A study of the Kuroshio's seasonal variabilities using an altimetric‐gravimetric geoid and TOPEX/POSEIDON altimeter data

Abstract

The first year of TOPEX/POSEIDON (T/P) altimeter data were used to study the seasonal variabilities of the Kuroshio Current. The intercomparison between the T/P and gauge sea levels at selected tide gauge stations around Taiwan shows that the two have a correlation of about 0.9 at the immediate vicinity of the deep ocean, and both show an annual cycle with an amplitude of 15 cm. A 3′ × 3′ geoid for the Western Pacific was constructed by least squares collocation using gravity anomalies and sea surface gradients derived from Seasat, Geosat, ERS 1, and T/P altimetry. To account for the oceanographic signal in the altimeter data, we derived a covariance function for the gradients of the sea surface topography (SST) based on a global averaging concept and a spherical harmonic expansion of the Levitus SST to degree 50. The accuracy of the geoid ranges from 5 to 40 cm. The SST values derived from the geoid and the data along T/P's descending tracks were fitted to the hyperbolic functions in a two‐step procedure to find the Kuroshio's seasonal axes and the parameters that describe its characteristics. The mean variabilities in the Kuroshio's path, maximum velocity, baroclinic transport, and width are 22 km, 19 cm, 8 Sv, and 27 km, respectively. The averaged percentage variability of all quantities is 25%. All the variabilities are relatively large near the northeast coast of Taiwan and the starting point of the Kuroshio Extension. The large‐scale circulations over the Western Pacific were obtained by median filtering the SST. At the 1334‐km spatial scale, the Pacific's subtropical gyre is clearly visible, and it shows different features over the four seasons and its kinetic energy shows possible correlations with the determined parameters. Over the South China Sea, a warm ring with a radius of 300–400 km and a center at 15°N, 113°E was detected in the spring and summer.