Wind influence on a coastal buoyant outflow

Abstract

[1] 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 (W s ) determines whether a plume's along-shelf flow is in a wind-driven or buoyancy-driven state. W s 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 (t tilt ) 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 W s values reveal that the coastal current is buoyancy-driven most of the time (jW s j < 1 on average). Wind events, however, overwhelm the buoyancy-driven flow (jW s j > 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 t tilt 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.

Evidence for the inherent unsteadiness of a river plume: Satellite observations of the Niagara River discharge

“…On the basis of typical Niagara River plume parameters, and assuming a value of K 5 2, u dis 5 0.15 m s 21 . Following Whitney and Garvine (2005), u wind . 2.65 3 10 22 U.…”

“…It is likely that this current limits the size of the bulge, whereas winds periodically remove it. Whitney and Garvine (2005) quantify the influence of alongshore winds on coastal river plumes in terms of a wind strength index, which is the ratio of velocity scales representative of the wind and buoyancy forcing,…”

Patos Lagoon outflow within the Río de la Plata plume using an airborne salinity mapper: Observing an embedded plume

“…Fong andGeyer, 2001. 2002;Whitney and Garvine, 2005) The Simionato and Nunez (2001) modeled the location of the Rio de la classification schemes span various forcing and boundary condiPlata salinity front using a 3-D baroclinic model to determine tions including the influence of varying discharge rate, tide and factors influencing its seasonal location. River discharge and the wind stress, and the effects of inlet geometry, shelf bottom slope, wind are the primary factors (M6ller et al, 2008).…”

Impact of operational model nesting approaches and inherent errors for coastal simulations

“…Wind influence acting on the ROFI can strain the vertical density gradient (Scully et al, 2005), either enhancing (offshore winds) or reducing or even reversing stratification (onshore winds). In addition, under stratified conditions, upwelling winds typically spread the plume offshore, while downwelling winds compress the plume towards the coast (Whitney and Garvine, 2005). In Liverpool Bay winds that allow the plume to spread into the bay are associated with a northerly direction and occur for only 14% of the year (Howarth et al, 2014).…”