The effects of the seasonal cycle on interannual SST variability in the Indian Ocean

Modulation of interannual sea surface temperature (SST) patterns by seasonal atmospheric forcings in the Indian Ocean (IO) region are studied via a hierarchy of experiments utilizing both a 4-1/2 layer ocean model in a realistic tropical Indian Ocean basin, and a version of this same ocean model coupled to a model of the atmospheric mixed layer (AML). While SST anomalies (SSTa) in the coupled AML-ocean model (AOM) exhibit a larger magnitude response to seasonal forcing than SSTa in the fully-forced ocean, results from both versions of the model demonstrate that the net effect of the seasonal cycle of atmospheric forcing on interannual SSTa can: (1) aid initiation of SSTa patterns associated with Indian Ocean dipole events in both the western and eastern basin; (2) alter the magnitudes of such events; and (3) accelerate termination of positive dipole events at the end of event years. Patterns of interannual SSTa response to seasonal forcing strongly indicate that dynamic forcing by seasonally reversing equatorial winds (in particular fall equatorial westerlies that are known to push down the thermocline in the eastern basin around November), as well as by seasonal southeasterlies associated with coastal upwelling off Sumatra-Java, play significant roles in modulation of SSTa associated with IO dipole events. This highlights the need to directly examine and compare the interannual SSTa responses to seasonal variability in several individual forcing fields, each of which is linked to specific physical processes that contribute to SST in the model.; To determine the specific physical processes leading to interannual IO SSTa modulation by seasonal variability, the coupled AOM is used to examine the effects on SSTa of each of the following in isolation: (1) thermocline variability and horizontal advection forced by seasonal wind stress variability; (2) vertical oceanic mixing due to seasonal wind speed; (3) changes in latent and sensible surface heat flux due to seasonal wind speed; (4) variation in long wave surface heat flux via seasonal cloud variability; and (5) changes in the radiative budget due to seasonal insolation. Meanwhile, the magnitude of cold eastern SSTa along the equator in these years appears to be mitigated (in order of decreasing importance) by: (1) thermocline deepening and eastward advection of surface water by equatorial fall westerly wind stress; (2) reduction of entrainment cooling and latent heat loss in the mixed layer due to weakening of interannual easterlies by seasonal fall westerlies; and, (3) accelerated seasonal insolation warming of the southeastern basin in austral summer due to an unusually thin mixed layer. Lastly, during the 1996 reverse dipole event, the processes listed above combine to amplify the positive (negative) SSTa in the eastern (western) basin associated with this event. (Abstract shortened by UMI.)...