Abstract
A method of estimating surface current velocities of the Kuroshio based on a single NOAA AVHRR image is developed and verified in reference to on-board measurements of surface current velocity during the period from 1986 to 1987. On the assumption that the geostrophic balance holds and that the effect of salinity on the density of the surface water along the path of the Kuroshio is negligibly small, the method is derived from the thermal wind equation in oceanography. The analysis of 10 years' ships observation data indicates the seasonal as well as spatial variabilities in the relationship between the gradient of geopotential anomaly and that of the surface water temperature. In the sea area off Shikoku (See Fig.1) the linear relationship based on regression equations is well established except in summer. It is found that the temperature difference of 1°C per 55.6 km across the flow of the Kuroshio corresponded to 2.1 m2/s2 of geopotential anomaly difference on an average, which turns out to be about 0.85 knots of geostrophic surface current speed. Though the proposed method is restricted to be applicable to the northern edge of the Kuroshio, where the sea surface topography gradient has a significant correlation with the sea surface temperature gradient, the fairly good agreement is attained between the estimated and the observed current velocities with the rms errors of about 0.5 knots for the maximum current speed of 3 to 4 knots and of about 14 degrees in current direction.
