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 Countercurrent (STCC) near the center of the Pacific's western subtropical gyre. The multiyear altimetry data reveal that the eastward-flowing STCC is a highly variable zonal current, whose area-averaged eddy kinetic energy level (338 cm(2) s(-2)) reaches half the eddy kinetic energy level of the Kuroshio 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 peak-to-peak amplitude of this seasonal eddy kinetic energy modulation exceeds 200 cm(2) s(-2). No such distinct annual cycle of the eddy kinetic energy 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-North Equatorial Current (NEC) system, it is shown that the seasonal modulation of the STCC's eddy field is a manifestation in the intensity of baroclinic instability. In spring the STCC-NEC system has a large vertical velocity shear and a weak vertical stratification, subjecting it to strong baroclinic instability. In fall, reduction in the vertical velocity shear between the STCC and its underlying NEC, and intensification of the upper-layer stratification weakens the baroclinic instability. In comparison with the STCC of 19 degrees-25 degrees N, the altimetry data reveal that the westward-flowing NEC existing between 10 degrees and 15 degrees N has a relatively low eddy kinetic energy level, despite being a stronger vertically sheared zonal current than the STCC. That the NEC is less eddy energetic is shown to be due to both its presence in a low-latitude band and its unidirectional flow. Both of these factors make it more difficult to reverse the potential vorticity gradient of the mean state (i.e., satisfying the necessary condition for the baroclinic instability) in the NEC than in the STCC-NEC system.