Effects of mesoscale turbulence on the wind-driven circulation in a closed basin with topography

Abstract

This work addresses the effects of time-dependent, mesoscale turbulence on the wind-driven ocean circulation in a closed basin with variable topography. The main results concern the so-called Neptune effect, which involves the generation of persistent flows correlated with topography, but in this case, such currents are formed in the presence of a continuous, stochastic forcing. Numerical simulations of a single-layer fluid with sloping bottom topography near the boundaries are performed. The forcing is a suitable combination of a steady, basin-scale wind that generates the classical western-intensified anticyclonic gyre, plus a shorter, time-dependent forcing that injects energy at a narrow range of scales. Two contrasting situations are considered. First, in the absence of large-scale forcing, the turbulence generates a cyclonic flow that follows the geostrophic contours around the basin. This configuration corresponds to the most probable state equivalent to that expected in statistical equilibrium. Second, the resulting mean circulation is studied when the large and small-scale forcing terms are considered together. The main consequence is the alteration of the anticyclonic gyre due to the turbulent-induced cyclonic circulation. This result implies that large-scale, semi-steady circulations might be altered according to the turbulence characteristics.