Kuroshio Extension dynamics from satellite altimetry and a model simulation

Abstract

Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy‐resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20°S and has a resolution of 0.125° latitude and 0.176° longitude. The model is synoptically driven by daily 1000‐mbar winds from the European Centre for Medium‐Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time‐longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasi‐permanent meanders seen in hydrographic climatology (cyclonic at 138°E and anticyclonic at 144°E, 150°E, and 160°E). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm2 s−2 along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s−1 in the region east of 155°E is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the cross‐stream differential in SSH from the simulation and Geosat observations.