Strait Flows Research Articles

Interannual Variability and Trend of the Bering Strait Throughflow Gleaned from Satellite Altimeters

Abstract Beginning with the TOPEX/Poseidon altimetry mission in 1992, along-track satellite sea surface height (SSH) estimates have been taken every 6-7 km at 10-day intervals at a cluster of satellite tracks across the Bering Strait near 66°N where the satellite tracks turn. Year-to-year geostrophic SSH estimates of the Bering Strait flow show that the flow in the warmer months is almost entirely confined to a 50m depth channel rather than the whole cross-sectional area, and that the peak-to-peak transport change is comparable to the approximate 0.8 Sverdrup mean. Energy flux calculations using the high-resolution SSH across Bering Strait, as well as an analysis of historical coastal Russian sea levels and wind stress along the Russian Arctic shelf and Bering Sea shelves, suggest that the interannual Bering Strait transport in the warmer water months can be described using wind-forced Arrested Topographic Waves (ATWs) driven by along-shelf wind stress on both the Russian Arctic and Bering Sea shelves. Most of the Bering Strait transport is barotropic in the 50m-depth channel, but analysis of SSH and in situ Alaskan coastal sea level shows that interannual transport variations in the narrow baroclinic Alaskan Coastal Current are not negligible. In disagreement with previous transport estimates, the satellite SSH analysis and almost all of the in situ current measurements do not indicate a significant long-term trend in barotropic Bering Strait northward transport in the warmer water late-summer/fall months.

Spatial variability of water mass transports in the Bransfield Strait based on direct current measurements

The Bransfield Strait connects the Bellingshausen and Weddell seas and is strongly affected by intense inflows of their water masses. In this work we analyzed Shipborne Acoustic Current Doppler Profiler (SADCP) measurements carried out in the Bransfield Strait from 2015 to 2022. The new dataset includes over 100 transects, crossing the main currents in the strait (namely, the Bransfield Current and the Transitional Weddell Water (TWW) flow). Based on these data, we studied spatial variability of water mass transports in the Bransfield Strait during austral summer season. As shown below in this research, the Bransfield Current was observed down to the 600 m depth. By considering deeper layers, we managed obtaining more precise assessments of mean water mass transports of the Bransfield Current. They were estimated at 1.4–2.1 Sv. We found that the transports of the Bransfield Current and TWW flow in the austral summer season (November–March) do not show significant variability. The water mass transport of the Bransfield Current and the TWW flow was stable within the central basin of the Bransfield Strait and differed in its western basin. We also found that in austral summer the heat transport into the Bransfield Strait varied between 0.2 and 0.4 Sv·°C and salinity transport was −0.07 to −0.06 Sv·psu, their sign reflecting a dominant northeastward flow of relatively warm and fresh water. In addition, we compared Lowered Acoustic Current Doppler Profiler (LADCP) and SADCP measurements and proved that the technical limitations of the SADCP do not significantly affect the estimates of the transports in this region.

Arctic and Sub‐Arctic Mechanisms Explaining Observed Increasing Northward Flow Through the Bering Strait and Why Models May Be Getting It Wrong

AbstractThe Pacific oceanic input to the Arctic via the Bering Strait (important for western Arctic ice retreat, water properties, and nutrient supply) has been increasing for three decades. Using satellite Ocean Bottom Pressure (OBP) and Dynamic Ocean Topography (DOT) data, we show that long‐term trends in mooring data for a well‐sampled sub‐period (2003–2014) relate to summer OBP and DOT drop in the Arctic's East Siberian Sea (ESS), in turn caused by stronger westward ESS winds, and increased fall westward winds in the Bering Sea. OBP/DOT differences imply strong (0.17 psu/year) ESS salinization, likely caused by hitherto unappreciated increased Pacific inflow to that region. We find ESS OBP trends are (erroneously) reversed in older data versions, and estimate that ESS salinization may significantly mediate Bering Strait flow increase. These facts may explain why models assimilating older OBP data, or with erroneous Bering Strait salinities, fail to simulate observed Bering Strait flow increase.

