This study focuses on evaluating the Lagrangian downward acceleration of fluid particles near the wavecrest as a dynamic criterion for identification of wave breaking in shallow water. The use of the downward acceleration as an indicator for wave breaking goes back to the work of Longuet-Higgins (1963), https://doi.org/10.1017/s0022112063000641 who showed theoretically that the acceleration near the crest of a regular wave is for the highest wave in deep water. In the subsequent literature, various other threshold values on the downward acceleration have been suggested based on theoretical or experimental investigations but a general agreement has not been established. Based on stereo imaging of tracer particles in the surf at Sylt, Germany, data analysis, and in-depth visual analysis, it is shown here that this criterion successfully classifies breaking and non-breaking waves in of the 99 wave events examined.

Abstract Nonlinear high‐frequency internal waves (HIWs) can occur at oceanic frontal interfaces but are rarely observed directly. This study presents field observations of HIWs near submesoscale fronts (SFs) in the northern Gulf of Mexico. A marine X‐band radar detected a series of organized bands, with a spacing of 50 ∼ 300 m, near SFs. The bands propagate toward SFs at speeds of 0.10–0.40 m/s, derived from consecutive radar backscatter intensity images. These fortuitous bands were identified as the surface expression of HIWs. This identification is supported by corresponding temperature undulations from a thermistor chain below a wave glider. The ship‐based observations provided vertical density and current (buoyancy and shear) profiles at a large frontal scale. These profiles, taken beneath and upstream of the HIW signature, are employed in the Taylor‐Goldstein equation using the mixed layer depth as a surrogate bottom, which predicts the shear instability growth rate at different wavelengths and corresponding phase speeds. The theoretically predicted fastest‐growing mode wavelength and its phase speed closely align with the observed HIWs after adjustments to the background shear and buoyancy uncertainty ranges. These findings support a shear–stratification instability framework for HIW generation along the shoaling thermohaline front. As HIWs influence local mixing and energy dissipation, they may bear the significance of the frontal energy budget and forward energy cascade from the submesoscale currents to isotropic turbulence in the upper ocean.

We show empirical evidence that the mean spatial gradient of the Doppler velocity, measured by a coherent X-band marine radar at grazing incidence, serves as a reliable proxy for measuring the so-called wave-related Reynolds number-defined as the product of air-side friction velocity and significant wave height divided by the kinematic viscosity of the surface water. Comparisons to approximately 180 h of in situ observations spanning various environmental conditions in the Southern North Sea yield an overall squared correlation coefficient of 0.93 and a normalized root-mean-square error of 16%. These results suggest that coherent marine radar offers a practical and relatively cost-efficient tool for observing key parameters related to ocean surface dynamics and air-sea interactions, such as whitecap coverage.

Abstract. As part of BOWTIE (German: Beobachtung von Ozean und Wolken - Das Trans ITCZ Experiment), the German research vessel FS Meteor navigated the moist tropics of the Atlantic Ocean for 40 days in summer 2024, with an east-west trajectory. The journey started in the port of Mindelo, Cape Verde, on August 16, and finished in the port of Bridgetown, Barbados, on September 24. The objective was to measure properties of the atmosphere, upper-ocean, and air-sea interface within the Intertropical Convergence Zone (ITCZ), under a variety of wind, convection, and sea surface temperature regimes. Using a set of 29 instruments/platforms, BOWTIE sampled the near-surface conditions of the atmosphere and ocean with high temporal resolution. This included continuous measurements of: the 2-D wind field within the lowest 2 km of the atmosphere, near-surface ocean currents, cloud and precipitation properties. Profiles of the ocean state and atmospheric thermodynamics and kinematics were obtained both continuously and at discrete intervals. Furthermore, dedicated stations sampled biochemical properties of the upper-ocean. Complementing BOWTIE observations, FS Meteor hosted further dedicated field campaigns for 3D cloud and precipitation properties, as well as intensive measurements of the atmospheric boundary layer. This manuscript provides an overview of the extensive instrumentation and data collected during BOWTIE. In addition, it addresses two key aspects based on these observations. First, it examines the range and uncertainties of selected quantities measured by multiple instruments, including column-integrated water vapor, rain detection, surface ocean currents, and sea surface temperature. Second, it illustrates the diversity of sampled weather regimes through two representative cases: calm doldrum conditions and a gusty, precipitating convective state.

