High-resolution atmospheric forcing for regional oceanic model: the Iroise Sea

This study was aimed at modeling, as realistically as possible, the dynamics and thermodynamics of the Iroise Sea by using the Model for Applications at Regional Scale (MARS), a regional ocean 3D model. The horizontal resolution of the configuration in use is 2 km with 30 vertical levels. The 3D model of the Iroise Sea is embedded in a larger model providing open boundary conditions. As regards the atmospheric forcing, the originality of this study is to force the regional ocean model with the high-resolution (6 km) regional meteorological model, Weather Research and Forecasting (WRF). In addition, as the air surface temperature is highly sensitive to the sea surface temperature (SST), this regional meteorological model is improved by taking into account a regional climatologic SST to compute meteorological parameters. By allowing a better coherence between the SST and the temperature of the atmospheric boundary layer while giving a more realistic representation of heat fluxes exchanged at the air/sea interface, this forcing constitutes a noticeable improvement of the Iroise Sea modeling. The different sensitivity tests discussed here pinpoint the importance of entering, in WRF, SST data of sufficiently high quality before the computation of meteorological forcing when the aim is a study of dynamics and thermodynamics far away from the coast. On the other hand, when the target is the reproduction of coastal small-scale features in Iroise Sea modeling, the resolution of the meteorological forcing and the quality of SST are both paramount. The simulation of reference was carried out throughout the Summer and Autumn of year 2005 to allow comparisons with a campaign of surface current measurements by high-frequency radars conducted at the same period.     

Application of effective medium approximation theory to ocean remote sensing under wave breaking

Based on the effective medium approximation theory of composites, the empirical model proposed by Pandey and Kakar is remedied to investigate the microwave emissivity of sea surface under wave breaking driven by strong wind. In the improved model, the effects of seawater bubbles, droplets and difference in temperature of air and sea interface (DTAS) on the emissivity of sea surface covered by whitecaps are discussed. The model results indicate that the effective emissivity of sea surface in-creases with DTAS increasing, and the impacts of bubble structures and thickness of whitecaps layer on the emissivity are included in the model by introducing the effective dielectric constant of whitecaps layer. Moreover, a good agreement is obtained by comparing the model results with the Rose’s ex-perimental data.     

Sea breezes drive currents on the inner continental shelf off southwest Western Australia

In southwest Western Australia, strong and persistent sea breezes are common between September and February. We hypothesized that on the inner continental shelf, in the absence of tidal forcing, the depth, magnitude, and lag times of the current speed and direction responses to sea breezes would vary though the water column as a function of the sea breeze intensity. To test this hypothesis, field data were used from four sites were that were in water depths of up to 13 m. Sites were located on the inner continental shelf and were on the open coast and in a semi-enclosed coastal embayment. The dominant spectral peak in currents at all sites indicated that the majority of the spectral energy contained in the currents was due to forcing by sea breezes. Currents were aligned with the local orientation of the shoreline. On a daily basis, the sea breezes resulted in increased current speeds and also changed the current directions through the water column. The correlation between wind–current speeds and directions with depth, and the lag time between the onset of the sea breeze and the response of currents, were dependent on the intensity of the sea breezes. A higher correlation between wind and current speeds occurred during strong sea breezes and was associated with shorter lag times for the response of the bottom currents. The lag times were validated with estimates of the vertical eddy viscosity. Solar heating caused the water column to stratify in summer and the sea breezes overcame this stratification. Sea breezes caused the mixed layer to deepen and the intensity of the stratification was correlated to the strength of the sea breezes. Weak sea breezes of <5 m s⁻¹ were associated with the strongest thermal stratification of the water column, up to 1°C between the surface and bottom layers (6 and 10 m below the surface). In comparison, strong sea breezes of >14 m s⁻¹ caused only slight thermal stratification up to 0.5°C. Apart from these effects on the vertical structure of water column, the sea breezes also influenced transport and mixing in the horizontal dimension. The sea breezes in southwest Western Australia rotated in an anticlockwise direction each day and this rotation was translated into the currents. This current rotation was more prominent in surface currents and in the coastal embayment compared to the open coast.     

