Pathways and impacts of ENSO-induced oceanic variability in the southern Indian Ocean investigated with high-resolution model simulations

Abstract

Pacific Ocean water masses can reach the Atlantic through the Indonesian Throughflow (ITF) and Agulhas Current (AC) leakage, constituting an essential route of the global ocean conveyor belt. However, whether the oceanic variability induced by El Niño-Southern Oscillation (ENSO) can efficiently penetrate through this route remains largely uncertain. In this study, we utilize experiments of a high-resolution (0.1° in horizontal resolution) ocean model to revisit pathways of Pacific-origin oceanic signals and their impacts on the Southern Indian Ocean (SIO) circulation. The experiments are performed for the 2014–2022 period to cover the 2015–2016 super El Niño and the 2020–2022 consecutive La Niña. Our results reveal a main pathway of Pacific-origin baroclinic Rossby waves stretching from the Maritime Continent to Madagascar, with ENSO signatures weakening rapidly as proceeding westward. The Pacific-origin oceanic signals explain ∼95 % of the strength variability in the ITF outflow at 116°E, ∼66 % of that in the South Equatorial Current (SEC) and ∼17 % of that in the Northeast Madagascar Current (NEMC) during 2014–2022. Through this “oceanic channel” mechanism, the 2015–2016 El Niño led to a strengthened NEMC in 2016 and a weakened Southeast Madagascar Current (SEMC) in 2017. By contrast, this oceanic channel effect is generally negligible for the AC system which is dominated by ocean internal variability. These results constrain the extent of ENSO's modulation effect on the SIO circulation and imply that effective ocean-channel communication between the Pacific and the Atlantic has to occur on decadal or longer timescales.