The Florida Current: Mean Jet Structure, Meandering, and Velocity Fluctuations Observed with HF Radar.

摘要

High-resolution ocean surface current velocity measurements from high frequency (HF) radar are used to map the Florida Current jet structure, and quantify its fluctuations, in more detail than has previously been possible. Whereas earlier HF radar studies in the Straits of Florida focused on individual events, this research takes the next step by using a 2 year timeseries to quantify and characterize the mean horizontal jet profile, its meandering and structural variability, and the space-time structure of the velocity fluctuations. This dissertation is organized into three research chapters:;In the first chapter, the 2 year mean horizontal profile of the Florida Current is constructed at high spatial (1 km) and temporal (20 min) resolution. To improve the mean calculation, each 2-D map in time is converted from geographical to stream coordinates, where grid points are shifted relative to the jet core (cross-stream/downstream). The core time-mean velocity is 162 cm s -1, compared to 136 cm s-1 in the geographical frame. This difference is due to meandering, which smears energy across grid points in the geographical frame, producing a diffuse jet profile with weaker cross-stream gradients. At 25.4°N, the mean position of the jet is 44 km offshore, over the 650 m isobath. Lateral meandering has a standard deviation of 8 km and a range of 60 km, accounting for 45% of mean eddy kinetic energy. Jet width exhibits an annual cycle in the Straits of Florida, with a boreal summer maximum and late winter minimum. The summer peak is accompanied by a maximum in volume transport and local meridional wind stress. The winter minimum precedes a peak in core intensity, lateral shear and surface transport. Sub-seasonal fluctuations of these variables peak at 7-10 days and 3 weeks, but exhibit large inter-annual variability.;The second chapter presents two case studies that demonstrate the power of HF radar to: (1) reveal new information regarding flow field kinematics of previously studied features; and (2) measure transient phenomena that have been historically difficult to capture with ship and moored point measurements, or to resolve with satellite imagery. In the first case study, the kinematic properties of a cyclonic vortex are investigated. In contrast to conditions recorded in a period of no eddy activity, the vorticity field revealed a complex structure, with significant contributions from strain, and a large Rossby number indicative of submesoscale dynamics. Strong horizontal current divergence near the core of the eddy was associated with anomalously cold water brought to the surface by upwelling, observed in satellite SST imagery. The particle dispersion metric IROS peaked during the event, indicating cross-shelf exchange of water properties between offshore and coastal regions. In the second case study, a near-inertial signal on the jet's anticyclonic flank was investigated for the first time. The strongly sheared Florida Current partially masked the structure of the signal, which manifested as a succession of clockwise-rotating eddies in the observed surface currents. The wave trough was not evident when embedded in a laterally sheared northward background flow. The dominant frequency was shifted by ~13% below ƒ in the average, which is consistent with a near-inertial wave propagating in a background regime with negative vorticity. Near-inertial energy peaked in the negative vorticity trough along the jet's eastern flank, indicative of wave trapping in the horizontal.;In the final chapter, the characteristic temporal and spatial scales of the fluctuations are calculated based on the flow field's correlation properties. The Florida Current dominates the ocean circulation in this region, and strongly determines the character of the fluctuations; the strongly sheared northward flow meridionally extends decorrelation length scales and polarizes fluctuating motions in the along-stream direction. The dominant periods of variability are quantified, along with their time dependency that reveals a seasonal variation in periodicity. The slope of the mean kinetic energy wavenumber spectrum is k-3, which is consistent with interior quasi-geostrophy theory. This result implies that nonlocal dynamics are dominant in driving local transport and dispersion. Eddy-mean flow interaction is investigated through the conservation of eddy kinetic energy equation, variance ellipses and the Reynolds stress terms. The map of the barotropic energy exchange term reveals a 2-D pattern, where south of 25.5°N there is an upgradient (downgradient) flux in the cyclonic (anticyclonic) shear zone, and vice versa north of 25.5°N. The magnitude of the divergence of energy flux is significant, however, suggesting there is not an equal exchange of energy between the eddy and mean, but rather an export out of the open domain.