Abstract
The upper ocean environment is especially critical as it acts at the direct link
between society and the ocean as a whole. The air-sea interface is responsible for the collection and transport of buoyant materials, like oil and plastic pollution, as well as almost all of the exchanges between the atmosphere and deeper ocean. The fate of biogeochemical materials in the upper ocean can have many implications for oceanic ecosystems as well as carbon storage in the ocean interior. In this dissertation I present analyses performed on a variety of field measurements, revolving around the deployment of large arrays of Lagrangian drifters, with the goal of furthering the understanding of the upper ocean dynamics at play in these key environmental issues. Very near surface ocean currents are dominated by wind and wave forcing which in turn have large impacts on the transport of buoyant materials in the ocean. Surface currents, however, are under resolved in most operational ocean models due to the difficulty of measuring ocean currents close to, or directly at, the air-sea interface with many modern instrumentations. In part, this dissertation presents observations of ocean currents at two depths within the first meter of the surface, acquired by utilizing trajectory data from both drogued and undrogued CARTHE drifters, which have draft depths of 60 cm and 5 cm, respectively. Trajectory data of dense, co-located drogued and undrogued drifters, were collected during the LAgrangian Submesoscale ExpeRiment (LASER) that took place from January to March of 2016 in the Northern Gulf of Mexico. Examination of the drifter data reveals that the drifter velocities become strongly wind- and wave-driven during periods of high wind, with the pre- existing regional circulation having a smaller, but non-negligible, influence on the total drifter velocities. During these high wind events, we deconstruct the total drifter velocities of each drifter type into their wind- and wave-driven components after subtracting an estimate for the regional circulation, which pre-exists each wind event. In order to capture the regional circulation in the absence of strong wind and wave forcing, a Lagrangian variational method is used to create hourly velocity field estimates for both drifter types separately, during the hours preceding each high wind event. Synoptic wind and wave output data from the Unified Wave INterface-Coupled Model (UWIN-CM), a fully coupled atmosphere, wave and ocean circulation model, are used for analysis. We find the wind-driven component of the drifter velocities to exhibit a rotation to the right with depth between the velocities measured by undrogued and drogued drifters. We also find that the average wind-driven velocity of undrogued drifters (drogued drifters) is ∼3.4-6.0 % (∼2.3-4.1 %) of the wind speed and is deflected ∼5o-55o (∼30o-85o) to the right of the wind, reaching higher deflection angles at higher wind speeds. Results provide new insight to the vertical shear present in wind-driven surface currents under high winds, which have vital implications for any surface transport problem. Additionally, much of the vertical transport near the surface of the ocean, which plays a critical role in the transport of dissolved nutrients and gases, is thought to be associated with ageostrophic submesoscale phenomena. Vertical velocities are challenging not only to model accurately, but also to measure because of how difficult they are to locate in the surface waters of the ocean. Using the massive drifter releases during the LASER campaign in the Gulf of Mexico, along with those from the Coherent Lagrangian Pathways from the Surface Ocean to the Interior (CALYPSO) experiment in the Mediterranean Sea, we investigate the generation of submesoscale structures along two different mesoscale fronts. To this purpose we further develop a novel method to project Lagrangian trajectories to Eulerian velocity fields, in order to calculate horizontal velocity gradients at the surface, which are used as a proxy for vertical transport. The velocity reconstruction uses a squared-exponential covariance function, which characterizes velocity correlations in horizontal space and time, and determines the scales of variation using the data itself. SST and towed CTD measurements support the findings revealed by the drifter data. Due to the production of a submesoscale instability eddy in the Gulf of Mexico, convergence magnitudes of up to ∼20 times the planetary vorticity, f, are observed, the value of which is almost 3 times larger than that found in the mesoscale dominated Western Mediterranean Sea.
The results presented in this dissertation further the understanding of wind-driven vertical velocity shear at the very surface of the ocean, which can provide first re- sponders to oil spills and/or search and rescue missions, in addition to those studying plastic debris in the upper ocean, with information to make more accurate predictions of surface transport. The characteristic kinematic properties within the submesoscale and mesoscale features analyzed here can also serve as baseline examples to help improve operational modeling efforts over a variety of spatiotemporal scales.