Diagnosing mesoscale vertical motion from horizontal velocity and density data

Abstract

The mesoscale vertical velocity is obtained by solving a generalized omega equation (ω 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 ζ/f ≈ -0.9 in the WAG and +0.9 in the AJ (where ζ is the vertical vorticity and f is the Coriolis parameter). The generalized ω equation includes the ageostrophic horizontal flow. It is found that the most important "forcing" term in this equation is (fζph + ∇h e) · ∇h2uh, where ζ ph is the horizontal (pseudo) vorticity and e is the buoyancy. This term is related to the horizontal advection of vertical vorticity by the vertical shear velocity, uhz · ∇hζ. 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) ω 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 e) in the ω equation. © 2005 American Meteorological Society.