Along-track and gridded altimetric observations of sea level are colocated and combined with data from drifter observations and wind reanalysis to reconstruct global instantaneous near-surface horizontal momentum balance. This reconstruction includes not only geostrophic terms but also Lagrangian accelerative terms and turbulent stress terms. The methodology developed quantifies the degree of closure, distinguishes statistically balanced components from errors and estimates compensation between pairs of terms. The links between statistically derived closure diagnoses and dynamical ones are established. Overall, the variance of the residual acceleration is about 20% of the sum of individual acceleration variances. We carry out a detailed exploration of the misclosure, which is dominated by unbalanced variance in drifter observations (resolution mismatch accounts for 41% of the total error) followed by instrumental and spatial colocation errors. Except for the turbulent stress term, errors are sufficiently small to ensure safe interpretation of statistically derived balanced contributions as dynamical ones. Although geostrophy is the leading order equilibrium, ageostrophic contributions associated with nonlinear balanced motions, internal tides, and near-inertial waves account for one third of the global balanced acceleration variance. Momentum balance reconstructions and the methodology developed here for that purpose hold promise for validating Surface Water Ocean Topography sea level observations, for quantifying our ability to estimate the ocean circulation from these observations, and for improving our understanding of ocean near-surface dynamics.

CIGoM Buoy Network in the Gulf of Mexico was implemented and operated between 2016 and 2019. While two type of buoys were designed and built, in this work we focus on spar buoy (BOMM, as Spanish acronym for Oceanography and Marine Meteorology Buoy) to directly estimate ocean surface wave full directional spectrum as well as the momentum flux between the ocean and the atmosphere. A detailed description of the buoy and main characteristics of sensors deployed in three BOMM is given. We aim to better understand the O-A momentum flux under non-equilibrium conditions, specifically when sudden wind changes occurred typically associated with atmospheric fronts passages over the region of interest. The influence of swell is analyzed in detail for cases when locally generated waves are being developed under moderate wind speeds. Of particular importance is the remote observations of the wave field under those conditions, by means of Synthetic Aperture Radar images of the ocean surface. We approach the subject by making use of a quasi-linear inversion algorithm (Krogstad et al., 1994) with the procedure already advanced and proposed by Vachon et al. (1994), in order to estimate the directional wave spectrum. The relation between SAR derived wave spectra and the directly estimated momentum fluxes are addressed.

Accurate nearshore bathymetry estimation remains a critical challenge, impacting coastal forecasting evolution assessments through the inaccuracies in both in-situ and remote sensing surveys. This article introduces the Satellite Derived Bathymetry (SDB) temporal correlation method, showcasing its ability in deriving accurate nearshore bathymetry from one minute spaceborne videos. The approach utilises correlation of pixel intensity time series, shifted in time and space, extracted from a frame stack within a defined correlation window. The resulting correlation is then projected using the Radon Transform to infer wave characteristics (celerity and wavelength) for the estimation of depth through wave linear dispersion. Moreover, the adaptation of the correlation window based on a first wavelength estimation provided a more focused assessment of the wavefield that reveals morphological features such as sandbars in the bathymetric estimation. The method’s capabilities using adapted correlation window is illustrated through its application to a metric resolution Jilin satellite video (57 s at 5 Hz) along the Saint-Louis coast in Senegal. Through this demonstration, the temporal correlation method is among the first SDB methods to successfully capture the submerged sandbar along a beach. Comparison against in-situ measurements conducted three years prior to the video acquisition shows a good agreement with a bias of 0.97 m within the initial 2 km of the cross-shore profile. Furthermore, the application of previously developed sky-glint surface elevation analysis on video pixel intensity, prior to the bathymetry estimation, significantly reduces the bias to 0.44 m in the Saint-Louis estimation. This article highlights the potential applications of future Earth observation satellite missions that will capture image sequences (or videos) such as CO3D (CNES/Airbus).

