Variability of the Kuroshio Current south of Sagami Bay as observed using long‐range coastal HF radars

Abstract

Ocean surface currents between Cape Nojima and Hachijo Island have been mapped since fall 2000 using a pair of SeaSonde long‐range HF radars. During September 2000 through December 2001, the no large meander (NLM) nearshore (n) and offshore (o) modes were observed. The surface current maps allowed new details to be observed in the structure of these modes and the transitions between them. Two noteworthy phenomena include blocking of the surface currents when the Kuroshio was in an extreme nearshore position behind the Zenisu Ridge and the “Kyucho” pattern when the current retroflexed from the NLMo position. In early 2004, a large meander (LM) developed and was well established in its historical position south of Honshu by August 2004. Subsequently, the LM continued propagating downstream, but stopped when it encountered the Izu Ridge. It then moved back to its original position near 137.5°E, and the process repeated itself. This east‐west oscillation resulted in two common positions for the LM south of Sagami Bay: from its more westerly position (LMw, for west) the Kuroshio had a very favorable approach to the deep gap between Miyake Island and the Zenisu Ridge, and the currents were strongly toward the northeast through the region. When the meander moved up against the ridge (LMe, for east), a strong along‐ridge current toward the northwest was evident, which turned sharply northeast when it encountered the shallow Zenisu Ridge. The dominant timescale for this within‐mode variability was about 30 days.