Wind Forcing Conditions Research Articles

Spatio-Temporal Variation in Suspended Sediment during Typhoon Ampil under Wave–Current Interactions in the Yangtze River Estuary

Suspended sediment plays a major role in estuary morphological change and shoal erosion and deposition. The impact of storm waves on sediment transport and resuspension in the Yangtze River Estuary (YRE) was investigated using a 3D coupling hydrodynamic-wave model with a sediment transport model during Typhoon Ampil. This model has been validated in field observations of water level, current, wave, and sediment concentration. The model was run for tide only, tide + wind, tide + wind and wave forcing conditions. It was found that: (1) typhoons can increase the suspended sediment concentration (SSC) by enhancing bed shear stress (BSS), especially in the offshore area of the YRE, and there is hysteresis between SSC and BSS variation; (2) exponential and vertical-line types are the main vertical profile of the SSC in the YRE and typhoons can strengthen vertical mixing and reconstruct the vertical distribution; and (3) waves are the dominating forcing factor for the SSC in the majority of the YRE through wave-induced BSS which releases sediment from the seabed. This study comprehensively investigates the spatio-temporal variation in SSC induced by Typhoon Ampil in the main branch of the YRE, which provides insights into sediment transport and resuspension during severe storms for estuaries around the world.

Extreme waves in the Caribbean Sea: spatial regionalization and long-term analysis

The extreme wave height distribution in the Caribbean Sea is studied using a new method based on the maximum basin-wide aggregate of significant wave height, Hs, values per month. Besides, by means of the Self-Organizing Maps (SOM) technique, we identify coherent geographical regions with similar extreme wave height variability in the Caribbean Sea. Our findings revealed three primary regions: the eastern side with comparatively lower values, the central region with intermediate values, and the western side with the highest extreme wave heights. The study also examines the wind forcing conditions driving the spatial and temporal variability of the extreme waves, highlighting the influence of the low-pressure belt dynamics as well as the role played by the Caribbean Low-Level Jet (CLLJ) index, and the impact of cold fronts and hurricanes on extreme wave heights. Additionally, we explore the relationship between the extreme wave height distribution and climatic indices, such as the Atlantic Multidecadal Oscillation (AMO), the Atlantic Meridional Mode (AMM), the North Atlantic Oscillation (NAO) and the Oceanic Niño (ONI). The results reveal that the spatial distribution of extreme wave heights in the Caribbean Sea is mostly ruled by the influence of the CLLJ, with correlations close to 80%. In addition, significant correlations were observed between the extreme wave heights and the ENSO in the central Caribbean, as well as positive correlations between the extreme wave heights and NAO in the eastern part of the basin, and significant values of correlation with the negative phases of AMO and AMM in the whole basin. We show that, unlike conventional (or broadly used) methods deployed to identify extreme wave height, such as percentile 99th, Hs99, our methodology allows a further assessment of the wind and climate forcing conditions associated with the extreme wave events. Although, we acknowledge that the method here presented has limitations to capture extreme wave height outliers, it has the advantage of being used concomitant with the wind forcing to develop multivariate wave climate analysis at basin scale, and could be extended to a more local scale when studying coastal processes.

Hydrography, inorganic nutrients and chlorophyll a linked to sea ice cover in the Atlantic Water inflow region north of Svalbard

