Mixed layer dynamics: exploring the impact of storms in the Mid Atlantic Bight
Creators: name: Watkins, Clifford Evan
Publisher: name: No Publisher Supplied
Year: 2021

description: This dissertation uses a combination of modeling with observations and new analysis techniques to better understand and predict how the ocean surface boundary layer and stratified coastal ocean interact with the winds at the scale of individual tropical cyclones to seasonal behavior. Understanding and predicting the dynamics of the shallow, coastal ocean response to tropical cyclones as well as its feedback on the storm, especially in the mid-latitudes, remains a gap in the field. First by using large eddy simulations, the physical processes of mixing and entrainment for are shown for Hurricane Irene (2011, where surface cooling due to entrainment ahead of the storm’s eye center led to the rapid weakening of the storm. We show how momentum is transferred from wind-induced shear to the pycnocline via large aspect-ratio coherent structures, and how the ocean surface boundary layer responds to the rapid rotation of winds during eye passage. This study showed the novel transition between distinct regimes of mixing from surface-enhanced coherent structures due to the coherent structures to shear across the pycnocline at eye passage. Secondly, we develop a novel model for determining the mixed-layer depth using a combination of high-frequency (HF) radar and model-derived 10-m winds. We show the shelf-wide behavior of the surface mixed layer depth (MLD) in response to both a tropical cyclone and seasonal forcing in the Mid-Atlantic Bight (MAB). By comparing the modeled MLDs to in situ observations through the life cycle of Irene, we show that the storm-driven deepening can be quantified using a novel cost function to solve an inverse solution to Pollard and Millard’s (1970) slab model of inertial motion. Using this, we map the rapid ahead-of-eye deepening of the mixed layer moving from Southern MAB northward toward New York City ahead of Irene at the leading edge of the wind field that entrained the cooler waters under the seasonal thermocline and consistent with the northward storm track. This method allows for analysis of the spatial structure of the seasonal cycle in MLD, where the transition from highly stratified to a cooler single-layer system moves from north to south in response to the storm-forcing and diminishing solar intensity of Autumn. And thirdly, we use 13 years of an integrated ocean observing system that consists of Teledyne Webb Slocum gliders, a regional-scale HF Radar network, buoys, and NOAA satellites to investigate the evolution of the MAB under the influence of 11 tropical cyclones. By separating the tracks into onshore, coastal, and offshore, we show that ahead-of-eye cooling, which can affect tropical cyclone intensity, is linked with the advection of Cold Pool waters to the surface via upwelling for onshore storms and wind-driven vertical mixing for along shore storms. The results of this study highlight the need for continued combined ocean observing systems and further work on understanding the ocean-atmosphere coupled system for near-coast regions potential feedbacks on storm intensity.