Variability and Energetics of the Kuroshio Extension and Its Recirculation Gyre from the First Two-Year TOPEX Data

Abstract

Altimetry data from the first 21-month TOPEX mission (September 1992--June 1994) are analysed to investigate the sea surface height (SSH) fluctuations in the Kuroshio Extension and its southern recirculation gyre regions (25$\,^{\circ}$--40$\,^{\circ}$N, 136$\,^{\circ}$E--180$\,^{\circ}$). To separate the time-dependent (SSH) signals associated with the Kuroshio Extension from those associated with the westward recirculating flows, and to study the energetics of these currents, the author first estimated the mean SSH profiles along individual ground tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian shaped and by successively fitting the synthetic current's height profile to the time-dependent SSH data. The mean SSH field, including the influence from the recirculating flows, is then derived through the constraint from climatological hydrographic data.During the 21-month period of the TOPEX mission, the eddy kinetic energy (EKE) of the Kuroshio Extension had relatively uniform values during three separate stages, each of which lasted longer than 6 months. A significant drop in the EKE level is found in the Kuroshio Extension after the end of 1993. In contrast, the EKE level in the southern recirculation region increased steadily over the two-year period. The energetics analysis shows that this EKE increase is due to the energy transfers from the mean flow field to the eddy field through barotropic instabilities. These barotropic eddy processes are found to be less important for the EKE changes in the Kuroshio Extension and its northern areas. On both seasonal and interannual timescales, the large-scale path fluctuations of the Kuroshio Extension are found to correlate significantly with the surface transport fluctuations: a more northerly Kuroshio Extension tends to correspond to a larger surface transport. Over the two-year period, both the eastward-flowing Kuroshio Extension and the westward recirculating flows weakened steadily. This decline in the intensity of the recirculation gyre is caused by the energy transfers from the mean flow field to the eddy field occurring in the region south of the Kuroshio Extension.