Study of the air-sea interactions at the mesoscale: the SEMAPHORE experiment

• Main Authors: L. Eymard, S. Planton, P. Durand, C. Le Visage, P. Y. Le Traon, L. Prieur, A. Weill, D. Hauser, J. Rolland, J. Pelon, F. Baudin, B. Bénech, J. L. Brenguier, G. Caniaux, P. De Mey, E. Dombrowski, A. Druilhet, H. Dupuis, B. Ferret, C. Flamant, P. Flamant, F. Hernandez, D. Jourdan, K. Katsaros, D. Lambert, J. M. Lefèvre, P. Le Borgne, B. Le Squere, A. Marsoin, H. Roquet, J. Tournadre, V. Trouillet, A. Tychensky, B. Zakardjian
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications
• Subjects: Science; Q; Physics; QC1-999; Geophysics. Cosmic physics; QC801-809; Northeast Atlantic
• Online Access: https://doaj.org/article/5c5879c3a0ac443a8e655d602b8c5467

The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherche Expérimentale) experiment has been conducted from June to November 1993 in the Northeast Atlantic between the Azores and Madeira. It was centered on the study of the mesoscale ocean circulation and air-sea interactions. The experimental investigation 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). Observations from meteorological operational satellites as well as spaceborne microwave sensors were used in complement. The main studies undertaken concern the mesoscale ocean, the upper ocean, the atmospheric boundary layer, and the sea surface, and first results are presented for the various topics. From data analysis and model simulations, the main characteristics of the ocean circulation were deduced, showing the close relationship between the Azores front meander and the occurrence of Mediterranean water lenses (meddies), and the shift between the Azores current frontal signature at the surface and within the thermocline. Using drifting buoys and ship data in the upper ocean, the gap between the scales of the atmospheric forcing and the oceanic variability was made evident. A 2 °C decrease and a 40-m deepening of the mixed layer were measured within the IOP, associated with a heating loss of about 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. Above the surface, turbulent measurements from ship and aircraft were analyzed across the surface thermal front, showing a 30% difference in heat 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 ...