On the Kuroshio Extension near the Emperor Seamounts

Abstract

In June–July 1982 and November 1983, studies were carried out of the vertical structure of the Kuroshio Extension along 167°E and the path of the current as it approached the Emperor Seamount chain; in addition, records from moored current meters at three locations (two at 167°E and one in the Main Gap of the Emperor Seamount chain) were obtained. The Kuroshio Extension in both 1982 and 1983 turned cyclonically (meander wave length of 450 km) as it approached the chain; the axis of the surface current was located in deep water (>3.5 km) in 1982 and in relatively shallow water (0.5 km) in 1983. In 1982 the horizontal temperature gradient decreased, and the density surfaces down to 2000 m ascended as the Kuroshio Extension flowed over the Emperor Seamounts. In neither year was there evidence of current splitting as it crossed the seamounts. In 1983, in contrast to 1982, there was no weakening of the horizontal temperature gradient, and the Kuroshio Extension went over a very shallow portion of Kinmei Seamount ( < 500 m). The current measurements made at the time of the hydrographic section show that the instantaneous Kuroshio extended to the bottom (abyssal eastward current of 2 cm s−1). The deep low‐frequency eddy kinetic energy values are intermediate between those measured at 152°E and those at 158°W, suggesting a large‐scale eastward decrease in deep eddy activity. Zonal mean flow near 3500 m at 167°E was essentially zero (0.1 cm s−1); the mean meridional components were larger (−2.1 and −0.7 cm s−1). At the southern side of the gap the deep flow was unusually steady (KE/KM < 1) and directed northeastward into the eastern basin of the North Pacific; low ratios (O(1)) were also observed in the lower thermocline compared with those at 167°E (O(10–100)). The frequency content of the gap kinetic energy differed from that measured at 167°E: the highest frequency band (2 hours < P < 2 days) was the most energetic, whereas at 167°E the mesoscale band (20 days < P < 150 days) was the most energetic. The flow in the gap resembles in many of its characteristics that observed in two other channel flows: the Vema Channel and the Charlie Gibbs Fracture Zone.