Search

Air bubbles in the upper ocean are generated mainly by waves breaking at the air–sea interface. As such, after the waves break, entrained air bubbles evolve in the form of plumes in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the link between bubble plume penetration depth, mechanical and thermal forcings, and the air–sea transfer velocity of CO2 , a field experiment was conducted from an oceanographic research tower in the north Adriatic Sea. Air bubble plumes were observed using high-resolution echosounder data from an upward-looking 1000 kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organized into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17 m depth under strong forcing. We verify that bubble plumes adapt rapidly to wind and wave conditions and have depths that scale approximately linearly with wind speed. Scaling with the wind–wave Reynolds number is also proposed to account for the sea-state severity in the plume depth prediction. Results show a correlation between measured bubble depths and theoretical air–sea CO2 transfer velocity parametrized with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air–sea interaction studies and pave the way for progress in CO2 gas exchange formulations. [ABSTRACT FROM AUTHOR]

The Mediterranean Sea is a primary source of food, ecosystem services and economic activities and one of the most active cydogenetic regions in the world, where the influence of orographic and morphological features of the relatively small basin plays an important role. Together with the explosive cyclogenesis, tropical-like cyclones (also called Mediterranean Hurricanes or Medicanes) are among the strongest types of storms that can be found in the Mediterranean basin, occurring predominantly in the Ionian, Balearic and Tyrrhenian sub-basins. Similarly to tropical cyclones (Hurricanes or Typhoons), these cyclonic structures are characterized by strong rotating and translating wind fields, which often lead to a combination of remotely generated swell waves and locally generated wind waves, often referred to as crossing sea states. Despite the well-known potential of Medicanes to cause significant damage near islands and coastal zones, which is predicted to intensify as a result of climate change, to date the characterization of maximum individual waves generated during these events is still lacking. In this study, we carry out the first analysis of the large-scale geographical distribution of wave maxima within the wave fields generated during three recent Medicane events using the WAVEWATCH III® spectral wave model forced by ERA5 reanalysis winds, also investigating the influence of crossing sea states on the maximum wave amplitudes with novel statistical formulations developed for such conditions. Our results show that, as in the case of tropical cyclones, several regions of the cyclone field are characterized by crossing sea states, whose role in the formation of the maximum individual waves occurring near the eye of the storm was found to be confined. Furthermore, extreme wave predictions accounting for the local crossing conditions yield differences up to 5% compared to standard statistical distributions. [ABSTRACT FROM AUTHOR]

Accurate estimates of extreme waves are central for maritime activities, and stochastic wave models are the best option available for practical applications. However, the way currents influence the statistics of space–time extremes in spectral wave models has not been properly assessed. Here we demonstrate impacts of the wave modulation caused by one of the world's strongest open ocean tidal currents, which reaches speeds of at least 3 m s−1. For a bimodal swell and wind sea state, we find that most intense interactions occur when the wind sea opposes the tidal current, with an increase in significant wave height and spectral steepness up to 45% and 167%, respectively. The steepness modulation strengthens the second-order Stokes contribution for the normalized extreme crests, which increases between 5% and 14% during opposing wind sea and current. The normalized extreme wave heights have a strong dependence on the narrow-bandedness parameter, which is sensitive to the variance distribution in the bimodal spectrum, and we find an increase up to 12% with currents opposing the wind sea. In another case of swell opposing a tidal jet, we find the spectral steepness to exceed the increase predicted by a simplified modulation model. We find support in single-point observations that using tidal currents as forcing in wave models improves the representation of the expected maximum waves, but that action must be taken to close the gap of measurements in strong currents. Significance Statement: The purpose of this study is to investigate how a very strong tidal current affects the surface wave field, and how it changes the stochastic extreme waves formulated for a space–time domain. Our results suggest that the expected maximum waves become more realistic when tidal currents are added as forcing in wave models. Here, the expected extremes exceed traditional model estimates, i.e., without current forcing, by more than 10%. These differences have implications for maritime operations, both in terms of planning of marine structures and for navigational purposes. However, there is a significant lack of observations in environments with such strong currents, which are needed to further verify our results. [ABSTRACT FROM AUTHOR]

