The Wind- and Wave-Driven Inner-Shelf Circulation

Abstract

The inner continental shelf, which spans water depths ofa few meters to tens of meters, is a dynamically defined region that lies between the surf zone (where waves break) and the middle continental shelf (where the along-shelf circulation is usually in geostrophic balance). Many types of forcing that are often neglected over the deeper shelf-such as tides, buoyant plumes, surface gravitywaves, and cross-shelfwind stress-drive substantial circulations over the inner shelf. Cross-shelf circulation over the inner shelf has ecological and geophysical consequences: It connects the shore to the open ocean by transporting pollutants, larvae, phytoplankton, nutrients, and sediment. This review of circulation and momentum balances over the inner continental shelf contrasts prior studies, which focused mainly on the roles of along-shelfwind and pressure gradients, with recent understanding of the dominant roles of cross-shelf wind and surface gravity waves.

Shelf response to intense offshore wind

“…A peak at the diurnal frequency was evident in the depth-254 averaged along-shelf and cross-shelf directions. Energy in the near-inertial band, which dominated in observations over the mid-shelf and slope [Rippeth et 256 al., 2002;Jordà, 2005], was not significant in the along-shelf due to the prevalence of the frictional terms [Lentz and Fewings, 2012]. The near-inertial 258 band contained a large fraction of the total energy of the surface cross-shelf flow with values exceeding the diurnal (sea-breeze effect) frequency.…”

“…The Ekman transport fraction (slope of 366 Equation 4) ranges from 16% during NE2 to 58% during NE4 (Table 1). Cross-shelf circulation in the inner-shelf during cross-shelf winds is 370 characterized by a downwind transport in the upper part of the boundary layer [Tilburg, 2003;Lentz and Fewings, 2012]. The downwind transport is 372…”

“…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].…”

“…[Horwitz and Lentz, 2014] explained that in shallower areas wind momentum is able to mix the entire water column faster than the time-56 scale of the rotation influence resulting in a reduced effect of the along-shelf wind stress to drive cross-shelf transport [Ekman, 1905;Lentz and Fewings, 58 2012]. Thus, the cross-shelf pressure gradient (or sea-level slope in the depthaveraged momentum balance) due to cross-shelf winds are balanced by wind 60 stress in the region onshore of where the surface boundary layer extends to the bottom [Lentz and Fewings, 2012]. 62…”

“…Considering the differences between 474 our results and the mentioned papers, the diversity and complexity of the processes involved in the cross-shelf response to cross-shelf winds under 476 typically stratified conditions is evident and will benefit for simultaneous stratification and velocity information. 478 Depth-averaged cross-shelf velocity is not constrained to be zero because the 480 circulation is not along-shelf uniform [Tilburg and Garvine, 2003;Kirincich and Barth, 2008;Lentz and Fewings, 2012]. 3D circulation may generate 482 divergences in the along-shelf flows driving substantial cross-shelf transport and increasing the spatial complexity of the flow [Lentz and Fewings, 2012].…”