Shelf Waves Research Articles

Wind-driven intensification of the Tsushima Warm Current along the Japanese coast detected by sea level difference in the summer monsoon of 2013

This study found an unusually large variation in the coastal sea level difference (SLD) in the southwestern Japan Sea from July to October 2013. The time and horizontal scales were about 4 months and 1000km along the coast, respectively. During the exceptionally hot summer of 2013, strong monsoon winds blew through the Tsushima Straits into the southwestern Japan Sea, because the North Pacific High was located farther west than average. The southwesterly winds averaged 7ms−1 parallel to the coast in July 2013, resulting in Ekman transport toward the coast. The most noticeable SLD was measured between Omijima and Mishima, normal to the 250km rectilinear coastal line. The velocity of the first branch of the Tsushima Warm Current increased to about 50cms−1 between Omijima and Mishima from late July to early August. Although the SLD seemed to simultaneously increase and decrease along the Japanese coast with large time scale, the variation in coastal SLD propagated along the Japanese coast, reaching Niigata-nishi in the eastern Japan Sea. The propagation speed in the wide continental shelf of the southern Japan Sea was much higher than that in the narrow eastern shelf, which was consistent with the behavior of continental shelf waves.

Wave responses from oil reservoirs in the Arctic shelf zone

We consider the appearance of scattered PP-waves, SS-waves, scattered converted PS-waves, and SP-waves reflected from the reservoirs in the Arctic shelf zone and wave responses from them in the water column when the source and receiver are located near the water surface. We obtained a numerical solution of the direct problems of seismic prospecting in the shelf zone with a reservoir and without it. The solution was obtained using the grid-characteristic method, which makes possible to model the wave processes in detail.

Dispersion Relation for Continental Shelf Waves When the Shallow Shelf Part Has an Arbitrary Width: Application to the Shelf West of Norway

AbstractThe dispersion relation for continental shelf waves (CSWs) in a shelf region with an unbounded flat outer ocean, a convex-upward exponential shelf, and an interior flat region of arbitrary width D is derived. The calculations allow for a nonzero divergence of the wave motion. Some consequences of these findings are discussed for the shelf west of Norway, where the shelf at mid-Norway is quite wide, while at Lofoten it is much narrower. Furthermore, north of Lofoten, along the opening to the Barents Sea, the shelf becomes extremely wide. In this region the conditions are nearly met for a double Kelvin wave. The paper discusses how CSWs generated along the common storm track outside southwest Norway modify as they travel along the shelf. The analytical results are compared with results from a numerical barotropic ocean model with realistic shelf topography.

Multiple Drivers of Intense Coastal Upwelling at Cape Town

ABSTRACT Jury, M.R., 2017. Multiple drivers of intense coastal upwelling at Cape Town. Cape Town, South Africa is well known for its intense coastal upwelling during austral summer. This study provides new insights using daily 4–9 km resolution satellite and ocean reanalysis datasets in the period 2009–2015. A key purpose of this work is to highlight the value of these finely detailed products in coastal research. The most intense case of 23–24 January 2011 revealed a cold plume <10°C extending offshore into a filament. SE winds accelerated cyclonically around Cape Town, and the enhanced wind vorticity lifted <10°C water over the shelf. Yet a statistical analysis found that sea surface temperature and equatorward winds are correlated at <−0.10. A significant outcome could only be achieved in summer (December–February) due to opposing thermal and kinematic influences. The intense upwelling of 23–24 January 2011 was supported by the approach of a midlatitude upper anticyclone and coupled shelf wave. Regress...

Localised continental shelf waves: geometric effects and resonant forcing

Alongshore variations in coastline curvature or offshore depth profile can create localised regions of shelf-wave propagation with modes decaying outside these regions. These modes, termed localised continental shelf waves ($\ell$CSWs) here, exist only at certain discrete frequencies lying below the local maximum frequency, and above the far-field maximum frequency, for propagating shelf waves. The purpose of this paper is to obtain these frequencies and construct, both analytically and numerically, and discuss $\ell$CSWs for shelves with arbitrary alongshore variations in offshore depth profile and coastline curvature. If the shelf curvature changes by a small fraction of its value over the shelf section of interest or an alongshore perturbation in offshore depth profile varies slowly over the same length scale then $\ell$CSWs can be constructed using WKBJ theory. Two subcases are described: (i) if the propagating region is sufficiently long that the offshore structure of the $\ell$CSW varies appreciably alongshore then the frequency and alongshore structure are found from a sequence of local problems; (ii) if the propagating region is sufficiently short that the alongshore change in offshore structure of the $\ell$CSW is small then the alongshore modal structure is given in an explicit, uniformly valid form. A separate asymptotic theory is required for curvature perturbations to shelves that are otherwise straight rather than curved. Comparison with highly accurately numerically determined $\ell$CSWs shows that both theories are extremely accurate, with the WKBJ theory having a significantly wider range of applicability. An idealised model for the generation of $\ell$CSWs is also suggested. A localised time-periodic wind stress generates an evanescent continental shelf wave in the far field of a localised mode where the coast is almost straight and the response on the shelf is obtained numerically. If the forcing frequency is close to that of an $\ell$CSW then the wind stress excites energetic motions in the region of maximum curvature, creating a significant localised response possibly far from the forcing region.