Air bubbles in the upper ocean are generated mainly by wave breaking at the air-sea interface. As such, after the waves break, entrained air bubbles evolve in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the relationships between bubble penetration depth, mechanical and thermal forcings, and air-sea transfer velocity of CO2, a field experiment was conducted from an oceanographic research platform in the North Adriatic Sea. Air bubble plumes were measured using high-resolution echosounder data from an up-looking 1000-kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organised into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17-m depth under strong forcing. We verify that bubble depths adapt rapidly to wind and wave conditions and scale approximately linearly with wind speed. A scaling with the wind/wave Reynolds number is proposed to account for the sea-state severity in the depth prediction. Results also show a strong connection between measured bubble depths and theoretical air-to-sea CO2 transfer velocity parametrised with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed, cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air-sea interaction studies and pave the way for progress in CO2 gas exchange formulations. [ABSTRACT FROM AUTHOR]

Wave crests of unexpected height and steepness pose a danger to activities at sea, and long-term field measurements provide important clues for understanding the environmental conditions that are conducive to their generation and behavior. We present a novel dataset of high-frequency laser altimeter measurements of the sea surface elevation gathered over a period of 18 years from 2003 to 2020 on an offshore platform in the central North Sea. Our analysis of crest height distributions in the dataset shows that mature, high sea states with high spectral steepness and narrow directional spreading exhibit crest height statistics that significantly deviate from standard second-order models. Conversely, crest heights in developing sea states with similarly high steepness but wide directional spread correspond well to second-order theory adjusted for broad frequency bandwidth. The long-term point time series measurements are complemented with space–time stereo video observations from the same location, collected during five separate storm events during the 2019/20 winter season. An examination of the crest dynamics of the space–time extreme wave crests in the stereo video dataset reveals that the crest speeds exhibit a slowdown localized around the moment of maximum crest elevation, in line with prevailing theory on nonlinear wave group dynamics. Extending on previously published observations focused on breaking crests, our results are consistent for both breaking and nonbreaking extreme crests. We show that wave crest steepness estimated from time series using the linear dispersion relation may overestimate the geometrically measured crest steepness by up to 25% if the crest speed slowdown is not taken into account. Significance Statement: Better understanding of the statistics and dynamical behavior of extreme ocean surface wave crests is crucial for improving the safety of various operations at sea. Our study provides new, long-term field evidence of the combined effects of wave field steepness and directionality on the statistical distributions of crest heights in storm conditions. Moreover, we show that the dynamical characteristics of extreme wave crests are well described by recently identified nonlinear wave group dynamics. This finding has implications, for example, for wave force calculations and the treatment of wave breaking in numerical wave models. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of climate change on coastal dynamics along the Adriatic Italian coast, with reference to the period 2021–2050 considering the IPCC RCP 4.5 and 8.5 scenarios. The wave datasets are obtained by forcing a spectral wave model with ERA5 wind fields corrected with a procedure that makes them suitable for the investigated semi-enclosed basin where local meteorological events occur at scales of a few kilometers. The wave climate changes between the historic run (1981–2010) and the future scenarios are studied in terms of mean values, percentile and extreme waves in 120 virtual buoys along the coasts. Moreover, a morphological equivalent wave is computed for all the datasets to highlight the consequences of climate change on coastal sediment transport. Along the Adriatic Italian coast, a small decrease in the significant wave heights is found, both for mean and extreme values, and the sediment transport is reduced. However, significant deviations along the coast are highlighted and the longshore sediment transport even reverses its direction in some locations for the future scenarios. [ABSTRACT FROM AUTHOR]

