Circulation in the Alboran Sea as determined by quasi-synoptic hydrographic observations. Part II: Mesoscale ageostrophic motion diagnosed through density dynamical assimilation

Abstract

The 3D velocity field in the Alboran Sea (Western Mediterranean) is diagnosed through density dynamical assimilation in a primitive equation (PE) model with mesoscale resolution. The ageostrophic motion is computed from fields produced by short-term backward and forward integrations of the PE model initialized with quasi-synoptic CTD data. A weight function based on a low-pass digital filter (Digital Filter Initialization method) is applied to the resulting time series of model variables to obtain the final, dynamically balanced, density and 3D velocity fields. The diagnosed ageostrophic motion is interpreted by comparing the vertical velocity field with that obtained from the quasigeostrophic (QG) ω equation. The two methods produce very similar results with maximum vertical motions in the range of 10-20 m d-1 associated with the differential advection of relative vorticity in small-scale jet meanders [upward (downward) motion upstream (downstream) of the ridges]. Small local differences between PE and QG vertical velocities (typically 1-2 m d-1) are attributed to known limitations of the QG theory and to differences between the analyzed and the dynamically initialized density fields. The horizontal ageostrophic motion in the western Alboran gyre (WAG) is in the same direction as the geostrophic motion above 100 m but in the opposite direction below 100 m. While the horizontal ageostrophic motion in the WAG can imply inflow or outflow depending on the position of local meanders, the gyre-scale ageostrophic circulation is characterized by convergence above 100 m and divergence below 100 m, implying an average downward motion of less than 1 m d-1. The general success of this assimilation approach could provide an alternative to QG diagnosis in mesoscale dynamics.