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Bubbles directly link sea surface structure to the dissipation rate of turbulence in the ocean surface layer through wave breaking, and they are an important vehicle for air–sea transfer of heat and gases and important for understanding both hurricanes and global climate. Adequate parameterization of bubble dynamics, especially in high winds, requires simultaneous measurements of surface waves and breaking-induced turbulence; collection of such data would be hazardous in the field, and they are largely absent from laboratory studies to date. We therefore present data from a series of laboratory wind-wave tank experiments designed to observe bubble size distributions in natural seawater beneath hurricane conditions and connect them to surface wave statistics and subsurface turbulence. A shadowgraph imager was used to observe bubbles in three different water temperature conditions. We used these controlled conditions to examine the role of stability, surface tension, and water temperature on bubble distributions. Turbulent kinetic energy dissipation rates were determined from subsurface ADCP data using a robust inertial-subrange identification algorithm and related to wind input via wave-dependent scaling. Bubble distributions shift from narrow to broadbanded and toward smaller radius with increased wind input and wave steepness. TKE dissipation rate and shear were shown to increase with wave steepness; this behavior is associated with a larger number of small bubbles in the distributions, suggesting shear is dominant in forcing bubbles in hurricane wind-wave conditions. These results have important implications for bubble-facilitated air–sea exchanges, near-surface ocean mixing, and the distribution of turbulence beneath the air–sea interface in hurricanes. Significance Statement: Bubbles are a vehicle for the flux of heat, momentum, and gases between the atmosphere and ocean. These fluxes contribute to the energy budgets of hurricanes, climate, and upper-ocean biology. Few to no simultaneous measurements of surface waves, bubbles, and turbulence have been made in hurricane conditions. To improve numerical model representation of bubbles, we performed laboratory experiments to parameterize bubble size distributions using physical variables including wind and waves. Bubble distributions were found to become broadbanded and shift toward smaller radius with increased wind stress and wave steepness. Turbulence dissipation rate and shear were shown to increase with wave steepness. Our results give the first physically based bubble distribution parameterization from naturally breaking waves in hurricane-force conditions. [ABSTRACT FROM AUTHOR]

The Coastal Land–Air–Sea Interaction (CLASI) project aims to develop new "coast-aware" atmospheric boundary and surface layer parameterizations that represent the complex land–sea transition region through innovative observational and numerical modeling studies. The CLASI field effort involves an extensive array of more than 40 land- and ocean-based moorings and towers deployed within varying coastal domains, including sandy, rocky, urban, and mountainous shorelines. Eight Air–Sea Interaction Spar (ASIS) buoys are positioned within the coastal and nearshore zone, the largest and most concentrated deployment of this unique, established measurement platform. Additionally, an array of novel nearshore buoys and a network of land-based surface flux towers are complemented by spatial sampling from aircraft, shore-based radars, drones, and satellites. CLASI also incorporates unique electromagnetic wave (EM) propagation measurements using a coherent array, drone receiver, and a marine radar to understand evaporation duct variability in the coastal zone. The goal of CLASI is to provide a rich dataset for validation of coupled, data assimilating large-eddy simulations (LES) and the Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). CLASI observes four distinct coastal regimes within Monterey Bay, California (MB). By coordinating observations with COAMPS and LES simulations, the CLASI efforts will result in enhanced understanding of coastal physical processes and their representation in numerical weather prediction (NWP) models tailored to the coastal transition region. CLASI will also render a rich dataset for model evaluation and testing in support of future improvements to operational forecast models. [ABSTRACT FROM AUTHOR]

Here we have determined the nature of turbulent flow associated with oceanic nonbreaking waves, which are on average much more prevalent than breaking waves in most wind conditions. We found this flow to be characterized by a low turbulence microscale Reynolds number of 30 < Reλ < 100. We observed that the turbulent kinetic energy dissipation rate associated with nonbreaking waves ϵ, ranged to 3 · 10−4 W/kg for a wave amplitude 50 cm. The ϵ, under nonbreaking waves, was consistent with ϵ=2νTSij2; Sij is the large‐scale (energy‐containing scales) wave‐induced mean flow stress tensor. The turbulent Reynolds stress associated with nonbreaking waves was consistent with experimental data when parameterized by an amplitude independent constant turbulent eddy viscosity, 10 times larger than the molecular value. Given that nonbreaking waves typically cover a much larger fraction of the ocean surface (90–100%) than breaking waves, this result shows that their contribution to wave dissipation can be significant. Plain Language Summary: Considering that surface waves cover most of the ocean, the precise determination of the rate at which surface waves dissipate energy is necessary to properly quantify climate, weather, or ocean dynamic processes at the air‐sea interface and within the upper layer of the ocean. The upper‐ocean mixing intensity is often related to breaking surface waves, while the turbulence generated by nonbreaking surface waves is poorly understood and thus not well represented. Our laboratory experiments used microstructure and optical measurements to observe micro velocity shears and temperature fluctuations associated with passing nonbreaking solitary surface waves. Here we report measurements of the energy dissipation associated with these nonbreaking surface waves. We present an analytical approach to quantify the nonbreaking wave turbulence strength from large‐scale (energy‐containing) flow measurements. Key Points: Nonbreaking surface waves magnify background turbulent fluctuationsNonbreaking wave‐enhanced turbulence is characterized by a relatively low Reynolds numberThe dissipation caused by nonbreaking waves is proportional to the square of the mean wave flow stress tensor [ABSTRACT FROM AUTHOR]