Ocean surface wave full directional spectrum is estimated directly from measurements obtained with a spar buoy and from synthetic aperture radar images of the sea surface. These two techniques complement each other to provide us with a rather comprehensive view of the dynamical behaviour of surface waves. We focus our study in sea state conditions under varying winds, when frequently mixed sea and swell systems are encountered. These conditions are characterized by non-equilibrium wind-wave systems. Direct measurements of ocean-atmosphere momentum fluxes obtained from dedicated air-sea interaction spar buoy are also analyzed. The aim is to better understand the ocean-atmosphere momentum transfer behaviour and uppermost ocean currents under rapidly varying wind field. Atmospheric cold front passage through the measuring buoy imposed a unique wind-wave system information, especially under the occurrence of cases when swell propagation opposes locally generated wind-waves. Of particular importance is the analysis of the wave field making use of synthetic aperture radar images of the sea surface. The wave and wind fields to both sides of the atmospheric front are analyzed. From the buoy measurements fetch-limited wind sea growth is also determined, where slanting fetch is to be considered as very relevant. In particular, wind acceleration effect on wave growth is addressed, during specific cases when wind direction prevailed relatively constant. Wind-wave growth rate is somewhat greater than stationary conditions, as it can be observed also in some laboratory experiments at least for the early stages of the growth process.

GlobCurrent provides a variety of datasets aiming to describe global ocean circulation, especially when dealing with large-scale phenomena. It includes surface Stokes drift and geostrophic, Ekman, and total (geostrophic plus Ekman) currents. GlobCurrent uses the CNES-CLS13 mean dynamic topography estimation as well as data from sea surface drifters and wind reanalysis to improve the computation of ocean currents from altimetry data, which represents a significant advance in describing the total ocean current. The aim of this work is to compare the surface GlobCurrent estimates with a coupled ocean–wave numerical simulation (POLCOMS-WAM), drifting buoys, and altimeter observations when dealing with a Tehuano event, i.e., intense (larger than 20 m s ^-1 ) and short duration (around 3–5 days) low-level winds blowing over the Gulf of Tehuantepec, Mexico. There is a good agreement between the wind-driven currents (Ekman currents plus Stokes drift) field from GlobCurrent and that estimated by POLCOMS–WAM, with the largest magnitudes ∼ 0.8 m s ^-1 in the region influenced by the highest winds’ speed. The geostrophic circulation patterns in the Gulf of Tehuantepec are similarly reproduced by GlobCurrent and POLCOMS-WAM. However, some differences were observed in the presence of an anticyclonic eddy located in the western part of the study area. Numerical results exhibit a more symmetrical eddy with geostrophic current speeds that, in agreement with along-track observations, exceed the 1 m s ^-1 . Instead, the geostrophic eddy in GlobCurrent shows velocities of about 0.8 m s ^-1 . As observed through drifting buoys in 2000, numerical results show that the anticyclonic eddy west of the GoT has strong ageostrophic currents related to a cyclogeostrophic balance, which is not included in GlobCurrent. This regional case study provides a guideline for future improvements of GlobCurrent products, in particular for the estimation of geostrophic and total currents.

This paper studies the error that occurs when measuring surface currents with a current meter mounted on a buoy or a mooring line whose horizontal and vertical motions respond to the presence of waves. The error is defined with respect to an Eulerian reference measurement where the sensor does not move. First, we present the subject with a theoretical analysis in the case of a monochromatic wave. That idealized model allows us to study particular sensor or mooring line motions. Second, a realistic numerical model is implemented to reconstruct the current field with a high resolution near the surface. Wave orbital velocities are generated with a random phase model. An Ekman-type current, uniform in the horizontal but with a vertical shear, is also incorporated. The results indicate that the error in the current measurement is highly dependent on the sensor motion induced by waves. The error magnitude is proportional to the wave momentum or Stokes drift and depends on the wave development state and the wind-generated current’s magnitude. The error obtained in the current measurement is analyzed by considering that the buoy only responds to low-frequency waves up to a maximum frequency. That maximum frequency is referenced concerning the peak frequency of the third moment of the spectrum (i.e., the Stokes drift spectrum). It allows us to classify the current time average into three ranges with respect to the maximum frequency: (1) Eulerian average, (2) wave-following average, and (3) intermediate case of undulating average where results cannot be generalized. The measurement error is most important in the region above the wave troughs. However, the error is also considerable in the region confined below the wave troughs and down to the Stokes drift e-folding depth. The error is particularly relevant in conditions of developed and energetic waves (Hs>3 m), where the surface Stokes drift can reach values above 0.1 m/s. It should be noted that measurement error can exceed the value of the Stokes drift at the sensor depth for certain mooring line motions. Those results should help better interpret in situ near-surface current measurements obtained from various devices.

