A Nonconservative β-Spiral Determination of the Deep Circulation in the Eastern South Pacific

Abstract

The horizontal mean circulation and diapycnal flux divergence field in the South Pacific between Tahiti and the East Pacific Rise are obtained from a nonconservative ?-spiral inverse method applied to high quality hydrographic data. The diapycnal flux divergence term is found to be an essential part of the deep vorticity balance and proper resolution of this term is critical to the success of the ?-spiral calculation. The subthermocline circulation in this region of the South Pacific consists of three vertical regimes. Between the thermocline and approximately 1800?2000 m a cyclonic circulation pattern exists. A zone of minimum motion is found between 1800 and 2000 m. Below this zone, northward flow along the rise and westward flow in the north indicate an anticyclonic gyre, centered on 15°S, that extends approximately 1000?2000 km to the west of the East Pacific Rise. The deep vorticity balance associated with this flow system is primarily between meridional advection of planetary vorticity and the nonconservative diapycnal flux divergence term. Below the zone of minimum motion, the water mass characteristics, flow, diapycnal flux divergence, and vorticity balance are consistent with those of a large-scale circulation actively driven by hydrothermal buoyancy flux from the East Pacific Rise.