This study analyzes high-resolution ship data collected in the Gulf of Mexico during the Lagrangian Submesoscale Experiment (LASER) from January to February 2016 to produce the first reported measurements of dissipative heating in the explicitly nonhurricane atmospheric surface layer. Although typically computed from theory as a function of wind speed cubed, the dissipative heating directly estimated via the turbulent kinetic energy (TKE) dissipation rate is also presented. The dissipative heating magnitude agreed with a previous study that estimated the dissipative heating in the hurricane boundary layer using in situ aircraft data. Our observations that the 10-m neutral drag coefficient parameterized using TKE dissipation rate approaches zero slope as wind increases suggests that TKE dissipation and dissipative heating are constrained to a physical limit. Both surface-layer stability and sea state were observed to be important conditions influencing dissipative heating, with the stability determined via TKE budget terms and the sea state determined via wave steepness and age using direct shipboard measurements. Momentum and enthalpy fluxes used in the TKE budget are determined using the eddy-correlation method. It is found that the TKE dissipation rate and the dissipative heating are largest in a nonneutral atmospheric surface layer with a sea surface comprising steep wind sea and slow swell waves at a given surface wind speed, whereas the ratio of dissipative heating to enthalpy fluxes is largest in near-neutral stability where the turbulent vertical velocities are near zero. [ABSTRACT FROM AUTHOR]

Surface wind stress is a crucial driver of upper-ocean processes, impacting air–sea gas flux, wind-wave development, and material transport. Conventional eddy covariance (EC) processing requires imposing a fixed averaging window on the wind velocity time series in order to estimate the downward flux of momentum. While this method has become the standard means of directly measuring the wind stress, the use of a fixed averaging interval inherently constrains one's ability to resolve transient signals that may have net effects on the air–sea interactions. Here we utilize the wavelet transform to develop a new technique for directly quantifying the wind stress magnitude from the wavelet coscalogram products. The time averages of these products evaluated at the scale of maximum amplitude are highly correlated with the EC estimates (R2 = 0.99; 5-min time windows), suggesting that stress is particularly sensitive to the dominant turbulent eddies. By taking advantage of the new method's high temporal resolution, transient wind forcing and its dominant scales may be explicitly computed and analyzed. This technique will allow for more general investigations into air–sea dynamics under nonstationary or spatially inhomogeneous conditions, such as within the nearshore region. [ABSTRACT FROM AUTHOR]

The Lagrangian Submesoscale Experiment (LASER) involved the deployment of ;1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR--drifter pairs, the root-mean-square error for the current speed is 4 cms21 and that for the current direction is 128. The MR samples currents at a greater effective depth than the CARTHE drifters (1-5mvs~0.4m). The mean MR--drifter differences are consistent with a wave-and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 158). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP. [ABSTRACT FROM AUTHOR]

Estimation of near-surface current is essential to the estimation of upper-ocean material transport. Wind forcing and wave motions are dominant in the near-surface layer [within O(0.01) m of the surface], where the highly sheared flows can differ greatly from those at depth. This study presents a new method for remotely measuring the directional wind and wave drift current profile near to the surface (between 0.01 and 0.001 m for the laboratory and between 0.1 and 0.001 m for the field). This work follows the spectral analysis of high spatial (0.002 m) and temporal resolution (60 Hz) wave slope images, allowing for the evaluation of near-surface current characteristics without having to rely on instruments that may disturb the flow. Observations gathered in the 15 m × 1 m × 1 m wind-wave flume at the University of Miami's Surge-Structure-Atmosphere Interaction (SUSTAIN) facility show that currents retrieved via this method agree well with the drift velocity of camera-tracked dye. Application of this method to data collected in the mouth of the Columbia River (MCR) indicates the presence of a near-surface current component that departs considerably from the tidal flow and may be steered by the wind stress. These observations demonstrate that wind speed-based parameterizations alone may not be sufficient to estimate wind drift and to hold implications for the way in which surface material (e.g., debris or spilled oil) transport is estimated when atmospheric stress is of relatively high magnitude or is steered off the mean wind direction. [ABSTRACT FROM AUTHOR]

A new structured-light method for 3D imaging has been developed which can simultaneously estimate both the geometric shape of the water surface and the geometric shape of underwater objects. The method requires only a single image and thus can be applied to dynamic as well as static scenes. Experimental results show the utility of this method in non-invasive underwater 3D reconstruction applications. The performance of the new method is studied through a sensitivity analysis for different parameters of the suggested method. [ABSTRACT FROM AUTHOR]

