Coastal Wave Research Articles

Monitoring Coastal Waves with ICESat-2

The coastal zone faces an ever-growing risk associated with climate-driven change, including sea level rise and increased frequency of extreme natural hazards. Often the location and dynamism of coastal regions makes them a formidable environment to adequately study with in-situ methods. In this study we use Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) to make measurements of basic wave parameters and wave directionality in the coastal zones of the Hawaiian Islands and North Carolina, USA. Our goal was to leverage as much of the full resolution data available in the ATL03 data product to generate wave metrics out from shore up to ~25 km. Using a combination of statistical and signal processing methods, including cross-correlation and wavelets, we can use ICESat-2 to generate basic wave metrics, including significant wave heights with an accuracy of ±0.5 m. In some profiles we can identify wave shoaling, which could be useful to infer bathymetry and coastal dynamics. In areas with complex wave dynamics, the nature of how ICESat-2 measures elevations (parallel laser altimetry beams) can make extracting some wave parameters, especially wavelength and directionality, more challenging. These wave metrics can provide important data in support of validating wave and tidal models and may also prove useful in extended ICESat-2 applications like bathymetric corrections and satellite-derived bathymetry.

Carbonate sediment dynamics in the Abu Dhabi lagoon - implications for low-angle inner-to-middle ramp models

Carbonate ramps are prominent features of continental margins and intrashelf basins throughout much of Earth's history. Among the limited number of recent analogues of ancient carbonate ramps, the Persian/Arabian Gulf stands out due to its morphologically complex array of open coastal to sabkha environments. This study presents detailed field and laboratory data on spatial facies variability linked to different hydrodynamic regimes spanning from open coastal to inner lagoon settings in Abu Dhabi. Sediment transport and deposition are determined by waves and tidal currents, with spatially and temporally complex variations in the hydrodynamic regime, resulting in a complex facies mosaic. The open coastal domain is subject to persistent north-westerly trade (Shamal) winds and episodic winter storms, generating waves that affect the deposition of ooids, peloids, lithic clasts and bioclasts. In the outer lagoon, these waves interact with tidal currents and thus become less dominant towards the middle lagoon. Tidal currents are more important in the restricted inner lagoon. The outer lagoonal sediments primarily consist of ooids, bioclasts and lithic clasts and are peloid- and micrite-lean. Bimodal tidal currents, locally with velocities of two or more metres per second, are present in the relatively open regime of the middle lagoon and accumulate bar complexes dominated by skeletal carbonates. Seagrass and biofilms locally stabilise muddy carbonates and limit sediment entrainment even where significant tidal currents exist. In a landward direction, and with decreasing hydrodynamic levels, carbonate mud (micrite) is increasingly abundant, reflecting the lower hydrodynamic conditions in the restricted regime of the mid-and inner lagoon. Data shown here provide a facies distribution model in the shallow coastal portions of what might be one of the few modern analogues of a ‘marginal epeiric’ sea, shedding light on the complex interplay between coastal morphology, wave and tide energy and biological processes. This study discusses how hydrodynamics and seafloor bio-stabilisation influence the spatial carbonate facies of this highly dynamic ramp system by comparing data shown here with data from the Kuwait and Doha coasts, which have different orientations to the prevailing winds. The quantitative and refined composite inner ramp model shown here has wider significance for coastal dynamics and carbonate stratigraphy.

Adjoint-based high-order spectral method of wave simulation for coastal bathymetry reconstruction

Bathymetry is an important factor affecting wave propagation in coastal environments but is often challenging to measure in practice. We propose a method for inferring coastal bathymetry from spatial variations in surface waves by combining a high-order spectral method for wave simulation and an adjoint-based variational data assimilation method. Recursion-formed adjoint equations are derived to obtain the sensitivity of the wave surface elevation to the underlying bottom topography to any desired order of nonlinear perturbation. We also develop a multiscale optimisation method to eliminate spurious high-wavenumber fluctuations in the reconstructed bathymetry data caused by sensitivity variations over the different length scales of surface waves. The proposed bottom detection method is validated with a realistic coastal wave environment involving complex two-dimensional bathymetry features, non-periodic incident waves and nonlinear broadband multidirectional waves. In numerical experiments at both laboratory and field scales, the bathymetry reconstructed from our method agrees well with the ground truth. We also show that our method is robust against imperfect surface wave data in the presence of limited sampling frequency and noise.

