On the origin and pathway of the saline inflow to the Nordic Seas: insights from models

The behaviours of three high-resolution ocean circulation models of the North Atlantic, differing chiefly in their description of the vertical coordinate, are investigated in order to elucidate the routes and mechanisms by which saline water masses of southern origin provide inflows to the Nordic Seas. An existing hypothesis is that Mediterranean Overflow Water (MOW) is carried polewards in an eastern boundary undercurrent, and provides a deep source for these inflows. This study, however, provides an alternative view that the inflows are derived from shallow sources, and are comprised of water masses of western origin, carried by branches of the North Atlantic Current (NAC), and also more saline Eastern North Atlantic Water (ENAW), transported northwards from the Bay of Biscay region via a ‘Shelf Edge Current’ (SEC) flowing around the continental margins. In two of the models, the MOW flows northwards, but reaches only as far as the Porcupine Bank (53°N). In third model, the MOW also invades the Rockall Trough (extending to 60°N). However, none of the models allows the MOW to flow northwards into the Nordic Seas. Instead, they all support the hypothesis of there being shallow pathways, and that the saline inflows to the Nordic Seas result from NAC-derived and ENAW water masses, which meet and partially mix in the Rockall Trough. Volume and salinity transports into the southern Rockall Trough via the SEC are, in the various models, between 25 and 100% of those imported by the NAC, and are also a similarly significant proportion (20–75%) of the transports into the Nordic Seas. Moreover, the highest salinities are carried northwards by the SEC (these being between 0.13 and 0.19 psu more saline at the southern entrance to the Trough than those in the NAC-derived waters). This reveals for the first time the importance of the SEC in carrying saline water masses through the RockallTrough and into the Nordic Seas. Furthermore, the high salinities found on density surfaces appropriate to the MOW in the Nordic Seas are shown to result from the wintertime mixing of the saline near-surface waters advected northwards by the SEC/NAC system. Throughout, we have attempted to demonstrate the extent to which the models agree or disagree with interpretations derived from observations, so that the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

Oceanic density/pressure gradients and slope currents

Eastern boundary currents are some of the most energetic features of the global ocean, contributing significantly to meridional mass, heat and salt transports. We take a new look at the form of an oceanic slope current in equilibrium with oceanic density gradients. We depth-integrate the linearised x and y momentum and continuity equations, assume an equilibrium force balance in the along-slope direction (no along-slope variation in the along-slope flow) and zero cross-slope flow at a coastal boundary. We relate the bottom stress to a bottom velocity via a simple boundary friction law (the precise details are easily modified), and then derive an expression for the slope current velocity by integrating upwards including thermal wind shear. This provides an expression for the slope current as a function of depth and of cross-slope coordinate, dependent on the oceanic density field and surface and bottom stresses. This new expression for the slope current allows for more general forms of oceanic density fields than have been treated previously. Wind stress is also now considered. The emphasis here is on understanding the simplified equilibrium force balance rather than the evolution towards that balance. There is a direct relationship between the slope current strength, friction and along-slope forcing (e.g. wind); also between the total along-slope forcing and bottom Ekman transport, illustrating that “slippery” bottom boundaries in literature are a direct consequence of unrealistically assuming zero along-slope pressure gradient. We demonstrate the utility of the new expression by comparison with a high resolution hydrodynamic numerical model

Morphological response to a North Sea bed depression induced by gas mining

Gas mining leads to saucer-like surface depressions. In the North Sea, gas is currently mined at several offshore locations. The associated bed depression has a similar spatial extent as offshore tidal sandbanks, which are large-scale bed patterns covering a significant part of the North Sea bottom. The morphological time scales of bed depressions and tidal sandbanks are similar, so that significant interaction between these features is expected. In this paper we allow the bed depression to become morphologically active. A simple depression model based on a homogeneous soil is tuned with data of a bed depression near the Dutch barrier island of Ameland. Next, this subsidence model is included in a morphodynamic model. We show that this model is able to explain tidal sandbanks, which represent natural bed behavior. Here we approximate the solution by an expansion up to first order. The zeroth-order solution of the model is a flat bed with a spatially uniform, time-independent current. The first-order solution is investigated using a Fourier transformation. In general, we observe significant interaction between the bed depression and the natural sandbank formation process. The process of induced bed depression triggers and intensifies the natural morphological behavior of the offshore seabed. The model also shows essential differences between modeling a morphodynamically active marine bottom depression and a bottom depression below the threshold for sediment motion. The maximum bed level depression in the active case is significantly larger, and the circular shape of depression contours is affected by stretching toward the preferred orientation of the tidal sandbank formation process

