Observations of Cross-Shelf Flow Driven by Cross-Shelf Winds on the Inner Continental Shelf

Abstract

Six-yr-long time series of winds, waves, and water velocity from a cabled coastal observatory in 12 m of water reveal the separate dependence of the cross-shelf velocity profile on cross-shelf and along-shelf winds, waves, and tides. During small waves, cross-shelf wind is the dominant mechanism driving the cross-shelf circulation after tides and tidal residual motions are removed. The along-shelf wind does not drive a substantial cross-shelf circulation. During offshore winds, the cross-shelf circulation is offshore in the upper water column and onshore in the lower water column, with roughly equal and opposite volume transports in the surface and bottom layers. During onshore winds, the circulation is nearly the reverse. The observed profiles and cross-shelf transport in the surface layer during winter agree with a simple two-dimensional unstratified model of cross-shelf wind stress forcing. The cross-shelf velocity profile is more vertically sheared and the surface layer transport is stronger in summer than in winter for a given offshore wind stress.During large waves, the cross-shelf circulation is no longer roughly symmetric in the wind direction. For onshore winds, the cross-shelf velocity profile is nearly vertically uniform, because the wind-and wavedriven shears cancel; for offshore winds, the profile is strongly vertically sheared because the wind-and wave-driven shears have the same sign. The Lagrangian velocity profile in winter is similar to the part of the Eulerian velocity profile due to cross-shelf wind stress alone, because the contribution of Stokes drift to the Lagrangian velocity approximately cancels the contribution of waves to the Eulerian velocity.

Ocean–atmosphere–wave characterisation of a wind jet (Ebro shelf, NW Mediterranean Sea)

“…These circulation patterns are consistent with other investigations (e.g. Horwitz and Lentz, 2014;Fewings et al, 2008;Dzwonkowski et al, 2011) where a well-developed two-layer flow due to intense cross-shelf winds tends to occur when the turbulent layers overlap (water depth in the inner shelf is of the order of metres to tens of metres according to Lentz and Fewings, 2012). In the mid-and outer shelf, the flow tends to be oriented in the along-shelf direction due to the prevalence of the regional response to the wind jet and the slope current.…”

Shelf response to intense offshore wind

“…The ratio in the U.S. East Coast for similar depths (35-55m) ranged from 0.08 to 0.25 [Fewings et al, 2008]. Meanwhile, results near the Martha's Vineyard 96…”

“…To compare the relative importance of the cross-shelf winds at several sites 86 along the U.S. East Coast in the depth-averaged cross-shelf momentum balance, Fewings et al [2008] defined the ratio R: the size of the cross-shelf 88 winds relative to the Coriolis term. The ratio R (computed as R=(τ x s )/(ρ o fhv da ), where τ x s represents cross-shelf wind stress, ρ o is water density, f is the Coriolis 90 parameter, h is the water depth and v da is depth-averaged along-shelf current) at the NES suggests that the cross-shelf wind stress was an important term in 92 the momentum balance when v da is weak.…”

“…Modeling [Tilburg, 2003;Horwitz and Lentz, 2014] and 44 observational [Lentz, 2001;Fewings et al, 2008;Dzwonkowski et al, 2011;Horwitz and Lentz, 2014] studies have shown that cross-shelf wind stress can 46 result in cross-shelf flow in areas where the surface and bottom boundary layers overlap (i.e., inner-shelf). In most cases, the cross-shelf wind term of the 48 momentum balance equation is smaller than the Coriolis term [Lentz and Fewings, 2012] and is relatively unimportant over the mid-and outer-shelf 50 [Fewings et al, 2008]. In these regions, the cross-shelf circulation tends to be dominated by along-shelf winds generating upwelling/downwelling flow [Crépon 52 et al, 1984;Lentz, 1992;Lentz, 2001;Keubel Cervantes et al, 2003;Dzwonkowski et al, 2011] consistent with early theoretical considerations 54 [Ekman, 1905].…”

“…Fewings et al, [2008] noticed an 462 increased surface transport observed at 12 m during the summer due to crossshelf winds south of Martha's Vineyard (Massachusetts, USA). In the NES, a 464 similar behavior was observed with comparable wind stresses than NW6 (for instance NW1 or NW9) producing more cross-shelf transport under likely 466 stratified conditions.…”