This study demonstrates the feasibility of using coherent X-band radar for wave field measurements and short-term predictions. The research focuses on three main objectives: reconstruction of the sea surface, propagation of the wave field using linear wave theory, and validating results through in situ measurements. Radar data were collected from the offshore research platform Forschungsplattform in Nord und Ostsee 3 (FINO-3) in the North Sea and compared with measurements from a collocated wave buoy. Ten datasets were analyzed, covering significant wave heights from 1.9 to 6.4 m. The method's performance was assessed by deriving the probability density function (PDF) for prediction times of up to 20 wave periods and evaluating key metrics such as the correlation coefficient, nondimensional root mean square error (NDRMSE), and surface similarity parameter (SSP). The results showed a most probable correlation coefficient of 0.89 for surface reconstruction and at least 0.80 for predictions extending to ten peak wave periods. These findings highlight the effectiveness of coherent X-band radar in wave field monitoring and short-term prediction.

Reliable, cost efficient, and continuous observations of nearshore hydrodynamics are often required for the design and maintenance of coastal structures as well as to understand coastal change. In the last decades, advances in digitization and computational efficiency for signal processing have led to an increased use of marine radars as a tool for hydrographic applications, such as the retrieval of bathymetry, surface currents, winds and sea state. Many marine radar products are based on a three dimensional fast Fourier transformation (3D-FFT) of the image sequences obtained from a scanning radar. These methods have been extensively validated in deep to intermediate water depths. In the nearshore, and increasingly shallow waters, validation studies are rare and the available studies mainly focus on the retrieval of bathymetry. Validated radar measurements of spatially varying wave and current fields are not yet available. The present study is thus focussed on the assessment of the limitations of radar hydrography in a nearshore environment.

A large portion of the kinetic energy found within the ocean originates from the growth of ocean surface waves under the action of wind. However our understanding of wind wave dynamical coupling mechanisms remains incomplete. Competing theories exist but direct observational evidence is lacking, due to the technical challenges involved in measuring wind and wave dynamics in the vicinity of the highly energetic wavy ocean surface. Here, direct observations of airflow dynamics in the first millimeters to meters above ocean surface waves are shown. These were achieved using laser imaging techniques on the Floating Instrument Platform FLIP in the Pacific Ocean. The results show that two dynamical wind-wave coupling regimes coexist. Short (~1 m wavelength), strongly wind-forced waves travel more slowly than the wind and cause intermittent airflow separation events. On average, these slow waves are coupled with the airflow via a sheltering mechanism, while longer (~100 m), faster waves induce orbital motions in the airflow.

Mesoscale structures are key dynamical features of the ocean. They are associated with a variety of short lived and small-scale dynamics linked to physical, biological, and chemical processes at the submesoscale, such as cascading energy, impacting ocean stratification, and guiding ocean carbon and oxygen uptake. In the high latitudes, the spatial extent of the mesoscale is only tens of kilometres, making it challenging to observe the submesoscale processes. In August-September 2022, an extensive submesoscale-resolving multiplatform experiment was conducted across an Irminger Ring in the Labrador Sea. The experiment leveraged two underwater electric gliders equipped with nitrate, microstructure shear, chlorophyll fluorescence, oxygen, and turbidity sensors, operated in concert with a variety of ship operated instruments including underway-CTD’s, a moving vessel profiler, Thermosalinograph, ADCPs and a X-band radar system. Observations were acquired both, along the peripheries and within the core of the eddy, and offered insight into submesoscale dynamics of the ring. Making use of nearly concurrent turbulence and nutrients observations, we estimated the vertical flux pattern across the eddy’s frontal and interior regions. From the recorded and expected glider vehicle motion a vertical water velocity could be inferred and compared with the nutrient flux pattern. The stability of the ring was tracked with surface drifters, for weeks after the ship and glider survey ended, and a link between the disintegration of the ring and an atmospheric event was investigated

Surf zone hydrodynamics is characterized by the dynamic interplay of various spatial and temporal scales related to different mechanical processes taking place across the surf zone. Among all the wave processes in the surf zone, wave breaking, and the associated mass transport and energy dissipation is the most significant. Wave breaking not only exerts significant forces on coastal structures but also triggers nearshore currents, and sediment transport. While our understanding of wave breaking has improved in recent decades, the precise mechanism triggering wave breaking remains unresolved. This study focuses on utilizing the Lagrangian downward acceleration of fluid particles near the wavecrest as a dynamic criterion for identification of breaking in shallow water waves. Using a dedicated stereo-imaging system in a 2019 field campaign on the island of Sylt off the coast of Germany, we were able to track tracer particles during various wave conditions (Bjørnestad et al. 2021). In the present work, the data are analyzed with the goal of testing the dynamic breaking criterion.