Changes in the inflow of Atlantic Water (AW) and its properties to the Arctic Ocean bring more warm water, contribute to sea ice decline, promote borealisation of marine ecosystems, and affect biological and particularly primary productivity in the Eurasian Arctic Ocean. One of the two branches of AW inflow follows the shelf break north of Svalbard, where it dominates oceanographic conditions, bringing in heat, salt, nutrients and organisms. However, the interplay with sea ice and Polar Surface Water (PSW) determines the supply of nutrients to the euphotic layer especially northeast of Svalbard where AW subducts below PSW. In an effort to build up a time series monitoring the key characteristics of the AW inflow, repeat sampling of hydrography, macronutrients (nitrate, phosphate and silicate), and chlorophyll a (chl a) was undertaken along a transect across the AW inflow at 31°E, 81.5°N since 2012 — first during late summer and in later years during early winter. Such time series are scarce but invaluable for investigating the range of variability in hydrography and nutrient concentrations. We investigate linkages between late summer hydrographic conditions and nutrient concentrations along the transect and the preceding seasonal dynamics of surface chl a and sea ice cover in the region north of Svalbard. We find large interannual variability in hydrography, nutrients and chl a, indicating varying levels of nutrient drawdown by primary producers over summer. Sea ice conditions varied considerably between the years, impacting upper ocean stratification, light availability and potential wind-driven mixing, with a strong potential for steering chl a concentration over the productive season. Early winter measurements show variable efficiency of nutrient re-supply through vertical mixing when stratification was low, related to autumn wind forcing and sea ice conditions. While this re-supply elevates nutrient levels sufficiently for primary production, it likely happens too late in the season when light levels are already low, limiting the potential for autumn blooms. Such multidisciplinary observations provide insight into the interplay between physical, chemical and biological drivers in the marine environment and are key to understanding ongoing and future changes, especially at this entrance to the central Arctic Ocean.

Correction of GLS turbulence closure for wave-resolving models with stratification

We build on the work of Mayer and Madsen (2000) and Larsen and Fuhrman (2018) to correct for overmixing in wave potential flow regions due to the linear instability of two-equation closure systems. We extend it to the Generic Length Scale (GLS) turbulent closure approach, while testing it for the first time on a stratified nearshore problem. The modified turbulence equations affect only the nonbreaking wave potential flow region where the vorticity is less than 10% of the strain rate and thus can preserve stratification on the innershelf under favorable heat and wind forcing conditions. This work is a necessary step before engaging in full 3D wave-resolving studies of surf–shelf exchange.

A Comparison of Wave Spectra during Pre-Monsoon and Post-Monsoon Tropical Cyclones under an Intense Positive IOD Year 2019

The impact of Indian Ocean Dipole (IOD) events on the generation and intensity of tropical cyclones under the influence of monsoons is explored. The standardized sea surface temperature (SST) anomalies are computed for the pre-monsoon and post-monsoon months for the Bay of Bengal (BOB) and Arabian Sea (AS) from 1971 to 2020 and relationships are analyzed with the frequency of tropical cyclones for the neutral, positive and negative IOD years. Ocean states are sensitive to cyclonic conditions exhibiting a complex spectral distribution of the wave energy. Due to a tropical cyclone, the surface waves remain under high wind forcing conditions for prolonged periods generating a huge amount of energy. The spectral wave model SWAN (Simulating WAves Nearshore) is used to generate the energy density spectra during FANI (26 April–5 May 2019), which was a pre-monsoon extreme severe cyclonic storm, and BULBUL (5–12 November 2019), which was a post-monsoon very severe cyclonic storm in the BOB region. This study aims to estimate the intensity of wave energy during tropical cyclones in the pre- and post-monsoon months for 2019 (an extremely positive IOD year).

Synoptic and Seasonal Variability of Small River Plumes in the Northeastern Part of the Black Sea

Small river plumes are typical features at many coastal regions in the World Ocean. These water masses have relatively small areas and volumes; however, due to their energetic dynamics localized in a thin surface layer, they strongly affect coastal circulation, water quality, and ocean-atmosphere interaction. In this study, we investigate external factors, which govern synoptic and seasonal variability of small river plumes, and, therefore, affect land-ocean fluxes of fluvial water and biogeochemically important material. We use numerical modeling to simulate small river plumes at the northeastern part of the Black Sea. We describe the response time of small river plumes to changes in river discharge and wind forcing conditions, which determines variability of river plumes at different time scales. We reveal that the influence of river plumes on coastal processes depends not only on total annual river discharge volume, but also on temporal distribution of high-discharge and low-discharge periods. Seasonal and synoptic features of local atmospheric circulation could strongly modify the relation between river plume characteristics and river discharge rate. The results obtained in this study are important for better assessment of delivery and fate of river-borne suspended and dissolved matter, as well as floating litter in coastal areas.