Diurnal sea breeze effects on inner-shelf cross-shore exchange

Cross-shore exchange by strong (cross-shore wind stress, τ_sx>0.05 Pa) diurnal (7–25 h) sea breeze events are investigated using two years of continuous wind, wave, and ocean velocity profiles in 13 m water depth on the inner-shelf in Marina, Monterey bay, California. The diurnal surface wind stress, waves, and currents have spectral peaks at 1, 2, and 3 cpd and the diurnal variability represents about 50% of the total variability. During sea breeze relaxation (−0.05<τ_sx<0.05 Pa), a background wave-driven inner-shelf Eulerian undertow profile exists, which is equal and opposite to the Lagrangian Stokes drift profile, resulting in a net zero Lagrangian transport at depth. In the presence of a sea breeze (τ_sx>0.05 Pa), a uniform offshore profile develops that is different from the background undertow profile allowing cross-shore Lagrangian transport to develop, while including Lagrangian Stokes drift. The diurnal cross-shore current response is similar to subtidal (>25 h) cross-shore current response, as found by Fewings et al. (2008). The seasonality of waves and winds modify the diurnal sea breeze impact. It is suggested that material is not transported cross-shore except during sea breeze events owing to near zero transport during relaxation periods. During sea breeze events, cross-shore exchange of material appears to occur onshore near the surface and offshore near the sea bed. Since sea breeze events last for a few hours, the long-term cross-shore transport is incremental each day.     

Sea surface temperature modelling in the Sea of Iroise: assessment of boundary conditions

The present study investigates the sensitivity of the COupled Hydrodynamical–Ecological model for REgioNal and Shelf seas (COHERENS) to predict sea surface temperature (SST) patterns in the Sea of Iroise (western end of French Brittany) in relation to the spatial and temporal resolutions of open boundary conditions (OBCs). Two sources of daily operational OBCs of temperature are considered, derived from (1) the Mercator Global Ocean and (2) the Iberian Biscay Irish analysis and forecasting systems delivering predictions at spatial resolutions of 1/12° and 1/36°, respectively. Coastal model performance is evaluated by comparing SST predictions with recently available field data collected (1) along the route of a vessel travelling between the coast and the isle of Ushant and (2) at two offshore stations. The comparison is extended to SST spatial distribution derived from remote-sensing observations. The influence of OBC spatial resolution is exhibited in the north-eastern area of the Sea of Iroise in relation to the intrusion of cold surface waters. OBC temporal resolution is found to have a lower impact advocating for the implementation of climatological temperature forcings to predict major SST patterns in the Sea of Iroise.     

Tidal-driven dynamics and mixing processes in a coastal ocean model with wetting and drying

A three-dimensional sigma coordinate numerical model with wetting and drying (WAD) and a Mellor–Yamada turbulence closure scheme has been used in an idealized island configuration to evaluate how tidally driven dynamics and mixing are affected by inundation processes. Comprehensive sensitivity experiments evaluate the influence of various factors, including tidal amplitudes (from 1- to 9-m range), model grid size (from 2 to 16 km), stratification, wind, rotation, and the impact of WAD on the mixing. The dynamics of the system involves tidally driven basin-scale waves (propagating anticlockwise in the northern hemisphere) and coastally trapped waves propagating around the island in an opposite direction. The evolutions of the surface mixed layer (SML) and the bottom boundary layer (BBL) under different forcing have been studied. With small amplitude tides, wind-driven mixing dominates and the thickness of the SML increases with time, while with large-amplitude tides, tidal mixing dominates and the thickness of the BBL increases with time. The inclusion of WAD in the simulations increases bottom stress and impacts the velocities, the coastal waves, and the mixing. However, the impact of WAD is complex and non-linear. For example, WAD reduces near-coast currents during flood but increases currents during ebb as water drains from the island back to the sea. The impacts of WAD, forcing, and model parameters on the dynamics are summarized by an analysis of the vorticity balance for the different sensitivity experiments.     

