Modeling and Analysis of Ageostrophic Circulation over the Azores Oceanic Front during the SEMAPHORE Experiment

Abstract

In the conventional quasigeostrophic (QG) form of the ? equation developed by Hoskins et al., the unique forcing of vertical velocity is the geostrophic deformation. As the QG or even the semigeostrophic (SG) hypotheses are not adapted to study the frontal dynamics in the atmospheric boundary layer, this paper proposes a generalized expression of the Hoskins et al. form of the vertical velocity. Two thermal and three dynamical sources of the vertical velocity are identified. These forcings allow for identification of each of the physical processes acting simultaneously on the ageostrophic circulation in the boundary layer. This new form of the ? equation is used to explain wind increase in the atmospheric boundary layer over the warm waters of the sea surface temperature (SST) front observed during a fair anticyclonic day of the SEMAPHORE experiment (1993) and simulated with a nonhydrostatic mesoscale atmospheric model. Since the SST gradients are weak (of the order of 1.5°C 100 km?1), the surface turbulent heat forcing is not 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 oceanic thermal front disturb significantly the atmospheric flow in the marine atmospheric boundary layer in such conditions, the degree of coupling between the turbulent heat forcing and the net forcing directly linked to the atmospheric flow is examined. Their strong anticorrelations (r < ?0.9) below 200 m indicate that the ageostrophic circulation and the turbulent heat fluxes are in interregulation in this atmospheric layer, which can be assimilated to an internal boundary layer for the flow. This interregulation works in such a fashion to minimize the atmosphere thermal wind imbalance through an adaptation of the atmospheric flow, but also, to some extent, of the surface turbulent heat fluxes themselves.