The impact of the high space-temporal variability of the wind field during the moderate and intense storm stages of a tropical cyclone on the wave field as computed by the numerical model WaveWatch III is investigated in this work. The realistic wind fields are generated by a high-resolution implementation of the HWRF model in the Gulf of Mexico and stored over 15 min intervals. The spatial structure of the wind field computed by HWRF is highly variable in space and time, although its mean structure is very similar to that described for parametric hurricanes already specified in the previous studies. The resulting storm-generated wave fields have a persistent structure, with wave maxima present in the forward quadrants of the storm and in the rear quadrant II. This structure is determined by the strong winds and the extended fetch condition in quadrants I and II, as well as by the translation speed of the storm. When a shorter time interval is analyzed (e.g., a 3 h period, when the storm becomes a category 1 hurricane), the structure of the mean wind field may differ greatly from the mean field calculated with a sufficiently longer period; however, the spatial distribution of the wave field around the hurricane tends to maintain its typical spatial structure. The use of wind fields with reduced time variability (e.g., with a 3 h moving average) does not change the structure of the mean wave field, but reduces the mean wave height values by up to 10%.

It is generally accepted that the typical average wind field in a hurricane is well described by a parametric Holland model. The spatial distribution of winds can be determined from data provided by the National Hurricane Center with a 6-hour regularity. The wave field can be calculated by a numerical model from the temporal interpolation of these wind fields. However, a characteristic of wind fields calculated by parametric models is that their spatial and temporal variability are relatively smooth compared to wind fields computed by a high-resolution dynamic model, such as the HWRF. This study analyses the effect of the high temporal and spatial variability of the wind field in a hurricane and its effect on the variability of the wave field calculated by the third-generation wave model Wavewatch III. The wind fields obtained from the use of the HWRF model correspond to the simulation of Hurricane Isaac while crossing through the Gulf of Mexico between 27 and 29 August 2012. The wind fields have a spatial resolution of approximately 2 km and a temporal resolution of 15 minutes. This information is used by an implementation of the Wavewatch III model in the Gulf of Mexico to obtain wave fields with a spatial resolution of 2 km and a temporal resolution of 15 minutes. We find that, despite the high temporal and spatial variability, the mean wind field has a spatial pattern similar to that described by the parametric models. In contrast, the wave field computed by the wave model is much more stable and has a typical structure already described in other studies: large values of significant height in the region of the first quadrant of the hurricane and at the front of its path, with maximum values at distances between one and two times the radius of maximum winds. Smaller values of significant height are found behind the hurricane's track, especially in the third quadrant. The structure of the wave field is largely determined by the translation speed of the storm and the extended fetch effect in the quadrants to the right of the hurricane's path.

There is great interest in acquiring directional ocean surface wave direct measurements in order to better determine sea state conditions in open waters as well as in harbors and nearshore sites. Typical applications range widely over coastal and oceanic engineering, naval architecture and safety at sea, for design and construction of vessels and infrastructure, as well as for maintenance and marine operations. In this work we explore the influence of the buoy motion and we are able to detect some turbulence characteristics of the near surface flow. Full motion of the buoy structure is recorded by an Inertial Motion Unit within the velocimeter case, and after applying motion corrections directional wave and some turbulence characteristics are analyzed. The buoy responde is readily defined and the final results are compared with corresponding measurements from a bottom fixed acoustic Doppler current profiler. Details of the groupinness behaviour of the wave field in a nearshore site are given, showing some enhancement of turbulence intensity during the passage of relatively high wave groups. Some attempts to quantify the kinetic energy dissipation rate are explained. Final results show similar turbulence intensity values from the buoys measurements when compared with those from the fixed ADCP.