Shallow seismicity, triggered seismicity, and ambient noise tomography at the long-dormant Uturuncu Volcano, Bolivia

Using a network of 15 seismometers around the inflating Uturuncu Volcano from April 2009 to 2010, we find an average rate of about three local volcano-tectonic earthquakes per day, and swarms of 5–60 events a few times per month with local magnitudes ranging from −1.2 to 3.7. The earthquake depths are near sea level, more than 10 km above the geodetically inferred inflation source and the Altiplano Puna Magma Body. The Mw 8.8 Maule earthquake on 27 February 2010 triggered hundreds of earthquakes at Uturuncu with the onset of the Love and Rayleigh waves and again with the passage of the X2/X3 overtone phases of Rayleigh waves. This is one of the first incidences in which triggering has been observed from multiple surface wave trains. The earthquakes are oriented NW–SE similar to the regional faults and lineaments. The b value of the catalog is 0.49, consistent with a tectonic origin of the earthquakes. We perform ambient noise tomography using Love wave cross-correlations to image a low-velocity zone at 1.9 to 3.9 km depth below the surface centered slightly north of the summit. The low velocities are perhaps related to the hydrothermal system and the low-velocity zone is spatially correlated with earthquake locations. The earthquake rate appears to vary with time—a seismic deployment from 1996 to 1997 reveals 1–5 earthquakes per day, whereas 60 events/day were seen during 5 days using one seismometer in 2003. However, differences in analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates. The rate of seismic activity at Uturuncu is higher than at other well-monitored inflating volcanoes during periods of repose. The frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable.     

Sand wave migration near the southeastern corner of Martha's Vineyard,Massachusetts, USA

Sand waves of approximately 2 m in height were observed to migrate nearly 40 m with counterclockwise rotation between two bathymetric surveys performed three months apart near the southeastern corner of Martha's Vineyard, Massachusetts. The region is characterized by strong tidal currents, intermittent energetic surface wave events, and shallow water with local depth ranging from 2 to 7 m. This study uses the process-based model, Delft3D, with a three-dimensional approach to examine the sand wave dynamics by incorporating surface waves, winds, currents, and bathymetric observations. The model successfully simulates sand wave migration in comparisons to observations. Model sensitivity analyses show that the sand wave migration reduces by 65% with the absence of the surface waves. The modeled sand wave migration speed is correlated with the tidal current Shields parameter, and sharp increases in migration speed occur when the wave-driven Shields parameter increases in response to energetic surface wave events. The combined effect of tides, surface waves, and bathymetry is the origin of the rotational aspect of the sand wave, using the Shields parameter as an indicator of tidal currents and surface wave influence on sand wave dynamics.     

Application of a sigma coordinate sea model to the calculation of wind-induced currents

A three-dimensional numerical sea model is formulated in terms of sigma coordinates in the vertical. The vertical grid spacing in the model is arbitrary and can be refined to give enhanced resolution in high shear regions (e.g., close to the sea surface in wind-driven flows, and/or across the thermocline in stratified flows). A method of accurately determining surface currents and indicating how fine a grid is required in the surface layer is described.The problem of determining a suitable formulation of vertical eddy viscosity to use in a model of wind-induced flow in a tidal sea is considered in detail. A formulation in which surface eddy viscosity depends upon the roughness of the sea surface and the transfer of momentum to depth by surface waves appears reasonable. Below the surface layer turbulence is related to the current at depth.Idealized calculations are performed to demonstrate the accuracy and stability of the sigma coordinate model. Results of these calculations indicate that the formulation of eddy viscosity developed in this paper can explain the high surface shears reported in lake measurements of wind-induced surface currents, and the lack of shear under strong wind conditions in the open sea (GORDON, 1982, Journal of Geophysical Research, 87, 1939–1951).Surface current to surface wind ratio are also computed.     

Statistical model on the surface elevation of waves with breaking

In the surface wind drift layer with constant momentum flux, two sets of the consistent surface eleva- tion expressions with breaking and occurrence conditions for breaking are deduced from the first in- tegrals of the energy and vortex variations and the kinetic and mathematic breaking criterions, then the expression of the surface elevation with wave breaking is established by using the Heaviside function. On the basis of the form of the sea surface elevation with wave breaking and the understanding of small slope sea waves, a triple composite function of real sea waves is presented including the func- tions for the breaking, weak-nonlinear and basic waves. The expression of the triple composite func- tion and the normal distribution of basic waves are the expected theoretical model for surface elevation statistics.     

