Modeling and analysis of ageostrophic circulation over the azores oceanic front during the SEMAPHORE experiment

In the conventional quasigeostrophic (QG) form of the omega equation develo ped by Hoskins ct al., the unique forcing of vertical velocity is the geost rophic deformation. As the QG or even the semigeostrophic (SG) hypotheses a re not adapted to study the frontal dynamics in the atmospheric boundary la yer, this paper proposes a generalized expression of the Hoskins et at. for m of the vertical velocity. Two thermal and three dynamical sources of the vertical velocity are identified. These forcings allow for identification o f each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer. This new form of the w equation is used to explain wind increase in the atmospheric boundary layer over the warm wa ters of the sea surface temperature (SST) front observed during a fair anti cyclonic day of the SEMAPHORE experiment (1993) and simulated with a nonhyd rostatic mesoscale atmospheric model. Since the SST gradients are weak (of the order of 1.5 degreesC 100 km(-1)), the surface turbulent heat forcing i s wt a dominant factor and all the five forcings of vertical velocity have rather the same intensity. In order to answer the question of how and over what thickness does the oce anic thermal front disturb significantly the atmospheric flow in the marine atmospheric boundary layer in such conditions, the degree of coupling betw een the turbulent heat forcing and the net forcing directly linked to the a tmospheric flow is examined. Their strong anticorrelations (r < -0.9) below 200 m indicate that the ageostrophic circulation and the turbulent heat fl uxes are in interregulation in this atmospheric layer, which can be assimil ated to an internal boundary layer for the flow. This interregulation works in such a fashion to minimize the atmosphere thermal wind imbalance throug h an adaptation of the atmospheric flow, but also, to some extent, of the s urface turbulent heat fluxes themselves.