BAROCLINIC INSTABILITIES OF NONZONAL OCEAN CURRENTS WITH APPLICATION TO THE KUROSHIO EXTENSION CURRENT

Using the TRANSPAC XBT data at a depth of 300 m, the regional variability of energy, time scales, length scales, and phase propagation of internal temperature fluctuations in the mid-latitude North Pacific was examined. The results showed that the regional variability in the eastern half of the basin is substantially different from that in the western half. The energy in the western North Pacific is high, particularly along the main axis of the Kuroshio Extension Current (KEC), and decreases toward the east. It also decreases towards the north and south, with an e-folding length scales of about 1000 km. Time scales are small near the western boundary and increase eastward. Both zonal and meridional length scales are large near the western boundary and decrease eastward. Phase propagation along the KEC appeared to be eastward, while that in the outer regions north and south of the KEC seemed to be westward with poleward components to the north and to the south, respectively.;In an attempt to explain these observed characteristics, three independent studies of baroclinic instabilities of nonzonal (including zonal) currents have been conducted; local baroclinic instability for the energetic fluctuations near the axis of the KEC, and radiation of energy from a traveling disturbance and radiating instability for the fairly energetic fluctuations in the region away from the KEC. The results suggests that these instabilities based on nonzonal (rather than zonal) currents are realistic mechanisms responsible for the observed spatial and temporal characteristics in the mid-latitude western North Pacific.