A STUDY OF THE OMEGA-EQUATION FOR DIAGNOSING VERTICAL MOTIONS AT OCEAN FRONTS

Estimation of vertical velocity is a key issue for understanding ocean physics and transport of biogeochemical tracers. We examine the accur acy of estimating vertical velocity in fronts with the omega equation. The diagnostic performance of the omega equation is evaluated by usin g vertical velocities obtained from simulation of frontal instabilitie s in a primitive equation model as a reference. We use two traditional quasigeostrophic methods in which the how is either a geostrophic flo w computed from density or a nondivergent flow derived from vorticity and also test two new formulations: a quasigeostrophic method using th e total flow held and the semigeostrophic omega equation. Results show that all four formulations correctly diagnose the vertical velocity p attern. However, estimates provided by the traditional quasigeostrophi c formulations have systematic bias. In contrast, the two new techniqu es, which are practically equivalent, produce unbiased vertical veloci ty diagnostic at fronts. These results point out the importance of inc luding higher order dynamics than quasigeostrophy to take into account the ageostrophic advection in the front. Since adequate filtering of ADCP data is not yet available to obtain a suitable total flow, the se migeostrophic omega equation is proposed as the most valuable tool to compute vertical velocities from high resolution density measurements.