Evidence for SST-Forced Anomalous Winds Revealed from Simultaneous Radiosonde Launches from Three Ships across the Kuroshio Extension Front

Simultaneous launches of radiosondes were conducted from three research vessels aligned meridionally across a sea surface temperature (SST) front on the flank of the Kuroshio Extension. The soundings carried out every 2 h over 5 days in early July 2012 provided a unique opportunity in capturing unambiguous data on anomalous easterly winds derived from a pronounced meridional SST gradient. The data indicate that a meridional contrast in surface heat fluxes from the underlying ocean enhanced the air temperature anomaly across the SST front, which was observed from the surface up to 300-m altitude. Correspondingly, high and low pressure anomalies that reached 800-m altitude formed on the north and south sides of the SST front, respectively. These temperature and pressure anomalies were maintained even during the passage of synoptic-scale disturbances. Although the free-tropospheric winds are overall westerly, winds below the 1000-m level were easterly due to geostrophic anomalies driven by the northward pressure gradient near the surface. During periods of the northerlies at the surface, especially over the warmer side of the SST front, the wind direction changed in a clockwise direction from 1500 m to the surface, in the opposite sense to the Ekman spiral. The vertical wind shear is apparently in the thermal wind balance ascribed to the meridional contrast in air temperature derived from the SST anomaly.