Coastal near-inertial wave bursts detected by OSCR

Surface current observations from High Frequency (HF) radar have revealed that not only are the low-frequency and tidal currents well resolved by the measurement, higher frequency motions are also contained within the records such as near-inertial oscillations. These motions are associated with oscillations in the internal wave continuum and usually dominate the internal wave energy. A complicating feature in the coastal regime is oceanic frontal structure that influences inertial motions through the relative vorticity fields. These background vorticities may be considerably larger than in the deep ocean case due to the larger density contrast between the fresher, cooler coastal water and the more saline, warmer water masses associated with the subtropical gyre circulation. Synoptic observations of the horizontal flow structure from HF radar provides the spatial context to assess the role of coastal fronts on the near-inertial motions. Observations from an Ocean Surface Current Radar (OSCR) during the Office of Naval Research (ONR)/Naval Research Laboratory (NRL) High Resolution Remote Sensing Experiment, the ONR-sponsored Duck-94 experiment, and U.S. Coast Guard (USCG)-sponsored Ocean Pollution Research Center (OPRC) experiments and NOAA Southeast Caribbean Recruitment Studies (SEFCAR) in the Florida Keys have revealed the presence of internal waves in surface current expressions. The data from the Florida Keys are used to demonstrate that the near-inertial motions were detected in surface current records acquired from an OSCR

Upper Ocean Structure: Responses to Strong Atmospheric Forcing Events

Over the past several years, progress in measurement and modeling of the oceanic response to hurricane forcing is given with a focus on the Gulf of Mexico basin for tropical cyclones (TCs). In terms of the upper ocean impacts on intensity, considerable attention has focused on the cold wake structure beginning just in back of the eye-wall of TCs. Cooling levels of more than 3°C often result in the negative feedback primarily due to vertical mixing at the base of the ocean mixed layer through wind-driven current shears. This effect has been modeled extensively assuming that the basic state (or pre-TC condition) is at rest. However, the background oceanic state, and in particular in the western parts of the oceanic basins in the Northern Hemisphere the flows are energetic. The ensuing upper ocean thermal response tends to be considerably weaker (less surface cooling) to strong TC forcing where the warm layers extend to greater depth and currents transport warm water poleward (Loop Current, Florida Current, and Gulf Stream). Observed wind-driven shears do not develop that essentially keep sea surface temperatures (SSTs) and oceanic heat content levels high during TC passage. This less-negative feedback implies that a more sustained ocean-to-atmosphere heat and moisture flux to the TC and represents an important mechanism for observed deepening of recent severe TCs. Recently developed assimilation schemes for models and new measurement tools for oceanic observations are given, focusing on future avenues for basic and applied research in improving our understanding of these processes during TC passage

Wavelet analysis of surface current vector fields measured by high frequency Doppler radar

Fourier spectral methods have been widely applied to coastal zone current measurements. However in cases such as riverine tides or estuarine outflow currents exhibit non-stationary characteristics which invalidate the basic assumptions of these methods. Wavelet analysis techniques can be used to determine the temporal evolution of current variance over a range of frequency scales and therefore can provide an improved understanding of event-driven dynamics. Morlet continuous-wavelet transforms were applied to multiple vector time-series measurements from a High Frequency (HF) Doppler radar and moored ADCPs near the mouth of Chesapeake Bay in 1996 and 1997 as well as wind measurements at the Chesapeake Light tower. The time-varying clockwise (CW) and counter-clockwise (CCW) wavelet spectra were computed from each vector time-series. The horizontal, vertical and temporal evolution of high energy scales could then be visualized. Significant short-term intensifications of 30-60 hour CW energy in the region of the outfall plume were observed that were highly coherent with local wind forcing

Behavior of the ocean radar cross-section at low incidence, observed in the vicinity of the Gulf Stream

Observational results derived from the airborne C-band scatterometer RESSAC looking at small incidence angles (/spl les/21/spl deg/) were acquired in the vicinity of the Gulf Stream during the Surface Wave Dynamics Experiment in February and March 1991. Although the instrument was primarily designed to study the directional spectra of long waves, it is also possible to process the data to examine the behaviour of the radar cross-section /spl sigma//sup 0/ versus incidence /spl theta/ and azimuth /spl phi/. Although the classical physical or empirical models of /spl sigma//sup 0/(/spl theta/,/spl phi/) predict that the anisotropy of /spl sigma//sup 0/ versus azimuth increases with wind speed for a given incidence angle /spl theta/, it is found that under some conditions, this behaviour does not occur. By contrast, the anisotropy shows strong minima on certain occasions. Given the high-resolution upper ocean current measurements from Airborne expendable Current Profilers deployed on 5 March 1991, the regions of anisotropic minima could be identified from the three sets of observations (March 4, 5, and 7) downwind of the core of the Gulf Stream with a maximum current of 2 m s/sup -1/, and downwind of the current speed maxima of an anticyclonic-rotating warm core ring juxtaposed along the North Wall. On three other days (February 27, March 2 and 6), no clear anisotropic minima of was found, because the upper ocean current effects were obscured by variability in surface wind field.<