Unified Wind-Wave Growth and Spectrum Functions for All Water Depths: Field Observations and Model Results

Abstract Wind-wave development is governed by the fetch- or duration-limited growth principle that is expressed as a pair of similarity functions relating the dimensionless elevation variance (wave energy) and spectral peak frequency to fetch or duration. Combining the pair of similarity functions, the fetch or duration variable can be removed to form a dimensionless function of elevation variance and spectral peak frequency, which is interpreted as the wave energy evolution with wave age. The relationship is initially developed for quasi-neural stability and quasi-steady wind forcing conditions. Further analyses show that the same fetch, duration, and wave-age similarity functions are applicable to unsteady wind forcing conditions, including rapidly accelerating and decelerating mountain gap wind episodes and tropical cyclone (TC) wind fields. Here it is shown that with the dimensionless frequency converted to dimensionless wavenumber using the surface wave dispersion relationship, the same similarity function is applicable in all water depths. Field data collected in shallow to deep waters and mild to TC wind conditions and synthetic data generated by spectrum model computations are assembled to illustrate the applicability. For the simulation work, the finite-depth wind-wave spectrum model and its shoaling function are formulated for variable spectral slopes. Given wind speed, wave age, and water depth, the measured and spectrum-computed significant wave heights and the associated growth parameters are in good agreement in forcing conditions from mild to TC winds and in all depths from deep ocean to shallow lake. Significance Statement This paper presents a growth function and spectrum model to describe wind-wave development in all water depths. Their applicability covers a wide range of wind forcing conditions including steady, accelerating, decelerating, and tropical cyclone events. Support for the unified spectrum model and growth function is presented with field observations and numerical computations.

Spreading and accumulation of river-borne sediments in the coastal ocean after the environmental disaster at the Doce River in Brazil

This study is focused on the fate of a large volume of mine slurry discharged from the Doce River (DR) to the coastal ocean after the worst environmental disaster in Brazilian which occurred in November 2015. We used Eulerian (ROMS) and Lagrangian (STRiPE) numerical models, as well as satellite remote sensing data, to study the spreading and seafloor accumulation of fine river-borne sediments during the initial six months following the disaster. We show that the regions of intense sediment accumulation were determined by spreading patterns of the surface-advected DR plume. The river discharge rate governed the plume surface area, while its position depended on local wind forcing conditions. The spreading of sediments carried by the DR plume was dominated by southward transport caused by prevailing upwelling-favorable northeasterly winds during the study period. Under high discharge conditions, river-borne sediments were transported over 100 km southward from the DR mouth and reached the outer shelf. In contrast, sediments were arrested near the mouth during drought periods and remained on the inner shelf. As a result, fine river-borne sediments accumulated on the seafloor, mainly in the large shallow shelf area southward from the DR mouth. Conversely, only a small fraction of residue was deposited northward. Thus, the Environmental Protection Area (EPA) of Costa das Algas, located 40 km southward from the DR, potentially exhibited more susceptibility to sediment arrival. On the other hand, their influence on Abrolhos Marine National Park, located 200 km northeastward from the DR mouth, was presumably minimal.

Response of a Small River Plume on Wind Forcing

Wind forcing is the main driver of river plume dynamics. Direction and magnitude of wind determine position, shape, and size of a river plume. The response of river plumes on wind forcing was simulated in many numerical modeling studies; however, in situ measurements of this process are still very scarce. In this study, we report the first direct measurements of frontal movement of a small river plume under variable wind forcing conditions. Using quadcopters, we performed nearly continuous daytime aerial observations of the Bzyb river plume located in the non-tidal Black Sea. The aerial remote sensing was accompanied by synchronous in situ measurements of wind forcing. We assessed spreading patterns of the plume and evaluated movement velocity of its outer border with unprecedentedly high spatial (∼10 m) and temporal (∼1 min) resolution, which was not available in previous studies based on in situ measurements and satellite observations. Based on the collected data, we evaluated the time of response of plume spreading dynamics on changes in wind forcing conditions. The advection velocity of the outer plume border shows linear relation to wind speed with very small response time (10–20 min). The reversal between upstream/downstream plume spreading occurs during several hours under moderate wind forcing conditions. These reversals involve only near-field part of the plume, which cause detachment of the far-field part of the plume. The obtained results are crucial for understanding and simulating spreading dynamics of small river plumes worldwide.

