Meridional Location of the Pacific Ocean Subtropical Gyre

Abstract

The observed subtropical gyre in the North Pacific shows a shift in meridional location with depth. At shallow levels the density deviation peaks at around 15°N while at deep levels the peak is more like 30°N. It is argued here using analytical solutions to the beta-plane shallow-water equations that such a shift can be explained by the effects of oceanic dissipation processes. These solutions show that the highly damped solution is approximately proportional to Ekman pumping whereas the lightly damped case tends toward the classical Sverdrup solution. In the North Pacific, Ekman pumping peaks near 15°N while the Sverdrup solution peaks at 30°N. It is further demonstrated that 1) density deviations in the upper ocean are more highly influenced by higher order baroclinic modes than those in the deep, which are influenced by the lower modes, and 2) constant dissipation effectively acts much more strongly on the higher order baroclinic modes because of their slower speeds and smaller Rossby radii. These two factors thus explain the observed shift in the gyre with depth.