Wind‐induced Kuroshio warm water intrusion into Sagami Bay

Abstract

The mechanisms of the periodic Kuroshio warm water intrusion (KWWI) into Sagami Bay through the Oshima West Channel (OWC) and the fluctuation of the current field in the bay were investigated using high‐frequency oceanic radar observation data, satellite imagery, and meteorological data. During the period from 15 December 2000 to 16 January 2001 the Kuroshio took a nonlarge meander path, looping south around the Kii Peninsula and then turning sharply north toward the Izu Peninsula (IP). Kuroshio warm water (KWW) then flowed into Sagami Bay periodically through the OWC on a synoptic timescale of 8–11 days. Statistical analysis of the data indicated that the occurrence of the KWWI and the variability in the Kuroshio front were highly coherent with wind velocity field fluctuations corresponding to the passage of cyclones across northern Japan. When southward to southwestward winds dominated in and around Sagami Bay, the Kuroshio front approached the southern coast of the IP, and then the KWW strongly intruded into the bay through the OWC. When eastward to northeastward winds prevailed, the front moved offshore about 10–20 km, ending the KWWI. The distances of the Kuroshio front movements were consistent with the internal radius of the deformation estimated from conductivity‐temperature‐depth data collected off the IP, suggesting that the Kuroshio front movement was induced by the onshore‐offshore surface Ekman transport associated with the wind velocity field regime shift corresponding to the passage of cyclones. In addition, examination of the vorticity dynamics in the northern part of the bay based on radar‐derived surface current data indicated that the cyclonic circulation generated in the bay was enhanced due to the transport of the positive relative vorticity by the KWWI plus the horizontal convergence accompanied by the stretching of the vorticity.