Disturbance of sedimentary processes in tidal salt marshes invaded by exotic vegetation

In situ moorings were conducted at salt marsh and bare flat to reveal the environmental disturbance of sedimentary processes in the intertidal flat. Spartina alterniflora (Spartina), an invasive species, grew up to 156 cm in the Ganghwa tidal flat from June to November 2019. This rapid growth has resulted in the dense salt marsh, which complicated hydrodynamics and associated sediment processes. Stems and leaves could effectively trap fine-grained sediments (17.24–20.42 μm) at the bed, increasing the differences in bed elevation between the two sites to up to 5.11 cm. The cohesive sediments accumulated in Spartina communities were resuspended differently by stem-scale turbulence generated from the disturbance of stems and leaves, depending on wind forcing and vegetation conditions. The vegetated sediments, under low wind speeds (<4 m s−1), were hardly resuspended in the water column, compared to those in the bare flat, resulting in sedimentation. Under high wind speeds (>6 m s−1), stem-scale turbulence was sufficiently strengthened to surpass the sedimentation of suspended sediments; thus, it resuspended additional bed sediments without a loss of the trapped sediment, unlike in the bare flat. The flocculation of suspended sediments in Spartina communities was mainly controlled by stem-scale turbulence. The flocs were confined to a size of 40 μm (settling velocity: 0.17 mm s−1) and developed an approaching spherical shape. After the cut-off of Spartina, a favorable condition for the flocs to grow by 57 μm (settling velocity: 0.23 mm s−1) was established with decrease in stem-scale turbulence. These larger flocs were able to develop into a ramified spherical structure. Despite diminishment of stem-scale turbulence, the disturbed sediments were outflowed, resulting in an abrupt decrease in bed elevation (0.12 cm day−1). The results suggest that the sediment accumulated in Spartina communities remained erodible due to frequent disturbances without sufficient consolidation.

Rapid-response observations on barrier islands along Cape Fear, North Carolina, during Hurricane Isaias

Hurricane Isaias struck the Cape Fear Region of North Carolina around 23:00 EDT on 3 August 2020, making landfall at Ocean Isle Beach as a Category 1 storm with peak wind speeds of 80 mph. An array of nearshore Sofar Spotter wave buoys captured the wave field at two beaches off the coasts of Bald Head Island (south-facing and east-facing beaches) and Masonboro Island. Local variations in significant wave height and peak wave direction were observed along the Lower Cape Fear Region, due to large shoal features impacting the regional wave climate. A cross-shore transect of five pressure sensors was installed at the north end of Masonboro Island 2.5 days prior to landfall to measure storm surge, wave runup, and variation of gravity/ infragravity wave energy across the barrier island. The three fast-sampling wave gauges along the backshore became buried before Hurricane Isaias peak storm surge, and the two gauges on and behind the dune were never inundated. A low-cost (&lt; $250) Storm Surge Observation Camera (SSOC) prototype captured storm surge and coastal erosion at Kure Beach, in conjunction with pre- and post-storm RTK GPS beach profile surveys. Kure Beach experienced more than 1.0 m of vertical erosion of the berm, while Masonboro Island experienced around 0.1 m of accretion across the backshore, despite nearly identical wave and wind forcing conditions at the two beaches separated by ~20 km. Pre-storm berm height and width (higher and wider at Kure Beach), as well as foreshore slope (steeper, 1:9, at Kure Beach), are likely factors influencing significant erosion at Kure Beach, while slight accretion was observed at Masonboro Island.

2D Parametric Model for Surface Wave Development Under Varying Wind Field in Space and Time

AbstractA fully consistent 2D parametric model of wave development under spatially and/or time varying winds is developed. Derived coupled equations are written in their characteristic form to provide practical means to rapidly assess how the energy, frequency and direction of dominant surface waves are developing and distributed under varying wind forcing conditions. For young waves, nonlinear interactions drive the peak frequency downshift, and the wind energy input and wave breaking dissipation are governing the wave energy evolution. With a prescribed wind wave growth rate, proportional to (u*/c) squared, wave breaking dissipation must follow a power‐function of the dominant wave slope. For uniform wind conditions, this choice for the growth rate imposes solutions to follow fetch laws, with exponents q = −1/4, p = 3/4 correspondingly. This set of exponents recovers the Toba's laws, and imposes the wave breaking exponent equal to 3. A varying wind direction can then drive spectral peak direction changes, leading to the occurrence of focusing/defocusing wave groups over localized areas where wave‐rays merge and cross. Significant (but finite) local variations of the energy are then expected under varying wind forcing. Propagating away from a stormy area, wave rays generally diverge, leading to dispersive swell systems. Examples of practical applications of this model are provided in (Kudryavtsev et al., 2021, companion paper).