Oceanic responses to Hurricane Igor over the Grand Banks: A modeling study

AbstractA three‐dimensional (3‐D) baroclinic finite‐volume ocean model (FVCOM) was developed to examine the oceanic response to Hurricane Igor over the Grand Banks of Newfoundland. Hurricane Igor generated a storm surge of almost 1 m at St. John's and about 0.8 m at three nearby coastal tide gauge stations (Bonavista, Argentia and St. Lawrence). The surge magnitude from the 3‐D baroclinic model agrees approximately with tide‐gauge observations at all four stations, slightly better than that from an alternative 3‐D barotropic case. The sudden drop of sea surface temperature caused by the storm, approximately as observed by buoys, is well simulated by the baroclinic model with a k‐ε turbulence closure. A sensitivity simulation with the Mellor‐Yamada turbulence closure significantly underestimates sea surface cooling. It is shown that the sea surface cooling is mainly associated with turbulent mixing, and to a lesser degree with Ekman upwelling. The model solution shows that the largest surge occurred between Bonavista and St. John's. Further analysis suggests the generation of a free continental shelf wave after the storm made landfall, with the peak surge propagating from St. John's to St. Lawrence.

Oscillation of hydrophysical fields on the shelf and continental slope caused by nonstationary wind

Barotropic and baroclinic oscillations typical of the spring period have been studied based on the three-dimensional thermohydrodynamic model of the Black Sea. This season can be considered transient between the termination of the effect of strong winter winds and the onset of seasonal thermocline formation. The three-dimensional numerical model with a free surface and boundary conditions, corresponding to the real wind field for April 2006 and to the heat and salt flows on the surface, has been used. The horizontal and vertical spatial resolutions of the model are 1.5 km and 27 levels, respectively; the time discreteness is 1 h. This makes it possible to perform detailed modeling in the South Coastal Crimean sea sector. The spatial-time structure of the level, kinetic energy, and temperature fields has been studied using the spectral analysis method. The maximal energy-carrying frequency has been interpreted. Manifestations of Poincare and Kelvin waves, as well as captured shelf waves, have been distinguished in the submesoscale spectral region. A comparison with previous studies has been performed. It is indicated that the stratification variability caused by synoptic processes affects spectral characteristics of wave processes on smaller scales, making its contribution to the Black Sea water mixing.

Growth and dissipation of typhoon-forced solitary continental shelf waves in the northern South China Sea

This study aims to gain insight into the sea level response to typhoon in the northern South China Sea using typhoon records from 1978 to 1997, simultaneous tidal gauge records and satellite altimeter along-track sea level data. We find that the ocean sea level response signatures in the deep water agree well with a single sea level soliton, of which the average soliton amplitude is (34 ± 6) cm and the average characteristic half width is (115 ± 12) km. Evolution of the single soliton as propagating across a shoaling bottom topography obeys solutions of the perturbed Korteweg and de Vries equation. The solutions reveal that forced by the hyperbolic-tangent-shaped topography, the single soliton evolves to a solitary wave packet consisting of a large leading soliton and smaller following waves in an oscillatory tail as approaching the shallow water. This phenomenon is verified by observed sea level signatures of 19 storm surges. The correlation coefficient between theoretical solution and observations is as high as 0.91. Meanwhile, the soliton amplitude grows with a soliton amplitude growth ratio (SAGR), which depends on the deep water and shallow water depths. However, without consideration of viscous effects, the SAGR model overestimates the soliton amplitude growth. Thus the vorticity equation for viscous fluid is adopted to solve this issue. The results indicate that turbulent viscosities are responsible to soliton amplitude decay concurring with the growth and causes dependence of the storm surge amplitude on the typhoon incident angle.

