Transient Adjustment of Circulation in a Midlatitude Abyssal Ocean Basin with Realistic Geometry and Bathymetry

Abstract

The early stages in the adjustment of the circulation in a midlatitude abyssal basin with realistic geometry and bathymetry are studied using an inverted 1½-layer model of the Eastern Mediterranean Sea as a natural test basin. The model is forced with a localized sidewall mass source and a compensating distributed uniform mass sink. The basin is spun up by topographic waves with planetary vorticity gradients playing a minor role. A shallow sill in the middle of the basin dynamically separates the abyssal Eastern Mediterranean Sea into two regions. Despite a constant mass flux into the basin, the resulting energy input is time dependent and is correlated with nonlocal dynamics. A positive feedback develops between the source region and the interior circulation during the early adjustment; for a higher interface height level in the source region, the amount of energy injected by the mass source increases. A series of simulations with filtered bathymetries show that the presence of high-wavenumber topographic features can alter the dynamical balances of the adjustment and quasi-steady circulations. Whereas Laplacian dissipation dominates in the presence of high-wavenumber bathymetry, bottom drag becomes the dominant dissipation mechanism when the small-scale topography is filtered out. The timescales for the adjustment are shown to depend on the spectra of the bathymetry.