Studying the Dynamics and Biological Significance of the Hudson River Using an Ocean Observatory

The Lagrangian Transport and Transformation Experiment (LaTTE) was designed to quantify how physical, biological and chemical processes transform material in a buoyant river plume and to link these processes to wind forced changes in the plume structure. The three LaTTE field programs include a May 2004 pilot, a full scale effort in April 2005, and a final study planned for May 2006. In each field study, dye is released and tracked by two vessels for physical, biological and chemical sampling of the tagged water mass. The field study and data assimilation segments of LaTTE rely on a research-based coastal ocean observatory to provide a temporal and spatial context for these intensive process studies. The observatory includes a shelf-wide observational backbone (international satellites, nested HF Radars, and autonomous underwater gliders) that was locally enhanced with high-resolution relocatable moorings in the New York Bight apex for the process studies. During the experiments, a shore based operations center combined real-time datasets with forecasts from a high-resolution atmospheric model (WRF) and hindcasts from an ocean model (ROMS) to provide adaptive sampling guidance to the research vessels. Results from the April 2005 pilot and the May 2006 process study will be reviewed. During the strong outflows of April 2005, the ebb tide squirts flowing onto the shelf were observed to respond to a strong sea breeze, forming a recirculating eddy just south of the Harbor entrance. The eddy served as an incubator for biological productivity, resulting in high phytoplankton concentrations leading to depleted bottom dissolved oxygen in a location consistent with historical observations. Only a portion of the fresh river water entering the recirculation zone exited as the expected coastal current along the New Jersey shelf. Most of the freshwater was observed to flow cross-shelf along the southern flank of the Hudson Shelf Valley, consistent with historical remote sensing data. This newly observed transport pathway can have potentially significant impacts on material transport from the Hudson River plume onto the continental shelf. In 2006, wind driven circulation resulted in the plume advecting south along New Jersey and eventually detaching into two pieces. In 2006, wind driven ROMS forecasts were successful at predicting the transport of the river as validated by the drifters and glider data