Vertical heat flux and lateral mass transport in nonlinear internal waves

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 37 (2010): L08601, doi:10.1029/2010GL042715.Comprehensive observations of velocity, density, and turbulent dissipation permit quantification of the nonlinear internal wave (NLIW) contribution to vertical heat flux and lateral mass transport over New Jersey's shelf. The effect of NLIWs on the shelf heat budget was significant. On average, heat flux in NLIWs was 10 times larger than background at the pycnocline depth. NLIWs were present at midshelf <10% of the time, yet we estimate that they contributed roughly one−half the heat flux across the pycnocline during the observation period, which was characterized by weak to moderate winds. Lateral transport distances due to the leading 3 waves in NLIW packets were typically inline equation(100 m) but ranged several kilometers. The month-averaged daily onshore transport (per unit alongshelf dimension) by NLIWs is estimated as 0.3 m2s−1. This is comparable to a weak downwelling wind, but sustained over an entire month.This work was funded by the Office of Naval Research

Signal propagation related to the North Atlantic Overturning

Changes of the meridional overturning circulation (MOC) due to surface heat flux variability related to the North Atlantic Oscillation (NAO) are analyzed in various ocean models, i.e., eddying and non‐eddying cases. A prime signature of the forcing is variability of the winter‐time convection in the Labrador Sea. The associated changes in the strength of the MOC near the subpolar front (45°N) are closely related to the NAO‐index, leading MOC anomalies by about 2–3 years in both the eddying and non‐eddying simulation. Further south the speed of the meridional signal propagation depends on model resolution. With lower resolution (non‐eddying case, 4/3° resolution) the MOC signal propagates equatorward with a mean speed of about 0.6 cm/s, similar as spreading rates of passive tracer anomalies. Eddy‐permitting experiments (1/3°) show a significantly faster propagation, with speeds corresponding to boundary waves, thus leading to an almost in‐phase variation of the MOC transport over the subtropical to subpolar North Atlantic

What proportion of riverine nutrients reaches the open ocean?

Globally, rivers deliver significant quantities of nitrogen (N) and phosphorus (P) to the coastal ocean each year. Currently, there are no viable estimates of how much of this N and P escapes biogeochemical processing on the shelf to be exported to the open ocean; most models of N and P cycling assume that either all or none of the riverine nutrients reach the open ocean. We address this problem by using a simple mechanistic model of how a low-salinity plume behaves outside an estuary mouth. The model results in a global map of riverine water residence times on the shelf, typically a few weeks at low latitudes and up to a year at higher latitudes, which agrees well with observations. We combine the map of plume residence times on the shelf with empirical relationships that link residence time to the proportions of dissolved inorganic N (DIN) and P (DIP) exported and use a database of riverine nutrient loads to estimate the global distribution of riverine DIN and DIP supplied to the open ocean. We estimate that 75% of DIN and 80% of DIP reaches the open ocean. Ignoring processing within estuaries yields annual totals of 17 Tg DIN and 1.2 Tg DIP reaching the open ocean. For DIN this supply is about 50% of that supplied via atmospheric deposition, with significant east-west contrasts across the main ocean basins. The main sources of uncertainty are exchange rates across the shelf break and the empirical relationships between nutrient processing and plume residence time

Preface: Developments in the science and history of tides

This special issue marks the 100th anniversary of the founding of the Liverpool Tidal Institute (LTI), one of a number of important scientific developments in 1919. The preface gives a brief history of how the LTI came about and the roles of its first two directors, Joseph Proudman and Arthur Doodson. It also gives a short overview of the research on tides at the LTI through the years. Summaries are given of the 26 papers in the special issue. It will be seen that the topics of many of them could be thought of as providing a continuation of the research first undertaken at the LTI. Altogether, they provide an interesting snapshot of work on tides now being made by groups around the world

