Diagnosing Mesoscale Vertical Motion from Horizontal Velocity and Density Data

Abstract

The mesoscale vertical velocity is obtained by solving a generalized omega equation (? 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 uh in the AJ reached maxima of 1.2 m s?1 for the three surveys, with extreme Rossby numbers of ?/f ≈ ?0.9 in the WAG and +0.9 in the AJ (where ? is the vertical vorticity and f is the Coriolis parameter). The generalized ? equation includes the ageostrophic horizontal flow. It is found that the most important ?forcing? term in this equation is (?f?ph + ?h?) · ?2huh, where ?ph is the horizontal (pseudo) vorticity and ? is the buoyancy. This term is related to the horizontal advection of vertical vorticity by the vertical shear velocity, uhz · ?h?. Extreme values of the diagnosed vertical velocity w were located at 80?100 m with max{w} ? [34, 45] and min{w} ? [?64, ?34] m day?1. Comparison with the quasigeostrophic (QG) ? 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 (Qh ≡ ?huh · ?h?) in the ? equation.