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Analyses of current time series longer than 200 days from 33 sites over the Middle Atlantic Bight continental shelf reveal a consistent mean circulation pattern. The mean depth-averaged flow is equatorward, alongshelf, and increases with increasing water depth from 3 cm s Ϫ1 at the 15-m isobath to 10 cm s Ϫ1at the 100-m isobath. The mean cross-shelf circulation exhibits a consistent cross-shelf and vertical structure. The near-surface flow is typically offshore (positive, range Ϫ3 to 6 cm s Ϫ1). The interior flow is onshore and remarkably constant (Ϫ0.2 to Ϫ1.4 cm s Ϫ1). The near-bottom flow increases linearly with increasing water depth from Ϫ1 cm s Ϫ1 (onshore) in shallow water to 4 cm s Ϫ1 (offshore) at the 250-m isobath over the slope, with the direction reversal near the 50-m isobath.A steady, two-dimensional model (no along-isobath variations in the flow) reproduces the main features of the observed circulation pattern. The depth-averaged alongshelf flow is primarily driven by an alongshelf pressure gradient (sea surface slope of 3.7 ϫ 10 Ϫ8 increasing to the north) and an opposing mean wind stress that also drives the near-surface offshore flow. The alongshelf pressure gradient accounts for both the increase in the alongshelf flow with water depth and the geostrophic balance onshore flow in the interior. The increase in the near-bottom offshore flow with water depth is due to the change in the relative magnitude of the contributions from the geostrophic onshore flow that dominates in shallow water and the offshore flow driven by the bottom stress that dominates in deeper water.

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.

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.

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A two-dimensional numerical model is used to study the response to upwelling-and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf ͙t transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratification suppresses vertical mixing, causing a small reduction in stress at mid depth that drives a weak cross-shelf circulation over the inner shelf. For constant stratification, the inner shelf stratification and cross-shelf circulation are stronger. During downwelling on an initially stratified shelf, the inner shelf becomes unstratified because the very weak cross-shelf circulation forces lighter water under denser, driving convection which enhances the vertical mixing. As a result the stress is nearly constant throughout the water column and the cross-shelf circulation is slightly weaker than in the initially unstratified case. The downwelling response is essentially the same for the constant stratification and the two-layer cases. Model runs including the evolution of a passive tracer indicate that the inner shelf region acts as a barrier to cross-shelf transport of tracers from the coastal boundary to farther offshore and vice versa, due to strong vertical mixing and weak cross-shelf circulation in this region.

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The temperature in the coastal ocean off the northeastern U.S. during the first half of 2012 was anomalously warm, and this resulted in major impacts on the marine ecosystem and commercial fisheries. Understanding the spatiotemporal characteristics of the warming and its underlying dynamical processes is important for improving ecosystem management. Here, we show that the warming in the first half of 2012 was systematic from the Gulf of Maine to Cape Hatteras. Moreover, the warm anomalies extended through the water column, and the local temperature change of shelf water in the Middle Atlantic Bight was largely balanced by the atmospheric heat flux. The anomalous atmospheric jet stream position induced smaller heat loss from the ocean and caused a much slower cooling rate in late autumn and early winter of 2011–2012. Strong jet stream intraseasonal oscillations in the first half of 2012 systematically increased the warm anomalies over the continental shelf. Despite the importance of advection in prior northeastern U.S. continental shelf interannual temperature anomalies, our analyses show that much of the 2012 warming event was attributed to local warming from the atmosphere.