Low-frequency sea level variability in the southern Indian Ocean and its impacts on the oceanic meridional transports

摘要

Sea levels in the southern Indian Ocean (SIO) display significant interannual to decadal variability. Off the northwest Australian coast, it has been demonstrated that sea level variability is mostly modulated by remote wind forcing from the tropical Pacific through equatorial and coastal waveguides. In this study, a linear reduced gravity model is used to investigate relative contributions of local wind forcing and remote forcing from the Pacific to the sea level variability of the SIO, with a focus on the western SIO. North of the South Equatorial Current bifurcation latitude (17°S), model simulated sea levels are well correlated with altimeter observations at the dissipation timescale of about 3 years, suggesting that sea level variability on interannual-to-decadal timescales could well be explained by nondispersive baroclinic Rossby wave adjustment. The large sea level variability of the western SIO is primarily caused by westward-propagating Rossby waves driven by wind stress curl in 70°E-95°E, with a minor influence from the remote Pacific forcing. To the south, sea level variability at around 20°S displays lower amplitude due to weaker wind variations at this latitude band, and the modeled sea level variability is weaker than observations. There is a close linkage between the cross-basin sea level difference at 15°S and the interior meridional ocean transport across this latitude on decadal timescales, as assessed with outputs from a data-assimilation model. Thus, the meridional overturning cell of the SIO is influenced by both remote forcing from equatorial Pacific and local winds in the SIO.