LONG-TERM TEMPERATURE AND CIRCULATION CHANGES OFF EASTERN AUSTRALIA

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 degrees x 5 degrees bins (with some exceptions to allow f or 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 coher ent over scales as large as the Cord Sea. Since there is a high correl ation between the temperature at 450 dbar (T-450) with steric height i n the Tasman and Coral Seas, we have focused our study on the behavior of T-450 The 1985-1989 pentad appears to be cooler at 450 m depth tha n the preceding 15 years over most of the region of study (by more tha n 1 degrees C in some bins). Applying an empirical orthogonal function analysis to the data shows that the first two modes explain 47% and 2 5% of the variance, respectively. The first mode consists of a warming from 1970 to 1976 followed by a steady cooling period to the latter p art of the 1980's (below the starting temperature); its spatial struct ure shows that it is centered just to the west of New Caledonia and ha s a Coral Sea-wide scale, reminiscent of a weakening of the Sverdrup w ind-driven gyre. The second mode has a more complex spatial character which is most pronounced in the EAC region. It is qualitatively simila r to that found in a recent study of the effect of blocking the Indone sian Throughflow, in a global ocean model. It shows a warming from 197 0 which plateaus from 1976 to 1983 and then decreases slightly. There are some statistically significant changes in the T-S relations for th e region, but the associated steric height variations are at the noise level. Futhermore, the long-term vertical profile of temperature chan ge 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 st eric height with T-450, the data suggest that there were quite large c hanges in the circulation over this period. We estimate that the zonal geostrophic inflow to and outflow from the Coral and Tasman Seas at 1 62 degrees E weakened by 15 and 10 Sv respectively, in the period betw een the 1975-1979 and 1985-1989 pentads. This is large compared to a s imilarly estimated long term mean inflow of 27 Sv. However, little evi dence of broad-scale, in-phase El Nine-Southern Oscillation related ch anges are found south of about 10 degrees S.