This paper proposes a method to infer the future change in the wind-wave climate using reanalysis wind corrected to statistically match data from a regional climate model (RCM). The method is applied to the sea surface wind speed of the reanalysis ERA5 from the European Centre for Medium-Range Weather Forecasts. The correction is determined from a quantile mapping between ERA5 and the RCM at any given point in the geographical space. The issues that need to be addressed to better understand and apply the method are discussed. Corrected ERA5 wind fields are eventually used to force a spectral wave numerical model to simulate the climate of significant wave height. The correction strategy is implemented over the Adriatic Sea (a semi-enclosed basin of the Mediterranean Sea) and includes the present-day period (1981–2010) and the near-future period (2021–2050) under the two IPCC RCP4.5 and RCP8.5 concentration scenarios. Evaluation against observations of wind and waves gives confidence in the reliability of the proposed approach. Results confirm the evolution toward an overall decrease in storm wave severity in the basin, especially under RCP8.5 and in its northern area. It is expected that the methodology may be applied to other reanalyses, RCMs (including multi-model ensembles), or seas with similar characteristics. [ABSTRACT FROM AUTHOR]

In November 2011, an Atlantic depression affected the Mediterranean basin, eventually evolving into a Tropical-Like Cyclone (TLC or Mediterranean Hurricane, usually designated as Medicane). In the region affected by the Medicane, mean sea level pressures down to 990 hPa, wind speeds of hurricane intensity close to the eye (around 115 km/h) and intense rainfall in the prefrontal zone were reported. The intensity of this event, together with its long permanence over the sea, suggested its suitability as a paradigmatic case for investigating the sensitivity of a numerical modeling system to different configurations, air-sea interface parameterizations and coupling approaches. Toward this aim, a set of numerical experiments with different parameterization schemes and levels of coupling complexity was carried out within the Coupled Ocean Atmosphere Wave Sediment Transport System (COAWST), which allows the description of air-sea dynamics by coupling an atmospheric model (WRF), an ocean circulation model (ROMS) and a wave model (SWAN). The sensitivity to different initialization times and Planetary Boundary Layer (PBL) parameterizations was firstly investigated by running a set of WRF standalone (atmospheric-only) simulations. In order to better understand the effect of coupling on the TLC formation, intensification and trajectory, different configurations of atmosphere-ocean coupling were subsequently tested, eventually including the full coupling among atmosphere, ocean and waves, also changing the PBL parameterization and the formulation of the surface roughness. Results show a strong sensitivity of both the trajectory and the intensity of this TLC to the initial conditions, while the tracks and intensities provided by the coupled modeling approaches explored in this study do not introduce drastic modifications with respect to those resulting from a fine-tuned standalone atmospheric run, though they provide by definition a better physical and energetic consistency. Nevertheless; the use of different schemes for the calculation of the surface roughness from wave motion, which reflects the description of air-sea interface processes, can significantly affect the results in the fully coupled runs. [ABSTRACT FROM AUTHOR]

Stereo wave imaging of the sea surface elevation has become an effective instrumentation to gather small- and medium-range 3-D wind wave data. Indeed, fruitful applications of stereo techniques have provided new insights into directional wave spectra, space–time distributions of wave maxima, and small-scale wave statistics. So far, however, stereo systems have been deployed mainly on fixed structures (e.g. oceanographic platforms or lighthouses) in order to simplify the installation and maintenance procedures. Nonetheless, advances in stereo calibration and processing suggest that stereo deployments are also feasible onboard moving vessels, thus broadening the impact of these observations on the study of wind waves. In this context, this study aims at discussing how the stereo processing designed to gather reliable wave data from fixed structures should be managed to operate on a moving structure. In particular, estimate of stereo cameras orientation and position with respect to the mean sea plane is of utmost importance. We discuss this aspect by using a synthetic sea state and stereo data collected during an oceanographic campaign onboard a research vessel. Results suggest that, without complementary data sources for ship motion compensation, the sea surface elevation field should include at least about sixteen spatial (2-D) waves to gather a robust estimate of the mean sea plane and consequently realistic wave parameters (e.g. the significant wave height). In this respect, our results provide also insight into the uncertainty of estimates in case of a limited number of 2-D waves is collected by the stereo system. Finally, applications of stereo wave imaging on a moving structure are discussed, with particular emphasis on the collection of space–time wave fields for assessment of numerical models and operational wave observation onboard vessels. [ABSTRACT FROM AUTHOR]

