Surface circulation in the Tasman and Coral Seas: climatological featiures derived from bathy-thermograph data

Abstract

Surface current climatology for 1960-87 in the Coral and Tasman seas for 20-50°S,150-175°E is investigated using geostrophic current values calculated from basin-wide expendable bathy-thermograph temperature sections coupled with a synthesized salinity profile and dynamic height regressions. The 180 sections lie on routes between Australia, New Zealand and Fiji, including 50 sections between Sydney and the northern tip of New Zealand along 34°S. The analysis shows that bottom topography, seasonality and mesoscale effects are all important factors in descriptions of the Tasman Front and Tasman-Coral Sea flows.

Preferred locations for both cold and warm features are found along 34°S,152-172°E, correlated with positions of ridges and basins respectively, indicating that currents along all of this section are strongly affected by bottom topography. Positions of seamount chains, ridges and rises appear to be the determining factor for zonal mesoscale length scales and meander positions. Summer and winter temperature sections and flow patterns for 34°S are similar. Current speeds between Sydney and Lord Howe Rise were higher in summer and spring, and highest for 152-154°E in the south-flowing East Australian Current. At least one warm feature was always present between Sydney and Lord Howe Rise for these data, but usually two, sometimes three and occasionally four. The first warm feature (usually the first meander of the East Australian Current) was preferentially found in either of two areas. Ratios of surface widths of warm meanders to depths of the 15°C isotherm at the meander centres along 34°S indicate that the depths of the 15°C isotherm may be inferred from surface data alone in particular areas. This could allow quantitative estimates of subsurface temperature structure, and hence surface current strengths, to be inferred from satellite infrared imagery.

Between Sydney and Cook Strait (34°S,152°E to 40°S,172°E) summer and winter temperature regimes vary greatly. In summer the 15°C isotherm is nearly always subsurface, but in winter it outcrops as far west as 158°E. Direct East Australian Current influence is apparently confined to west of 160°E on this line, and the Tasman Front is usually north of this line.