Modeling the wind-driven variability of the south Indian Ocean

This article describes the results of numerical experiments carried out wit h a general circulation ocean model to investigate the effect of the season al cycle of the wind forcing on the Agulhas transport. Two cases are descri bed. The first was initialized with temperature and salinity values obtaine d by horizontally averaging Levitus climatology. The second experiment was designed to isolate the spatial and temporal structure of the barotropic mo de. The model, therefore, was initialized with constant values of temperatu re and salinity. Both experiments were started from rest, forced at their s urface with Hellerman and Rosenstein wind stress climatology, and spun up u ntil dynamical equilibrium. According to the experiments there are two dist inct modes of variability in the south Indian Ocean. These modes appear to be separated by the topographic ridges that run south of Madagascar. On the western side of the basin there is a dominant mode with a maximum during s pring-summer and a minimum during fall-winter. East of Madagascar there is a marked decrease of the circulation in fall and relative maximums during l ate summer and late winter. The midlatitude time variability, east of 45 de grees E, appears to be dominated by advection and wave propagation. West of 45 degrees E there is dominance by local wind forcing. A comparison betwee n baroclinic and barotropic experiments indicates that although their annua l mean structure is markedly different, their monthly anomalies, south of 3 0 degrees S, are quite similar. This result, which agrees with previous the oretical and experimental studies, indicates that the seasonal adjustment i n the south Indian Ocean is mostly accomplished by the westward propagation of barotropic planetary waves. This propagation is inhibited by the bottom topography of the Madagascar Ridge and Southwest Indian Ridge (similar to 45 degrees E). These topographic features appear to isolate the Agulhas Cur rent in the western region from the large-scale gyre farther east at season al timescales.