Roles of the Indo-Pacific subsurface Kelvin waves and volume transport in prolonging the triple-dip 2020–2023 La Niña

The rare triple-dip 2020–2023 La Niña event has resulted in a series of extreme climate events across the globe. Here, we reveal the role of tropical Indo-Pacific oceanic interactions in driving the first triple-dip La Niña of the twenty-first century. Specifically, we found that the eastern Indian Ocean subsurface warming anomalies were associated with the re-intensification of the subsequent La Niña event. The subsurface warming anomaly signals were propagated eastward by equatorial and coastal subsurface Kelvin waves from the eastern Indian Ocean to the western Pacific Ocean through the Indo-Pacific oceanic pathway, which contributes to the accumulation of heat content and deepens the thermocline in the western tropical Pacific. The westward Indonesian Throughflow (ITF) transported more heat during multi-year La Niña events from the western Pacific Ocean to the eastern Indian Ocean than during single-year events, resulting in the injection of more warm water into the eastern Indian Ocean. The combination of subsurface Kelvin wave propagation and increased ITF volume transport in the Indo-Pacific region acted to prolong the heat content in the western Pacific during the decay phase of La Niña, ultimately leading to the rare triple-dip 2020–2023 La Niña event.

The Effect of an Exponentially Decaying Upper-Ocean Vertical Mixing on the Pacific Tropical Sea Surface Temperature

Abstract We investigate the mechanisms with which the sea surface temperature (SST) in the tropical Pacific responds to the perturbation of an exponential form to the background vertical mixing of the upper ocean. For a surface value of 0.005 m2 s−1 and a scale depth of 10 m (as typically used in the so-called nonbreaking wave parameterization), it is found that only ocean temperature within the equatorial eastern Pacific (EEP) is directly impacted; surface cooling and thermocline warming anomalies are produced. These signals propagate poleward as coastal Kelvin waves and then westward as equatorial Rossby waves. The surface cooling is severely damped while the thermocline warming is able to reach the western coast. This warm anomaly is brought up to the surface by equatorial upwelling more strongly around 110°W than at other places. In the coupled model, such equatorial warming induces an El Niño–like large-scale warming through Bjerknes feedback. Increasing the surface value of vertical mixing by a factor of 10 does not increase the equatorial surface warming while increasing the scale depth to 20 m does. Increasing the scale depth generates thermocline warming also in the subtropical region, which then propagates to the equatorial thermocline and enhances the warming there. Moreover, the off-equatorial cooling is enhanced, which makes the final warming anomaly narrower meridionally compared to an El Niño pattern.

Intraseasonal oscillations of the Andaman Sea thermocline

Variations in the upper ocean thermal structure have significant implications for air-sea interaction and upper-ocean ecosystem processes. Vertical profiles of temperature measured by a moored buoy located at 10.5°N, 94°E in the Andaman Sea and simulation by an Indian Ocean model are used in this study to characterise the intraseasonal variations (ISV) in the Andaman Sea (AndS) thermal structure and identify their sources. The seasonal variations in the upper ocean thermal structure show a strong semi-annual cycle driven by monsoons. The sub-surface temperature shows significant intraseasonal oscillations within a band of 40–110 days, which are de-coupled from that in the mixed layer, which has dominant periodicity in the band of 90–120 days. Thermocline ISV are seasonally modulated with a primary peak during August–September and a secondary peak during February–March, with significant year-to-year variations. A cross-wavelet analysis shows that ISV in the 40–60 days period is in phase with that at the eastern boundary and they are locally forced by the passage of eddies. The 60–110 day band is out of phase with the eastern boundary and is forced by Rossby waves radiated from the equatorially generated intraseasonal coastal Kelvin waves.

