Mean flow and variability in the Kuroshio Extension from Geosat altimetry data

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Abstract
Using altimeter data from the Geosat Exact Repeat Mission (ERM), we investigated the mean flow and temporal and spatial variations of the Kuroshio Extension in the region of 140°–180°E and 30°–40°N. Mean surface height profiles were estimated along individual tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian‐shaped and by successively fitting this synthetic current's height profile to the residual height data. Using the mean profiles from ascending and descending tracks, we derived the mean surface height field by an inverse method and obtained the absolute surface height fields for the first 2.5 years of the Geosat ERM. Both the mean and the instantaneous height fields thus derived compared well with the available hydrographic data and the SST patterns from the NOAA satellites. The mean surface height difference across the Kuroshio Extension attains its maximum around 146°E between the two quasi‐stationary meanders, and its decrease thereafter is mainly due to large‐scale recirculations on the southern side of the Kuroshio Extension. The ratio of the eddy kinetic energy over the mean kinetic energy has a nearly constant value of 1.5–2.0 along the Kuroshio Extension path. Propagation of mesoscale fluctuations in the height fields is generally westward except for the upstream region of the Kuroshio Extension. Effects of deep mean flow and baroclinic shear are found to be important in explaining the observed propagation speeds. In the upstream region of 141°E and 154°E, annual variations in the surface height difference across the Kuroshio Extension(δh) have a September maximum with an average amplitude of 0.2m. For large‐scale interannual fluctuations, anomalies in δh are found to be significantly correlated with those of the current axis positions: a larger surface height difference corresponds to a more northerly position of the Kuroshio Extension. The interannual changes in δh are possibly related to the 86/87 ENSO event in the low‐latitude Pacific Ocean.