A comprehensive analysis of the wavefield evolution under accelerated wind conditions provides an essential contribution to understanding the wind-wave generation process. A set of experiments was carried out in a large wind-wave facility where it is possible to reproduce high wind speed conditions to study the wind acceleration effect in the wind-wave development. The facility was equipped with high-frequency sampling devices that provide accurate air turbulence and water surface displacement measurements. From this deployment, it was possible to describe the evolution of the wave characteristics under different magnitudes of constant wind acceleration in detail. This study analyzes the wind acceleration effect in the early stages of wind-wave generation and evolution. The increase of spectrum energy saturation level and the downshift of the peak frequency are processes associated with the spectral shape evolution under low acceleration wind conditions. Under high wind acceleration conditions, the spectral shape did not vary with wind speed and fetch. Despite under low acceleration wind conditions, the wavefield is more developed than under high acceleration wind conditions; there was no direct relation between wind acceleration and the wavefield efficiency to grow. Besides, during these early instants, it was observed that a more developed wave field, associated with low acceleration wind conditions, could slow down the increase of drag coefficient with wind speed.

Sea surface roughness is affected by surface current gradients, which provides a means of monitoring from satellite sharp oceanic fronts. This paper is the second report of an experiment designed to compare observations of sea surface roughness and surface currents at an unprecedented accuracy, owing to the conjunction of numerous deployed drifters and roughness instruments. About 200 drifters sampled a thin 10km elongated submesoscale front, also monitored by a high density of roughness instruments: satellite synthetic aperture radar, satellite, and airborne multiangle sunglint radiometers. The first paper focused on the retrieval of the current gradient direction (convergence and cyclonic vorticity) at the front, using roughness observations at multiple angle from airplane. This second paper focuses on the retrieval of the current gradient magnitude and scale, using roughness observations at different scales, from airplane and from satellite. Two main results are obtained: (i) Trajectories of selected drifters show that the front is only 50m wide and unambiguously exhibits convergence and cyclonic vorticity up to 100f (with f the Coriolis frequency). This far exceeds previously documented values for submesoscale deep ocean fronts. (ii) Correct estimation of such extreme current gradients using surface roughness hinges on instruments with sufficiently high spatial resolution. Lower-resolution roughness sensors can still detect the front, as demonstrated from observations and a simplified model, but cannot properly estimate the current gradient magnitude and the frontal width. Those results provide guidelines for monitoring intense current gradients from space using sea surface roughness. Plain Language Summary Ocean surface currents are not uniform horizontally but involve structures of many different sizes. After focusing on large-scale currents (about 1,000km) and eddies (about 100km), attention is nowadays turning toward fine-scale (about 1km) fronts and filaments. Those fine structures are indeed particularly important for the exchanges between ocean surface and deeper ocean, for the accumulation of drifting pollution, and for the concentration of biological production and marine predators. Fishermen well know that oceanic fronts can be revealed by visual inspection of the ocean surface roughness. Exploring this idea, this paper investigates how oceanic fronts can be monitored using images of sea surface roughness. Images from satellites and airplanes were taken during an experiment where hundreds of surface drifters were released and got caught within an intense front. Two new lessons are learned from those observations: (1) The front was only 50m wide, which is much sharper than commonly assumed for fronts of this type. (2) The front is thus associated with extreme current shear, which can only be detected by roughness sensors with sufficiently high spatial resolution. Those are important findings to improve monitoring of intense oceanic fronts from satellite.

Direct measurements have been conducted from a spar buoys deployed in the Gulf of Mexico, and in the vicinity of Todos Santos Island, offshore Ensenada BC, Mexico, in order to better understand ocean surface wave modulated processes under a variety of oceanographic and meteorological conditions. Full ocean surface wave directional spectrum is estimated from sea surface elevation data acquired with an array of capacitance wires, to represent directional spectrum as a function of frequency and direction, as well as a function of the wave number components Kx and Ky. Momentum transfer between ocean and the atmosphere is calculated directly through the eddy correlation method applied to wind velocity components acquired with a sonic anemometer. Momentum transfer variability is analysed to study its dependance on the surface wave conditions, with special emphasis on mixed sea states. Comparison between single peak spectra results with those cases where bi-modal spectra were present are performed in order to detect wind stress variability effects. Ocean-atmosphere transfer of momentum is studied and explained in terms of the shape and evolution of the surface wave spectrum. This research is funded by SENER-CONACYT 249795 and 201441 projects.

