Characteristics of ocean surface winds in the lee of an isolated island observed by synthetic aperture radar

Characteristics of ocean surface winds around an isolated island are examined in relation to atmospheric stability using a synthetic aperture radar (SAR) and rawinsonde sounding observations. The SAR-derived winds on 22 May 2009 indicate a low-level jet extending over 30 km behind the island. Around the time of SAR acquisition, winds intensified on the leeward side in association with the stabilization of stratified flows, which suggests the connection of the SAR-derived jet with downslope winds. A number of SAR-derived winds elucidate typical wind patterns and their transitions depending on the nondimensional mountain height ĥ.For cases of large ĥ (>2), a wake is formed in the lee of the island and low-level jets produce strong wind shear on both sides of the wake. For cases of relatively small ĥ (<1.75), although a weak wind region is formed behind the mountain, no wind jets develop. As a transition of the above two cases (1 < ĥ< 2.5), a low-level jet develops in the lee of the island, as in the case on 22 May 2009. These wake configurations and their ĥ-dependent transitions seem to correspond to major regimes for hydrostatic flow over topography with constant upstream speed and buoyancy frequency: small-amplitude waves (ĥ< 1.75), wave breaking (1 < ĥ< 2.5), and flow splitting (ĥ> 2.5). It is noted that the ocean surface winds behind the island are very sensitive to ĥ around the transition point, changing up and down. The occurrence of each wind pattern shows clear seasonal features dependent on atmospheric stability.