The oceanographic campaign CARPET2014 (Characterizing Adriatic Region Preconditionig EvenTs), (30 January-4 February 2014) collected the very first turbulence data in the Gulf of Trieste (northern Adriatic Sea) under moderate wind (average wind speed 10ms-1) and heat flux (net negative heat flux ranging from 150 to 400Wm-2). Observations consisted of 38 CTD (Conductivity, Temperature, Depth) casts and 478 microstructure profiles (grouped into 145 ensembles) with three sets of yoyo casts, each lasting for about 12 consecutive hours. Averaging closely repeated casts, such as the ensembles, can lead to a smearing effect when in the presence of a vertical density structure with strong interfaces that can move up or down between subsequent casts under the influence of tides and internal waves. In order to minimize the smearing effect of such displacements on mean quantities, we developed an algorithm to realign successive microstructure profiles to produce sharper and more meaningful mean profiles of measured turbulence parameters. During the campaign, the water column in the gulf evolved from well-mixed to stratified conditions due to Adriatic waters intruding at the bottom along the gulf's south-eastern coast. We show that during the warm and relatively dry winter, the water column in the Gulf of Trieste, even under moderate wind forcing, was not completely mixed due to the influence of bottom waters intruding from the open sea. Inside the gulf, two types of water intrusions were found during yoyo casts: one coming from the northern coast of the Adriatic Sea (i.e. cooler, fresher and more turbid) and one coming from the open sea in front of the Po Delta (i.e. warmer, saltier and less turbid). The two intrusions had different impacts on turbulence kinetic energy dissipation rate profiles. The former, with high turbidity, acted as a barrier to winddriven turbulence, while the latter, with low sediment concentrations and a smaller vertical density gradient, was not able to suppress downward penetration of turbulence from the surface. [ABSTRACT FROM AUTHOR]

The self-organizing map (SOM) technique is considered and extended to assess the extremes of a multivariate sea wave climate at a site. The main purpose is to obtain a more complete representation of the sea states, including the most severe states that otherwise would be missed by a SOM. Indeed, it is commonly recognized, and herein confirmed, that a SOM is a good regressor of a sample if the frequency of events is high (e.g., for low/moderate sea states), while a SOM fails if the frequency is low (e.g., for the most severe sea states). Therefore, we have considered a trivariate wave climate (composed by significant wave height, mean wave period and mean wave direction) collected continuously at the Acqua Alta oceanographic tower (northern Adriatic Sea, Italy) during the period 1979-2008. Three different strategies derived by SOM have been tested in order to capture the most extreme events. The first contemplates a pre-processing of the input data set aimed at reducing redundancies; the second, based on the post-processing of SOM outputs, consists in a two-step SOM where the first step is applied to the original data set, and the second step is applied on the events exceeding a given threshold. A complete graphical representation of the outcomes of a two-step SOM is proposed. Results suggest that the post-processing strategy is more effective than the pre-processing one in order to represent the wave climate extremes. An application of the proposed two-step approach is also provided, showing that a proper representation of the extreme wave climate leads to enhanced quantification of, for instance, the alongshore component of the wave energy flux in shallow water. Finally, the third strategy focuses on the peaks of the storms. [ABSTRACT FROM AUTHOR]

