begins to turn more strongly north and becomes sha llower and weaker. Potential vorticity distributions indicate that thi s current acts as an impediment toward the northward spreading of Anta rctic Intermediate Water, But why the South Pacific Current east of Ne w 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 now east of Australia, to the axis of the subtropical gyre in the South Pacific shifting mor e rapidly southward with depth than those elsewhere, thus causing grea ter reductions in the underlying zonal velocities, and to strong polew ard eddy heat and salt fluxes in the other two basins leading to small er cross-STF gradients in the Pacific.