Separation of quasi-semiannual Rossby waves from the eastern boundary of the Indian Ocean.
Résumé
Westward propagating features, identified as semiannual Rossby waves, have been observed in the southern subtropical Indian Ocean between 20S and 35S. A previous study based on ERS satellite scatterometer wind stress data in this region has shown that the Rossby waves were not forced by local Ekman pumping in the ocean interior nor by local variations in wind stress near the Australian coast. In the present study, we investigate a third possible source, in the form of remotely forced semiannual coastal waves propagating along the Australian coast. Altimetric and tide gauge data show consistent semiannual anomalies along the northwestern and western coast, with a phase lag between the northernmost station and about 25S, producing a poleward phase speed of ∼0.4 m s-1. South of the critical latitude for semiannual Rossby wave radiation at 27S, we still observe a significant sea level signal along the coast at the semiannual frequency that appears nearly in phase between the stations. Semiannual sea level anomalies lead the semiannual wind forcing by about one month, confirming that the signal is not locally forced. These semiannual coastal variations are in phase with the offshore radiation of Rossby waves observed between 20S and 35S. We suggest that, south of the critical latitude of 27S, the observed coastal wave signal could interact with the poleward coastal Leeuwin Current and that this wave-current interaction could stimulate nonlinear instabilities of this density-driven current and may be a key mechanism for the Rossby wave radiation in the subtropical Indian Ocean.