Diagnosing mesoscale vertical motion from horizontal velocity and density data

The mesoscale vertical velocity is obtained by solving a generalized omega equation (omega equation) using density and horizontal velocity data from three consecutive quasi-synoptic high-resolution surveys in the Alboran Sea. The Atlantic Jet (AJ) and the northern part of the Western Alboran Gyre ( WAG) were observed as a large density anticyclonic front extending down to 200-230 m. The horizontal velocity u(h) in the AJ reached maxima of 1.2 m s(-1) for the three surveys, with extreme Rossby numbers of zeta/f approximate to -0.9 in the WAG and +0.9 in the AJ ( where zeta is the vertical vorticity and f is the Coriolis parameter). The generalized omega equation includes the ageostrophic horizontal flow. It is found that the most important forcing term in this equation is (f zeta(ph) + del(h rho)) . del(2)(h)u(h), where zeta(ph) is the horizontal ( pseudo) vorticity and rho is the buoyancy. This term is related to the horizontal advection of vertical vorticity by the vertical shear velocity, u(hz) . del(h zeta). Extreme values of the diagnosed vertical velocity w were located at 80-100 m with max{w} subset of [34, 45] and min{w} subset of [-64, -34] m day(-1). Comparison with the quasigeostrophic (QG) omega equation shows that, because of the large Rossby numbers, non-QG terms are important. The differences between w and the QG vertical velocity are mainly related to the divergence of the ageostrophic part of the total Q vector (Q(h) = del(h)u(h) . del(h rho)) in the omega equation.