Modelling secondary circulations in stratified bi-directional flows: A potential mechanism for flux transfer from lower to upper fluid layers

Results are presented for new laboratory-scale numerical simulations of bi-directional stratified flows generated within an idealized trapezoidal sill-channel topography, focussing on the lateral (or secondary) flow structure across the channel both in non-rotating and rotating frames of reference. The simulations utilise the non-hydrostatic Bergen Ocean Model (BOM), a three-dimensional general ocean circulation model. The results from the BOM simulations are verified by large-scale experimental data obtained in the LEGI Coriolis rotating platform in Grenoble, within which velocity and density fields for bi-directional stratified flows were measured through particle image velocimetry (PIV) and micro-conductivity density probes, respectively. The BOM simulations reproduce the main dynamic flow patterns and structure of the large-scale exchange flows generated through the trapezoidal sill-channel, with the lower layer saline intrusion flux shown to reduce, due to partial blockage, as the upper freshwater flow is increased (i.e. net barotropic forcing). Non-rotational BOM simulations show upward (and downward) flow along the centreline of the trapezoidal sill-channel in the upper (and lower) layers, with symmetrical counter-rotating circulation cells forming both above and below the density interface. By contrast, equivalent BOM simulations within a rotating frame of reference show the secondary flow circulation in the lower dense water layer to be dominated by Ekman dynamics, while two co-rotating cells are formed in the upper freshwater layer above the inclined density interface. In these rotational cases, significant upward and downward velocities (i.e. strong upwelling and downwelling) are generated adjacent to the inclined sill-channel side walls (in comparison to non-rotating cases). This secondary flow effect may be especially relevant for exchange flows in estuaries, fjords, deep-ocean channels and sea straits, where the channel width exceeds the Rossby radius of deformation and, thus, Earth rotation effects become important dynamically. In particular, any potential vertical transport pathway associated with this upwelling and downwelling at the inclined channel side slopes may have strong managerial implications in coastal areas, whereby tracers (e.g. contaminants, salinity, reduced DO waters, fine sediments) can be transferred from deep waters to the near surface.

Assessment of Indonesian Throughflow transports from ocean reanalyses with mooring-based observations

Abstract. The transport of heat and freshwater from the Pacific Ocean to the Indian Ocean via the Indonesian seas is commonly referred to as the Indonesian Throughflow (ITF). The interaction between the ITF and large-scale phenomena occurring from intraseasonal to decadal timescales reflects its connection to the global ocean and the climate system, indicating the need for monitoring the ITF region. In situ observations in this region are highly valuable, but they are temporally and spatially insufficient for near-real-time monitoring. Ocean reanalyses have the potential to serve as near-real-time monitoring tools and to extend time series backward in time, yet a comprehensive quality assessment of their realism in this region with challenging bathymetry has been lacking so far. We focus on oceanic transports diagnosed from the Copernicus Marine Service (CMEMS) Global Reanalysis Ensemble Product (GREP) and the higher-resolution product GLORYS12V1, totaling six reanalysis products. They are validated against in situ observations taken from two different monitoring programs, namely International Nusantara Stratification and Transport (INSTANT 2004–2006) and Monitoring the Indonesian Throughflow (MITF 2006–2011 and 2013–2017), resulting in a total time series of about 11.5 years in the major inflow passage of the Makassar Strait and shorter sampled time series in the Lombok Strait, the Ombai Strait, and the Timor Passage. Results show that there is reasonable agreement between reanalysis-based transports and observations in terms of means, seasonal cycles, and variability, although some shortcomings stand out. The lower-resolution products do not represent the spatial structure of the flow accurately. They also tend to underestimate the integrated net flow in the narrower straits of Lombok and Ombai, an aspect that is improved in GLORYS12V1. Reanalyses tend to underestimate the effect of seasonal Kelvin waves on the transports, which leads to errors in the mean seasonal cycle. Interannual variations of reanalyzed transports agree well with observations, but uncertainties are much larger on sub-annual variability. Finally, as an application of physically consistent and observationally constrained fields provided by ocean reanalyses, we study the impact of the vertically varying pressure gradient on the vertical structure of the ITF to better understand an apparent two-layer regime of the flow.