Controlled experiments were conducted in the Air-Sea Interaction Saltwater Tank (ASIST) at the University of Miami to investigate air-sea moist enthalpy transfer rates under various wind speeds (range of -1 0.6-39 m s scaled to equivalent 10-m neutral winds) and water-air temperature differences (range of 1.3°-9.2°C). An indirect calorimetric (heat content budget) measurement technique yielded accurate determinations of moist enthalpy flux over the full range of wind speeds. These winds included conditions with significant spray generation, the concentrations of which were of the same order as field observations. The moist enthalpy exchange coefficient so measured included a contribution from cooled reentrant spray and therefore serves as an upper limit for the interfacial transfer of enthalpy. An unknown quantity of spray was also observed to exit the tank without evaporating. By invoking an air volume enthalpy budget it was determined that the potential contribution of this exiting spray over an unbounded water volume was up to 28%. These two limits bound the total enthalpy transfer coefficient including spray-mediated transfers. [ABSTRACT FROM AUTHOR]

A quantitative description of wind-wave momentum transfer in high wind conditions is necessary for accurate wave models, storm and hurricane forecasting, and models that require atmosphere--ocean coupling such as circulation and mixed layer models. In this work, a static pressure probe mounted on a vertical wave follower to investigate relatively strong winds ( U10 up to 26.9 m s−−1 and U10/ Cp up to 16.6) above waves in laboratory conditions. The main goal of the paper is to quantify the effect of wave shape and airflow sheltering on the momentum transfer and wave growth. Primary results are formulated in terms of wind forcing and wave steepness ak, where a is wave amplitude and k is wave number. It is suggested that, within the studied range ( ak up to 0.19), the airflow is best described by the nonseparated sheltering theory. Notably, a small amount of spray and breaking waves was present at the highest wind speeds; however, their effect on the momentum flux was not found to be significant within studied conditions. [ABSTRACT FROM AUTHOR]

Wave-height observations derived from single-site high-frequency (HF) radar backscattered Doppler spectra are generally recognized to be less accurate than overlapping radar techniques but can provide significantly larger sampling regions. The larger available wave-sampling region may have important implications for observing system design. Comparison of HF radar–derived wave heights with acoustic Doppler profiler and buoy data revealed that the scale separation between the Bragg scattering waves and the peak energy-containing waves may contribute to errors in the single-site estimates in light-to-moderate winds. A wave-height correction factor was developed that explicitly considers this scale separation and eliminates the trend of increasing errors with increasing wind speed. [ABSTRACT FROM AUTHOR]

The present work is an observational analysis of the small-scale variations induced by Chesapeake Bay outflow on coastal surface circulation off Virginia. The variation in freshwater discharge into Chesapeake Bay is investigated as a triggering mechanism for local coastal jets. While it is known that an extra amount of freshwater received from the tributaries will lead to an increased total outflow, the effect on the outflow surface signature (the upper meter) has not been extensively explored. Our analysis is based on surface currents, wind data, and freshwater discharge records. The surface currents were derived from two high-frequency Doppler radars deployed on the Virginia coast during September and October 1996 and 1997. The wind data were obtained from a nearby weather Coastal-Marine Automated Network station, and the freshwater discharge data were from the U.S. Geological Survey stations located at the mouth of each tributary. The freshwater discharge into the bay observed during 1996 decreased from 4200 to 1000 m3/s in 10 days, while for the 1997 period it varied between 500 and 5500 m3/s in 1 week. The present analysis shows that sudden variations of freshwater flux will drive the plume further offshore before its normal southward turning, in the same way upwelling-favorable winds do as reported in all the available literature of the area. Strong freshwater variations on weekly scales are then shown to be an important factor in determining the extent of the surface plume displacement and the fate of the shelf currents or jets. [ABSTRACT FROM AUTHOR]

This paper is dedicated to wave-induced turbulence unrelated to wave breaking. The existence of such turbulence has been foreshadowed in a number of experimental, theoretical, and numerical studies. The current study presents direct measurements of this turbulence. The laboratory experiment was conducted by means of particle image velocimetry, which allowed estimates of wavenumber velocity spectra beneath monochromatic nonbreaking unforced waves. Observed spectra intermittently exhibited the Kolmogorov interval associated with the presence of isotropic turbulence. The magnitudes of the energy dissipation rates due to this turbulence in the particular case of 1.5-Hz deep-water waves were quantified as a function of the surface wave amplitude. The presence of such turbulence, previously not accounted for, can affect the physics of the wave energy dissipation, the subsurface boundary layer, and the ocean mixing in a significant way. [ABSTRACT FROM AUTHOR]

