High‐frequency forcing and subtidal response of the Hudson River plume

Abstract

The Lagrangian Transport and Transformation Experiments (LaTTE) document the physical, biological, and chemical evolution of the Hudson River plume during the spring seasons of 2004, 2005, and 2006. While plume variability due to river discharge, subinertial frequency variability in winds, and ambient shelf circulation are important, the observations show that the plume reacts directly to higher‐frequency forcing as well. Mooring records during 2005 and 2006 show that fortnightly variability in tidal mixing is manifested as fortnightly changes in plume stratification. Diurnal variability related to forcing by the sea‐land breeze system (SLBS) is apparent in the Hudson River plume during the 2005 experiment. The SLBS, while episodic, accounts for ∼15% of the kinetic energy in surface currents in the New York Bight apex during the summer months with individual SLBS events providing up to 50% of the total kinetic energy. Simulations of the plume, using the Regional Ocean Modeling System (ROMS), demonstrate there is a subtidal response to high‐frequency forcing. Spring‐neap variability in tidal mixing modifies the estuary outflow Rossby and Froude numbers, resulting in increased transport (80% of river discharge) in the New Jersey coastal current during spring tides with lower transport (60% of river discharge) during neap tides. SLBS variability results in greater storage of river discharge water in the recirculating bulge region and increases the net transport of freshwater along the Long Island coast while significantly reducing freshwater supplied to the New Jersey coastal current to as little as 30% of the total river discharge.