Wind influence on a coastal buoyant outflow

Abstract

This paper investigates the interplay between river discharge and winds in forcing coastal buoyant outflows. During light winds a plume influenced by the Earth's rotation will flow down shelf (in the direction of Kelvin wave propagation) as a slender buoyancy‐driven coastal current. Downwelling favorable winds augment this down‐shelf flow, narrow the plume, and mix the water column. Upwelling favorable winds drive currents that counter the buoyancy‐driven flow, spread plume waters offshore, and rapidly mix buoyant waters. Two criteria are developed to assess the wind influence on a buoyant outflow. The wind strength index (Ws) determines whether a plume's along‐shelf flow is in a wind‐driven or buoyancy‐driven state. Ws is the ratio of the wind‐driven and buoyancy‐driven along‐shelf velocities. Wind influence on across‐shelf plume structure is rated with a timescale (ttilt) for the isopycnal tilting caused by wind‐driven Ekman circulation. These criteria are used to characterize wind influence on the Delaware Coastal Current and can be applied to other coastal buoyant outflows. The Delaware buoyant outflow is simulated for springtime high–river discharge conditions. Simulation results and Ws values reveal that the coastal current is buoyancy‐driven most of the time (∣Ws∣ < 1 on average). Wind events, however, overwhelm the buoyancy‐driven flow (∣Ws∣ > 1) several times during the high‐discharge period. Strong upwelling events reverse the buoyant outflow; they constitute an important mechanism for transporting fresh water up shelf. Across‐shelf plume structure is more sensitive to wind influence than the along‐shelf flow. Values of ttilt indicate that moderate or strong winds persisting throughout a day can modify plume width significantly. Plume widening during upwelling events is accompanied by mixing that can erase the buoyant outflow.