Abstract: Physical processes affecting the dispersion of passive particles (e.g., coral larvae, pollutants) in the Upper Florida Keys are investigated through in situ observations (acoustic Doppler current profilers and surface drifters) and numerical ocean circulation modeling (horizontal resolution: 800m, vertical resolution: 0.1–1m). During the study period in August 2006 (set to coincide with an annual coral spawning event), Lagrangian trajectories in the vicinity of the reef tract indicate that alongshelf advection was mainly poleward and due to the subtidal flow of the Florida Current, while cross-shelf advection was mainly onshore and due to wind-driven currents. Tidal currents resulted in predominantly alongshelf displacements, but did not contribute significantly to net passive particle transport on a weekly timescale. Typical advection distances were of the order of 10 to 50km for pelagic durations of 1 week, with significant variability linked to geographical location. In contrast, the direction of transport from the offshore reefs remained essentially constant (i.e., potential dispersion pathways were limited). In addition, Lagrangian trajectories and progressive vector diagrams in the vicinity of the reef tract indicate that alongshelf variations in the cross-shelf velocity gradient associated with the FC are relatively weak on an alongshore scale of ca. 50km. For August 2006, the highest particle concentrations typically occur inshore of the reef tract, thereby suggesting that onshore transport associated with wind-driven currents contributes significantly to the local retention of passive organisms (and other tracers) in the Upper Florida Keys. Overall, the results illustrate the necessity of conducting targeted in situ observations and numerical model predictions to quantify the physical processes affecting reef-scale advection, especially in an effort to understand local retention and dispersion mechanisms for larval marine organisms. [Copyright &y& Elsevier]

Tides, wind and the Florida Current affect the dynamics of the South Florida continental shelf over a range of scales. Two field experiments in the winter (LARGO1) and summer (LARGO2) employed a high-frequency (HF) Ocean Surface Current Radar (OSCR), moored current meters and a boat-mounted Acoustic Doppler Current Profiler (ADCP) to study the circulation over the southeast Florida Shelf. The OSCR measured near-surface currents over an approximately 30 km by 30 km grid, with 1-km horizontal resolution, covering the shelf and extending offshore of the shelf break. By using observed 20-min surface vector field time series to backward track patches with defined volumes of shelf water, their recent origin was identified. The proportion of surface volume flux past various points on the shelf that originated offshore, along the shelf edge and inshore was then estimated. Onshore winds were linked to periods of transport of offshore water all the way to the inner shelf during both the summer and winter seasons. In LARGO2 the exchange of offshore water with water over the middle and inner shelf was closely linked to onshore and downcoast winds. Simulated particle trajectories revealed that Florida Current spin-off eddies caused off-shelf water to be advected onto the middle (LARGO1) or outer (LARGO2) shelf. However, because eddies had short residence times and small spatial scales their contribution to the overall exchange was much smaller than the wind. [Copyright &y& Elsevier]

High-Frequency Doppler radars synoptically mapped surface currents over the inner-shelf off the North Carolina Outer Banks during the Duck94 experiment. The radar system sampled for the month of October, 1994 over a region 45 to 66 km southeast of the Chesapeake Bay mouth up to 34 km offshore. Concurrently, salinity and temperature were measured at locations 0.5, 1.6, 5.3 and 16.6 km offshore along a cross-shelf transect. Salinities over the shelf ranged from 26.6 to 32.7, with the lower salinities resulting from the intermittent presence of the coastal buoyancy current from the Chesapeake Bay. Three events were observed in which downwelling favorable winds persisted for at least three days, relaxed and then rotated to upwelling favorable. Downwelling favorable winds produced coastal sea level set-up. When the buoyancy current was present, cross-shelf salinity gradients were established between fresher water near the coast and the more saline water offshore. The response of the buoyancy current was sensitive to the temporal evolution of the wind field from downwelling to upwelling favorable. Following downwelling favorable winds the wind typically shifted to an onshore direction and decreased in magnitude. When the alongshelf velocities decreased, the onshore Coriolis force also decreased and the buoyancy current exhibited significant alongshelf variability over scales less than 10 km. Offshore flowing jets and undulations in the flow developed that formed vortices with diameters of 5-10 km following two of the downwelling events. Offshore surface layer velocities were observed between 4 and 16 hours before the wind switched to upwelling favorable. Formation of the vortices was likely triggered by an imbalance in the crossshelf momentum due to nearshore water level set-up and the decreased onshore Coriolis component. [ABSTRACT FROM AUTHOR]

Abstract: Plastics and spilled oil pose a critical threat to marine life and human health. As a result of wind forcing and wave motions, theoretical and laboratory studies predict very strong velocity variation with depth over the upper few centimeters of the water column, an observational blind spot in the real ocean. Here we present the first‐ever ocean measurements of the current vector profile defined to within 1 cm of the free surface. In our illustrative example, the current magnitude averaged over the upper 1 cm of the ocean is shown to be nearly four times the average over the upper 10 m, even for mild forcing. Our findings indicate that this shear will rapidly separate pieces of marine debris which vary in size or buoyancy, making consideration of these dynamics essential to an improved understanding of the pathways along which marine plastics and oil are transported. [ABSTRACT FROM AUTHOR]

