Subtidal flow structure at the turning region of a wide outflow plume

A series of underway current velocity profiles and near-surface temperature and salinity measurements were combined with temperature and salinity profiles to characterize the subtidal flow structure at the turning region of a wide plume, the Chesapeake Bay outflow plume. In this context, "wide" refers to the ratio of lateral plume expansion to internal radius of deformation being greater than one. Observations were obtained in September and November of 1996 and in February and May of 1997 with the idea of capturing the variability in forcing conditions typically associated with these seasons. However, regional precipitation patterns yielded similar buoyancy forcing conditions for the four surveys and among the wettest years on record. This buoyancy forcing produced a well-delineated outflow plume that separated from the coast on its way out the estuary. The plume separation acted in conjunction with frictional effects to delineate an inshore front, in addition to the customarily described offshore front. The outflow plume was markedly constrained by the Chesapeake Channel, which was also the main conduit of shelf waters toward the estuary. The bathymetric influence was also evident in the surface salinity field, the mean flows and the volume fluxes. The offshore extent of the plume was found between the scale predicted by geostrophic dynamics (internal Rossby radius) and that predicted by cyclostrophic dynamics. Such offshore extent was most likely linked to the plume interactions with the bathymetrically steered up-estuary flow. This was corroborated by an analytical solution that explored the dynamical balance among pressure gradient, Coriolis accelerations and friction. In addition to being influenced by bathymetry, the Chesapeake Bay outflow plume was modified by local and remote effects related to atmospheric forcing.