Purpose. The approach represented in the article is applied to analysis of satellite scanner optical images of high spatial resolution for identifying and quantitative determining the characteristics of the sub-mesoscale dynamic processes in the ocean upper layer. Methods and Results. The Envisat AATSR and MERIS SAR-images are used as the satellite data, which permit to determine the ocean surface temperature and surface brightness in the visible range, respectively. Variations in the sea surface glitter contrasts are associated with modulations of the sea surface roughness (rms slope of short waves) on the currents. It is shown that the surface roughness contrasts correlate with the spatial inhomogeneities of the ocean surface temperature, tracing sub-mesoscale processes in the ocean (spiral eddies, filaments, local shears of currents). The described model of formation of surface manifestations is based on interaction between the Ekman current and the main flow vorticity. Conclusions. Possibility of detecting and quantitative assessing the intense current gradients in the vicinity of sub-mesoscale fronts is shown. These gradients are manifested in the optical satellite images through the ocean surface roughness modulations. The proposed approach makes it possible to study and to assess quantitatively the dynamic processes taking place in the vicinity of the sub-mesoscale fronts. These processes, in their turn, affect the exchange of momentum, heat and gases between the ocean and the atmosphere. The prospects of applying the sub-mesoscale variability defined from the satellite measurements, to development of the models and the systems for the ocean global observations and monitoring are discussed.

The future international Surface Water and Ocean Topography (SWOT) Mission, planned for launch in 2021, will make high-resolution 2D observations of sea-surface height using SAR radar interferometric techniques. SWOT will map the global and coastal oceans up to 77.6° latitude every 21 days over a swath of 120 km (20 km nadir gap). Today’s 2D mapped altimeter data can resolve ocean scales of 150 km wavelength whereas the SWOT measurement will extend our 2D observations down to 15-30 km, depending on sea state. SWOT will offer new opportunities to observe the oceanic dynamic processes at these scales, that are important in the generation and dissipation of kinetic energy in the ocean, and act as one of the main gateways connecting the interior of the ocean to the upper layer. The active vertical exchanges linked to these scales have impacts on the local and global budgets of heat and carbon, and on nutrients for biogeochemical cycles. This review paper highlights the issues being addressed by the SWOT science community to understand SWOT’s very precise SSH / surface pressure observations, and it explores how SWOT data will be combined with other satellite and in-situ data and models to better understand the upper ocean 4D circulation (x,y,z,t) over the next decade. SWOT’s new SAR-interferometry technology aims to observe ocean SSH scales down to 15-30 km in wavelength. At these scales, SSH includes “balanced” geostrophic eddy motions and high-frequency internal tides and internal waves. This presents both a challenge in reconstructing the 4D upper ocean circulation, or in the assimilation of SSH in models, but also an opportunity to have global observations of the 2D structure of these phenomena, and to learn more about their interactions. At these small scales, the ocean dynamics evolve rapidly, and combining SWOT 2D SSH data with other satellite or in-situ data with different space-time coverage is also a challenge. SWOT’s new technology will be a forerunner for the future altimetric observing system, and so advancing on these issues today will pave the way for our future.

The energy and momentum transfer between the atmosphere and the ocean has typically been studied for conditions where the waves have almost or already reached a local equilibrium with a uniform wind. The purpose of this work is to investigate the early stages of the generation of waves under non-stationary wind conditions and to describe the momentum and energy exchange at the air–water interface for non-equilibrium wind conditions. Some experiments with a characteristic wind acceleration were conducted in a large wind-wave facility at the Institut Pythéas (Marseille-France). Momentum fluxes were estimated through hot-wire anemometry and, the free surface displacement was measured along the wave tank by resistance and capacitance wire probes. Wind speed and water elevation measurements were acquired at a high sampling rate. During the experiments, the wind speed was increased with a constant acceleration over time, reaching a constant maximum intensity of 13 ms−1. Under accelerated wind conditions, the degree of wave field development associated with a certain value of wind speed depends on the wind acceleration. Accordingly, once the rough flow regime is established, the drag coefficient values associated with a certain wind speed also vary depending on wind acceleration. It was observed that higher wind speed is needed to reach a rough flow regime as the wind acceleration increases. Also, the momentum transfer is reduced as wind acceleration increases. Under the rough flow regime, a less developed wave field induces a higher increase of drag coefficient with wind speed.