A magnetic signature is created by secondary magnetic field fluctuations caused by the phenomenon of seawater moving in Earth's magnetic field. A laboratory experiment was conducted at the Surge Structure Atmosphere Interaction (SUSTAIN) facility to measure the magnetic signature of surface waves using a differential method: a pair of magnetometers, separated horizontally by one-half wavelength, were placed at several locations on the outer tank walls. This technique significantly reduced the extraneous magnetic distortions that were detected simultaneously by both sensors and additionally doubled the magnetic signal of surface waves. Accelerometer measurements and local gradients were used to identify magnetic noise produced from tank vibrations. Wave parameters of 4-m-long waves with a 0.56-Hz frequency and a 0.1-m amplitude were used in this experiment. Freshwater and saltwater experiments were completed to determine the magnetic difference generated by the difference in conductivity. Tests with an empty tank were conducted to identify the noise of the facility. When the magnetic signal was put through spectral analysis, it showed the primary peak at the wave frequency (0.56 Hz) and less pronounced higher-frequency harmonics, which are caused by the nonlinearity of shallow water surface waves. The magnetic noise induced by the wavemaker and related vibrations peaked around 0.3 Hz, which was removed using filtering techniques. These results indicate that the magnetic signature produced by surface waves was an order of magnitude larger than in traditional model predictions. The discrepancy may be due to the magnetic permeability difference between water and air that is not considered in the traditional model. [ABSTRACT FROM AUTHOR]

The capability of phased-array HF radar systems to sample the spatial distribution of wave energy is investigated in different storm scenarios and coastal configurations. First, a formulation introduced by D. E. Barrick to extract significant wave height Hs from backscatter Doppler spectra was calibrated and subsequently tested (to assess bias and uncertainty) with data from seven different buoy/gauge stations collected during three different field experiments. Afterward, Hs observations were obtained for selected sampling locations within the radar effective domain (in all experiments), and a filtering technique based on wavelet transform characterization and decomposition was applied. The accuracy of the filtered radar-derived observations was assessed by comparing these estimates to results from independently calibrated wave propagation models. It was found that the HF radar accurately measured the energy field induced by different storm events. The filtering technique minimized the contribution of unrealistic features introduced by the presence of defective sampling, which is intrinsic to radar remote sensing at this frequency, and it proved to be central for the use of the HF radar as a tool to identify wave energy trends and potential zones of wave energy concentration in coastal areas. These findings show that the sampling capabilities of radar systems may be greatly enhanced because reliable wave energy estimates can be obtained in addition to conventional surface current measurements. This is particularly important in locations such as harbor entrances where in situ measuring devices cannot be deployed. [ABSTRACT FROM AUTHOR]

Our unmanned aerial system (UAS) current mapping is based on optical video data of the sea surface. We use three-dimensional fast Fourier transform and least squares fitting to measure the surface waves' phase velocities and the currents via the linear dispersion relationship. Our UAS is a low-cost off-the-shelf quadcopter with inaccurate camera position and attitude measurements, which may cause spurious currents as large as the signal. We present a novel wave-based UAS heading and position correction, improving the image rectification accuracy by a factor of ~3.5 and the current measurements' temporal repeatability by factors of 1.8–4.8. This validation study maps the currents at high spatiotemporal resolution (5 m and 4 s) across the ~700-m-wide tidally dominated Bear Cut channel in Miami, Florida. The UAS currents are compared to flotsam tracks, obtained through automated UAS video object detection and tracking, drifter tracks, and acoustic Doppler current profiler measurements. The root-mean-square errors of the cross- and along-channel currents are better than 0.03 m s−1 for the flotsam comparison and better than 0.06 m s−1 for the drifter comparison; the latter revealed a 0.06 m s−1 along-wind bias due to wind- and wave-driven vertical current shear. UAS current mapping could be used to monitor river discharge, buoyant pollutants, or submesoscale fronts and eddies; the proposed wave-based heading and position correction enables its use in areas without ground control points. [ABSTRACT FROM AUTHOR]

Short-range infrared (IR) observations of ocean surface reveal complicated spatially varying and evolving structures. Here we present an approach to use spatially correlated time series IR images, over a time scale of one-tenth of a second, of the water surface to derive underlying surface velocity and turbulence fields. The approach here was tested in a laboratory using grid-generated turbulence and a heater assembly. The technique was compared with in situ measurements to validate our IR-derived remotemeasurements. The IR-measured turbulent kinetic energy (TKE) dissipation rateswere consistent with in situ--measured dissipation using a vertical microstructure profiler (VMP). We used measurements of the gradient of the velocity field to calculate TKE dissipation rates at the surface. Based on theoretical and experimental considerations, we have proposed two models of IR TKE dissipation rate retrievals and designed an approach for oceanic field IR applications. [ABSTRACT FROM AUTHOR]

Floating oil, plastics and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 x 20 km2 converged into a 60 x 60 m region within a week, a factor of more than 105 decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s-1 and 0.01 ms-1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material and create opportunities to more efficiently recover such material. [ABSTRACT FROM AUTHOR]

