Rossby wave propagation in the Indian Ocean associated with El Niño-Southern Oscillation

Transport variability in the south-western Indian Ocean (SWIO) affects local climate and the global overturning circulation through inter-ocean transport of heat. Here we examine remote drivers of SWIO variability, particularly the generation and propagation of Indian Ocean Rossby waves excited by wind and sea-level anomalies associated with the El Niño-Southern Oscillation (ENSO). These waves, a by-product of both atmospheric and oceanic ENSO teleconnections, can propagate climate signals over considerable spatial and temporal scales, modifying thermocline depth and air-sea interactions. We use 156 years of climate model data to identify ENSO-induced Rossby wave propagation across the southern tropical and sub-tropical Indian Ocean. We apply a simplified 1½-layer reduced-gravity model to reconstruct sea-level anomalies propagating along Rossby wave characteristics. An optimisation algorithm is used to understand the relative importance of baroclinic and barotropic waves, originating from either interior Ekman-pumping or eastern boundary forcing. Our optimised algorithm is skillful in explaining simulated sea-level variance and demonstrates the prominent role of baroclinic Rossby waves in modulating sea-surface heights. Partitioning anomalous sea-level signals into contributions from Rossby wave components also sheds light on the dynamical mechanisms responsible for modulating SWIO sea-level at different latitudes. Deconstruction of wave components shows eastern boundary-sourced signals crossing the ocean basin at most latitudes, but being swamped by destructive signals generated in the interior. At subtropical latitudes where the ENSO wind-stress curl response is weak, eastern boundary waves appear to propagate mostly uninterrupted across the basin, although associated sea-level anomalies are small compared with further north. Robust SWIO sea-level anomalies are diagnosed between latitudes 9.5 degrees south and 18.5 degrees south as a response to downwelling waves traced to anomalous anticyclonic wind-stress curl fields in the interior Indian Ocean forced by El Niño via the “atmospheric bridge”. As such, El Niño can influence SWIO regional climate two to four seasons after its peak. Our reconstruction confirms that these same ENSO-induced wind fields are also the most likely mechanism behind mid-basin destructive interference with Rossby waves triggered at the West Australian coast via the “oceanic bridge”