PhD Dissertation Defense, Lu Han: An observation-based study of shelf-open exchange off Cape Hatteras, NC

The convergence of different water masses on the shelf and along the shelfbreak, and cross-isobath shelf-open ocean exchanges contribute to the complex circulation near Cape Hatteras. To better understand the exchange between the continental shelf and the adjacent open ocean in this region, an integrated observational and modeling program, PEACH (Processes driving Exchange At Cape Hatteras) was conducted in 2017-2018. The focus of my dissertation is to provide a comprehensive examination of the processes of shelf water export around Cape Hatteras at synoptic, seasonal, and longer timescales using data from PEACH observational array.

Dense shelf water cascading down the continental slope is an important, yet little-studied pathway of carbon export and sequestration at Cape Hatteras. With detailed observations over and immediately adjacent to the continental shelf, we document for the first time a shelf water cascading event off Cape Hatteras carrying a large amount of carbon during January 2018 and study the characteristics and dynamics of shelf water cascading process. 

Using data from 9 bottom-mounted acoustic Doppler current profilers (ADCPs) deployed during PEACH, we examine the mean and variability of the circulation off Cape Hatteras. Two dominant spatial patterns in the velocity field are obtained from an empirical orthogonal function (EOF) analysis. The first mode exhibits a wind-driven along-shelf flow pattern, while the second mode shows a Gulf Stream-driven convergent flow pattern. 

We further provide the mean circulation field using surface velocity measured by nested high-frequency radars (HFR), which clearly captures the convergence of the shelf flows as they approach Cape Hatteras, the Gulf Stream and its separation from the upper slope. By comparing the HFR surface velocity with the ADCP-measured near-surface velocity and the geostrophic component of the surface velocity derived from along-track satellite altimetry data, we find an issue with the ability of the HFR to measure the offshore currents at the shelfbreak north of Cape Hatteras and the fastest current speeds in the Gulf Stream, possibly related to the range and angle from radar sites. Collectively, my research has led to improvements on current knowledge of the ocean circulation off Cape Hatteras and the processes and dynamics driving the exchange between the shelf and the open ocean.

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