Marine life and fisheries around offshore oil and gas structures in southeastern Australia and possible consequences for decommissioning

The Gippsland Basin is the location of Australia’s oldest offshore oil and gas (O&G) structures, with hydrocarbon production beginning in the 1960s. The Bass Strait flows over this area with fisheries providing seafood for the major population centers of Melbourne, Sydney and beyond. Since Australia’s maritime legislation restricts activities to outside of 500 meters from O&G structures as a security exclusion zone, these O&G structures may serve as de facto marine protected areas that may have spillover effects to local fisheries. Therefore, it is critical to understand the habitat value of O&G infrastructure to marine life in the Bass Strait and whether decommissioning of these structures affect local marine ecosystems and fisheries. We analyzed industry-collected remotely operated vehicle (ROV) imagery from 2008-2018 and compared this data with reported catch data from fishing vessels operating in this region collected by the Australian Fisheries Management Authority (AFMA) from 2008-2018. We assessed species richness and relative abundance on two platforms and two pipelines and compared the species composition with retained catch reported by commercial fishers operating in Commonwealth fisheries. We found diverse communities of fishes and invertebrates around O&G structures, with a different subset of species inhabiting pipelines than platforms. We found little overlap between the species that were targeted by commercial fishers and those found around O&G structures (10% overlap), however, species composition data from fisheries often groups species making the data coarse and under-representative of true species diversity. Fishery-independent data from ROV imagery or other methods greatly augments our understanding of deepwater marine communities, including those around O&G structures. Combining data sources provides a holistic look at these novel ecosystems and provides better insight into future decommissioning scenarios.

Sea level variability in the Gulf of Mexico since 1900 and its link to the Yucatan Channel and the Florida Strait flows

Sea level variability in the Gulf of Mexico since 1900 and its link to the Yucatan Channel and the Florida Strait flows

(Paleo)oceanography of semi-enclosed seas with a focus on the Mediterranean region; Insights from basic theory

To provide context for the interpretation of their sedimentary and paleoceanographic record, semi-enclosed seas are here investigated through the application of basic theory. The principles of conservation of water, salt and heat, in combination with a representation of flow through the seaway to the ocean, are used to chart how basin geometry, connectivity and atmospheric forcing together control basin-averaged salinity and temperature and the exchange flux. Data on present-day semi-enclosed seas of the wider Mediterranean region are used for illustration. First, ignoring the heat balance and with forcing constant in time, a dimensionless form of the governing equations is derived which clarifies the role of the various controlling parameters. This is applied to aspects of the Messinian Salinity Crisis. In the second part of the analysis the forcing is made a generalised periodic function of time. This informs us how basin salinity, its amplitude of variation and lag relative to the forcing, depend on basin and strait properties and varies with the period of forcing. Insights are applied to the precessional variation observed in the record of the Mediterranean Sea. In the third and fourth part of the analysis we include the balance of heat and basin-averaged temperature. Examination of the budget equations allows us to derive a relationship between the average air-sea heat flux and basin restriction. Finally, re-introducing strait flow, we study the interplay of basin temperature and salinity and establish under which conditions heat flux and temperature play a role, in addition to net evaporation and salinity.

Wavefronts and modal structure of long surface and internal ring waves on a parallel shear current

We study long surface and internal ring waves propagating in a stratified fluid over a parallel shear current. The far-field modal and amplitude equations for the ring waves are presented in dimensional form. We re-derive the modal equations from the formulation for plane waves tangent to the ring wave, which opens a way to obtaining important characteristics of the ring waves (group speed, wave action conservation law) and to constructing more general ‘hybrid solutions’ consisting of a part of a ring wave and two tangent plane waves. The modal equations constitute a new spectral problem, and are analysed for a number of examples of surface ring waves in a homogeneous fluid and internal ring waves in a stratified fluid. Detailed analysis is developed for the case of a two-layered fluid with a linear shear current where we study their wavefronts and two-dimensional modal structure. Comparisons are made between the modal functions (i.e. eigenfunctions of the relevant spectral problems) for the surface waves in homogeneous and two-layered fluids, as well as the interfacial waves described exactly and in the rigid-lid approximation. We also analyse the wavefronts of surface and interfacial waves for a large family of power-law upper-layer currents, which can be used to model wind generated currents, river inflows and exchange flows in straits. Global and local measures of the deformation of wavefronts are introduced and evaluated.

