Surface current variability east of Okinawa Island obtained from remotely sensed and in situ observational data

Abstract

Abstract. We investigated ocean surface current variability in the region east of Okinawa Island in the spring of 1998 using in situ and remotely sensed data including surface velocity estimates inferred from HF ocean radars. Most previous studies utilizing HF ocean radars were limited to shallow continental shelves. In contrast, most of the observation area in this study was in the open ocean. During the observation period the HF ocean radars sampled a region where anticyclonic and cyclonic mesoscale eddies were adjacent to each other. The HF-radar-derived ocean currents agree with those measured by a current meter. The surface currents were highly variable in both time and space and were affected by the eddy field in the offshore region. The current field was related to the near-surface water temperature. For example, a local temperature rise was often associated with northeastward flows. The surface currents in regions shallower than about 1000 m had a significant correlation with local wind forcing. Our HF ocean radars often detected a strongly convergent zone, which was different in character from those described in previous studies in that it was in the deep ocean. From other data such as National Oceanic and Atmospheric Administration advanced very high resolution radiometer imagery, we concluded that the convergent zone was due to mesoscale eddy fronts. IntroductionMeasurements of ocean surface currents are important for physical oceanography, fishery studies, and monitoring the coastal environment for contaminants such as oil pollution. One promising method that efficiently measures the evolving current field over broad spatial areas uses high-frequency (HF) ocean 31,057

Southward recirculation of the East China Sea Kuroshio west of Okinawa Island

“…The correlation coefficients were greater than 0.85 and the root-mean-square difference of velocity components between the sensors was about 10 cm s À1 [Hisaki et al, 2001]. Therefore, we can consider that the HF ocean radar-estimated currents are sufficiently accurate for our purposes.…”

“…The currents were spatially and temporally interpolated to 1.5 km regular grid points. Details of the HF ocean radar system are described by Hisaki et al [2001].…”

“…[11] The HF radar-estimated currents were compared with the currents measured by the current meter described by Hisaki et al [2001]. The correlation coefficients were greater than 0.85 and the root-mean-square difference of velocity components between the sensors was about 10 cm s À1 [Hisaki et al, 2001].…”

“…We analyzed ocean surface currents observed by HF ocean radars. HF ocean radar is used to measure ocean surface currents [e.g., Barrick et al, 1977;Prandle and Ryder, 1985;Takeoka et al, 1995;Hisaki et al, 2001;Hisaki and Naruke, 2003] and surface waves [e.g., Hisaki, 1996Hisaki, , 2002Hisaki, , 2004Hisaki, , 2005Hisaki, , 2006 over broad spatial areas. The method is to radiate high-frequency radio waves and analyze the backscattered signals from the ocean.…”

A bistatic HF radar for surface current mapping

“…The most commonly used are backscatter radars that use Doppler principle to measure current velocity [1,2,3]. A single backscatter radar can measure components of flow along the radial lines emanating from the radar site.…”