Hurricane Sandy storm surges observed by HY‐2A satellite altimetry and tide gauges

AbstractHurricane Sandy made landfall to the northeast of Atlantic City, New Jersey at 23:30 UTC on 29 October 2012 and caused large storm surges and devastating flooding along the New Jersey and New York coasts. Here we combine sea surface height measurements from the HaiYang‐2A (HY‐2A) satellite altimeter with coastal tide‐gauge data to study the features of the Hurricane Sandy storm surges. The HY‐2A altimeter captured the cross‐shelf profile of surge at the time of Sandy's peak surge, with a surge magnitude of about 1.83 m at the coast and a cross‐shelf decaying scale of 68 km. The altimetric surge magnitude agrees approximately with tide‐gauge estimate of 1.73 m at nearby Montauk. Further analysis suggests that continental shelf waves were generated during the passage of Sandy. The continental shelf wave observed by altimetry has a propagating speed of 6.5 m/s. The post landfall free shelf wave at Atlantic City observed by tide gauges has a propagating phase speed of 6.8 m/s and cross‐shelf e‐folding scale of 75 km. In contrast, the post landfall sea level oscillation at Montauk is not associated with a continental shelf wave. The study indicates that satellite altimetry is capable of observing and useful for understanding features of storm surges, complementing existing coastal tide gauges.

Coupled wind-forced controls of the Bering–Chukchi shelf circulation and the Bering Strait throughflow: Ekman transport, continental shelf waves, and variations of the Pacific–Arctic sea surface height gradient

We develop a conceptual model of the closely co-dependent Bering shelf, Bering Strait, and Chukchi shelf circulation fields by evaluating the effects of wind stress over the North Pacific and western Arctic using atmospheric reanalyses, current meter observations, satellite-based sea surface height (SSH) measurements, hydrographic profiles, and numerical model integrations. This conceptual model suggests Bering Strait transport anomalies are primarily set by the longitudinal location of the Aleutian Low, which drives oppositely signed anomalies at synoptic and annual time scales. Synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves, whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along-strait (meridional) pressure gradient. In particular, we show that storms centered over the Bering Sea excite continental shelf waves on the eastern Bering shelf that carry northward velocity anomalies northward through Bering Strait and along the Chukchi coast. The integrated effect of these storms tends to decrease the northward Bering Strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the Aleutian Basin and the Western Subarctic Gyre. Ekman suction then increases the water column density through isopycnal uplift, thereby decreasing the dynamic height, sea surface height, and along-strait pressure gradient. Storms displaced eastward over the Gulf of Alaska generate an opposite set of Bering shelf and Aleutian Basin responses. While Ekman pumping controls Canada Basin dynamic heights (Proshutinsky et al., 2002), we do not find evidence for a strong relation between Beaufort Gyre sea surface height variations and the annually averaged Bering Strait throughflow. Over the western Chukchi and East Siberian seas easterly winds promote coastal divergence, which also increases the along-strait pressure head, as well as generates shelf waves that impinge upon Bering Strait from the northwest.

DESIGN AND IMPLEMENTATION OF THE OCEANOGRAPHIC MODELING AND OBSERVATION NETWORK (REMO) FOR PHYSICAL OCEANOGRAPHY STUDIES AND OCEAN FORECASTING

This paper is concerned with the planning, implementation and some results of the Oceanographic Modeling and Observation Network, named REMO, for Brazilian regional waters. Ocean forecasting has been an important scientific issue over the last decade due to studies related to climate change as well as applications related to short-range oceanic forecasts. The South Atlantic Ocean has a deficit of oceanographic measurements when compared to other ocean basins such as the North Atlantic Ocean and the North Pacific Ocean. It is a challenge to design an ocean forecasting system for a region with poor observational coverage of in-situ data. Fortunately, most ocean forecasting systems heavily rely on the assimilation of surface fields such as sea surface height anomaly (SSHA) or sea surface temperature (SST), acquired by environmental satellites, that can accurately provide information that constrain major surface current systems and their mesoscale activity. An integrated approach is proposed here in which the large scale circulation in the Atlantic Ocean is modeled in a first step, and gradually nested into higher resolution regional models that are able to resolve important processes such as the Brazil Current and associated mesoscale variability, continental shelf waves, local and remote wind forcing, and others. This article presents the overall strategy to develop the models using a network of Brazilian institutions and their related expertise along with international collaboration. This work has some similarity with goals of the international project Global Ocean Data Assimilation Experiment OceanView (GODAE OceanView).