Elucidating Large‐Scale Atmospheric Controls on Bering Strait Throughflow Variability Using a Data‐Constrained Ocean Model and Its Adjoint

AbstractA regional data‐constrained coupled ocean‐sea ice general circulation model and its adjoint are used to investigate mechanisms controlling the volume transport variability through Bering Strait during 2002 to 2013. Comprehensive time‐resolved sensitivity maps of Bering Strait transport to atmospheric forcing can be accurately computed with the adjoint along the forward model trajectory to identify spatial and temporal scales most relevant to the strait's transport variability. The simulated Bering Strait transport anomaly is found to be controlled primarily by the wind stress on short time scales of order 1 month. Spatial decomposition indicates that on monthly time scales winds over the Bering and the combined Chukchi and East Siberian Seas are the most significant drivers. Continental shelf waves and coastally trapped waves are suggested as the dominant mechanisms for propagating information from the far field to the strait. In years with transport extrema, eastward wind stress anomalies in the Arctic sector are found to be the dominant control, with correlation coefficient of 0.94. This implies that atmospheric variability over the Arctic plays a substantial role in determining Bering Strait flow variability. The near‐linear response of the transport anomaly to wind stress allows for predictive skill at interannual time scales, thus potentially enabling skillful prediction of changes at this important Pacific‐Arctic gateway, provided that accurate measurements of surface winds in the Arctic can be obtained. The novelty of this work is the use of space and time‐resolved adjoint‐based sensitivity maps, which enable detailed dynamical, that is, causal attribution of the impacts of different forcings.

Control of Bering Strait Transport by the Meridional Overturning Circulation

AbstractIt is well established that the mean transport through Bering Strait is balanced by a sea level difference between the North Pacific and the Arctic Ocean, but no mechanism has been proposed to explain this sea level difference. It is argued that the sea level difference across Bering Strait, which geostrophically balances the northward throughflow, is associated with the sea level difference between the North Pacific and the North Atlantic/Arctic. In turn, the latter difference is caused by deeper middepth isopycnals in the Indo-Pacific than in the Atlantic, especially in the northern high latitudes because there is deep water formation in the Atlantic, but not in the Pacific. Because the depth of the middepth isopycnals is associated with the dynamics of the upper branch of the meridional overturning circulation (MOC), a model is formulated that quantitatively relates the sea level difference between the North Pacific and the Arctic/North Atlantic with the wind stress in the Antarctic Circumpolar region, since this forcing powers the MOC, and with the outcropping isopycnals shared between the Northern Hemisphere and the Antarctic circumpolar region, since this controls the location of deep water formation. This implies that if the sinking associated with the MOC were to occur in the North Pacific, rather than the North Atlantic, then the Bering Strait flow would reverse. These predictions, formalized in a theoretical box model, are confirmed by a series of numerical experiments in a simplified geometry of the World Ocean, forced by steady surface wind stress, temperature, and freshwater flux.

