Interannual Dynamics and Thermodynamics of the Indo–Pacific Oceans

Abstract

A global ocean circulation model, driven by observed interannual fluxes, is used to gain insight into how sea surface temperature anomalies (SSTAs, i.e., variations from the mean seasonal signal) in the tropical and subtropical Indian and Pacific Ocean are maintained and changed on interannual timescales. This is done by investigation of heat in the upper ocean at five selected sites and by comparison to observations based on expendable bathythermograph data and the TOGA/TAO moored buoys. A 6-yr simulation between 1985 and 1990 reveals that the model?s simulated interannual temperature variability in the upper 450 m of the ocean is in reasonable agreement with observations. However, the model overestimates the meridional extent and amplitude of SST variability in parts of the equatorial Pacific and Indian Oceans. The problem is associated with the choice of heat flux boundary condition: the ratio of air humidity to saturated humidity over freshwater at SST in the latent heat flux term is independent of the spatial scale of SSTA pattern, which implies a weaker negative feedback on SST change. In the central Pacific at (0°, 140°W), budgets for the surface mixed layer and over the top 300 m both show the primary causes of temperature change to be zonal and vertical advection, with their sum generally less than half of either term individually. At (0°, 110°W), the mixed layer is much thinner so that the temperature changes result from a small disturbance of a basic balance between the vertical convergence of heat flux and vertical and zonal advection. At both sites the zonal flow (and hence the zonal heat advection) is determined by a sum of several terms, none of which are small. It is therefore difficult to find a clear physical basis in the model for the Kessler?McPhaden empirical rule for SSTAs, which correlates highly with observed SSTAs. However, this rule suggests that differences between wind stress products that exceed 0.04 N m?2 over several months (as occurs at 140°W in 1989) could lead to differences in SSTAs of up to 4°C. This may help explain the occurrence of a short but intense La Niña episode that occurred in the model, but not in the observed SST. Comparison with earlier model results tends to confirm that FSU winds were in error in the east Pacific in late 1989 and suggests that the use of a realistic (thin) surface mixed layer exacerbates the problem by strengthening the sensitivity of SSTAs to wind errors. A simple time integral of the depth-averaged (0?350 m) current at 140°E, near the western boundary of the equatorial Pacific, shows a clear correlation with the zonal movements of the eastern edge of the warm pool, lagged by about six months. This is qualitatively as expected from ?delayed oscillator? theory and confirms that the basic current structure of our model is in close agreement with observations. Model and XBT observations show strong similarities in the depth of the 20°C isotherm and SSTA along the IX1 section from western Australia to Java during 1985?90. SST close to the southern end of this section (23°S, 112°E) is dominated by the annual signal with a superimposed weak interannual signal. The time rate of change of accumulated temperature anomalies in the top 450 m is dominated by anomalous cold vertical advection from late 1986 to early 1988 with the opposite happening from late 1988 to early 1990. Both signals indicate the arrival of the ENSO signal along the northwest Australian coast with a reduced (increased) thermocline thickness during the El Niño (La Niña) event. SSTA at 23°S, 112°E in the model is controlled by a balance between anomalous vertical advection and total diffusion; SSTA is not driven by local heat fluxes.