The barotropic response of the subpolar North Pacific to stochastic wind forcing

Abstract

In a recent observational study, Koblinsky et al. (1989) attempted to demonstrate that the topographic Sverdrup balance can account for mesoscale current fluctuations with periods between 10 and 100 days in the abyssal North Pacific. However, the topographic Sverdrup balance proved difficult to verify systematically over the North Pacific. In this paper the conditions required to obtain the topographic Sverdrup balance are investigated through numerical experiments with a stochastically forced, barotropic, quasi‐geostrophic numerical model of the subpolar North Pacific. In the numerical experiments the topographic Sverdrup balance emerges from area averages of the terms in the vorticity equation, but only for periods longer than about 40 days when the averages are taken over 4°×4° region. This period increases when averages are taken over progressively smaller regions. An examination of vorticity balances at individual points along sections through the model domain indicates that the topographic Sverdrup balance is not obtained at individual points, even for very long period motions. The possibility of verifying topographic Sverdrup dynamics with oceanic data is considered. It is argued that topographically generated small scales in the velocity field reduce the likelihood that the topographic Sverdrup balance can be systematically observed in velocity measurements from individual current meter moorings. The results suggest that the vorticity balance is complex and that topographic Sverdrup dynamics cannot be expected to provide an adequate explanation for the seasonal modulation of the abyssal eddy kinetic energy that is observed with moored instrumentation in the North Pacific.