Detection and attribution of intra-annual mass component of sea-level variations along the Norwegian coast

Reliable sea-level observations in coastal regions are needed to assess the impact of sea level on coastal communities and ecosystems. This paper evaluates the ability of in-situ and remote sensing instruments to monitor and help explain the mass component of sea level along the coast of Norway. The general agreement between three different GRACE/GRACE-FO mascon solutions and a combination of satellite altimetry and hydrography gives us confidence to explore the mass component of sea level in coastal areas on intra-annual timescales. At first, the estimates reveal a large spatial-scale coherence of the sea-level mass component on the shelf, which agrees with Ekman theory. Then, they suggest a link between the mass component of sea level and the along-slope wind stress integrated along the eastern boundary of the North Atlantic, which agrees with the theory of poleward propagating coastal trapped waves. These results highlight the potential of the sea-level mass component from GRACE and GRACE-FO, satellite altimetry and the hydrographic stations over the Norwegian shelf. Moreover, they indicate that GRACE and GRACE-FO can be used to monitor and understand the intra-annual variability of the mass component of sea level in the coastal ocean, especially where in-situ measurements are sparse or absent.

Study on Numerical Modeling for Pollutants Movement Based on the Wave Parabolic Mild Slope Equation in Curvilinear Coordinates

Nearshore waves and currents induced by breaking of obliquely incident waves are important dynamic factors that significantly affect pollutants movement at offshore zones. A combined numerical model in orthogonal curvilinear coordinates is developed to simulate pollutants movement in surf zones, including a wave transformation model based on the parabolic mild slope equation, a wave-induced current model, and a pollutant transport model driven by waves and currents. This combined model has been applied to pollutants movement laboratory cases, and comparison of the simulations with experimental measurements shows good agreement. The model has also been applied to simulation of the Gourlay experiment and it shows consistent results with the distribution of waves and offshore currents. This coupled numerical model has preliminarily improved the situation of rare numerical research on the nearshore pollutant transport in coastal wave and wave-induced current based on wave radiation stress theory in orthogonal curvilinear coordinates.

Effect of the structural complexity of a coral reef on wave propagation: A case study from Komaka Island, Japan

The structural complexity of coral reefs has an essential impact on coastal wave propagation. Nevertheless, bathymetry data, from which complex structures of several meters in size can be ascertained, have yet to be used for numerical modelling. To investigate the effects of coral reef structural complexity on coastal wave propagation, we first obtained 1-m resolution bathymetry data from a multibeam echosounder on Komaka Island, Japan. Then we conducted numerical simulations of storm waves over the obtained bathymetry. The simulation results using fine topographic resolution demonstrate that vorticity over the reef slope is significant due to the structural complexity of coral reefs, including spurs and grooves. This implies that complex features of several meters over the reef slope induce wave-driven currents, which is often a pathway for nutrient transport. However, the calculated wave energy densities over the reef flat are mostly the same, even when coarse bathymetry resolution is used. Thus, the overall reef slope, edge, and flat account for most of the dissipation process of storm waves, whereas complex structures of several meters in size have little impact. In contrast, the coral structures several meters in size contribute to the ecosystem by facilitating seawater rather than dissipating extreme waves.

The 2021 Atlantic Niño and Benguela Niño Events: external forcings and air–sea interactions

AbstractThis paper presents a comprehensive analysis of the extreme Atlantic and Benguela Niño events that occurred during the boreal spring–summer of 2021. We conducted sensitivity experiments with a regional ocean–atmosphere coupled model of the tropical Atlantic to investigate the phenology of these interannual events, unravel their triggering mechanisms, and quantify the contributions of local and remote processes. The results revealed that both the 2021 Atlantic and Benguela Niños were triggered by anomalous atmospheric fluxes at the model southern boundary (32° S), leading to a significant and persistent weakening of the South Atlantic Anticyclone. The associated poleward anomalous coastal wind off Africa reduced coastal upwelling and evaporation south of 15° S, initiating the Benguela Niño. Then, the relaxation of the equatorial trade winds forced a downwelling equatorial Kelvin wave, which warmed the eastern equatorial region, marking the onset of the Atlantic Niño. The equatorial event reached full maturity in July 2021 through ENSO-like air-sea interactions in the equatorial basin, enhanced by the atmospheric connection associated with low-level winds converging toward the distant coastal warming. While air–sea interactions in the tropical Atlantic acted as a negative feedback for the coastal warming, the ocean connection with the equatorial variability through the propagation of equatorially-forced downwelling coastal waves intensified the coastal warming, peaking end of May 2021. Overall, this research provides valuable insights into the complex dynamics of Atlantic and Benguela Niños, emphasizing the interconnectedness between these two systems. This has important implications for improving Earth system models which currently struggle to simulate these extreme events.

