CIRCULATION IN THE ALBORAN SEA AS DETERMINED BY QUASI-SYNOPTIC HYDROGRAPHIC OBSERVATIONS .2. MESOSCALE AGEOSTROPHIC MOTION DIAGNOSED THROUGH DENSITY DYNAMICAL ASSIMILATION

The 3D velocity field in the Alboran Sea (Western Mediterranean) is di agnosed through density dynamical assimilation in a primitive equation (PE) model with mesoscale resolution. The ageostrophic motion is comp uted from fields produced by short-term backward and forward integrati ons of the PE model initialized with quasi-synoptic CTD data. A weight function based on a low-pass digital filter (Digital Filter Initializ ation method) is applied to the resulting rime series of model variabl es 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 quasigeostroph ic (QG) omega equation. The two methods produce very similar results w ith maximum vertical motions in the range of 10-20 m d(-1) associated with thr differential advection of relative vorticity in small-scale j et meanders [upward (downward) motion upstream (downstream) of the rid ges]. Small local differences between PE and QG vertical velocities (t ypically 1-2 m d(-1))are attributed to known limitations of the QG the ory and to differences between the analyzed and the dynamically initia lized density fields. The horizontal ageostrophic motion in the wester n Alboran gyre (WAG) is in the same direction as the geostrophic motio n above 100 m but in the opposite direction below 100 m. While the hor izontal ageostrophic motion in the WAG can imply inflow or outflow dep ending 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.