Subinertial Dynamics on the Inner New Jersey Shelf during the Upwelling Season

Abstract

Subinertial dynamics on the inner New Jersey shelf is examined using time series of the forcing agents (atmospheric pressure, wind stress, and Hudson River streamflow), adjusted sea level (ASL) along the southern part of the Mid-Atlantic Bight, and mooring data collected during the summer of 1996. High-frequency (period 1‐3 days) transient wind-driven events were evident both in ASL and alongshelf current data. ASL events propagated southward with remarkably high speed (;10 m s21) in the manner of free coastally trapped waves (CTW). However, these transients were forced by the wind events within the study domain: both ASL and alongshelf current fluctuations were coherent with the local alongshore wind stress. ASL amplitude substantially increased downshelf (southward). These transient flows propagated from the corner in the coastline formed by the southern Long Island and northern New Jersey coasts. This bend of the coastline created a discontinuity in the alongshore wind stress component that caused the generation of CTW pulses at this location. During the period of observations, enhanced buoyant flows arrived at the site of the moorings. They were associated with increased Hudson River discharge. These buoyant flows and transient wind-driven events strongly interacted: transient wind-driven currents were dramatically amplified in the buoyant water while the buoyant water was spread offshore. Amplified transient currents were not associated with the enhanced vertical shear. Lower-frequency wind forcing generated upwelling events with typical duration of 8‐10 days. During the upwelling, temperature dropped through the whole water column, but the stratification remained significant (58‐ 68C in 8‐10 m of water). Even though upwelling-favorable winds dominated, record-mean currents in the upper layer were weak (2‐5 cm s21) due to the close competition between wind and buoyancy forcing.