Ocean Processes Feeder Report

We assess the physical state of the UK’s seas and so provide a context for the clean, healthy, safe, productive and biologically diverse aspects in the other Charting Progress 2 Feeder Reports. UK annual mean temperature has risen by approximately 1°C since the beginning of the 20th Century. 2006 was the warmest year in central England since records began in the seventeenth century. Sea-surface temperature has risen by between 0.5 and 1 degree C from 1871 to 2000. Warming since the mid 1980s has been more pronounced in regions 2, 5 and 6 (southern North Sea, Irish and Hebridean seas). Oceans are acidifying (pH decreasing) as carbon dioxide is absorbed. We have no baseline measurements of pH against which changes in UK waters can be judged, and it will be some time before we can make accurate judgements about the rate of acidification relative to natural annual and inter- annual cycles of pH. Mean sea level around the UK coast rose by about 1.4mm per year during the 20th century. Circulation, suspended particulate matter, turbidity, salinity and waves vary on daily to inter-annual timescales but show no significant trend over the last decade, except for a slight salinity decrease in region 2 (southern North Sea) and a slight increase in salinity in the (northern) regions 1, 7 and 8

Circulation

Circulation is important to distributions of salt, of deep-ocean heat and hence regional climate, of pollutants and of many species carried by the flow during their lifecycle. Currents affect offshore operations and habitats. Five sections from 1957 to 2004 suggest decline of the Atlantic Meridional Overturning Circulatin (AMOC) but this is within the range of large variability on time-scales of weeks to months. An overall trend has not been determined from the continuous measurements begun in 2004. Deep outflows of cold water from the Nordic seas are likewise too variable to infer any overall trend. Strong North Atlantic flow eastwards towards the UK may correlate with positive North Atlantic Oscillation (NAO) Index (i.e. prevailing westerly winds). Enhanced along-slope current around the UK may correlate with a negative NAO Index. Climate models’ consensus makes it very likely that AMOC will decrease over the next century, but not ‘shut down’ completely. Similar spatial and temporal variability (arising from complex topography and variable forcing) is likely in future

Ocean processes. In: Charting progress 2: the state of UK seas, Chapter 2, 14-26

The main changes in ocean processes over the past few decades are largely due to the effects of rising sea surface temperature, rising sea levels and ocean acidity. The changes are already affecting some sensitive ecosystems and could have significant long-term impact

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

Waves. In: Charting the progress 2: ocean processes feeder report, Section 3.6, 159-180

Waves affect marine operations and coastal communities; they can cause coastal erosion and structural damage. They influence stratification and enhance air-sea fluxes; in shallow waters they cause near-bed currents and suspend sediment, so affecting nearshore and benthic habitats, communities and demersal fish. Wave heights in winter (when largest) increased through the 1970s and 1980s: in the NE Atlantic (significant increase between the 1960s and early 1990s); in the North Sea (increase from 1973 to the mid-1990s); at Seven Stones off Land’s End (increase of about 0.02 m/y over 25 years to 1988). However, recent trends are not clear and may depend on region; some series appear to show a decrease. Winter wave heights correlate significantly with the North Atlantic Oscillation Index (a measure of the strength of westerly winds at UK latitudes), in the west and the Irish Sea; the correlation is particularly strong in the north west. In very shallow waters (e.g. near coasts) trends are reduced; wave heights are limited by water depth (as waves break); however, if sea levels (raised by climate change) increase depths nearshore, then larger waves may approach the shore. Climate change may affect storminess, storm tracks and hence wave heights. Some climate models suggest more frequent very severe storms but there is little confidence in predicted changes of wave height

Cross-shelf exchange in the northwestern Black Sea

The transports of water, heat, and salt between the northwestern shelf and deep interior of the Black Sea are investigated using a high-resolution three-dimensional primitive equation model. From April to August 2005, both onshore and offshore cross-shelf break transports in the top 20 m were 0.24 Sv on average, which is equivalent to the replacement of 60% of the volume of surface shelf waters (0–20 m) per month. Two main exchange mechanisms are studied: Ekman transport, and transport by mesoscale eddies and associated meanders of the Rim Current. The Ekman drift causes nearly uniform onshore or offshore flow over a large section of the shelf break, but it is confined to the upper layers. In contrast, eddies and meanders penetrate deep down to the bottom, but they are restricted laterally. During the strong wind events of 15–22 April and 1–4 July, some 0.66 × 1012 and 0.44 × 1012 m3 of water were removed from the northwestern shelf, respectively. In comparison, the single long-lived Sevastopol Eddy generated a much larger offshore transfer of 2.84 × 1012 m3 over the period 23 April to 30 June, which is equivalent to 102% of the volume of northwestern shelf waters. Over the study period, salt exchanges increased the average density of the shelf waters by 0.67 kg m−3 and reduced the density contrast between the shelf and deep sea, while lateral heat exchanges reduced the density of the shelf waters by 0.16 kg m−3 and sharpened the shelf break front

Tidally induced lateral dispersion of the Storfjorden overflow plume

We investigate the flow of brine-enriched shelf water from Storfjorden (Svalbard) into Fram Strait and onto the western Svalbard Shelf using a regional set-up of NEMO-SHELF, a 3-D numerical ocean circulation model. The model is set up with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the sh-coordinate system which is specially designed to resolve bottom boundary layer processes. In a series of modelling experiments we focus on the influence of tides on the propagation of the dense water plume by comparing results from tidal and non-tidal model runs. Comparisons of non-tidal to tidal simulations reveal a hotspot of tidally induced horizontal diffusion leading to the lateral dispersion of the plume at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into the deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography

Buoyancy arrest and shelf–ocean exchange

Author Posting. © American Meteorological Society, 2012. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 42 (2012): 644–658, doi:10.1175/JPO-D-11-0143.1.When steady flow in a stratified ocean passes between the continental slope and open ocean, its ability to cross isobaths is potentially limited by buoyancy arrest. If the bottom Ekman transport vanishes and there are no interior stresses, then steady linear flow on an f plane must be geostrophic and follow isobaths exactly. The influence of arrest on cross-shelf transport is investigated here to establish 1) whether there are substantial penetration asymmetries between cases with upwelling and downwelling in the bottom boundary layer; 2) over what spatial scales, hence in what parameter regime, buoyancy arrest is important; and 3) the effects of depth-dependent interior flow. The problem is approached using scalings and idealized numerical models. The results show that there is little or no asymmetry introduced by bottom boundary layer behavior. Further, if the stratification is weak or moderate, as measured by a slope Burger number s = αN/f (where α is the bottom slope, N is buoyancy frequency, and f is the Coriolis parameter), buoyancy arrest does not exert a strong constraint on cross-isobath exchange.This research was supported by the National Science Foundation Physical Oceanography program through Grant OCE-0849498.2012-10-0

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