Seasonal eddy field modulation of the North Pacific subtropical countercurrent: TOPEX/Poseidon observations and theory

Altimetry data from the first 5 1/4-yr TOPEX/Poseidon mission (October 1992 -December 1997) are analyzed focusing on the North Pacific Subtropical Coun tercurrent (STCC) near the center of the Pacific's western subtropical gyre . The multiyear altimetry data reveal that the eastward-flowing STCC is a h ighly variable zonal current, whose area-averaged eddy kinetic energy level (338 cm(2) s(-2)) reaches half the eddy kinetic energy level of the Kurosh io Extension. The eddy kinetic energy of the STCC has a well-defined annual cycle with a maximum in April/May and a minimum in December/January. The p eak-to-peak amplitude of this seasonal eddy kinetic energy modulation excee ds 200 cm(2) s(-2). No such distinct annual cycle of the eddy kinetic energ y is found in any other zonal current of the North Pacific Ocean. Using a 2 1/2-layer reduced-gravity model representing the vertically sheared STCC-N orth Equatorial Current (NEC) system, it is shown that the seasonal modulat ion of the STCC's eddy field is a manifestation in the intensity of barocli nic instability. In spring the STCC-NEC system has a large vertical velocit y shear and a weak vertical stratification, subjecting it to strong barocli nic instability. In fall, reduction in the vertical velocity shear between the STCC and its underlying NEC, and intensification of the upper-layer str atification weakens the baroclinic instability. In comparison with the STCC of 19 degrees-25 degrees N, the altimetry data reveal that the westward-fl owing NEC existing between 10 degrees and 15 degrees N has a relatively low eddy kinetic energy level, despite being a stronger vertically sheared zon al current than the STCC. That the NEC is less eddy energetic is shown to b e due to both its presence in a low-latitude band and its unidirectional fl ow. Both of these factors make it more difficult to reverse the potential v orticity gradient of the mean state (i.e., satisfying the necessary conditi on for the baroclinic instability) in the NEC than in the STCC-NEC system.