Topographic influence on overflow dynamics: Idealized numerical simulations and the Faroe Bank Channel overflow

Abstract

Properties of the North Atlantic Deep Water (NADW) depend on mixing that occurs in the Denmark Strait (DS) and the Faroe Bank Channel (FBC) overflow regions. How the sill's topography in those regions may affect mixing processes and downstream variability is thus investigated using a high‐resolution terrain‐following ocean model. Model results agree with observations that show enhanced mixing and entrainment downstream from the sill; however, mixing seems to occur over a longer distance downstream from the FBC sill and more abruptly downstream from the DS sill. Sensitivity experiments with various FBC sill widths demonstrate that the narrow sill is responsible for the enhanced mixing. The downstream flow variability, eddy propagation, and deep water properties are affected by sill width and background stratification. Similar to the laboratory overflow experiments of Cenedese et al. (2004), three distinct mixing regimes (characterized by the Froude number) are identified in the FBC simulations: a steady subcritical flow regime upstream from the sill, a supercritical wave‐like flow regime downstream from the sill, and an irregular eddy‐dominated regime farther away from the sill. Satellite altimeter data near the FBC show cyclonic anomalies propagating along the northern slope of the channel, resembling the surface eddies associated with the overflow variability in the model. A cross‐channel circulation over the FBC sill, driven by frictional bottom boundary layers, resulted in convergence/divergence zones near the southern/northern slopes and pinching/spreading of isotherms across the channel, similar to the observation‐based mechanism proposed by Johnson and Sanford (1992).