In January-February 2020, the EUREC4A-OA/ATOMIC experiment took place in the Northwest Tropical Atlantic with the overall goal of understanding the role of fine-scale processes in internal ocean dynamics and air-sea interaction. Four oceanographic ships, the French Atalante, the German Maria S. Merian and Meteor, and the US Ron Brown, were closely coordinated with airborne observations and autonomous ocean platforms (gliders, ©Saildrones, Argo floats, and drifters) to simultaneously measure the ocean and atmosphere from east of Barbados to the northern border of French Guyana. The multiple observations of the ocean, atmosphere, and their interface have revealed more complex ocean dynamics than expected, in particular a strong interaction between the Amazon River outflow (despite its reduced winter discharge), the North Brazil Current (NBC), and several mesoscale eddies (including the highly energetic NBC rings). This leads to even richer submesoscale dynamics that shape an important fraction of the air-sea exchange of heat, momentum, and CO2, and efficiently isolates the NBC northward flow waters from intense and continuous interactions with the atmosphere. Owing to the many complementary observations from ships and autonomous platforms, we have been able to quantify some of these processes, including the diurnal cycle and the 3D dynamics of different mesoscale eddies, as well as to map and quantify different terms of the air-sea fluxes and their impacts on the marine atmospheric boundary-layer water budget. The results have been widely used not only to validate numerical simulations of the region, but also to guide their analyses and to improve various numerical parameterizations. The collection of these observations was the result of an important international coordination between many different groups of ocean and atmospheric scientists. In addition, the special strategy for targeted data collection of meso- and submesoscale processes relied on daily planning of the field experiment and on detailed analysis of the near-real-time satellite data and the observations already obtained during the experiment, which was essential for providing the right snapshots of the ocean and atmosphere for the quantification of many processes. The lessons learned from this experiment will be implemented and extended in the upcoming major high-resolution oceanographic endeavor, the WHIRLS experiment, which will take place in June-July 2025, southwest of Africa.

This contribution is structured in two parts: a concise summary of the key findings from the original study (Bonde- hagen et al. (2024)) and an exploration of some open-ended questions concerning tracer paths in the surfzone. The Boussinesq-type model BOSZ is validated against field measurements, demonstrating its effectiveness in capturing wave-driven surf zone dynamics. The influence of tidal elevation, wave direction, and directional spread on vortex size and strength is examined, highlighting the interplay between wave propagation and bathymetry. Building on these find- ings, the remainder of this paper delves deeper into vortex behavior using Lagrangian tracers and presents preliminary insights into chaotic motion in the surf zone, further advancing our understanding of nearshore wave dynamics.

We are interested in the interaction of ocean waves with steep coastal topography such as encountered in some coastal profiles for example in the United States, New Zealand and Norway. In previous works, it has been shown that under such conditions, shoaling ocean waves may experience significant amplification in the last 50 to 100 meters before they run up on the shore, leading to potentially hazardous run-up events even under relatively calm conditions. In the present work, we are reporting on a remotely accessible observational system which was deployed on the Norwegian Coast near the city of Haugesund. We report on the data analysis and statistical correlation of large run-up events with certain sea states and weather conditions.References:[1] Bjørnestad, M. and Kalisch, H., 2020. Extreme wave runup on a steep coastal profile. AIP Advances, 10(10).[2] Kalisch, H., Lagona, F. and Roeber, V., 2023. Sudden wave flooding on steep rock shores: a clear but hidden danger. Natural Hazards, pp.1-21.

An improved understanding of wave breaking is still a hot topic owing to its relevance in the coupling of ocean and atmosphere. Multiple communities are focusing on numerical simulations of the fully coupled two-phase flow, the validation of such models remains challenging. Herein, we demonstrate how coherent marine radars can help to shed light on how different wave and wind parameters influence the evolution of waves towards breaking in the nearshore. The interpretation of the results is undermined by SWASH simulations of shoaling waves for different wave spectra and two beaches and simulations of radvarimages of these waves. Data of three independent measurement campaigns shows that the shoaling characteristics are strongly influenced by the wave steepness, relative depth the Ursell number and the wind. The influence of individual parameters cannot be isolated, but must be understood in its entirety.

