| Satellite altimeters can measure the sea-surface dynamic topography down to the oceanic mesoscale and provide irreplaceable information for ocean climate monitoring. The altimeter data can be utilized much more efficiently by being combined with in-situ oceanographic data. Here are shown two examples; one is the surface flow field of the North Pacific derived by combined use of altimeter and surface drifter data, and the other is the Kuroshio transport estimated from altimeter data by using empirical relationship between the transport and sea-surface height difference, which has been obtained from in-situ intensive observations. Example 1: The surface flow field. In the altimeter data processing, the information on the temporal mean topography is lost. Here, the mean surface flow field is estimated by combining anomalies of sea-surface dynamic topography obtained from altimeter data with in-situ surface velocities estimated from trajectories of surface drifters, as follows. Sea-surface velocity may be divided into the temporal mean and anomaly. The velocity anomalies are derived from satellite altimeter data with geostrophic approximation. Where a drifter measured the surface velocity, the temporal mean velocity can be estimated by subtracting the altimeter-derived velocity anomaly at that time from the drifter-measured surface velocity. The method is applied to the surface flow field of the North Pacific, using TOPEX/POSEIDON and ERS-1/2 altimeter data (CLS gridded data), and WOCE-TOGA surface drifter data obtained from October 1992 through August 2001. The temporal mean velocity field is estimated with a resolution of quarter degrees in both latitude and longitude. The instantaneous velocity field is estimated by summing up the temporal mean velocity field and velocity anomalies, every ten days during the nine-year period of 1993-2001. The instantaneous flow field shows energetic variability of the Kuroshio Extension vividly. Example 2: The Kuroshio transport. In 1993-1995, the ASUKA (Affiliated Surveys of the Kuroshio off Cape Ashizuri) Group carried out intensive observations of the Kuroshio south of Japan, including direct current measurements and frequently repeated hydrographic surveys along a subsatellite track of TOPEX/POSEIDON. Velocity sections are obtained by geostrophic calculation using the hydrographic survey data, referenced to velocities observed at mid and abyssal depths. Estimated transports of the Kuroshio have a high correlation with sea-surface height differences across the Kuroshio. Having this relationship and using the altimeter data, we obtained a time series of Kuroshio transport over nine years (1993-2001) at ten-day intervals. The Kuroshio transport, excluding contributions by local recirculations, is estimated to be 43 Sv on average. The range of annual fluctuation is found to be much smaller than that of transport expected from the interior wind stress curl. |
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