Characteristics of the East Sea (Japan Sea) circulation depending on surface heat flux and its effect on branching of the Tsushima Warm Current

The effects of the surface heat flux and wind forcing on the East Sea (Japan Sea) circulation and the branching of the Tsushima Warm Current were investigated using a three-dimensional circulation model (RIAMOM). For the numerical experiments with surface heat flux, regardless of wind forcing, the Tsushima Warm Current, the Nearshore Branch, and the East Korea Warm Current were well simulated in the southern part of the East Sea. Furthermore, the cold water mass of 1–2 °C was formed at a depth of 200–250 m because of the convection of the upper layer and ventilation that actively occurs in the northern part of the East Sea. However, in the experiments without surface heat flux, the East Korea Warm Current was not simulated. The Tsushima Warm Current through the Korea/Tsushima Strait flows mostly northeastward along the Japanese coast as the Nearshore Branch. Moreover, the cold water mass was not formed at the northern part of the East Sea, thus the 1–2 °C cold water was distributed below 300–400 m depth with weak ventilation. The upper layer in the southern part of the East Sea, ranging from the surface to the bottom of the pycnocline, was thicker than the case of the surface heat flux forcing. Since the Tsushima Warm Current flowing through the Korea/Tsushima Strait is dominantly affected by the continental slope of the Japanese coast, the current does not lead to formation of the East Korea Warm Current as a western boundary current. If the effect of the continental slope of the Japanese coast is not applied, the Tsushima Warm Current flows only along the Korean coast as a western boundary current. The numerical experiments reveal that the surface heat flux serves as a critical external forcing which controls the topographic and planetary beta effects on the upper layer circulation in the southern part of the East Sea.Therefore, if the surface heat flux is changed due to global warming in the future, the distribution of surface currents is also expected to change in the southern part of the East Sea.

Impact of the water input from the eastern Qiongzhou Strait to the Beibu Gulf on Guangxi coastal circulation

Based on a comparison of synchronized temperature and salinity data collected in the eastern Qiongzhou Strait and at coastal marine stations, this study finds that, in summer, the variation in salinity near the Weizhou Island in Guangxi is similar to that in the eastern and central portions of the Qiongzhou Strait. Additionally, the Beihai Station in Guangxi exhibits a small salinity variation, whereas the Longmen and Bailongwei Stations, both of which are located far from the Qiongzhou Strait, mainly exhibit continental hydrological characteristics in summer. Moreover, a comparison of the multi-year ocean current data from the Qiongzhou Strait and ocean current observations from the Weizhou Island Station and recently installed current-measuring stations shows that the residual current in the Qiongzhou Strait flows westward in winter and summer. The numerical simulation results also indicate that water from the eastern Qiongzhou Strait enters the Beibu Gulf. The characteristics of the temperature and salinity distributions and analyses of the residual currents further confirm that the western Guangdong coastal current is the main source of the westward transport of water in the Qiongzhou Strait. The primary driver of the formation of the western Guangdong coastal current is the westward flow of freshwater from the Zhujiang (Pearl) River. This water enters the Beibu Gulf via the Qiongzhou Strait and enhances the formation of the cyclonic circulation in the northern Beibu Gulf. In summer, the strong influence of the southwesterly wind leads to the formation of a strong northward coastal current along the western coast of the Beibu Gulf. This process promotes the transport of low-salinity diluted water toward the open ocean and the formation of larger-scale cyclonic circulation around Weizhou Island in the eastern Beibu Gulf. The results of this study regarding the effects of the water inflow from the eastern Qiongzhou Strait to the Beibu Gulf on the Guangxi coastal circulation directly challenge conventional conclusions concerning the transport direction of water through the Qiongzhou Strait in winter and summer.

Rethinking Tasmania’s Regionality from an Antarctic Perspective: Flipping the Map

Tasmania hangs from the map of Australia like a drop in freefall from the substance of the mainland. Often the whole state is mislaid from Australian maps and logos (Reddit). Tasmania has, at least since federation, been considered peripheral—a region seen as isolated, a ‘problem’ economically, politically, and culturally. However, Tasmania not only cleaves to the ‘north island’ of Australia but is also subject to the gravitational pull of an even greater land mass—Antarctica. In this article, we upturn the political conventions of map-making that place both Antarctica and Tasmania in obscure positions at the base of the globe. We show how a changing global climate re-frames Antarctica and the Southern Ocean as key drivers of worldwide environmental shifts. The liquid and solid water between Tasmania and Antarctica is revealed not as a homogenous barrier, but as a dynamic and relational medium linking the Tasmanian archipelago with Antarctica. When Antarctica becomes the focus, the script is flipped: Tasmania is no longer on the edge, but core to a network of gateways into the southern land. The state’s capital of Hobart can from this perspective be understood as an "Antarctic city", central to the geopolitics, economy, and culture of the frozen continent (Salazar et al.). Viewed from the south, we argue, Tasmania is not a problem, but an opportunity for a form of ecological, cultural, economic, and political sustainability that opens up the southern continent to science, discovery, and imagination.