We have performed large-eddy simulations (LES) to study the effect of complex land topography on the atmospheric boundary layer (ABL) in coastal areas. The areas under investigation are located at three beaches in Monterey Bay, CA, USA. The sharp-interface immersed boundary method is employed to resolve the land topography down to grid scale. We have considered real-time and what-if cases. In the real-time cases, measurement data and realistic land topographies are directly incorporated. In the what-if cases, the effects of different scenarios of wind speed, wind direction, and terrain pattern on the momentum flux at the beach are studied. The LES results are compared with simulations using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) and field measurement data. We find that the land topography imposes a critical influence on the ABL in the coastal area. The momentum fluxes obtained from our LES agree with measurement data. Our results indicate the importance of capturing the effects of land topographies in simulations. [ABSTRACT FROM AUTHOR]

Targeted observations of submesoscale currents are necessary to improve science's understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory settings. Results showed that the drifter accurately follows the currents in the upper 0.60 m, that it presents minimal wave rectification issues, and that its wind-induced slip velocity is less than 0.5% of the neutral wind speed at 10 m. In experiments conducted in both coastal and deep ocean conditions under wind speeds up to 10 m s−1, the trajectories of the traditional Coastal Ocean Dynamics Experiment (CODE) and the CARTHE drifters were nearly identical. Following these tests, 1100 units were produced and deployed during the LASER campaign, successfully tracking submesoscale and mesoscale features in the Gulf of Mexico. It is hoped that this drifter will enable high-density sampling near metropolitan areas subject to stress by the overpopulation, such as lakes, rivers, estuaries, and environmentally sensitive areas, such as the Arctic. [ABSTRACT FROM AUTHOR]

In order to more fully understand the specific hydrodynamic relationship between young wind-generated gravity-capillary waves and longer gravity waves, a laboratory experiment was devised to observe changes in short wave spectral behavior over the phase of a long wave. This paper endeavors to expand on the body of laboratory wave modulation data and extend the investigation in support of the radar remote sensing of ocean surface waves. Measurements were made in the University of Miami’s surge–structure–atmosphere interaction facility in the air–sea interaction saltwater tank wind-wave tank, with 10 m referenced wind speeds ranging between 5 and 23 m/s and paddle-generated wave steepnesses “ak” varying between 0.05< \mathrm ak <0.3 . A polarimetric camera was used to capture high sampling frequency maps of wave slope, yielding spatiotemporal information about short wind-wave behavior [provided as temporal variations in the wavenumber spectrum, where k~\approx \mathscr O (100–1000) rad/m]. The simultaneous and colocated long wave phase was measured via a side-looking camera. Hydrodynamic modulation transfer function (MTF) phases are found to be in general agreement with established values (between 2 and 10 radians) at the given wind speeds. The positive phase of the modulation places it immediately downwind of the long wave crest, with MTF magnitudes strongest for high wavenumbers at the lowest wind speeds. The results are also presented to show the modulation of gravity-capillary and pure capillary waves as variations in mean square slope over the long wave phase, with peak roughness enhancement found to move upwind of the long wave crest with increasing wind forcing. [ABSTRACT FROM AUTHOR]

A unique spatiotemporal perspective of evolving surface currents off the northern California coast is provided with Lagrangian Coherent Structures (LCSs) determined from attracting Finite-Time Lyapunov Exponents (FTLEs). The FTLEs are calculated from hourly 2-D surface current velocities obtained with HF radars. Attracting FTLE field maxima can identify confluence and shear in flows which can be useful in mapping dynamics associated with fronts. FTLE and SST fields are compared during three time periods in 2009: late March, early September, and late September. During strong upwelling-favorable winds in late March the FTLE and SST fields were not strongly related indicating that frontal dynamics were not strongly influencing surface circulation. Exceptions to this occurred when FTLE ridges calculated from a shorter integration period captured the evolution of a cold water filament and when a FTLE ridge migrated offshore along with an upwelling front. During the two September cases an improved relationship between the FTLE and SST fields was found although occasionally they became shifted. The shifts occurred when the FTLE integration period spanned backwards in time through periods of relaxed and upwelling-favorable winds. This suggests that frontal dynamics captured by LCSs during relaxed winds can subsequently become advected after the onset of wind-forcing by a surface mixed layer decoupled from stably stratified water below the surface mixed layer. Additionally, the LCSs were found to be useful in mapping the origin and destination of surface trajectories, the confluence associated with a persistent eddy-like feature, and retention zones off coastal promontories. [ABSTRACT FROM AUTHOR]

Ocean prediction systems rely on an array of assumptions to optimize their data assimilation schemes. Many of these remain untested, especially at smaller scales, because sufficiently dense observations are very rare. A set of 295 drifters deployed in July 2012 in the north-eastern Gulf of Mexico provides a unique opportunity to test these systems down to scales previously unobtainable. In this study, background error covariance assumptions in the 3DVar assimilation process are perturbed to understand the effect on the solution relative to the withheld dense drifter data. Results show that the amplitude of the background error covariance is an important factor as expected, and a proposed new formulation provides added skill. In addition, the background error covariance time correlation is important to allow satellite observations to affect the results over a period longer than one daily assimilation cycle. The results show the new background error covariance formulations provide more accurate placement of frontal positions, directions of currents and velocity magnitudes. These conclusions have implications for the implementation of 3DVar systems as well as the analysis interval of 4DVar systems. [ABSTRACT FROM AUTHOR]