The diurnal cycles of near‐surface velocity and temperature, also known as diurnal jet and diurnal warm layer (DWL), are ubiquitous in the tropical oceans, affecting the heat and momentum budget of the ocean surface layer, air‐sea interactions, and vertical mixing. Here, we analyze the presence and descent of near‐surface diurnal shear and stratification in the upper 20 m of the equatorial Atlantic as a function of wind speed using ocean current velocity and hydrographic data taken during two trans‐Atlantic cruises along the equator in October 2019 and May 2022, data from three types of surface drifters, and data from Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) moorings along the equator. The observations during two seasons with similar mean wind speeds but varying surface heat fluxes reveal similar diurnal jets with an amplitude of about 0.11 m s−1 and similar DWLs when averaging along the equator. We find that higher wind speeds lead to earlier diurnal peaks, deeper penetration depths, and faster descent rates of DWL and diurnal jet. While the diurnal amplitude of stratification is maximum for minimal wind speeds, the diurnal amplitude of shear is maximum at 6 m depth for moderate wind speeds of about 5 m s−1. The inferred wind dependence of the descent rates of DWL and diurnal jet is consistent with the earlier onset of deep‐cycle turbulence for higher wind speeds. The DWL and the diurnal jet not only trigger deep‐cycle turbulence but are also observed to modify the wind power input and thus the amount of energy available for mixing.

A marine X-band radar system, developed by Helmholtz-Zentrum Hereon (Hereon) was deployed within view of the nearshore at the US Army Engineer Research and Development Center, Field Research Facility (FRF), in Duck, North Carolina, from October 2021 to August 2022. The radar deployment was a collaboration among researchers at the FRF, Hereon, and the University of Miami and was initiated as part of the During Nearshore Event Experiment (DUNEX), a large multi-institutional field experiment funded by the US Coastal Research Program. The Hereon radar successfully collected data during the main DUNEX field campaign (approximately October 2021) and continued to collect nearly continuously until August 2022. To facilitate use of Hereon radar data, this document describes the deployment, provides background and context, and presents metadata. Within, we describe in detail the Hereon radar system, the locations of two different installations, the time periods covered, sampling modes, environmental conditions and notable events, example data products, and potential pathways for future use of the data.

Significant wave height retrieval from X-band marine radars operating at grazing incidence is typically achieved by empirical algorithms that require extensive calibrations, which ideally are performed for every individual setup. Within this novel physics based approach coherent X-band marine radar data are being utilized to retrieve significant wave heights. In contrast to previously published methods, where the antenna had to be pointed into the main wave direction, this approach is utilized with a continuously rotating antenna. Radial velocities of surface scatterers are retrieved from the coherent radar data and converted under consideration of linear wave theory to surface elevations, which in turn are utilized to retrieve the significant wave height. The method was tested and applied to an extensive data set collected over a period of 29.5 days, covering four storms, at the offshore research platform Forschungsplattform in Nord und Ostsee 3 (FINO-3) in the southern North Sea. Comparison of radar retrieved significant wave heights to data obtained by a wave buoy in vicinity of FINO-3 resulted in a correlation of 0.99, a root mean square error of 0.31 m, and a bias of 0.09 m. In addition, sensitivity studies were carried out with respect to the quality of the signal, width of the dispersion filter and amount of data considered for calculations, to study the robustness of the method.

Wave-driven currents have a substantial impact on local circulation patterns in and across the surf zone, and are responsible for cross-shore and longshore exchange of mass and momentum over a broad range of spatial and temporal scales. Nearshore currents may drive sediment transport, lead to beach erosion, and also affect the spread of bacteria and other marine microorganisms, as well as the distribution of pollutants such as chemicals and microplastics. In addition, surf zone currents can cause hazardous conditions for beach-goers in the form of rip currents.It is known from previous work (Chen et al., 2003; Feddersen et al., 2011; Hally-Rosendahl and Feddersen, 2016) that Boussinesq-type models in combination with appropriate boundary conditions and wave breaking capabilities can function as powerful tools for the analysis of circulation patterns in the surf zone. In the present work, data from a recent field campaign reported on in Bjørnestad et al. (2021) are used to further validate the capability of Boussinesq systems to simulate nearshore dynamics.The numerical model is then used to study the influence of tidal elevation, peak direction and directional spread of the incoming wavefield on the quantity, extent, and circulatory magnitude of the nearshore circulation. In addition, fundamental features such as horizontal eddies are investigated, and comparisons are made to solid-body rotation and irrotational vortices.Overall, it is observed that local variations in the bathymetry across the surf zone are the controlling factor regarding the size of these circulations, and an increasing tidal level, which can be seen as a uniform offset to the bathymetry, favors the generation of larger vortex patterns. For a given tidal stage, the directional spread of the incoming wavefield has the most pronounced influence on the size and strength of the nearshore eddies while the peak direction has the strongest effect on the total number of circulations.