Diurnal coastal trapped waves propagating along the east coast of the Shimokita Peninsula, Japan

In this study, we investigated diurnal coastal trapped waves in the eastern coastal area of the Shimokita Peninsula near the Tsugaru Strait. The coastal trapped waves in this area have not yet been observed. We observed current velocities at three sites on the coast to clarify the propagation and seasonal features. We also used an ocean general circulation model to determine the detailed structure and the causes underlying the seasonal characteristics of the waves. The coastal trapped waves propagated southward along the coast from the strait, where significant tidal currents exist. Coastal trapped waves depend on cross-shelf length and stratification strength. The coastal trapped waves propagate as internal Kelvin waves from summer to early winter in the northern part of the peninsula (where the shelf is narrow) and as shelf waves in the southern part (where the shelf is wide). From late winter to spring, the coastal trapped waves practically disappear in the northern part of the peninsula owing to the vertical uniform density off the east coast and the small cross-shelf width in the northern part. In autumn, the tidal current flows north of the sill near Cape Shiriya at the eastern mouth of the straits owing to the northward shift of the Tsugaru Warm Current in the strait; thus, the coastal trapped waves along the Shimokita coast weaken.

Field Studies of Megagravel Transport: Relevance for Coastal Infrastructure Engineering

AbstractExtreme wave impacts on coastal infrastructure, including bridges, is a growing problem. Highest energy events are difficult to study because direct observations in real time are almost impossible. Also the non‐linear dynamics are difficult to model or to reproduce in scaled experiments. Coastal boulder deposits incorporating megagravel (y axis >4.1 m) provide natural analogues of infrastructure components such as bridge deck slabs; and field measurements of these wave‐transported blocks can address this knowledge gap. In this contribution we present geoscience data relating coastal topography, wave climate, and dislodgement of megagravel up to 620 tonnes. We hope that this will generate new synergies among coastal geocientists and engineers.

ON THE BEHAVIOR OF A TETHERED CYLINDER ARRAY UNDER IRREGULAR WAVES

An interesting option for coastal wave attenuation can be provided by tethered floating breakwaters (see e.g. Agerton, 1976 and Seymour, 1979). This type of structure can be designed to be effective, while determining a very small impact on the environment (Dai, 2018). In principle, a regular lattice of reversed pendula (i.e. the breakwater) can be seen as an approximation to a mechanical metamaterial with interesting properties related to its periodic structure. The tethered floating breakwater we are studying has been designed as a finite lattice of submerged inverse pendula. The objective is to determine the efficiency of a regular two-dimensional lattice of spherical pendula. To isolate the one-dimensional behavior, a single array of reversed cylindrical pendula anchored to the bottom and excited by long crested waves has been tested.

COMPARISON OF FIELD AND FORECAST METOCEAN DATA IN THE GERMAN BIGHT

Offshore wind energy can contribute significantly to achieve greenhouse gas neutrality in the near future. But the planning, construction and operation of offshore wind parks is associated with high costs. In particular, the operation and maintenance (OandM) costs account for a major fraction and in turn are also dependent on appropriate weather windows in which working vessels can operate. A precise knowledge of how wind, sea state and currents affect each other, as well as the most accurate possible prediction of the development, is essential to maximize these periods and reduce the resulting costs. For the German Bight, the high-resolution coastal wave model (CWAM) is often used for this purpose. The CWAM is based on WAM (Hasselmann et al. 1988) and was developed by the German Meteorological Service (DWD) and German Maritime and Hydrographic Agency (BSH) in cooperation with the Helmholtz-Zentrum Hereon. Compared to other WAM models it offers the advantages of higher resolution, as well as the consideration of current speeds and tide-dependent water depths. The BSH also operates multiple measuring stations in the German Bight, which enable real-time investigation of sea state parameters. The general objective of this study is to compare historical data sets from in situ measurements in the German Bight with numerical forecast data and to identify which factors influence the accuracy of the CWAM and why deviations occur.

