Abstract

The response of the deep equatorial ocean to an oscillatory baroclinic western boundary current is investigated in a continuously stratified primitive equation model. The symmetry of the current about the equator is such that mixed Rossby-gravity (MRG) waves are excited in the western part of the equatorial ocean. Depending on the forcing frequency, short to long scale (when compared to equatorial Rossby radius) monochromatic MRG waves are selected. The subsequent MRG wave destabilization generally leads to a much higher vertical mode response than the forced MRG wave mode. In a channel, short MRG waves are destabilized by shear instability (Hua et al., 2007). In a basin, the destabilization occurs in the vicinity of the western boundary and leads to the formation of finite amplitude, nonlinear jets in the entire equatorial basin. The space and time pattern of the jets correspond to low-frequency oscillating equatorial basin-modes, the period of which is set by the dominant vertical mode of the response. The vertical scale of the jets is a function mainly of the forcing period and is independent of the forcing vertical mode, as long as the excited MRG waves are in a sufficiently short regime to be unstable.As a result, an oscillatory western boundary current leads to a permanent equatorial zonal circulation, unlike a steady western boundary current. But most importantly, MRG wave destabilization appears to be a plausible formation mechanism for the observed Equatorial Deep Jets. The spatial and dynamical characteristics of the zonal circulation achieved with a 60-day forcing period are indeed compatible with the observations in the Atlantic Ocean.

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