The reconstruction of ocean surface wave characteristics, i.e. directional energy distribution and dispersion, from ocean sunglitter images is discussed. Wave spectral analysis is performed, through the definition of linear transfer functions, to relate image brightness variations and sea surface slopes. Surface-current Doppler shift modifies the wave dispersion relation and can be obtained from cross-correlating two images and tracking glint changes over a known time. The technique is tested on photographs, taken from an aircraft in the Gulf of Mexico, and from a drone in the Black sea. The method is further extended to satellite sunglitter imagery, taking advantage of the technical peculiarities of Sentinel- 2 sensors helping to infer both the brightness gradient in the along-track direction. Wave dispersion information through the inter-channel time delay allows to reconstruct the current vector. The results demonstrate encouraging prospects of using the sunglitter imagery for the sea state monitoring, including formation of abnormally high waves and the study of wave-current interactions at different scales.

Reconstruction and evolution of two-dimensional spectra of surface waves in the Gulf of Mexico are derived from airborne sun-glitter imagery. As the proposed method is based on a linear transfer function deduced from the shape of the sunglitter brightness, the absolute wavenumber elevation spectrum does not require any additional assumption or information about sky brightness, wind or wave energy. The detailed description of the airborne image processing method is given. As demonstrated, retrieved spectra agree well with nearby NDBC buoy data, both for spectrum shape, level and energy angular distribution. The 180-degree wave direction ambiguity, inherent to image-derived spectra, is eliminated by using cross-correlation analysis between two consecutive images. A case study corresponding to the spectral evolution with increasing distance from shore in slanting fetch conditions is then considered. Energy level and peak position transformation are consistent with established approximations and laws of wind-sea development. The technical requirements (flight altitude, image resolution, view angles, etc) and applicability of the suggested methodology are also discussed. These results demonstrate the potential efficiency of high resolution sea state monitoring from drones or light aircrafts using sunglitter imagery.

High-resolution satellite images of the ocean surface in and around the sunglint often provide unique observations of sub-mesoscale upper ocean surface processes. Local anomalies of wind, waves, currents or surfactants appear on the images as local anomalies of brightness. A quantitative interpretation of those brightness anomalies must relate them to slope properties of the wave field, which are to the lowest order described by the mean square slope (mss).The prevailing paradigm for such interpretation is that of the critical zenith angle. It states that, for sub-critical zenith view angle, brightness and mss anomalies have opposite signs, and this defines the so-called inversion region. This prevailing paradigm implicitly builds on the assumption that the mss decomposition between upwind and crosswind components is conserved. The mss anomalies are then isotropic and can be reduced to a scalar (i.e. one-dimensional) quantity. In such a case, one single sunglint image would be sufficient to retrieve the mss anomaly. This isotropic case likely applies for surface wave changes induced by varying wind speed or by surfactants.Yet, satellite and airborne observations at multiple view angles recently revealed anisotropic mss anomalies, e.g. with mss increase in the upwind direction and decrease in the crosswind direction. This anisotropic behavior likely characterizes wave modulations by anisotropic surface current gradients. This paper details the expected properties of such anisotropic mss modulations. It is shown that: 1) The classical concept of critical angle does not systematically hold, neither for frontal current shear nor for internal wave divergence. 2) At least two sunglint images at different zenith and azimuth angles are needed to retrieve the mss anomalies, and a single observation is not sufficient. 3) A satellite with radiometers looking at multiple zenith angles is capable of providing a geometry favorable to retrieve mss anomalies. An illustration is discussed with internal waves observed by the Multi-angle Imaging SpectroRadiometer (MISR), where the upwind and crosswind components of the retrieved mss anomalies are anisotropic. Those results provide guidelines to interpret available observations and to help refine strategy for future satellite missions.

Doppler radars at all incidence angles measure mean velocities and spreads that have complex relations to oceanic motions, with opportunities to measure winds, waves, and currents. Here, we extend previous theoretical models of backscatter and Doppler using a Kirchhoff approximation and the physical optics model. We show that in Ka-band, around 12° incidence, range-resolved measurements of Doppler and backscatter provide unambiguous estimations of the wave spectrum and surface current. This property is illustrated with numerical examples and airborne data from the Air Surface Water Ocean Topography instrument. The same measurement conditions can be exploited for global ocean mapping from the low Earth orbit sensor satellite configuration.