Scatterometer observations of wind variations induced by oceanic islands: Implications for wind-driven ocean circulation Cédric Chavanne, Pierre Flament, Rick Lumpkin, Bénédicte Dousset, and Abderrahim Bentamy Abstract. Scatterometer data at 25-km resolution are used to investigate the effects of the Hawaiian and Cabo Verde islands on the mean atmospheric flow. A wake of weak winds, flanked by accelerated winds, appears for each major island of both archipelagos. The resulting wind stress curl displays dipole-like structures, with positive values on the northern side and negative values on the southern side of the lee, extending several island diameters downwind. These curl anomalies reach a magnitude of 2 10 –6 Pa·m –1 and correspond to Ekman pumping velocities of 3 m·day –1 for Hawaii and 4 m·day –1 for Cabo Verde. They spin up cyclonic eddies on the north side and anticyclonic eddies on the south side of the lee of each island. The response of the ocean circulation is investigated using a simple Sverdrup balance. Two counter-rotating Sverdrup gyres are spun up west of the island of Hawaii and extend to the western boundary of the Pacific Ocean. They result in an eastward zonal transport confined between 19° and 20°N. East of 170°W, the surface expression of this transport coincides with the Hawaiian Lee Counter Current. Similar gyres are anticipated to form in the Atlantic Ocean, but remain to be observed. These results suggest that strong mesoscale patterns in the wind field occurring in the lee of high-topography features must be resolved to force global ocean circulation models. Résumé. Des données diffusiométriques sont utilisées à une résolution de 25 km pour étudier les effets des îles d’Hawaii et du Cap Vert sur l’écoulement atmosphérique. Un sillage de vents faibles, bordé de vents accélérés, est associé à chaque île des deux archipels. Le rotationnel de la tension du vent a une structure dipolaire, avec des valeurs positives au nord et négatives au sud du sillage, s’étendant de plusieurs fois le diamètre de l’île sous le vent. Les anomalies de rotationnel atteignent une amplitude de 2 10 –6 Pa·m –1 , correspondant à des vitesses de pompage d’Ekman de l’ordre de 3 m·jour –1 pour Hawaii et 4 m·jour –1 pour le Cap Vert. Elles engendrent des tourbillons cycloniques et anticycloniques au nord et au sud du sillage de chaque île. La réponse de la circulation océanique est étudié à l’aide d’une simple balance de Sverdrup. Deux gyres océaniques de Sverdrup se forment à l’ouest de l’île d’Hawaii, et s’étendent jusqu’au bord ouest du Pacifique. Un transport zonal vers l’est en résulte, confiné entre 19°N et 20°N. A l’est de 170°W, l’expression de ce transport coïncide avec le Hawaiian Lee Counter Current. La formation de gyres similaires est prévue dans l’Atlantique, mais n’a pas encore été observée. Les résultats suggèrent que les structures du vent à méso-échelle se formant dans le sillage d’îles montagneuses doivent être inclues dans le forçage des modèles de circulation océanique. 474 Introduction Oceanic islands with high topography drastically affect both the atmospheric and oceanic flows. Sharp horizontal wind shear lines often form on the edges of the island wake (Figure 1a), resulting in rapid variations of the Ekman transport in the ocean, strong upwelling and downwelling, and spin-up of mesoscale eddies (Barton, 2001). The input of vorticity from the wind stress to the ocean and the blocking effect of the island ridge on the impinging currents affect the large-scale oceanic circulation. These processes are enhanced when the wind is steady and capped by an atmospheric inversion layer, as in the trade-wind belt. Using National Aeronautics and Space Administration scatterometer (NSCAT) measurements at 25 km resolution, mean sea surface wind fields were computed in the vicinity of various islands (Chavanne et al., 2000; Hermes, 2000). Some of the most pronounced interactions with the atmospheric flow are found for Hawaii in the Pacific Ocean and Cabo Verde in the Atlantic Ocean. The Hawaiian archipelago presents a 600-km- long series of high vertical obstacles to the wind. It is situated south of the atmospheric North Pacific High, between 19°N and 22°N, in steady trade winds of 6 m·s –1 , with an inversion typically at 2000 m altitude (Chen and Feng, 1995), thus below 466 © 2002 CASI Can. J. Remote Sensing, Vol. 28, No. 3, pp. 466–474, 2002 Received 24 December 2001. Accepted 18 March 2002. C. Chavanne, 1 P. Flament, 2 and A. Bentamy. Département d’Océanographie Spatiale, Institut Français de Recherche pour l’Exploitation de la Mer, 29280 Plouzané, France. R. Lumpkin. Department of Oceanography, Florida State University, Tallahassee, FL 32306, U.S.A. B. Dousset. Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, U.S.A. 1 Corresponding author. Present address: Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, U.S.A. (e-mail: cedric@hawaii.edu). 2 Present address: Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, U.S.A.