Interconnectivity Between Volume Transports Through Arctic Straits

AbstractArctic heat and freshwater budgets are highly sensitive to volume transports through the Arctic‐Subarctic straits. Here we study the interconnectivity of volume transports through Arctic straits in three models; two coupled global climate models, one with a third‐degree horizontal ocean resolution (High Resolution Global Environmental Model version 1.1 [HiGEM1.1]) and one with a twelfth‐degree horizontal ocean resolution (Hadley Centre Global Environment Model 3 [HadGEM3]), and one ocean‐only model with an idealized polar basin (tenth‐degree horizontal resolution). The two global climate models indicate that there is a strong anticorrelation between the Bering Strait throughflow and the transport through the Nordic Seas, a second strong anticorrelation between the transport through the Canadian Arctic Archipelago and the Nordic Seas transport, and a third strong anticorrelation is found between the Fram Strait and the Barents Sea throughflows. We find that part of the strait correlations is due to the strait transports being coincidentally driven by large‐scale atmospheric forcing patterns. However, there is also a role for fast wave adjustments of some straits flows to perturbations in other straits since atmospheric forcing of individual strait flows alone cannot lead to near mass balance fortuitously every year. Idealized experiments with an ocean model (Nucleus for European Modelling of the Ocean version 3.6) that investigate such causal strait relations suggest that perturbations in the Bering Strait are compensated preferentially in the Fram Strait due to the narrowness of the western Arctic shelf and the deeper depth of the Fram Strait.

On Neutral and Unstable Modes and on the Role of Reynolds Stress in the Stability Problem of Density Stratified Shear Flows in Channels with Variable Bottom

The stability problem of inviscid incompressible stratified shear flows in sea straits of arbitrary cross sections is formulated without the Boussinesq approximation. Series solutions of the stability equation are found by the method of Frobenius and these solutions are used to study the existence of neutral modes when the Richardson number is larger than one quarter. The energy aspect of the problem is studied by discussing the role of the Reynolds stress and its variation in the vertical direction for both unstable modes and neutral modes adjacent to the unstable modes. The interfacial conditions at layers of discontinuity of the basic velocity, density or topography are derived and these are used in the study of instability of two examples of basic flows.

Process‐Specific Contributions to Anomalous Java Mixed Layer Cooling During Positive IOD Events

AbstractNegative sea surface temperature (SST) anomalies associated with positive Indian Ocean Dipole (pIOD) events first appear in the seasonal upwelling zone along the southern coast of Java during May–July. The evolution of anomalous SSTs in this coastal region is analyzed by computing a temperature budget using output from a strongly eddy‐active ocean general circulation model. The seasonal cooling south of Java in May–July is driven by a reduction in incoming shortwave radiation and by vertical mixing, consistent with earlier studies in the region; however, the model budget also shows an advective contribution that drives anomalous cooling at the onset of pIOD events. To identify which process(es) are responsible for the anomalous advective cooling during pIOD events, a novel process index regression method is used to estimate the contributions of wind stress, equatorial Kelvin waves, mesoscale eddies, and Lombok Strait flow to anomalous cooling south of Java. Using this method, wind stress forcing along the west coast of Sumatra is found to make the most substantial contribution to anomalous cooling south of Java, with lesser contributions from equatorially sourced Kelvin waves and local wind stress. Mesoscale eddies redistribute heat from the Lombok Strait outflow, and have an anomalous warming effect on the eastern side of the upwelling region. The process‐specific temperature budget south of Java highlights the importance of wind stress forcing west of Sumatra relative to equatorial and local forcing, and explains most of the mixed layer temperature anomaly evolution associated with advection during pIOD events.