Reliable forecasts for the dispersion of oceanic contamination are important for coastal ecosystems, society, and the economy as evidenced by the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 and the Fukushima nuclear plant incident in the Pacific Ocean in 2011. Accurate prediction of pollutant pathways and concentrations at the ocean surface requires understanding ocean dynamics over a broad range of spatial scales. Fundamental questions concerning the structure of the velocity field at the submesoscales (100 m to tens of kilometers, hours to days) remain unresolved due to a lack of synoptic measurements at these scales. Using high-frequency position data provided by the near-simultaneous release of hundreds of accurately tracked surface drifters, we study the structure of submesoscale surface velocity fluctuations in the Northern Gulf of Mexico. Observed two-point statistics confirm the accuracy of classic turbulence scaling laws at 200-m to 50-km scales and clearly indicate that dispersion at the submesoscales is local, driven predominantly by energetic submesoscale fluctuations. The results demonstrate the feasibility and utility of deploying large clusters of drifting instruments to provide synoptic observations of spatial variability of the ocean surface velocity field. Our findings allow quantification of the submesoscale-driven dispersion missing in current operational circulation models and satellite altimeter-derived velocity fields. [ABSTRACT FROM AUTHOR]

Evolution of nonlinear wave groups to breaking under wind forcing was studied by means of a fully nonlinear numerical model and in a laboratory experiment. Dependence of distance to breaking and modulation depth (height ratio of the highest and the lowest waves in a group) on wind forcing was described. It was shown that in the presence of a certain wind forcing both distance to breaking and modulation depth decrease; the latter signifies slowing down of the instability growth. It was also shown that wind forcing significantly reduces the energy loss in a single breaking event. [ABSTRACT FROM AUTHOR]

Abstract: Fecal indicator microbes, such as enterococci, are often used to assess potential health risks caused by pathogens at recreational beaches. Microbe levels often vary based on collection time and sampling location. The primary goal of this study was to assess how spatial and temporal variations in sample collection, which are driven by environmental parameters, impact enterococci measurements and beach management decisions. A secondary goal was to assess whether enterococci levels can be predictive of the presence of Staphylococcus aureus, a skin pathogen. Over a ten-day period, hydrometeorologic data, hydrodynamic data, bather densities, enterococci levels, and S. aureus levels including methicillin-resistant S. aureus (MRSA) were measured in both water and sand. Samples were collected hourly for both water and sediment at knee-depth, and every 6 h for water at waist-depth, supratidal sand, intertidal sand, and waterline sand. Results showed that solar radiation, tides, and rainfall events were major environmental factors that impacted enterococci levels. S. aureus levels were associated with bathing load, but did not correlate with enterococci levels or any other measured parameters. The results imply that frequencies of advisories depend heavily upon sample collection policies due to spatial and temporal variation of enterococci levels in response to environmental parameters. Thus, sampling at different times of the day and at different depths can significantly impact beach management decisions. Additionally, the lack of correlation between S. aureus and enterococci suggests that use of fecal indicators may not accurately assess risk for some pathogens. [Copyright &y& Elsevier]

A dual-station high-frequency Wellen Radar (WERA), transmitting at 16.045 MHz, was deployed along the west Florida shelf in phased array mode during the summer of 2003. A 33-day, continuous time series of radial and vector surface current fields was acquired starting on 23 August ending 25 September 2003. Over a 30-min sample interval, WERA mapped coastal ocean currents over an ≈40 km × 80 km footprint with a 1.2-km horizontal resolution. A total of 1628 snapshots of the vector surface currents was acquired, with only 70 samples (4.3%) missing from the vector time series. Comparisons to subsurface measurements from two moored acoustic Doppler current profilers revealed RMS differences of 1 to 5 cm s-1 for both radial and Cartesian current components. Regression analyses indicated slopes close to unity with small biases between surface and subsurface measurements at 4-m depth in the east–west (u) and north–south (υ) components, respectively. Vector correlation coefficients were 0.9 with complex phases of -3° and 5° at EC4 (20-m isobath) and NA2 (25-m isobath) moorings, respectively. Complex surface circulation patterns were observed that included tidal and wind-driven currents over the west Florida shelf. Tidal current amplitudes were 4 to 5 cm s-1 for the diurnal and semidiurnal constituents. Vertical structure of these tidal currents indicated that the semidiurnal components were predominantly barotropic whereas diurnal tidal currents had more of a baroclinic component. Tidal currents were removed from the observed current time series and were compared to the 10-m adjusted winds at a surface mooring. Based on these time series comparisons, regression slopes were 0.02 to 0.03 in the east–west and north–south directions, respectively. During Tropical Storm Henri’s passage on 5 September 2003, cyclonically rotating surface winds forced surface velocities of more than 35 cm s-1 as Henri made landfall north of Tampa Bay, Florida. These results suggest that the WERA measured the surface velocity well under weak to tropical storm wind conditions. [ABSTRACT FROM AUTHOR]