Mixing processes in lakes: mechanisms and ecological relevance

In Lake Baldegg, Switzerland (surface area 5.3 km², maximum depth 66 m) the analysis of data from moored instrument systems (atmospheric boundary layer, lake temperature distribution, bottom currents) was correlated to the long-term development of vertical mixing as seen from profiles of natural isotopes (radon-222, tritium and helium-3) and chemical species. The investigation shows: 1. Vertical mixing coefficients below 25 m are small. Consequently the vertical concentration distribution of sediment emanating species in the deep hypolimnion is controlled by the bottom topography. 2. Renewal of deep hypolimnic water is significant even during stratification. 3. Weakly damped internal waves characterize the internal dynamics during stratification. 4. Horizontal bottom currents play an important role in the hypolimnion mixing and can be correlated to internal waves during stratification.     

Wave-induced stress and estimation of its driven effect on currents

Since the 1940s, the significant progress has beenmade both in theory and in numeric study for oceanwaves. Beginning from the theory of generalized windwave spectra proposed by Wen[1], some distinctiveachievements have been obtained in China. A numberof original academic papers, for instance, were pub-lished in the late decades[2-10]. However, the study oflarge-scale effect of ocean waves has not been noticed,except for the introduction of information entropy ofocean wave height field to desc…     

Ship-induced effects on bottom-mounted acoustic current meters in shallow seas

The echo-amplitude of a 23-m-deep bottom-mounted acoustic Doppler current profiler (ADCP) shows regular spikes up to 30 dB above background level when a ship passes nearby, due to deep penetration of bubble clouds. This is evidenced from regularly occurring spikes in echo-data that are simultaneous with ferry crossings in a narrow sea-strait. The bubbles can nearly reach the bottom and are comparable in magnitude to near-bottom scattering off suspended material in vigorous tidal currents exceeding 1 m s⁻¹ in magnitude. The bubble clouds mask the sea surface from the echo-amplitude, which hampers the use of an ADCP for estimating atmospheric parameters and near-surface currents, under such conditions. The echo-spikes associated with the ferry are confirmed with coinciding dips in bottom pressure up to 1200 N m⁻² and with deviations up to 10° in the ADCP's heading due to pressure waves and magnetic field disturbances from under the ferry and from its rear, respectively.     

Two-layer Ekman model for wind-induced shear flow near the sea surface

A simple two-layered model for steady wind-induced shear flow near the sea surface has been formulated. Basic assumptions of Ekman's theory are retained, including horizontal uniformity, infinite depth and constant (but differing) vertical eddy viscosities in the respective layers. Employing information coming from observational data, the parameters of the model (depth of the surface layer and the two eddy viscosities) are evaluated and optimized. The results thereby obtained favour the presence of a high shear layer, about 1 m thick, immediately below the sea surface. The eddy viscosity in that layer increases approximately linearly with wind speed but remains comparatively small, being generally less than 20 cm² s⁻¹ for wind speeds less than 20 m s⁻¹. In contrast, the eddy viscosity directly below the layer is two orders of magnitude larger and increases more steeply with wind speed.     

Submesoscale features and their interaction with fronts and internal tides in a high-resolution coupled atmosphere-ocean-wave model of the Bay of Bengal

Large freshwater fluxes into the Bay of Bengal by rainfall and river discharges result in strong salinity fronts in the bay. In this study, a high-resolution coupled atmosphere-ocean-wave model with comprehensive physics is used to model the weather, ocean circulation, and wave field in the Bay of Bengal. Our objective is to explore the submesoscale activity that occurs in a realistic coupled model that resolves mesoscales and allows part of the submesoscale field. Horizontal resolution in the atmosphere varies from 2 to 6 km and is 13 km for surface waves, while the ocean model is submesoscale permitting with resolutions as high as 1.5 km and a vertical resolution of 0.5 m in the upper 10 m. In this paper, three different cases of oceanic submesoscale features are discussed. In the first case, heavy rainfall and intense downdrafts produced by atmospheric convection are found to force submesoscale currents, temperature, and salinity anomalies in the oceanic mixed layer and impact the mesoscale flow. In a second case, strong solitary-like waves are generated by semidiurnal tides in the Andaman Sea and interact with mesoscale flows and fronts and affect submesoscale features generated along fronts. A third source of submesoscale variability is found further north in the Bay of Bengal where river outflows help maintain strong salinity gradients throughout the year. For that case, a comparison with satellite observations of sea surface height anomalies, sea surface temperature, and chlorophyll shows that the model captures the observed mesoscale eddy features of the flow field, but in addition, submesoscale upwelling and downwelling patterns associated with ageostrophic secondary circulations along density fronts are also captured by the model.     

