Scatterometer observations of wind variations induced by oceanic islands: Implications for wind-driven ocean circulation

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 19degrees and 20degreesN. East of 170degreesW, 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.