Influence of the Barotropic Mean Flow on the Width and the Structure of the Atlantic Equatorial Deep Jets

A representation of an equatorial basinmode excited in a shallow-watermodel for a single high-order baroclinic vertical normal mode is used as a simple model for the equatorial deep jets. The model is linearized about both a state of rest and a barotropicmean flow corresponding to the observedAtlantic Equatorial Intermediate Current System. It was found that the eastward mean flow associated with the North and South Intermediate Counter Currents (NICC and SICC, respectively) effectively shields the equator from off-equatorial Rossby waves. The westward propagation of these waves is blocked, and focusing on the equator due to beta dispersion is prevented. This leads to less energetic jets along the equator.On the other hand, thewestward barotropicmean flow along the equator reduces the gradient of absolute vorticity and hence widens the cross-equatorial structure of the basin mode. Increasing lateral viscosity predominantly affects the width of the basin modes’ Kelvin wave component in the presence of themean flow, while theRossby wave is confined by the flankingNICC and SICC. Independent of the presence of themean flow, the application of sufficient lateralmixing also hinders the focusing of off-equatorial Rossby waves, which is hence an unlikely feature of a low-frequency basin mode in the real ocean.