STUDY OF THE AIR-SEA INTERACTIONS AT THE MESOSCALE - THE SEMAPHORE EXPERIMENT

The SEMAPHORE (Structure des Echanges Mer-Atmosphere, Proprietes des H eterogeneites Oceaniques: Recherche Experimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic betwee n the Azores and Madeira. It was centered on the study of the mesoscal e ocean circulation and air-sea interactions. The experimental investi gation was achieved at the mesoscale using moorings, floats, and ship hydrological survey, and at a smaller scale by one dedicated ship, two instrumented aircraft, and surface drifting buoys, for one and a half month in October-November (IOP: intense observing period). Observatio ns from meteorological operational satellites as well as spaceborne mi crowave sensors were used in complement. The main studies undertaken c oncern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the va rious topics. From data analysis and model simulations, the main chara cteristics of the ocean circulation were deduced, showing the close re lationship between the Azores front meander and the occurrence of Medi terranean water lenses (meddles), and the shift between the Azores cur rent frontal signature at the surface and within the thermocline. Usin g drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was mad e evident. A 2 degrees C decrease and a 40-m deepening of the mixed la yer were measured within the IOP, associated with a heating loss of ab out 100 W m(-2). This evolution was shown to be strongly connected to the occurrence of storms at the beginning and the end of October. Abov e the surface, turbulent measurements from ship and aircraft were anal yzed across the surface thermal front, showing a 30% difference in hea t fluxes between both sides during a 4-day period, and the respective contributions of the wind and the surface temperature were evaluated. The classical momentum flux bulk parameterization was found to fail in low wind and unstable conditions. Finally, the sea surface was invest igated using airborne and satellite radars and wave buoys. A wave mode l, operationally used, was found to get better results compared with r adar and wave-buoy measurements, when initialized using an improved wi nd field, obtained by assimilating satellite and buoy wind data in a m eteorological model. A detailed analysis of a 2-day period showed that the swell component, propagating from a far source area, is underesti mated in the wave model. A data base has been created, containing all experimental measurements. It will allow us to pursue the interpretati on of observations and to test model simulations in the ocean, at the surface and in the atmospheric boundary layer, and to investigate the ocean-atmosphere coupling at the local and mesoscales.