Seasonal Eddy Field Modulation of the North Pacific Subtropical Countercurrent: TOPEX/Poseidon Observations and Theory

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Abstract
Altimetry data from the first 5¼-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 cm2 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 cm2 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 2½-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... Abstract Altimetry data from the first 5¼-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 cm2 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 cm2 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 2½-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...

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