Wind driven by a sea surface temperature front observed by 3-ship simultaneous atmospheric sounding in the Kuroshio Extension

Simultaneous GPS radiosonde soundings from three research vessels was carried out across a sea surface temperature (SST) front of the Kuroshio Extension every hour for five days in July 2012. A north to south array of the three vessels -- a north cold SST side, near the SST front and a south warm SST side -- provides direct conclusive evidence that the SST front excites local atmospheric circulations, which could not be offered by any single-ship soundings.

Large meridional difference of atmospheric heating from the underlying sea strengthened the north-south contrast of the air temperature, which extended up to the altitudes of 300 m. Associated SST-driven high and low air pressure anomalies up to 800 m were respectively formed in the north and south sides. These features were maintained even in the passage of a synoptic-scale disturbance. Easterly wind was persistently observed from the surface up to 1000 m although it was westerly in the overlying free atmosphere. This easterly is roughly maintained by the norward Coriolis force and the SST-driven southward pressure gradient force. The SST-driven easterly wind can bring about cold summer in Japan. A clockwise spiral of wind direction change from about 1000 m to the surface was observed in the northerly period in the free atmosphere. This spiral is in reverse to the Ekman spiral, and is accounted by the thermal wind balance owing to the SST-driven air temperature contrast. A see-breeze like meridional circulation under the altitudes of 300 m was also observed.