Cross-shelf circulation and momentum and heat balances over the inner continental shelf near Martha's Vineyard, Massachusetts

Abstract

The water circulation and evolution of water temperature over the inner continental shelf are investigated using observations of water velocity, temperature, density, and bottom pressure; surface gravity waves; wind stress; and heat flux between the ocean and atmosphere during 2001-2007. When waves are small, cross-shelf wind stress is the dominant mechanism driving cross-shelf circulation. The along-shelf wind stress does not drive a substantial crossshelf circulation. The response to a given wind stress is stronger in summer than winter. The cross-shelf transport in the surface layer during winter agrees with a two-dimensional, unstratified model. During large waves and onshore winds the crossshelf velocity is nearly vertically uniform, because the wind-and wave-driven shears cancel. During large waves and offshore winds the velocity is strongly vertically sheared because the wind-and wave-driven shears have the same sign.The subtidal, depth-average cross-shelf momentum balance is a combination of geostrophic balance and a coastal set-up and set-down balance driven by the crossshelf wind stress. The estimated wave radiation stress gradient is also large. The dominant along-shelf momentum balance is between the wind stress and pressure gradient, but the bottom stress, acceleration, Coriolis, Hasselmann wave stress, and nonlinear advection are not negligible. The fluctuating along-shelf pressure gradient is a local sea level response to wind forcing, not a remotely generated pressure gradient.In summer, the water is persistently cooled due to a mean upwelling circulation. The cross-shelf heat flux nearly balances the strong surface heating throughout midsummer, so the water temperature is almost constant. The along-shelf heat flux divergence is apparently small. In winter, the change in water temperature is closer to that expected due to the surface cooling. Heat transport due to surface gravity waves is substantial.Thesis Supervisor:

Observations and a Model of Undertow over the Inner Continental Shelf

“…However, fu and Ϫfu st are not the dominant terms in the subtidal alongshelf momentum balance at MVCO (Fewings 2007). The along-shelf wind stress and along-shelf pressure gradient tend to balance and have subtidal standard deviations that are 2-3 times larger than the other terms, which include temporal acceleration, nonlinear advective terms, Coriolis, Hasselmann wave stress, and bottom stress.…”

“…Wave height measurements at the 7-m site are used for that site because there is a complete time series and wave heights decrease substantially between the 12-m and 7-m sites during a few large wave events (H sig Ͼ 3.5 m) when the 7-m site was in the surf zone (H sig Ͼ h/2). Wind observations for both MVCO and SWWIM are primarily from a 10-m shore mast (12.5 m MSL) with some data gaps filled using winds from a second site farther onshore (Fewings et al 2008). All SWWIM time series were averaged and/or interpolated onto the MVCO time base with samples every 20 minutes.…”

“…However, an onshore and southward wind stress The long time series at MVCO allows us to separate wind-and wave-driven cross-shelf flows. The wind stress has a substantial impact on the vertical structure of the cross-shelf flow at MVCO (Fewings et al 2008). During moderate waves (1 m Ͻ H sig Ͻ 2 m) and weak wind stresses (defined as | s | Ͻ 0.03 N m Ϫ2 ), the mean cross-shelf flow profile has maximum flow and larger vertical shear near the surface, and smaller shear and weak flow near the bottom (Fig.…”

“…9, triangles). Fewings et al (2008) examine the response of the cross-shelf circulation to wind stress and to combined wind stress and wave forcing using the MVCO observations. Since the focus here is on wave-driven cross-shelf flows, the remainder of the analyses concentrated on the MVCO current profiles during times when the wind stress is weak ( When the wind stress is weak and the waves are small (H sig Ͻ 0.75 m), the depth-averaged flow is approximately zero (section 4a), but there is still a vertically sheared cross-shelf flow with offshore flow in the upper half of the water column and onshore flow in the lower half of the water column (Fig.…”

“…1b), consistent with a dynamical balance between the Coriolis force and the Hasselmann (or Stokes-Coriolis) wave stress (Hasselmann 1970) due to the influence of the earth's rotation on surface gravity waves. The response of the cross-shelf flow at the Martha's Vineyard site to wind forcing and combined wind and wave forcing is presented in a companion paper (Fewings et al 2008).…”