Local and remote forcing of decadal sea level and thermocline depth variability in the South Indian Ocean

Abstract

AbstractAnalysis is performed on a set of diagnostic numerical experiments designed to isolate local Indian Ocean forcing versus remote forcing from the Pacific via the Indonesian throughflow on decadal variability of subsurface temperature, sea level, and thermocline depth of the South Indian Ocean. It is found that the vertical structure of decadal temperature variability varies from decade‐to‐decade, with maximum variation peaking in the vicinity of the thermocline. The decadal‐scale temperature variations in the tropical southwestern Indian Ocean between 5°S and 17°S are primarily associated with the vertical displacements of the thermocline. Prior to the early 1990s, decadal variations in sea level and thermocline depth can be described in terms of a baroclinic Sverdrup balance, forced by Ekman pumping velocity associated with windstress curl acting on the Indian Ocean. Beginning in the early 1990s, decadal variability of the equatorial Pacific trades forces thermocline variations that modify the sea level and thermocline depth across the tropical South Indian Ocean basin. Farther south, between 20°S and 30°S, oceanic internal variability makes significant contributions to decadal variability of the thermocline. The anomalies along the western coast of Australia are primarily driven by regional forcing acting on the Indian Ocean prior to the 1990s, and signals originating from the equatorial Pacific make a greater contribution thereafter.