Nonlinear Effects and Run-up of Coastal Waves Generated by Billiards with Semi-rigid Walls in the Framework of Shallow Water Theory

Nonlinear Effects and Run-up of Coastal Waves Generated by Billiards with Semi-rigid Walls in the Framework of Shallow Water Theory

The During Nearshore Event Experiment (DUNEX): A collaborative coastal community experiment to address coastal resilience

The During Nearshore Event Experiment (DUNEX) was a large-scale coastal field effort focused on improving understanding of during-storm nearshore processes to ultimately develop predictive technologies, engineering solutions, and actions to enhance coastal resilience. The experiments were conducted on the North Carolina coast by a multidisciplinary group of over 30 research scientists from 18 academic and federal institutions supporting over 30 graduate students and deploying over 300 instruments from 2019 to 2021. The overarching goal of DUNEX was to gather information collaboratively to improve understanding of the interactions of coastal water levels, waves, currents, beach and dune evolution, soil behavior, vegetation, and groundwater during major coastal storms that affect infrastructure, habitats, and communities. In the short term, these high-quality field measurements will lead to better understanding of during-storm processes and impacts and will enhance U.S. academic coastal research programs by providing opportunities for students to learn about field data collection and to potentially analyze data as part of their studies. Longer-term, DUNEX data and outcomes will improve the ability to predict extreme event physical processes and impacts, validate coastal processes numerical models, and improve coastal resilience strategies and communication methods for coastal communities impacted by storms. The purpose of this paper is to describe the motivation for and science goals of the experiment, how stakeholder needs led to these goals, collaborations amongst researchers, and the knowledge gained that will lead to tools to improve coastal resilience. Herein, we first describe how researchers worked with stakeholders to structure their community-driven needs into science-based requirements. Next, we summarize how federal, academic, and stakeholder researchers worked together to design and execute a multi-organizational experiment aligned with those requirements. Finally, we articulate early findings and lessons learned from the experiment. This paper does not summarize all the research findings from DUNEX, as analyses are still ongoing. An American Geophysical Union (AGU) Special Collection on Coastal Storm Research will be published in 2025 including outcomes from DUNEX research.

Temporal changes in dependence between compound coastal and inland flooding drivers around the contiguous United States coastline

Flooding in low-lying coastal zones arises from coastal (storm surge, tides, and waves), fluvial (excessive river discharge), and pluvial (excessive surface runoff) drivers. We analyse changes in compound flooding potential around the contiguous United States (CONUS) coastline stemming from select combinations of these flooding drivers using long observational records with at least 55 years of data. We assess temporal changes in the tail (extremal) dependence (χ) using a 30-year sliding time window. Periods of strong tail dependence are found for the windows centered between the 1960s and 1980s/1990s at several locations for surge-discharge (S-Q) and surge-precipitation (S–P) combinations. Changes in dependence are associated with large-scale climate indices such as the Arctic Oscillation (AO) and El Nino Southern Oscillation indices (Niño 1.2 and Niño 3), among others. The significance of potential changes in the dependence structure is subsequently tested using Kullback–Leibler (KL) divergence. We find that changes are mostly not significant. Finally, we perform a complete multivariate statistical analysis exemplarily for one selected pair of variables at one location (S-Q in Washington, DC), allowing for varying dependence strength and structure as well as changes in the marginal distributions. Combined changes with increase in the dependence and marginals exacerbate the predicted compound flood potential. The comprehensive analysis presented here provides new insights into how and where compound flooding potential has changed with time, demonstrates associated links with large-scale climate indices, and highlights the effects of changes in the dependence and marginals in a multivariate statistical framework.

