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Joachim Ribbe and Matthias Tomczak

Abstract

South of Tasmania, the Fine Resolution Antarctic Model (FRAM) shows a narrow band of westward flow in the surface layer. The authors argue that this is caused by the closure of the Indonesian passage in FRAM. This is supported by numerical experiments carried out by Hirst and Godfrey and by recent radiocarbon observations in the Great Australian Bight (Ribbe et al.). The FRAM surface temperature and salinity distribution exhibits distinct anomalies in the southeast Indian Ocean, in good agreement with anomalies observed in the Hirst and Godfrey model. Indonesian Throughflow water advects heat into the Indian Ocean; its absence in FRAM results in a lack of thermal energy to warm the Indian Ocean in the model. The surface salinity anomaly is most likely caused by an overestimated Ekman transport. The weakened heat and salinity transport in FRAM restricts surface convection in the midlatitude region to approximately 350 m.

The effect of the Indonesian Throughflow closure in FRAM is even more dramatic for the circulation around Australia and Tasmania. Hirst and Godfrey’s results suggest that in the case of an open Indonesian passage, the flow in the surface layer is eastward, that is, from the Indian to the Pacific Ocean. Preliminary analysis of radiocarbon observations from the Great Australian Bight supports this, showing a better correlation with Indian Ocean data than Pacific Ocean data. FRAM shows westward flow, inconsistent with these observations.

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Lothar Stramma, Ray G. Peterson, and Matthias Tomczak

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.

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Matthew H. England, J. Stuart Godfrey, Anthony C. Hirst, and Matthias Tomczak

Abstract

Realistic representation of the low-salinity tongue of Antarctic Intermediate Water (AAIW) has been achieved in a coarse-resolution ocean general circulation model. The authors find that this water mass is not generated by direct subduction of surface water near the polar front. Instead, the renewal process is concentrated in the southeast Pacific Ocean off southern Chile. The outflow of the East Australian Current progressively cools (by heat loss to the atmosphere) and freshens (by assimilation of polar water, carried north by the surface Ekman drift) during its slow movement across the South Pacific toward the AAIW formation zone. Further, deep, warm advection near Chile enables more convective overturn, resulting in very deep mixed layers from which AAIW is fed into the South Pacific and also into the Malvinas Current. Along with this isolated region of AAIW renewal, the model relies on alongisopycnal mixing of fresh surface water from the polar front to capture a realistic circumpolar tongue of low salinity water at 1000-m depth.

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