| In this study a hierarchy of numerical models and a suite of diagnostic numerical experiments are used to characterize the spatial and temporal structure of sea level and thermocline depth variations in the southern Indian Ocean. The key physical processes responsible for driving the multi-timescale variability are then identified. Particular emphasis is placed on the Thermocline Ridge of the Indian Ocean (TRIO), a region in the southwestern tropical Indian Ocean where variations in thermocline depth are suggested to affect sea surface temperature and thus climate.;On seasonal-to-decadal timescales, the key physical processes that contribute to variability of the TRIO are timescale dependent and largely arise from wind forcing acting on the Indian Ocean. Seasonally, variability of the TRIO results from a combination of local Ekman pumping and the arrival of Rossby waves forced by winds east of region. Interannually, variability is associated with westward-propagating Rossby waves forced by the Ekman pumping velocities east of the ridge. Remote oceanic forcing from the Pacific has significant influence on the seasonal and interannual thermocline variability in the eastern basin, but a weak impact in the tropical ocean interior. On decadal timescales, sea level and thermocline depth variability peaks in the southwestern Indian Ocean in both the tropics and subtropics. Prior to the early 1990s, low frequency variations in sea level and thermocline depth in the tropics, can be described as a baroclinic Sverdrup balance, forced by windstress acting on the Indian Ocean. Beginning in the early 90's, decadal variability of the equatorial Pacific trade winds forces thermocline variations that modify the sea level and thermocline depth across the entire tropical southern Indian Ocean basin. Farther south, nonlinearities of the flow appear to dominate the low frequency variability in the ocean interior. On multi-decadal timescales, the TRIO has experienced a shoaling trend spanning 1961--2000. This shoaling is consistent with regional enhanced upward Ekman pumping velocity and spin-up of the southern Indian Ocean subtropical cell. |
