Long‐term temperature and circulation changes off eastern Australia

Abstract

Long‐term changes in the thermocline circulation have been observed in the southwest Pacific Ocean from a combined data set of hydrographic and expendable bathythermograph data. The data were divided up into a network of 5° × 5° bins (with some exceptions to allow for coastal features) and mean values of temperature at several depths were obtained for the four pentads between 1970 and 1989. Differences in temperature between pairs of pentads show features which are coherent over scales as large as the Coral Sea. Since there is a high correlation between the temperature at 450 dbar (T450) with steric height in the Tasman and Coral Seas, we have focused our study on the behavior of T450. The 1985–1989 pentad appears to be cooler at 450 m depth than the preceding 15 years over most of the region of study (by more than 1°C in some bins). Applying an empirical orthogonal function analysis to the data shows that the first two modes explain 47% and 25% of the variance, respectively. The first mode consists of a warming from 1970 to 1976, followed by a steady cooling period to the latter part of the 1980's (below the starting temperature); its spatial structure shows that it is centered just to the west of New Caledonia and has a Coral Sea‐wide scale, reminiscent of a weakening of the Sverdrup wind‐driven gyre. The second mode has a more complex spatial character, which is most pronounced in the EAC region. It is qualitatively similar to that found in a recent study of the effect of blocking the Indonesian Throughflow in a global ocean model. It shows a warming from 1970, which plateaus from 1976 to 1983 and then decreases slightly. There are some statistically significant changes in the T‐S relations for the region, but the associated steric height variations are at the noise level. Futhermore, the long‐term vertical profile of temperature change in the top 450 m is shown to be similar to that associated with the energetic changes due to eddy movements. Using the relationship of steric height with T450, the data suggest that there were quite large changes in the circulation over this period. We estimate that the zonal geostrophic inflow to and outflow from the Coral and Tasman Seas at 162°E weakened by 15 and 10 Sv respectively, in the period between the 1975–1979 and 1985–1989 pentads. This is large compared to a similarly estimated long‐term mean inflow of 27 Sv. However, little evidence of broad‐scale, in‐phase El Niño‐Southern Oscillation‐related changes are found south of about 10°S.