Annual and Interannual thermocline variability of the tropical Southern Indian Ocean: remote versus local forcing

The impacts of annual and interannual remote forcing from the Pacific Ocean, via the oceanic-bridge on the Southern Indian Ocean thermocline variability are investigated. Particular emphasis is placed on variability of the thermocline ridge (50E-75E, 5S-10S), due to its importance in climate variations. Two experiments are performed using an ocean general circulation model (OGCM). In one experiment (MR), both the Indian and Pacific oceans are forced by observed atmospheric conditions for the period 1970-2001. In the second experiment (EXP), the atmospheric forcing that acts upon the Pacific is fixed to a spatial and temporal mean, while the forcing that acts upon the Indian Ocean is allowed to change. The experiment design is intended to isolate the transmission of atmospherically generated variability from the Pacific into the Southern Indian Ocean via the Indonesian Throughflow (ITF). In agreement with previous studies, OGCM experiments suggest that the annual cycle of variability over the thermocline ridge is dominated by: a semiannual signal that is present in the depth of the thermocline, sea surface height anomalies and the local Ekman pumping anomalies. Climatologically, the annual cycle of thermocline variability over the Southern Indian Ocean thermocline ridge is primarily controlled by local Ekman pumping. The remote forcing from the Pacific via ITF dampens the thermocline anomalies over the thermocline ridge, which in turn affects the amplitude of the annual cycle but not the phase. In contrast, on interannual timescales, thermocline variability is associated with the westward propagating Rossby wave and is driven by wind forcing acting upon the Indian Ocean. Composite analysis suggests that similarly signed anomalies originating from the Pacific minimally contribute to strength of the thermocline anomalies in the western Southern Indian Ocean. Interannual thermocline anomalies originate at the eastern boundary of the Indian Ocean and are enhanced by forcing from the overlying Ekman pumping anomalies as they propagate westward. For both the annual and interannual frequencies, influence from the Pacific via ITF is largely confined to the western boundary region of Australia. Thermocline variability north of 10S in the interior South Indian Ocean results primarily from winds that act upon the Indian Ocean.