In this paper, an observational space-time ensemble of sea surface elevations is investigated in search of the highest waves of the sea state. Wave data were gathered by means of a stereo camera system, which was installed on top of a fixed oceanographic platform located in the Adriatic Sea (Italy). Waves were measured during a mature sea state with an average wind speed of 11 m s−1. By examining the space-time ensemble, the 3D wave groups have been isolated while evolving in the 2D space and grabbed 'when and where' they have been close to the apex of their development, thus exhibiting large surface displacements. The authors have selected the groups displaying maximal crest height exceeding the threshold adopted to define rogue waves in a time record, that is, 1.25 times the significant wave height ( H s). The records at the spatial positions where such large crests occurred have been analyzed to derive the empirical distributions of crest and wave heights, which have been compared against standard statistical linear and nonlinear models. Here, the maximal observed wave crests have resulted to be outliers of the standard statistics, behaving as isolated members of the sample, apparently uncorrelated with other waves of the record. However, this study has found that these unexpectedly large wave crests are better approximated by a space-time model for extreme crest heights. The space-time model performance has been improved, deriving a second-order approximation of the linear model, which has provided a fair agreement with the empirical maxima. The present investigation suggests that very large waves may be more numerous than generally expected. [ABSTRACT FROM AUTHOR]

Wave observations and modeling have recently demonstrated that wave extremes of short-crested seas are poorly predicted by statistics of time records. Indeed, the highest waves pertain to wave groups at focusing that have space-time dynamics. Therefore, the statistical prediction of extremes of short-crested sea states should rely on the multidimensional random wave fields' assumption. To adapt wave extreme statistics to the space-time domain, theoretical models using parameters of the directional wave spectrum have been recently developed. In this paper, the influence of metocean forcings (wind conditions, ambient current, and bottom depth) on these parameters and hence on wave extremes is studied with a twofold strategy. First, parametric spectral formulations [Pierson-Moskowitz and Joint North Sea Wave Project (JONSWAP) frequency spectra with cos2 directional distribution function] are considered to represent the dependence of wave extremes upon wind speed, fetch, and space domain size. Afterward, arbitrary conditions are simulated by using the SWAN numerical model adapted to store the spectral parameters, and the effects on extremes of current- and depth-induced shoaling are investigated. Preliminarily, the space-time extremes prediction model adopted is assessed by means of numerical simulations of Gaussian random seas. Compared to the significant wave height of the sea state and for a given space domain size, results show that space-time extremes are enhanced by opposite currents, whereas they are weakened by increasing wind conditions (wind speed and fetch) and by depth-induced shoaling. In this respect, the remarkable contribution to wave extremes of the size of the space domain is substantiated. [ABSTRACT FROM AUTHOR]

One of the most promising techniques for the analysis of Spatio-Temporal ocean wave fields is stereo vision. Indeed, the reconstruction accuracy and resolution typically outperform other approaches like radars, satellites, etc. However, it is computationally expensive so its application is typically restricted to the analysis of short pre-recorded sequences. What prevents such methodology from being truly real-time is the final 3D surface estimation from a scattered, non-equispaced point cloud. Recently, we studied a novel approach exploiting the temporal dependence of subsequent frames to iteratively update the wave spectrum over time. Albeit substantially faster, the unpredictable convergence time of the optimization involved still prevents its usage as a continuously running remote sensing infrastructure. In this work, we build upon the same idea, but investigating the feasibility of a fully data-driven Machine Learning (ML) approach. We designed a novel Convolutional Neural Network that learns how to produce an accurate surface from the scattered elevation data of three subsequent frames. The key idea is to embed the linear dispersion relation into the model itself to physically relate the sparse points observed at different times. Assuming that the scattered data are uniformly distributed in the spatial domain, this has the same effect of increasing the sample density of each single frame. Experiments demonstrate how the proposed technique, even if trained with purely synthetic data, can produce accurate and physically consistent surfaces at five frames per second on a modern PC. [ABSTRACT FROM AUTHOR]

