The 26‐ and 50‐day oscillations in the western Indian Ocean: Model results

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Abstract
The circulation of the western Indian Ocean is examined using a reduced‐gravity model with one active layer and realistic basin geometry for the entire Indian Ocean north of 30°S. The Hellerman and Rosenstein monthly mean wind stress climatology is used to force the model. The numerical simulations reproduce the observed (Luyten and Roemmich, 1982) 26‐day waves along the equator and the 50‐day oscillations (Mysak and Mertz, 1984; Schott et al. 1988) between the equator and Madagascar. The 25‐ to 28‐day oscillations of the model meridional velocity component agree with observed values of period, amplitude, wavelength, group velocity, and phase of the seasonal modulation. The model oscillations, which are excited in August and persist into February–March, are shown to be the result of Yanai waves generated between the western boundary and 50°E. During the southwest monsoon, the Yanai waves are initiated by a complex barotropic instability associated with the southern gyre. During the early stages of the northeast monsoon, the 26‐day Yanai waves are generated by resonant forcing due to the intrusion into the equatorial waveguide of a standing, 800‐ to 900‐km‐wavelength meander of the eastward flow fed by the East African Coastal Current. Hence the simulation reveals that the 26‐day oscillations in the equatorial Indian Ocean are excited by mechanisms significantly different than that believed to be responsible for the 20‐ to 30‐day oscillations in the equatorial Atlantic and Pacific oceans. The numerical simulation also shows a 50‐day oscillation between the equator and Madagascar west of 50°E. This periodicity is due to Rossby waves generated by a barotropic instability associated with the East African Coastal Current beginning about April each year. No evidence of the 50‐day period oscillation is found in a corresponding linear simulation. Hence the barotropic instability of the oceanic currents in this region is offered as an alternative to direct wind forcing as the generating mechanism for the observed 40‐ to 60‐day oscillations in the western Indian Ocean.