Modelling regional and local-scale larval seeding strategies for abalone (H. midae) ranching in South Africa

Understanding connectivity between abalone populations and residence times for pelagic larvae within suitable settlement habitats is critical for sustainable management of wild abalone stocks and ranching schemes. In the Eastern Cape region of South Africa, poaching has severely depleted wild abalone Haliotis midae populations. Rebuilding stock through larval seeding is being considered as a potentially viable method, though is hampered by a limited understanding of abalone habitat connectivity through larval dispersal, both at a regional and local scale. To address this shortcoming, we apply a multi-scale approach to investigate potential larval dispersal along the coastline of Eastern Cape, South Africa, examining both intra- and inter-site connectivity, with the primary focus at the local nearshore scale (< 40 m water depth) where surf zone processes have a strong influence. Delft3D is applied at a regional scale (400-km domain) and XBeach is used at a local scale (5-km domain) to model a known abalone habitat at Cape Recife at high-resolution (5-m cell size). Models were validated using field observations. Complex topo-bathymetry data for the local-scale model domain around Cape Recife were obtained using a state-of-the-art low-cost multi-method surveying approach. At a regional scale, sites with abalone habitat have the potential to be connected across distances of 50–100 km. However, a large sandy embayment (Algoa Bay; 100-km arc length) is likely to present an obstacle to larval transport in all but the most extreme and sustained wind and wave forcing conditions. The central Cape Recife abalone ranching area has the potential to act either as a self-seeding or meta-population connected site, depending on forcing conditions. At the local scale around Cape Recife, a zone of shallow gullies, under lower-energy waves, was determined to be the optimal release point to maximise residency times within the preferred rocky, shallow subtidal habitat of H. midae. Competent larvae released in this zone are predicted to have an order of magnitude greater residency than randomly seeded larvae. This study provides site-specific connectivity and residence time information for the Eastern Cape abalone fishery, but more generally it provides a novel multi-scale method to aid in planning and managing of abalone larval seeding for ranching and stock enhancement programs that could be applied in other locations.

Hydrophysical Structure and Current Dynamics of the Kodor River Plume

This work is focused on the river plume generated by the Kodor River which is the largest river of Abkhazia and the northeastern coast of the Black Sea. Based on in situ hydrological data, quadcopter aerial images and satellite observations, hydrophysical structure and circulation in the Kodor plume was studied at different river discharge conditions. Discharge of the Kodor River, which inflows to sea from several deltaic branches forms a relatively shallow plume with large area, as compared to the Mzymta River, the largest estuarine river of the study region, at similar hydrological, meteorological, and oceanographic conditions. Due to its small vertical size and large salinity gradient at the border with the subjacent saline sea, the Kodor plume is characterized by quick response on wind forcing variability. The Kodor plume also has large spatial inhomogeneity, caused by its formation conditions (several closely located freshwater sources prone to abrupt discharge variability), as well as regional bathymetry (interaction between the plume and shoals). During low discharge period, the inflowing Kodor River has relatively low kinetic energy, which causes abrupt deceleration of the jet in the sea area adjacent to the river mouth. As a result, the inertia of the inflowing jet abruptly decreases and large velocity and salinity gradients are formed in vicinity of river estuary, which hinder formation of anticyclonic circulation at the near-field part of the plume. This feature induces transport of freshwater discharge off the river mouth and its accumulation in the far-field part of the small plume under moderate wind forcing conditions that is not typical for large river plumes. The obtained results are important for understanding general aspects of dynamics of buoyant plumes formed by rivers with small discharge rates.