The Kuroshio Intrusion Into the South China Sea at Luzon Strait Can Be Remotely Influenced by the Downstream Intrusion Into the East China Sea

AbstractThe intrusion of Kuroshio into the South China Sea through Luzon Strait has been of enormous interest in western boundary current studies. Here we show that this intrusion can be remotely influenced by the downstream intrusion of the Kuroshio northeast of Taiwan into the East China Sea, at a lead time of about 220 days. This remarkable finding is first revealed by a quantitative causality analysis which is rigorously established from first principles in physics; it is originally motivated by the observation of a wake southeast of Taiwan, whose influence on the loop current, also revealed by causality analysis, is traced downstream to the Kuroshio intrusion into the East China Sea. Further analysis suggests that the two important Kuroshio intrusions are connected via a coastal trapped wave mode with a period of 1.5 years propagating southward along the eastern Taiwan coast, as supported by both theoretical and numerical models. Upon approaching the Luzon Strait, its negative (positive) phase functions to intensify (reduce) the loop current. At a speed of 1.37 km day−1, the mode takes about 220 days for the disturbances northeast of Taiwan to travel along the coast to Luzon Strait, leading to the same time delay for the downstream‐upstream causal relation.

Climate‐Induced Decadal Ocean Wave Height Variability From Microseisms: 1931–2021

AbstractLong‐term coastal wave climate changes under global warming cannot be reliably assessed from relatively short duration buoy significant wave height (Hs) observations that began about 1980. Alternatively, microseisms at double the wave frequency (DFM) provide proxy records of near‐coastal wave activity. An empirical wave reconstruction methodology transforms DFM from UC Berkeley seismic station BKS to seismic Hs (sHs). Broadband digital seismic observations since 1992 at BKS provide DFM reference spectral levels. Earlier digitized archived BKS seismogram DFM levels are adjusted to give spectral levels that are consistent with current observations, yielding a combined 90‐year sHs record for winters (November–March) spanning the 1931–2021 epoch. Mean winter sHs derived from digitized 1980s seismograms are well‐correlated with nearby buoy Hs. Comparison of deep‐water extreme Hs with sHs demonstrates that sHs is dominated by near‐coastal wave activity. Much greater decadal and interannual variability occurred during the 1931–1970 epoch, including extended periods of exceptionally low sHs during 1939–1947 and 1957–1965 that have not occurred since global warming accelerated near 1970, suggesting that winter extratropical storm intensity has generally increased. Comparisons of winter sHs variability with anomalous sea level pressure (SLPa) patterns across the North Pacific demonstrate the association of coastal sHs with broad‐scale atmospheric variability, with differences between long‐term averages of winter SLPa indicating an Aleutian Low intensification, consistent with both increased mean sHs and occurrence of extreme events since 1970. Recent record global annual temperatures suggest that warming may be accelerating, resulting in stronger storms that intimates further increased winter Hs may be forthcoming.

Experimental hydrodynamic assessment of a cylindrical-type floating solar system exposed to waves

In this paper, an experimental investigation on the wave loads and structural motions of two semi-fixed semi-immersed horizontal cylinders type rafts in the free surface zone is conducted. The physical models are tested at the 1:4.5 scale and exposed to a range of regular and irregular waves in a wave flume at Queen's University Belfast. The physical models and experimental setup are discussed alongside an investigation of the hydrodynamic phenomena, surge forces, and dynamic responses that each structure exhibits in the coastal wave climates. Furthermore, an investigation into the wave attenuation by both models is carried out. The results show that the surge forces have a positive correlation with wave steepness for both models. Hydrodynamic phenomena such as wave runup and overtopping, radiative damping and reflected waves, constructive interference, diffraction and flow separation were identified during the experiments. A negative mean heave displacement is observed during the monochromatic sea states which could result in impact loading and submergence of the superstructure components and photovoltaic panels at full-scale. The results presented in this paper may be used to calibrate and verify numerical models that calculate the global responses and hydrodynamic forces. It may also benefit the design processes of geometrically similar floating solar technologies by providing data on surge loads, motion responses and hydrodynamic observations.