Observations of a coastal buoyant jet with HF Doppler radar

Currents on the inner shelf respond to a combination of forcing by intrusions of ocean fronts, eddies and internal waves as well as winds, surface gravity waves and topographic rectifications. To quantify the effect that the interaction of these processes has on inner shelf flows, a shore-based, 25.4 MHz (High Frequency) Ocean Surface Current Radar (OSCR), operated by the Radar Ocean Sensing Laboratory (ROSL) of the University of Miami, was deployed to measure the surface current vector field at high spatial and temporal resolution over the inner and mid shelf. Surface currents were mapped in near-real time over a domain bounded in the south by the US Army Waterways Experiment Station's Field Research Facility along the central mooring array of the Coastal Ocean Processes (CoOP) shelf experiment and extended about 27 km northward to include two additional CoOP transects. The surface current vectors were measured at a spatial resolution of about 1 km every 20 minutes during October 1994. Over the experimental period of about 30 days, quality data were acquired 96% of the time extending to the maximum theoretical range of 44 km. Time series or the surface current vectors were in close agreement with near-surface measurements from moored current meters. The surface current maps revealed numerous surface current features. One such feature was the buoyant coastal jet formed by the Chesapeake Bay outflow. A five day period when the buoyant jet was evident in the OSCR generated surface current maps is described to demonstrate the capabilities of HF radar to infer dynamical properties of coastal flow features from measured velocity fields

HF Radar Observation of Wave Directional Spectra in a Strong Current Regime

Dual Wellen HF Radar (WERA) systems have been observing near-surface currents and wave parameters over the Southeast Florida shelf since June 2004 as a part of the Southeast Atlantic Coastal Ocean Observing System (SEACOOS). The region of coverage includes the Florida Straits and the Florida Current (FC) which typically has maximum surface velocities approaching 2 ms"1. The echo-Doppler spectra are also routinely recorded and archived at both stations which allows post-processing to extract surface wave directional spectra using an iterative approach as implemented in Seaview Sensing reg software. Both WERA sites operated continuously during the passage of Hurricane Jeanne over the Florida Straits on 25 Sept 2004. Although it passed ~ 200 km to the north of the measurement domain, the local mean winds exceeded 20 ms -1 and rotated over 270deg. The near-surface currents during the passage of Hurricane Jeanne reflected the influence of the wind as well as the Florida Current. The effect of the wind on the near surface flow was seen in easterly and southerly flow over the shallow shelves near Florida and the Bahamas respectively as well as relatively slow flow in the center of the Florida current. Maximum current velocities were only 130 cms -1 50-60 cms -1 less than typical values. The interaction of these wave fields from differing directions with the high lateral shear of the western edge of the Florida Current was observed every 10 minutes.. The wind-wave component of the spectrum was observed to respond rapidly to the rotating wind-field, but effects of the horizontal shear were observed in the off-wind angle of the wind-wave peak. Tower frequencies were often observed at large angles to the local wind

An EOF analysis of HF Doppler radar current measurements of the Chesapeake Bay buoyant outflow

Surface currents measured by HF Doppler radar as part of a study of the Chesapeake Bay outflow plume are examined using a ‘real-vector’ empirical orthogonal function (EOF) analysis (Kaihatu et al., 1998). Based on about 23 days of nearly continuous data, the analysis shows that the first three EOF modes, judged to be the only significant modes, account for 76% of the variance in the data set. The buoyant outflow occurs primarily in the mean flow field. The first EOF mode is dominated by wind forcing and the second mode by across-shelf semi-diurnal tidal forcing. The third mode exhibits a large-scale horizontal shear and contains a curved region of weak relative flow which appears to delineate the offshore edge of the plume; also, the third-mode response varies over the spring-neap cycle, suggesting a modulation of the outflow plume by a tidal residual eddy. The analysis therefore has provided a useful, exploratory examination of this dataset of surface currents

Use of ‘velocity projection’ to estimate the variation of sea-surface height from HF Doppler radar current measurements

The technique of ‘velocity projection’ (J. Geophys. Res. 106 (2001) 6973) is used to estimate the sea-surface height field and its change over time from measurements of surface velocity made using a shore-based HF Doppler radar over a 30×30-km region of the continental shelf located near the mouth of the Chesapeake Bay (USA). Projected current profiles are compared with measured currents from an array of acoustic Doppler current profilers, and the consistency and sensitivity of the projections to model assumptions are also examined. Using projected values of the local surface slope, a model sea-surface η( x, y) is least-squares fit over the study region at each measurement time. The error associated with these fits provides an internal check on the validity of the projection results. The slope of the model sea-surface shows a set-up toward the mouth of the Chesapeake Bay during downwelling-favorable winds and a counterclockwise rotation over the tidal cycle that is consistent with linear, shallow-water dynamics. A time series of sea-level difference extracted from the η maps shows a dominant M 2 tidal signal that compares well with measurements of bottom pressure made at two moorings. With proper attention to limits of applicability, such projection-based sea-surface slope fields (as well as other projection results) may be useful in diagnostic calculations or as nowcasts for use with prognostic models

Resolving Coastal Ocean Eddy Activity in Surface Velocity Signatures from Wellen Radars and an Acoustic Doppler Current Profiler

A dual-station high frequency Wellen Radar (WERA), transmitting at 16.045 MHz, was deployed along the east Florida Shelf and is currently operated and maintained by the University of Miami's Rosenstiel School for Marine and Atmospheric Science. From September 2004 to June 2005, a bottom-mounted acoustic Doppler current profiler (ADCP) obtained subsurface current measurements within the radar footprint along the shelf break at 86-m depth. The RMS differences ranged from 0.1 to 0.25 ms -1 between the surface and 14 m depth indicating good data. Monthly time series analyses indicated numerous current reversals during the 9-month deployment. When utilized in conjunction with the ADCP subsurface measurements, WERA enables 3-dimensional snapshots of coastal oceanographic features. Given the high temporal and spatial resolution of the WERA system, an increased understanding can be gained in the coastal ocean regime aiding the ability of ocean models at predicting complicated features in the domain