Abstract: A three-dimensional ocean circulation model is used to investigate the hydrodynamics of a tidal inlet and deltas system in Southeast Florida, and to understand the consequences for suspended and bedload sediment transport patterns. The model reproduces observed tidal currents and provides insight about residual currents caused by spatial asymmetries in the inlet throat and tidal deltas during ebb and flood flows. A particle-tracking approach for suspended and bedload sediment transport is used to simulate deposition patterns for different particle sizes. The simulation results qualitatively correlate with the distribution of sediment characteristics within the tidal inlet and deltas system and demonstrate sensitivity to the choice of advection velocities (e.g., near-bottom versus depth-averaged) and regions of sediment origin. Furthermore, the distinction between suspended and bedload transport as a function of particle size indicates significant differences in deposition patterns and their potential connection to geomorphologic features of the tidal inlet and deltas system. [Copyright &y& Elsevier]

Ocean Surface Current Radar (OSCR) HF radar measurements of ocean waves and currents were made during the Shoaling Waves Experiment (SHOWEX) in the fall of 1999. During some periods, at some locations, good quality wave measurements were obtained. Limitations in the wave measurement capability due to OSCR hardware, deployment configuration, signal-to-noise ratio, and antenna sidelobes are identified and discussed. A short period of large currents in the presence of antenna pattern distortion is identified as the source of the main errors in the wave measurements. [ABSTRACT FROM AUTHOR]

ABSTRACT As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September-October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km x 44 km domain. Comparisons to subsurface current measurements at 1-2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and -3 cms s[sup-1] for cross-shelf and along-shelf currents, respectively, where slopes were theta(1) with correlation coefficients of 0.8. Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M[sub 2] tidal component having an amplitude of 37.5 cm and a phase of 357 degrees. The S[sub 2] tidal constituent had an amplitude of 7 cm and a phase of 49 degrees. By contrast, the diurnal band (K[sub 1], O[sub 1]) tidal constituents were considerably weaker with amplitudes of 1-5 cm. From 19 days of HF-derived surface... [ABSTRACT FROM AUTHOR]

Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data to near-surface current observations from 1 to 30 October 1994 at two moored subsurface current meter arrays (20 and 25 m) instrumented with vector-measuring current meters (VMCMs) and Seacat sensors during the Duck94 experiment. A dual-station ocean surface current radar (OSCR) mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 25 km 3 44 km domain using the HF(25.4 MHz) mode and directly overlooked these moorings. In response to wind, tidal, and buoyancy forcing over 29 days, surface current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore direction. Regression analyses between surface and subsurface measurements at 4 and 6 m indicated biases of 2--6 cm s[sup-1], slopes of O(1), and rms differences of 7--9 cm s[sup-1] . Episodic freshwater intrusions of about 30 practical salinity units (psu) were associated with a coastally trapped buoyant jet superposed on tidal currents. This tidal forcing consisted of diurnal (K1 ) and semidiurnal (M2 ) tidal constituents where the surface and subsurface (4 m) speeds were 3 and 8 cm s[sup-1] ,and 2 and 7 cm s[sup-1] , respectively. During the passage of a nor'easter, near-surface winds reached 14 m s[sup-1] , which induced vertical mixing that caused weak stratification in the water column. An abrupt wind change following this event excited near-inertial (~20.3 h) currents with amplitudes of about 20 cm s[sup-1] rotating clockwise with time and depth. Bulk current shears over 4- and 6-m layers were O(10[sup-2] s[sup-1] ) at the 25-m mooring where the correlation coefficients exceeded 0.8. Similar results were found at the 20-m mooring until the nor'easter when correlation coefficients decreased to 0.5 due to the superposition of storm-induced flows and the buoyant jet, causing the (...). [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

Despite significant improvement in computational and observational capabilities, predicting intensity and intensification of major tropical cyclones remains a challenge. In 2017 Hurricane Maria intensified to a Category 5 storm within 24 h, devastating Puerto Rico. In 2019 Hurricane Dorian, predicted to remain tropical storm, unexpectedly intensified into a Category 5 storm and destroyed the Bahamas. The official forecast and computer models were unable to predict rapid intensification of these storms. One possible reason for this is that key physics, including microscale processes at the air-sea interface, are poorly understood and parameterized in existing forecast models. Here we show that surfactants significantly affect the generation of sea spray, which provides some of the fuel for tropical cyclones and their intensification, but also provides some of the drag that limits intensity and intensification. Using a numerical model verified with a laboratory experiment, which predicts spray radii distribution starting from a 100 μm radius, we show that surfactants increase spray generation by 20–34%. We anticipate that bio-surfactants affect heat, energy, and momentum exchange through altered size distribution and concentration of sea spray, with consequences for tropical cyclone intensification or decline, particularly in areas of algal blooms and near coral reefs, as well as in areas affected by oil spills and dispersants. [ABSTRACT FROM AUTHOR]