Large internal waves in Massachusetts Bay transport sediments offshore

A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10 min and wavelengths of 200–400 m, and caused downward excursions of the thermocline of as much as 30 m. At the 85-m site, the current measured 1 m above bottom (mab) typically increased from near 0 to 0.2 m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6 mab increased from about 0.1 to 0.4–0.6 m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2 h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves.     

Variations of vertical velocity in the deep oceans simulated by a 1/10° OGCM

This paper analyzes variations of vertical velocity w simulated by the 1/10° Ocean General Circulation Model for the Earth Simulator (OFES). Strong w-variability is found in the deep oceans. When w is WKBJ-normalized, the standard deviation averaged over the Southern Ocean increases with depth and is larger than 8 × 10^− 3 cm/s throughout the water column below 1,500 m. Evidences are presented that link this w-variability to internal waves generated by quasi-steady currents over topography. The aliasing errors in lag-3-day correlations suggest a bottom generation of near-inertial waves. A scale analysis indicates that vertically propagating waves that can be resolved by the OFES model are waves with frequencies of the order of inertial frequency and wavelengths comparable to the order of the grid size. The vertical energy flux associated with these waves is substantial. When integrated globally, the vertical energy flux is upward in the upper 4 km and reaches maximum values of about 0.8 TW at about 1 to 2 km depth. Thus, the w-variability in the 1/10° OFES integration points not only to a strong bottom generation of near-inertial internal waves in the deep Southern Ocean but also to the possibility that the power provided by internal waves generated by non-tidal currents over topography can be comparable to the power provided by internal waves generated by tidal flows over topography.     

Numerical study of surface water circulation around Sekisei Lagoon, southwest Japan

This paper discusses the variability of surface currents around Sekisei Lagoon using a nested grid ocean circulation model. We developed a triple-nested grid system that consists of a coarse-resolution (1/60° or ∼1.85 km) model off Taiwan, an intermediate-resolution (1/300° or ∼370 m) model around the Yaeyama Islands, and a fine-resolution (1/900° or ∼123 m) model of Sekisei Lagoon. The nested grid system was forced by wind and heat flux calculated from six-hourly atmospheric reanalysis data and integrated over the period from May to July 2003. The coarse-resolution model was driven by lateral boundary conditions calculated from daily ocean reanalysis data to include realistic variation of the Kuroshio and mesoscale eddies with spatial scales of ∼500–700 km in the open ocean. The tidal forcing was included in the intermediate-resolution model by interpolating sea level data obtained from a data-assimilative tidal model. The results were then used to drive the fine-resolution model to simulate the surface water circulation around Sekisei lagoon. Model results show that (1) currents inside the lagoon are mainly driven by tide and wind; (2) there exists a strong southwestward current along the bottom slope in the southeast portion of the lagoon; the current is mainly driven by remote mesoscale eddies and at times intensified by the local wind; (3) the flow relaxation scheme is effective in reducing biases along the open boundaries. The simulated currents were used to examine the retention and dispersion of passive particles in the surface layer. Results show that the surface dispersion in the strong open ocean current region is significantly higher than that inside the lagoon.     

Note on Coriolis-Stokes force and energy

In this study, we consider the origin of the Coriolis-Stokes (CS) force in the wave-averaged momentum and energy equations and make a short analysis of possible energy input to the ocean circulation (i.e., Eulerian mean velocity) from the CS force. Essentially, we find that the CS force appears naturally when considering vertically integrated quantities and that the CS force will not provide any energy input into the system for this case. However, by including the “Hasselmann force”, we show some inconsistencies regarding the vertical structure of the CS force in the Eulerian framework and find that there is a distinct vertical structure of the energy input and that the net input strongly depends on whether the wave zone is included in the analysis or not. We therefore question the introduction of the “Hasselmann force” into the system of equations, as the CS force appears naturally in the vertically integrated equations or when Lagrangian vertical coordinates are used.     

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.