Coastal-Trapped Waves in the East China Sea Observed by a Mooring Array in Winter 2006

Abstract Using five mooring array observations in the coastal water of the East China Sea (ECS) in winter 2006, the authors identify three kinds of low-frequency waves using the ensemble empirical mode decomposition (EEMD) method. The analysis indicates that the periods of the waves varied from 2 to 10 days, which are consistent with coastal-trapped wave (CTW) modes: the Kelvin wave (KW) mode, the first shelf wave (SW1) mode, and the second shelf wave (SW2) mode. An analytical model is established and the dispersion relation of the waves from the analytical method agrees well with the observations. The wind-forced, coastal-trapped wave theory is then applied. The calculation shows that over a wide shelf, the forcing term of wind stress curl plays an important role in shaping the CTW. Numerical solutions reproduce the sea level variation and the alongshore current. The results show that the sea level variation mainly resulted from the KW mode, but the alongshore current resulted from both the KW and SW1 modes.

The inner shelf circulation on the Abrolhos Bank, 18°S, Brazil

The inner shelf circulation on the Abrolhos Bank is investigated using four years (2002–2005) of moored current and bottom pressure observations from two sites in conjunction with wind data from a nearby meteorological station. This is one of the longest projects monitoring current and sea level along the Brazilian coast. The time variability of the local circulation and main forcings are described. For the first time, both the seasonal and the interannual variabilities are addressed, as are the impact of remote forcing. The cross-shore pressure gradient in the region is mostly set up by along-shore winds, whereas the sub-inertial cross-shore momentum balance is essentially geostrophic, with smaller contributions from the cross-shore wind stress. The along-shelf momentum balance is ageostrophic and mainly occurs between the wind and bottom stresses. South-southwestward along-shore currents occur between October and January, whereas stronger north-northeastward currents are observed in fall and winter. This seasonal cycle is driven by the N–S migration of the South Atlantic High between the seasons. An increasing frequency of the southern winds and, consequently, northward currents are observed between 2002 and 2005 and are related to both the number of fronts reaching the region and the remote effect of fronts that did not cross the area. The cross-shore circulation is weak and mainly forced by the tides. It is suggested that long-period shelf waves that propagate into the region change the inner shelf current field and sea level.

On the shelf resonances of the English Channel and Irish Sea

Abstract. The resonances of the English Channel and Irish Sea are investigated using the methods of Webb (2012a) together with an Arakawa C-grid model of the region under study. In the semi-diurnal tidal band, the high tides of the Bristol Channel and Gulf of St. Malo are shown to be due to two shelf resonances which strongly couple the two regions. In the diurnal band, the response is complicated by the presence of continental shelf waves.

Wave power variability over the northwest European shelf seas

Regional assessments of the wave energy resource tend to focus on averaged quantities, and so provide potential developers with no sense of temporal variability beyond seasonal means. In particular, such assessments give no indication of inter-annual variability – something that is critical for determining the potential of a region for wave energy convertor (WEC) technology. Here, we apply the third-generation wave model SWAN (Simulating Waves Nearshore) at high resolution to assess the wave resource of the northwest European shelf seas, an area where many wave energy test sites exist, and where many wave energy projects are under development. The model is applied to 7years of wind forcing (2005–2011), a time period which witnessed considerable extremes in the variability of the wind (and hence wave) climate, as evidenced by the variability of the North Atlantic Oscillation (NAO). Our simulations demonstrate that there is much greater uncertainty in the NW European shelf wave resource during October–March, in contrast to the period April–September. In the more energetic regions of the NW European shelf seas, e.g. to the northwest of Scotland, the uncertainty was considerably greater. The winter NW European shelf wave power resource correlated well with the NAO. Therefore, provided trends in the NAO can be identified over the coming decades, it may be possible to estimate how the European wave resource will similarly vary over this time period. Finally, the magnitude of wave power estimated by this study is around 10% lower than a resource which is used extensively by the wave energy sector – the Atlas of UK Marine Renewable Energy Resources. Although this can partly be explained by different time periods analysed for each study, our application of a third-generation wave model at high spatial and spectral resolution significantly improves the representation of the physical processes, particularly the non-linear wave-wave interactions.