This research aims to discuss and validate the presence of internal wave signatures in sea surface radar backscatter images using a dataset collected by a shipboard X-Band radar and a vessel-mounted ADCP in equatorial waters. The internal wave event occurred on 19th May 2022 between 10:38-11:00 UTC when the research vessel Meteor (M181 TRATLEQ II cruise) was moving towards Brazilian waters reaching latitudes of about 0. 0' S and longitudes from 42.20.73' W to 42. 26.06' W. The nautical radar onboard also reported this event and we identify some backscatter features from internal waves even in the individual raw-radar images. The internal waves are also detected in composite images of sea surface radar backscatter images and near-surface wave current maps. The radar-derived wave current vectors are computed through an iterative procedure based on filtering the energy of the dispersion relationship to enhance the signal-to-noise ratio of predictions. We observe variations in wave current vectors when interacting with internal wave signatures. Finally, insitu velocity data from a vessel-mounted ADCP validates the presence of internal waves during the analysis period, where vertical displacements occur in zonal and meridional velocities between 100-600 m water depth. Hence, results suggest shipboard X-Band radars are able to detect internal wave features at sea surface under special conditions.

The North Brazil Current (NBC) flows offshore of the mouth of the Amazon River and seasonally sheds anticyclonic rings (NBC rings) that propagate northwestward and interact with the Amazon River plume (ARP). Mesoscale features have a high temporal variability that is hard to monitor from current weekly and monthly sea surface salinity (SSS) satellite fields. Novel SSS fields with a higher temporal resolution analyzed together with satellite geostrophic currents, chlorophyll-a, and wind speed and in-situ data from the "Microbiomes cruise" on the SV Tara in August-September 2021 revealed a late summer freshwater pathway, which was not well documented in earlier studies. By combining these datasets, we improved the characterization of summer ARP pathways. In 2021, the ARP was a succession of freshwater patches cut off from the main plume by the NBC rings. A patch of about 200.000 km(2) with salinity <33.5 pss was observed in September 2021, bringing 0.5 Sv of Amazon water northwestward in a period where the mean ocean currents lead to eastward transport. This patch was shallow, very stratified, and it created a surface steric-height anomaly that was identified as an anticyclonic feature in altimetric sea level products. Once separated from the NBC retroflection, it was mainly driven by Ekman currents. Other similar patches were observed during the 2021 summer, leading to a strong intermittency of the ARP transport. They strongly contributed to make 2021 the year with the largest northwestward freshwater transport in late summer within the 2010-2021 time-period investigated. This freshwater transport pathway is important for all plume-related phenomena, and show the ability of combined SMOS and SMAP data to accurately represent the day-to-day SSS variability.

Coastal seas, shelf seas and marginal ice zones are dominated by small-scale ocean surface dynamic processes that play a vital role in the transport and exchange of climate-relevant properties such as carbon, heat, water and nutrients between land, ocean, ice and atmosphere. Mounting evidence indicates that ocean scales below 10 km have far-ranging impacts on air–sea interactions, lateral ocean dispersion, vertical stratification, ocean carbon cycling and marine productivity – governing exchanges across key interfaces of the Earth system, the global ocean, and atmosphere circulation and climate. Yet, these processes remain poorly observed at the fine spatial and temporal scales necessary to resolve them. The Ocean Surface Current Airborne Radar (OSCAR) is a new airborne instrument with the capacity to inform these questions by mapping vectorial fields of total ocean surface currents and winds at high resolution over a wide swath. Developed for the European Space Agency (ESA), OSCAR is the airborne demonstrator of the satellite mission concept SeaSTAR, which aims to map total surface current and ocean wind vectors with unprecedented accuracy, spatial resolution and temporal revisit across all coastal seas, shelf seas and marginal ice zones. Like SeaSTAR, OSCAR is an active microwave synthetic aperture radar along-track interferometer (SAR-ATI) with optimal three-azimuth sensing enabled by unique highly squinted beams. In May 2022, OSCAR was flown over the Iroise Sea, France, in its first scientific campaign as part of the ESA-funded SEASTARex project. The campaign successfully demonstrated the capabilities of OSCAR to produce high-resolution 2D images of total surface current vectors and near-surface ocean vector winds, simultaneously, in a highly dynamic, macrotidal coastal environment. OSCAR current and wind vectors show excellent agreement with ground-based X-band-radar-derived surface currents, numerical model outputs and NovaSAR-1 satellite SAR imagery, with root mean square differences from the X-band radar better than 0.2 m s−1 for currents at 200 m resolution. These results are the first demonstration of simultaneous retrieval of total current and wind vectors from a high-squint three-look SAR-ATI instrument and the first geophysical validation of the OSCAR and SeaSTAR observing principle. OSCAR presents a remarkable new ocean observing capability to support the study of small-scale ocean dynamics and air–sea interactions across the Earth's coastal, shelf and polar seas.