Recent Change—North Sea

This chapter discusses past and ongoing change in the following physical variables within the North Sea: temperature, salinity and stratification; currents and circulation; mean sea level; and extreme sea levels. Also considered are carbon dioxide; pH and nutrients; oxygen; suspended particulate matter and turbidity; coastal erosion, sedimentation and morphology; and sea ice. The distinctive character of the Wadden Sea is addressed, with a particular focus on nutrients and sediments. This chapter covers the past 200 years and focuses on the historical development of evidence (measurements, process understanding and models), the form, duration and accuracy of the evidence available, and what the evidence shows in terms of the state and trends in the respective variables. Much work has focused on detecting long-term change in the North Sea region, either from measurements or with models. Attempts to attribute such changes to, for example, anthropogenic forcing are still missing for the North Sea. Studies are urgently needed to assess consistency between observed changes and current expectations, in order to increase the level of confidence in projections of expected future conditions

Ocean Shelf Exchange, NW European Shelf Seas: measurements, estimates and comparisons

Transports across the continental shelf edge enhance shelf-sea production, remove atmospheric carbon and imply an active boundary to ocean circulation. We estimate relatively large overall transport across three contrasted sectors of north-west European shelf edge: the Celtic Sea south-west of Britain, the Malin-Hebrides shelf west of Scotland, the West Shetland shelf north of Scotland. The estimates derive from measurements in the project FASTNEt (Fluxes across sloping topography of the North East Atlantic): drifters, moored current meters, effective “diffusivity” from drifter dispersion and salinity surveys, other estimates of velocity variance contributing to exchange. Process contributions include transport by along-slope flow, internal waves and their Stokes drift, tidal pumping, eddies, Ekman transports in the wind-driven surface layer and bottom boundary layer. Overall exchange across the shelf edge is estimated as several m2s− 1: if extrapolated globally even 1 m2s− 1 is large compared with oceanic transports and potentially important to shelf-sea and adjacent oceanic budgets. In our context, most exchange is in tides, and other motion with periods ~ one day or less, and so effective only for water properties that evolve on such short time-scales. Nevertheless, cross-slope fluxes, and exchange by low frequency motion (periods > two days), are large by global standards and also very variable. Deployment mean fluxes nearest the shelf break were in the range 0.3–4 m2 s− 1; mean exchanges from low-frequency motion were 0.8–3 m2 s - 1. Deeper longer-term moorings and drifters crossing 500 m depth gave much larger fluxes and exchanges up to 20 m2 s − 1. These transports’ significance depends on distinctive properties of the water, or its contents, and on internal shelf-sea circulation affecting further transport. For the NW European shelf, transports across the shelf edge enable its disproportionately strong CO2 “pump”. The complex context, and small scales of numerous processes enabling cross-slope transports, imply a need for models. Measurements remain limited in extent and duration, but widely varied contexts, particular conditions, events, processes and behaviours are now available to support model validation, especially around the northwest European continental shelf edge. Variability still renders observations insufficient for stable estimates of transports and exchanges, especially if partitioned by sector and season; indeed, there may be significant inter annual differences. Validated fine-resolution models give the best prospect of spatial and temporal coverage and of estimating present-day and potential future shelf-sea sensitivities to the adjacent ocean

On the Driving Mechanism of the Annual Cycle of the Florida Current Transport

The mechanisms involved in setting the annual cycle of the Florida Current transport are revisited using an adjoint model approach. Adjoint sensitivities of the Florida Current transport to wind stress reproduce a realistic seasonal cycle with an amplitude of ~1.2 Sv (1 Sv ≡ 106 m3 s−1). The annual cycle is predominantly determined by wind stress forcing and related coastal upwelling (downwelling) north of the Florida Strait along the shelf off the North American coast. Fast barotropic waves propagate these anomalies southward and reach the Florida Strait within a month, causing an amplitude of ~1 Sv. Long baroclinic planetary Rossby waves originating from the interior are responsible for an amplitude of ~0.8 Sv but have a different phase. The sensitivities corresponding to the first baroclinic mode propagate westward and are highly influenced by topography. Considerable sensitivities are only found west of the Mid-Atlantic Ridge, with maximum values at the western shelf edge. The second baroclinic mode also has an impact on the Florida Current variability, but only when a mean flow is present. A second-mode wave train propagates southwestward from the ocean bottom on the western side of the Mid-Atlantic Ridge between ~36° and 46°N and at Flemish Cap, where the mean flow interacts with topography, to the surface. Other processes such as baroclinic waves along the shelf and local forcing within the Florida Strait are of minor importance