Study of current fluctuations based on satellite altimetry data

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Measuring sea-surface height by the radar altimeter on board satellite is a very promising technique; it covers almost all the world oceans, there are no data loss due to the presence of clouds, and surface velocities can be estimated directly from the sea-surface dynamic topography (here abbreviated SSDT) which could be derived from the altimetry data. However, the error of the estimated satellite orbit height is too large, so far, to use the data in geophysical studies. In the present study, we develop a procedure to reduce this error by using optimal interpolation technique, estimate the temporal mean and fluctuation of the SSDT using the latest geoid data, and compare the results with those from the conventional oceanographic observations. We select the study area in the western North Pacific, and use the altimetry data by satellite SEASAT for about three months (July-October, 1978).In 1988, we developed the analysis procedure for the altimetry data, and estimated the temporal mean SSDT. The estimated mean elevation field relative to the geoid qualitatively shows the existence of the Kuroshio in a limited local area. But for the whole of the study area, it can not be used to estimate the mean SSDT (the accuracy of the geoid data is not satisfactory).In 1989, we estimated the temporal fluctuation SSDT, and compared the results with observed fluctuations of the Kuroshio axis, movements of the warm and cold water masses off Tohoku, and sea level fluctuations at island stations around Japan. The results show that the temporal fluctuation of the SSDT estimated from the satellite altimetry data represents the actual long-term fluctuations of the sea level very well.