The annual cycle of circulation in the southwest subtropical Pacific, diagnosed in an OGCM

William Kessler

NOAA / PMEL

 

ABSTRACT: An ocean GCM, interpreted in light of linear models and sparse observations, is used to diagnose the dynamics of the annual cycle of circulation in the western boundary current system of the southwest Pacific. The simple structure of annual wind stress curl over the South Pacific produces a large region of uniformly-phased, stationary thermocline depth anomalies such that the western subtropical gyre spins up and down during the year, directing flow anomalies alternately towards and away from the boundary at its northern end, near 10S. The response of the western boundary currents is to redistribute these anomalies northward towards the equator and southwards to the subtropical gyre; a redistribution that is determined principally by offshore thermocline depth anomalies due to linear Rossby processes, not to boundary dynamics. When the subtropical gyre and South Equatorial Current (SEC) are strong (in the second half of the year), the result is both increased equat orward transport of the New Guinea Coastal Current, and poleward transport anomalies along the entire Australian coast. Because of this opposite phasing of boundary current anomalies across 10S, annual migration of the bifurcation point of the total SEC, near 18S in the mean, has no significance regarding variability of transport from subtropics to equator.