Comparative oceanographic eddy variability during climate change in the Agulhas Current and Somali Coastal Current Large Marine Ecosystems

Twenty years (1993–2012) of satellite altimetry dataset, through maps of absolute dynamic topography and their derived geostrophic velocities, as well as monthly climatological maps of windstress and windstress-curl have been used to assess the oceanic variability in the western Indian Ocean region (WIO). We have performed a comparative study between the Agulhas Current Large Marine Ecosystem (ACLME) and the Somali Coastal Current Large Marine Ecosystem (SCCLME) during the current time period when significant levels of climate change related variability have been documented. In the WIO region, climate variability, and/or change are impacting the atmospheric and oceanic circulation, e.g: excessive sea surface warming and consequent air-sea heat flux exchanges in the SCCLME; and broadening and weakening of the Agulhas Current, with potential negative implication on Agulhas leakage, in the ACLME. The latter being caused by an acceleration of the mesoscale eddy field in response to changing atmospheric wind forcing conditions. Therefore, eddies need to be properly quantified, characterized and monitored if their contribution on climate is to be assessed. While four hotspot regions of enhanced mesoscale variability have been identified in the combined Agulhas Somali Currents Large Marine Ecosystem (ASCLME), the main focus of this study has been in the ACLME and in the SCCLME. The results suggest that the levels of variability in these two LMEs are controlled by different physical processes: seasonality appears to be important in the SCCLME, but conversely less important in the Agulhas LME. The census conducted on the eddy field revealed that more eddies have been formed in the ACLME than in Somali Coastal Current. The eddy mean properties have shown that the eddies in the ACLME are smaller in diameter, and higher in amplitude than those in the SCCLME. In addition, the eddies in the ACLME are maintained as coherent structures longer; have been more strongly nonlinear; and have propagated longer distances from their origins than those in the SCCLME. These indicate that the eddies in the ACLME have relatively much stronger implication on the overall retention and transport of hydrographic properties (e.g. heat and salt). Therefore, they could have stronger influence on oceanic lateral fluxes and ocean-atmosphere flux exchanges, thus may influence more strongly the environmental conditions (both in the ocean and atmosphere) in the combined ASCLME, than those in the SCCLME. Time-evolution of the eddies mean properties suggests that inter-annual changes, may have influenced the eddy field differently in the ACLME and the SCCLME: moderate correlation r = 0.6, p = 0.05 has been found between time and eddy mean radius in the ACLME, and also r = 0.6, p = 0.05 was found between time and EKE in the SCCLME. Therefore, eddy size (for the Agulhas LME) and energy content (for Somali LME) could serve as potential indicators of change in oceanographic conditions over time in these LMEs.

Spatial evolution of young wind waves: numerical modelling verified by experiments

Abstract

Response of Total and Eddy Kinetic Energy to the Recent Spinup of the Beaufort Gyre

AbstractThe Beaufort Gyre in the Arctic Ocean has spun up over the past two decades in response to changes of the wind forcing and sea ice conditions, accumulating a significant amount of freshwater. Here a simulation performed with a high-resolution, eddy-resolving model is analyzed in order to provide a detailed description of the total and eddy kinetic energy and their response to this spinup of the gyre. On average, and in contrast to the typical open ocean conditions, the levels of mean and eddy kinetic energy are of the same order of magnitude, and the eddy kinetic energy is only intensified along the boundary and in the subsurface. In response to the strong anomalous atmospheric conditions in 2007, the gyre spins up and the mean kinetic energy almost doubles, while the eddy kinetic energy does not increase significantly for a long time period. This is because the isopycnals are able to flatten and the gyre expands outwards, reducing the potential for baroclinic instability. These results have implications for understanding the mechanisms at play for equilibrating the Beaufort Gyre and the variability and future changes of the Arctic freshwater system.