On the Long Wave Stability of Shear Flows in Sea Straits of Arbitrary Cross Section

We consider an eigen value problem of hydrodynamic stability which deals with inviscid, incompressible, density stratified shear flows of variable topography. In this paper, we obtained a condition for long wave stability i.e., if the wave number k is less than or equal to critical wave number $$k_{c}$$ which is greater than zero then the flow is stable. This result is illustrated with examples. An instability region which depends on various parameters is derived for an unstable mode and an estimate for the growth rate of an unstable mode is also obtained.

Nonlinear processes generated by supercritical tidal flow in shallow straits

Numerical experiments have been carried out using a nonhydrostatic and non-Boussinesq regional oceanic circulation model to investigate the nonlinear processes generated by supercritical tidal flow in shallow straits. Our approach relies on idealized direct numerical simulations inspired by oceanic observations. By analyzing a large set of simulations, a regime diagram is proposed for the nonlinear processes generated in the lee of these straits. The results show that the topography shape of the strait plays a crucial role in the formation of internal solitary waves (ISWs) and in the occurrence of local breaking events. Both of these nonlinear processes are important turbulence producing phenomena. The topographic control, observed in mode 1 ISW formation in previous studies [Y. Dossmann, F. Auclair, and A. Paci, “Topographically induced internal solitary waves in a pycnocline: Primary generation and topographic control,” Phys. Fluids 25, 066601 (2013) and Y. Dossmann et al., “Topographically induced internal solitary waves in a pycnocline: Ultrasonic probes and stereo-correlation measurements,” Phys. Fluids 26, 056601 (2014)], is clearly reproducible for mode-2 ISW above shallow straits. Strong plunging breaking events are observed above “narrow” straits (straits with a width less than mode 1 wavelength) when the fluid velocity exceeds the local mode 1 wave speed. These results are a step towards future works on vertical mixing quantification and localization around complex strait areas.

Current power potential of a sea strait: The Bosphorus

The annual current energy potential of a sea strait, Bosphorus in Turkey, is evaluated by the calibrated and validated three dimensional numerical model results. The numerical approach is based on a number of unstructured flexible meshes (triangle or quadrilateral elements) and uses a cell-centered finite volume solution technique. Three-dimensional incompressible Reynolds averaged Navier-Stokes equations are solved invoking the assumptions of Boussinesq and hydrostatic pressure. The turbulence closure was implemented by using Smagorinsky and k-ε models in the horizontal and vertical domains, respectively. Water level and density differences acting on the model open boundaries combined with the meteorological structure of the region (wind speed, direction and atmospheric pressure difference) are the main forcing mechanisms of the numerical model as inputs. The strait flow is a typical example of a stratified flow among the world's straits, with a possible exception of having a negligible tidal oscillation. The results show that the complex geometry (both horizontally and vertically) of the strait combined with highly variable hydrological and meteorological conditions of the adjacent seas, the Marmara Sea and Black Sea, result in a considerable fluctuation in the kinetic energy potential. Cross-sectional variability of the kinetic energy is also notable both horizontally and vertically with increasing energy upwards and towards the shore. In spatial domain, although it is not the narrowest part of the strait, the highest kinetic energies are calculated at the southernmost part of the strait due to both a decrease in cross-sectional area and the presence of a sill on the bottom, a geometrical feature likely seen in straits that mainly control the flow structure (e.g., flow velocities). For a given cross section taken from a meandering part of the strait, the kinetic energy of the strait is higher at the outer and inner banks of the strait for the upper and lower layer flows, respectively. In time scale, the most energetic time period spans from the late spring to the end of summer related to the increase in water level difference between both ends of the strait due to long-term effects that represent seasonal variations (mainly the river inflows toward the Black Sea) and short-term effects (southward storms in the same direction with the upper layer flow). In winter, however, due to the southerly storms acting opposite to the surface layer flow, the kinetic energy potential of the strait drops to considerably low values occasionally.