福島第一原発からの放射性核種の初期分散に及ぼす沿岸ジェットとメソスケール渦の影響

A detailed oceanic downscaling model (Uchiyama et al., 2012) is utilized to better understand the oceanic initial dispersion of the leaked radionuclides during the accident at the Fukushima Daiichi Nuclear Power Plant (1F). Particular attention is paid to the influences of northward geostrophic coastal jet associated with an alongshore high SSH belt peaked at 50 km offshore, and mesoscale eddies shed by the jet in Sendai Bay and by the Kuroshio near the separation point. A spectral coherence analysis exhibits that alongshore wind stress is highly correlated with alongshore current on the shelf over the periods around 168 hours, suggesting that coastally trapped shelf waves are also responsible for the dispersion of the nuclide.

Observing storm surges from space: Hurricane Igor off Newfoundland

Coastal communities are becoming increasingly more vulnerable to storm surges under a changing climate. Tide gauges can be used to monitor alongshore variations of a storm surge, but not cross-shelf features. In this study we combine Jason-2 satellite measurements with tide-gauge data to study the storm surge caused by Hurricane Igor off Newfoundland. Satellite observations reveal a storm surge of 1 m in the early morning of September 22, 2010 (UTC) after the passage of the storm, consistent with the tide-gauge measurements. The post-storm sea level variations at St. John's and Argentia are associated with free equatorward-propagating continental shelf waves (with a phase speed of ~10 m/s and a cross-shelf decaying scale of ~100 km). The study clearly shows the utility of satellite altimetry in observing and understanding storm surges, complementing tide-gauge observations for the analysis of storm surge characteristics and for the validation and improvement of storm surge models.

A new approximation for the dynamics of topographic Rossby waves

ABSTRACTA new theory of non-harmonic topographic Rossby waves over a slowly varying bottom depth of arbitrary, 1-D, profile is developed based on the linearised shallow water equations on the f-plane. The theory yields explicit approximate expressions for the phase speed and non-harmonic cross-slope structure of waves. Analytical expressions are derived in both Cartesian and Polar coordinates by letting the frequency vary in the cross-shelf direction and are verified by comparing them with the numerical results obtained by running an ocean general circulation model (the MITgcm). The proposed approximation may be suitable for studying open ocean and coastal shelf wave dynamics.

Transport simulation of the radionuclide from the shelf to open ocean around Fukushima

We simulate radionuclide distribution in the ocean due to direct emission from the Fukushima Daiichi nuclear power plant (FNPP) for the period from 21 March to 6 May 2011. Dispersion of Cesium-137 is modeled by a transport equation with advection and diffusion by three dimensional ocean current. The ocean current data are provided from a data-assimilative, tide-resolving ocean general circulation model with horizontal resolution of 1/36°. Results show that the radionuclide expands from the shelf region into open ocean in April by the ocean currents and farther transported eastward along the Kuroshio Extension front in May. Sensitivity experiments demonstrate that the mesoscale geostrophic currents basically governed the transport processes in the open ocean after April. In the shelf region, on the other hand, the wind-driven currents facilitate the north–south extension of the radionuclide distribution through the repeated generations of the shelf waves. Influences of tide and river discharges cannot be neglected.

Ocean circulation promotes methane release from gas hydrate outcrops at the NEPTUNE Canada Barkley Canyon node

The NEPTUNE Canada cabled observatory network enables non‐destructive, controlled experiments and time‐series observations with mobile robots on gas hydrates and benthic community structure on a small plateau of about 1 km2 at a water depth of 870 m in Barkley Canyon, about 100 km offshore Vancouver Island, British Columbia. A mobile Internet operated vehicle was used as an instrument platform to monitor and study up to 2000 m2of sediment surface in real‐time. In 2010 the first mission of the robot was to investigate the importance of oscillatory deep ocean currents on methane release at continental margins. Previously, other experimental studies have indicated that methane release from gas hydrate outcrops is diffusion‐controlled and should be much higher than seepage from buried hydrate in semipermeable sediments. Our results show that periods of enhanced bottom currents associated with diurnal shelf waves, internal semidiurnal tides, and also wind‐generated near‐inertial motions can modulate methane seepage. Flow dependent destruction of gas hydrates within the hydrate stability field is possible from enhanced bottom currents when hydrates are not covered by either seafloor biota or sediments. The calculated seepage varied between 40–400 μmol CH4 m−2 s−1. This is 1–3 orders of magnitude higher than dissolution rates of buried hydrates through permeable sediments and well within the experimentally derived range for exposed gas hydrates under different hydrodynamic boundary conditions. We conclude that submarine canyons which display high hydrodynamic activity can become key areas of enhanced seepage as a result of emerging weather patterns due to climate change.