Circulation at the Delaware Bay mouth from the Lagrangian and Eulerian perspectives.

摘要

A consistent picture of the circulation of the Delaware Bay estuary and adjacent inner shelf established in the literature is based on a synthesis of mooring and hydrographic data complemented by elegant theoretical analyses. Because of the highly variable and episodic nature of the flow at the Delaware Bay mouth, a synoptic picture of the flow has been elusive. The advent of high-frequency (HF) radars for routine monitoring of coastal surface currents (developed over the past twenty years) offers a unique opportunity to provide synoptic descriptions of complex time-dependent flows like those that exist at estuarine mouths. Synoptic measurements lend themselves to examinations from a dual perspective, taking advantage of Lagrangian and Eulerian methods.;Here we analyze synoptic, high-resolution surface winds and currents in the Delaware Bay mouth. Two high-frequency radars measured the surface currents while the surface winds were extracted from a data-assimilating regional wind model. The large-scale characteristics noted by previous workers are clearly corroborated. Specifically the M2 tide dominates the surface currents, and the Delaware Bay outflow plume is clearly evident in the low frequency currents. Several new aspects of the surface circulation were also identified. These include a map of the spatial variability of the M2 tide (validating an earlier model study), persistent low-frequency cross-mouth flow, and a rapid response of the surface currents to a changing wind field. However, strong wind episodes did not persist long enough to set up a sustained Ekman response.;In addition to the Eulerian results presented here, several Lagrangian analyses were conducted. The approach used here calculates simulated trajectories from an archive of gridded synoptic velocities from HF radar measurements. Lagrangian methods are the best tool for examining the advective transport at the Delaware Bay mouth. There was an observed rapid separation of progressive vector diagrams (PVDs) with trajectories implying limited utility over long time scales. Synoptic Lagrangian maps (SLMs) such as residence time, escape fate, and origin showed clear dependence on tidal forcings. Blob simulations have provided a visualization tool able to examine deformation processes on a finite area of ocean.;The most significant result might be the examination of a persistent cross-mouth sub-tidal flow that may be a result of non-local advection from the New Jersey shelf. This feature is not well documented in the literature and we make a first attempt to describe its characteristics.