What Drives the Mean Along-Shelf Flow in the Northwest Atlantic Coastal Ocean?

A long-standing hypothesis is that the steady along-shelf circulation in the Northwest Atlantic (NWA) coastal ocean is driven by buoyancy input from continental freshwater runoff. However, the forcing from the freshwater runoff has not been adequately evaluated and compared with other potential driving mechanisms. This study investigates the roles of both wind stress and freshwater runoff in driving the mean along-shelf flow in the NWA coastal ocean and examines other potential drivers using a newly developed high-resolution regional model with realistic forcing conditions. The results reveal that wind stress has a larger impact than freshwater runoff on the overall mean circulation and along-shelf sea-level gradient on the NWA shelf. While the continental freshwater input consistently contributes to the equatorward along-shelf flow and higher sea level along the coast, wind stress is more effective for the setup of the broad-scale circulation pattern by driving the along-shelf flow on the Labrador Shelf and opposing the flow in the Mid-Atlantic Bight and on the Scotian Shelf. In addition to the local wind and continental runoff, the sub-Arctic inflow from higher latitude is an essential part of the NWA shelf circulation system. This remote driver directly contributes to the along-shelf flow and insulates the shelf flow from the Gulf Stream on the southern shelves.