Water exchange between the Sea of Azov and the Black Sea through the Kerch Strait

Abstract. The Sea of Azov is a small, shallow, and freshened sea that receives a large freshwater discharge. Under certain external forcing conditions low-salinity waters from the Sea of Azov flow into the north-eastern part of the Black Sea through the narrow Kerch Strait and form a surface-advected buoyant plume. Water flow in the Kerch Strait also regularly occurs in the opposite direction, which results in the spreading of a bottom-advected plume of saline and dense waters from the Black Sea into the Sea of Azov. In this study we focus on the physical mechanisms that govern water exchange through the Kerch Strait and analyse the dependence of its direction and intensity on external forcing conditions. Analysis of satellite imagery, wind data, and numerical modelling shows that water exchange in the Kerch Strait is governed by a wind-induced barotropic pressure gradient. Water flow through the shallow and narrow Kerch Strait is a one-way process for the majority of the time. Outflow from the Sea of Azov to the Black Sea is induced by moderate and strong north-easterly winds, while flow into the Sea of Azov from the Black Sea occurs during wind relaxation periods. The direction and intensity of water exchange have wind-governed synoptic and seasonal variability, and they do not depend on the rate of river discharge to the Sea of Azov on an intra-annual timescale. The analysed data reveal dependencies between wind forcing conditions and spatial characteristics of the buoyant plume formed by the outflow from the Sea of Azov.

The Impact of Nonbreaking Waves on Wind‐Driven Ocean Surface Turbulence

AbstractA comprehensive study on the properties of upper ocean turbulence and its response to a variety of wind and wave forcing conditions was conducted at the Surge Structure Atmosphere INteraction (SUSTAIN) facility, University of Miami, RSMAS. The dimensions of SUSTAIN wind‐wave‐current tunnel are 18 m long, 6 m wide, and 2 m high, uniquely allowing for freely forming turbulence unconstrained by side walls. The grid of input parameters covered a range of wave steepness (ak: 0–0.27) within each investigated wind speed (U10: 2–20 m/s), allowing for the isolation of wave effects on the turbulence driven by both wind and waves. The response of turbulence to each pair of wind‐wave parameters was measured in steady‐state conditions (after 20+ min of settling time) by a suite of nonintrusive turbulence visualization techniques, located at 10 m fetch. These included 3‐D visualization of water tracing dye entrainment by turbulence, underwater particle image velocimetry, and passive and active thermal imagery resolving surface skin temperature and velocity fluctuations. The resulting data set includes both qualitative 3‐D view of subsurface turbulent structures and precise quantitative turbulent kinetic energy (TKE) measurements mapped out in response to the grid of input parameters. In lower winds, TKE was found to grow substantially in response to increasing wave steepness, in line with the expected effect of increasing wave forcing. However, in higher winds the effect of increasing wave steepness on TKE was found to be negative. It is hypothesized to be due to the rise of airflow separation, effectively sheltering much of the water surface from turbulence production by wind friction.

Impact of wind-driven circulation on contaminant dispersion in a semi-enclosed bay

Mediterranean semi-enclosed bays are often exposed to high levels of contaminants originating from anthropogenic activities in the bay. To assess their fate and impact on the environment, it is essential to investigate coastal circulation regimes which may play an important role in the dispersion of contaminants across the bay and beyond. In this study, a high resolution coupled hydrodynamic - passive tracer model was combined with ADCP observations, to identify major circulation patterns and associated dissolved contaminant dispersion pathways in the contaminated semi-enclosed bay of Toulon (South of France, NW Mediterranean Sea). Two dominant circulation patterns and two derived ones could be identified, driven by winds (Mistral and easterly winds) and offshore water intrusions. Medium to strong Mistral events (> 6 m s−1) with a WNW direction cause a bi-layer pattern with surface waters flowing out of the bay and marine waters entering at depth. Less frequently, west Mistral winds of medium to strong strength (> 6 m s−1) may generate an anticyclonic circulation. During easterly wind conditions (> 6 m s−1), an inward flow can be observed which is sometimes reinforced by offshore water intrusions, probably from the local boundary current, the Northern Current (NC). Furthermore, dissolved contaminant dispersion pathways were simulated under typical wind forcing conditions with three point sources of copper (Cu) that were identified based on surface Cu observations. While most of the WNW Mistral wind events transport dissolved copper plumes across and out of the bay, contaminant dispersion can remain confined to the bay under certain west mistral conditions. Conversely, during easterly wind events, contaminants are exiting the bay as a narrow vein along the Saint-Mandrier peninsula, before probably converging on the NC offshore. Accordingly, this study demonstrates the important impact of hydrodynamic-driven processes on the dispersion of contaminants within a semi-enclosed bay.