Interannual dynamics and thermodynamics of the Indo-Pacific oceans

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 sub-tropi cal Indian and Pacific Ocean are maintained and changed on interannual time scales. This is done by investigation of heat in the upper ocean at five se lected sites and by comparison to observations based on expendable bathythe rmograph data and the TOGA/TAO moored buoys. A 6-yr simulation between 1985 and 1990 reveals that the model's simulated interannual temperature variab ility in the upper 450 m of the ocean is in reasonable agreement with obser vations. However, the model overestimates the meridional extent and amplitu de 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 patte rn, which implies a weaker negative feedback on SST change. In the central Pacific at (0 degrees, 140 degrees W), budgets for the surfa ce mixed layer and over the top 300 m both show the primary causes of tempe rature change to be zonal and vertical advection, with their sum generally less than half of either term individually. At (0 degrees, 110 degrees 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 Bur and vertical and zonal advection. At both sites the zonal flow (a nd 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 physica l basis in the model for the Kessler-McPhaden empirical rule for SSTAs, whi ch correlates highly with observed SSTAs. However, this rule suggests that differences between wind stress products that exceed 0.04 N m(-2) over seve ral months (as occurs at 140 degrees W in 1989) could lead to differences i n SSTAs of up to 4 degrees C. This may help explain the occurrence of a sho rt but intense La Nina episode that occurred in the model, but not in the o bserved SST. Comparison with earlier model results tends to confirm that FS U winds were in error in the east Pacific in late 1989 and suggests that th e 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 degre es E, near the western boundary of the equatorial Pacific, shows a clear co rrelation with the zonal movements of the eastern edge of the warm pool, la gged by about six months. This is qualitatively as expected from "delayed o scillator" theory and confirms that the basic current structure of our mode l is in close agreement with observations. Model and XBT observations show strong similarities in the depth of the 20 degrees 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 deg rees S, 112 degrees E) is dominated by the annual signal with a superimpose d weak interannual signal. The time rate of change of accumulated temperatu re anomalies in the top 450 m is dominated by anomalous cold vertical advec tion from late 1986 to early 1988 with the opposite happening from late 198 8 to early 1990. Both signals indicate the arrival of the ENSO signal along the northwest Australian coast with a reduced (increased) thermocline thic kness during the El Nino (La Nina) event. SSTA at 23 degrees S, 112 degrees E in the model is controlled by a balance between anomalous vertical advec tion and total diffusion; SSTA is not driven by local heat fluxes.