There is general consensus that accurate model predictions of extreme wave events during marine storms can substantially contribute to avoiding or minimizing human losses and material damage. Reliable wave forecasts and hindcasts, together with statistical analysis of extreme conditions, are then of utmost importance for monitoring marine areas. In this study, we perform an analysis of the limitations of the available short-term/range extreme-value distributions suitable for space-time maximum wave and crest heights. In particular, we propose an improvement of the theoretical distributions by including upper bounds on the maximum heights that waves may reach. The modification of the space-time probability distributions and its impact for extreme-value assessment is discussed in the paper. We show that unbounded space-time distributions are still effective provided that the surface area included in the analysis has sides smaller than O(102 m). For wider surfaces, the use of the bounded distributions is consistent with the expected saturation of maximum heights that ocean waves attain. [ABSTRACT FROM AUTHOR]

We perform a critical analysis of the present approach in wave modeling and of the related results. While acknowledging the good quality of the best present forecasts, we point out the limitations that appear when we focus on the corresponding spectra. Apart from the meteorological input, these are traced back to the spectral approach at the base of the present operational models, and the consequent approximations involved in properly modeling the various physical processes at work. Future alternatives are discussed. We then focus our attention on how, given the situation, to deal today with the estimate of the maximum wave heights, both in the long term and for a specific situation. For this, and within the above limits, a more precise evaluation of the wave spectrum is shown to be a mandatory condition. [ABSTRACT FROM AUTHOR]

The LATEMAR project aims at improving the modelling of large wave events during marine storms in the framework of the Copernicus Marine Environment Monitoring Service (CMEMS). Indeed, at present, CMEMS operational systems only release average and peak wave parameters, with no information on individual waves whatsoever. However, developments of the state-of-the-art third-generation wave models (WAVEWATCH III and SWAN, for instance) demonstrated that, using the directional wave spectrum and theoretical statistical models for wave extremes, forecasters can accurately infer the expected shape and likelihood of the maximum waves during marine storms. The main goal of the LATEMAR project is therefore to provide the wave models used by the CMEMS service (WAM and WAVEWATCH III) with the procedures to explicitly estimate the maximum wave heights for each sea state. Besides this, further goals of LATEMAR are the following: perform an extensive assessment of the modeled maximum wave events using observations collected from an oceanographic station; inter-compare WAM and WAVEWATCH III model performance in a test area (Mediterranean Sea); investigate the sensitivity on sea state parameters of the maximum waves. All model developments and evaluations resulting from this R&D project will be directly applicable to the wave model forecasting systems, extending thus the CMEMS product catalogue. [ABSTRACT FROM AUTHOR]

In this study, we use reanalysis datasets to characterize the maximum crest, crest-to-trough and envelope heights of sea states, on a global and long-term scale. In particular, we rely on the ERA-Interim directional wave spectra, which are used to estimate the parameters of the probability distributions of wave maxima. To represent the typical single-point observations we use time extreme theoretical models, while to take the three-dimensional geometry and short-crestedness of ocean waves into account, we adopt a space-time extreme model. In order to assess the confidence limits of the reanalysis-based wave maxima we verify them against buoy and stereo-video wave observations collected in the North Pacific Ocean. We provide the global assessment of maximum crest, crest-to-trough and envelope heights during typical and extreme conditions, showing where the largest values are attained, that is in the mid-latitude storm belts, in particular in the North Atlantic Ocean. With respect to previous wave climate studies that focused on the significant wave height only, in this study, besides quantifying the maximum wave heights in magnitude, we also highlight the role of wave parameters such as wave steepness and kurtosis (measures of nonlinearity) and spectral bandwidth (measure of irregularity). Finally, we stress the relevant contribution of the short-crestedness, which should be taken into account in assessing wave maxima for the safety of navigation, ship routing and marine structural design. [ABSTRACT FROM AUTHOR]