Absolute geostrophic velocity within the Subantarctic Front in the Pacific Ocean

Abstract

Velocity measurements from a shipboard acoustic Doppler current profiler (ADCP) are used as a reference for geostrophic current calculations on six sections across the Subantarctic Front (SAF) in the Pacific Ocean. The resulting cross-track velocity estimates near the bottom range from 4 to 10 cm s-1 to the east in the eastward jet at the SAF; in adjacent regions of westward surface flow, the near-bottom velocity is usually to the west. On one section where simultaneous lowered ADCP velocity profiles are available, they confirm the results from the shipboard ADCP. Annual mean velocity sections from the Parallel Ocean Program numerical model also show near-bottom velocities exceeding 5 cm s-1, with the same tendency for the zonal velocity component near the bottom to match the direction of the surface jets. Transport across the entire Antarctic Circumpolar Current (ACC) cannot be estimated accurately from ADCP-referenced geostrophic sections because even a very small cross-track bias integrates to a large error. A preliminary look at the 1992 model transport stream function shows that the effect of bottom-referencing varies from section to section; it can cause 40-Sv recirculations to be missed, and can cause net transport to be underestimated or overestimated by O(30 Sv). Copyright 2001 by the American Geophysical Union.