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Fofonoff-type, Inertial-Mode Steady States in a Model of the Equatorial Oceans

Richard GreatbatchGeophysical Fluid Dynamics Program Princeton University, Princeton, NJ 08542

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Toshio YamagataGeophysical Fluid Dynamics Program Princeton University, Princeton, NJ 08542

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Abstract

We examine the steady state response in the equatorial regions of a nonlinear, one-active-layer, reduced-gravity shallow water ocean model with meridional boundaries that is driven by uniform wind forcing. When the wind forcing is westward, the final steady state is one of no motion with a zonal pressure gradient balancing the wind forcing, as implied by linear theory. However, when the wind forcing is eastward, a steady state is reached in which a narrow eastward jet along the equator is balanced by a broader westward return flow, the solution resembling Fofonoff's free inertial mode. The entire circulation is confined within equatorial regions. It is shown that Rossby wave energy propagating eastwards from the western boundary is crucial for setting up the mode. This steady solution with motion was found even in a case with weak wind forcing. However, it is easily “wiped out” by friction (esqecially lateral mixing) in this case. In general, the solutions are found to be sensitive to the lateral mixing which can prevent the equatorial jet from extending all the way to the eastern boundary.

Abstract

We examine the steady state response in the equatorial regions of a nonlinear, one-active-layer, reduced-gravity shallow water ocean model with meridional boundaries that is driven by uniform wind forcing. When the wind forcing is westward, the final steady state is one of no motion with a zonal pressure gradient balancing the wind forcing, as implied by linear theory. However, when the wind forcing is eastward, a steady state is reached in which a narrow eastward jet along the equator is balanced by a broader westward return flow, the solution resembling Fofonoff's free inertial mode. The entire circulation is confined within equatorial regions. It is shown that Rossby wave energy propagating eastwards from the western boundary is crucial for setting up the mode. This steady solution with motion was found even in a case with weak wind forcing. However, it is easily “wiped out” by friction (esqecially lateral mixing) in this case. In general, the solutions are found to be sensitive to the lateral mixing which can prevent the equatorial jet from extending all the way to the eastern boundary.

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