ATMOSPHERIC BOUNDARY-LAYER RESPONSE TO SEA-SURFACE TEMPERATURES DURING THE SEMAPHORE EXPERIMENT

The sensitivity of the marine atmospheric boundary layer (MABL) subjec ted to sea surface temperatures (SST) during the Structure des Echange s Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment in 1993 has been studied. Atmosph eric analyses produced by the Action de Recherche, Petite Echelle, Gra nde Echelle (ARPEGE) operational model at the French meteorological we ather service assimilated data sets collected between October 7 and No vember 17, 1993, merged with the Global Telecommunication System (GTS) data. Analyses were validated against independent data from aircraft instruments collected along a section crossing the Azores oceanic fron t, not assimilated into the model. The responses of the mean MABL in t he aircraft cross section to changes in SST gradients of about 1 degre es C/100 km were the presence of an atmospheric front with horizontal gradients of 1 degrees C/100 km and an increase of the wind intensity from the cold to the warm side during an anticyclonic synoptic situati on. The study of the spatiotemporal characteristics of the MABL shows that during 3 days of an anticyclonic synoptic situation the SST is re markably stationary because it is principally controlled by the Azores ocean current, which has a timescale of about 10 days. However, the t emperature and the wind in the MABL are influenced by the prevailing a tmospheric conditions. The ocean does not appear to react to the surfa ce atmospheric forcing on the timescale of 3 days, whereas the atmosph eric structures are modified by local and synoptic-scale advection. Th e MABL response appears to be much quicker than that of the SSTs. The correlation between the wind and the thermal structure in the MABL is dominated by the ageostrophic and not by the geostrophic component. In particular, the enhancement of the wind on either side of the SST fro nt is mainly due to the ageostrophic component. Although the surface h eat fluxes are not the only cause of ageostrophy, the surface buoyancy flux Q(b) appears to be an important local source.