Right running head: Mixing in the Hudson Estuary Left running head: F. L. Hellweger et al. Title: Mixing in the Hudson Estuary: The Role of Estuarine Circulation and Tidal Trapping

The effect of estuarine circulation and tidal trapping on mixing in the Hudson Estuary is investigated by a numerical model simulation of a tracer release. The data set used in this study was obtained from a large-scale SF6 tracer release experiment conducted during July/August 2001. It consists of over 2,000 measurements taken over a period of two weeks and distance of 110 km with a typical resolution of 400 meters. The model is based on the threedimensional, time-variable, estuarine and coastal circulation modeling framework (ECOM), and consists of over 10,000 mass balance segments with a 600 m horizontal and 1 m vertical resolution in the study area. The ability of the model to reproduce the observed fate and transport of the tracer (model skill) is quantified using a new set of metrics. The model can reproduce the data over four orders of magnitude with a mean relative agreement (MRA) factor of 2.0. The modeled and measured longitudinal tracer concentration profiles (plumes) differ from the ideal Gaussian shape in two ways: (1) On a large scale the plume is asymmetric, with the downstream end stretching out farther. (2) Small-scale (1-2 km) peaks are present at the upstream and downstream ends of the plume. A sensitivity analysis is used to understand the processes responsible for these features. The model forcing functions (e.g., freshwater flow, boundary salinity, geometry) and process parameterizations (e.g., gas transfer velocity) are modified systematically and the resulting tracer profiles are compared to those from the unmodified model (base case). It is demonstrated that the large-scale asymmetry is due to salinity intrusion, which sets up an estuarine circulation. In the presence of an estuarine circulation, tracer in the surface layer is diluted to a larger degree with water from the lower layer causing increased longitudinal dispersion. Since the salinity intrusion is confined to the downstream end of the tracer plume, only that part of the plume is subject to a larger dispersion, Page 4 Hellweger, Blumberg, Schlosser, Ho, Caplow, Lall and Li which leads to the large-scale asymmetry. The small-scale peaks are due to tidal trapping. Small embayments along the estuary trap water and tracer as the plume passes by in the main channel. At a later time, when the plume in the main channel has passed, the tracer is released back to the main channel, causing a secondary peak in the longitudinal profile.