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

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

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

Leaky slope waves and sea level: unusual consequences of the beta-effect along western boundaries with bottom topography and dissipation

Coastal Trapped Waves (CTWs) carry the ocean’s response to changes in forcing along boundaries, and are important mechanisms in the context of coastal sea level and the meridional overturning circulation. Motivated by the western boundary response to high latitude and open ocean variability, we investigate how the latitude dependence of the Coriolis parameter (β-effect), bottom topography, and bottom friction, modify the evolution of western boundary CTWs and sea level using a linear, barotropic model. For annual and longer period waves, the boundary response is characterized by modified Shelf Waves and a new class of leaky Slope Waves that propagate alongshore, typically at an order slower than Shelf Waves, and radiate short Rossby waves into the interior. Energy is not only transmitted equatorward along the slope, but also eastward into the interior, leading to the dissipation of energy locally and offshore. The β-effect and friction result in Shelf and Slope Waves that decay alongshore in the direction of the equator, decreasing the extent to which high latitude variability affects lower latitudes, and increasing the penetration of open ocean variability onto the shelf - narrower continental shelves and larger friction coefficients increase this penetration. The theory is compared against observations of sea level along the North American east coast and qualitatively reproduces the southward displacement and amplitude attenuation of coastal sea level relative to the open ocean. The implications are that the β-effect, topography, and friction are important in determining where along the coast sea level variability hot spots occur

Idealised modelling of offshore-forced sea level hot spots and boundary waves along the North American East Coast

Hot spots of sea level variability along the North American East Coast have been shown to shift in latitude repeatedly over the past 95 years and connections with a number of forcing phenomena, including the North Atlantic Oscillation (NAO) and Atlantic Meridional Overturning Circulation (AMOC), have been suggested. Using a barotropic 1/12° NEMO model of the North American East Coast (to represent the upper ocean and a homogeneous shelf), we investigate the coastal sea level response to remote sea surface height (SSH) variability along the upper continental slope. Hilbert transform Complex EOF analysis is used to investigate the responses to interannual changes in the strength of the mean winds and an idealised NAO. Variability in the mean winds produces in-phase coastal sea level variability along the entire coastline and is driven by a SSH anomaly in the subpolar gyre. Variability due to the NAO forcing is in phase along the coast south of Cape Hatteras. Interannual coastal sea level variability at a given latitude is found to be driven by off-shore SSH anomalies originating many degrees of latitude (100s km) further north, and linear barotropic trapped wave theory is used to explain the mechanism. A comparison of the results from an analytical model with those from the numerical model is used to suggest that the boundary wave mechanism is also relevant for understanding the coastal response to interior sea level change over longer time periods. Nonlinear effects are found not to significantly modify the character of the linear solution

Tidal asymmetry and residual circulation over linear sandbanks and their implication on sediment transport : a process-oriented numerical study

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 112 (2007): C12015, doi:10.1029/2007JC004101.A series of process-oriented numerical simulations is carried out in order to evaluate the relative role of locally generated residual flow and overtides on net sediment transport over linear sandbanks. The idealized bathymetry and forcing are similar to those present in the Norfolk Sandbanks, North Sea. The importance of bottom drag parameterization and bank orientation with respect to the ambient flow is examined in terms of residual flow and overtide generation, and subsequent sediment transport implications are discussed. The results show that although the magnitudes of residual flow and overtides are sensitive to bottom roughness parameterization and bank orientation, the magnitude of the generated residual flow is always larger than that of the locally generated overtides. Also, net sediment transport is always dominated by the nonlinear interaction of the residual flow and the semidiurnal tidal currents, although cross-bank sediment transport can occur even in the absence of a cross-shore residual flow. On the other hand, net sediment divergence/convergence increases as the bottom drag decreases and as bank orientation increases. The sediment erosion/deposition is not symmetric about the crest of the bank, suggesting that originally symmetric banks would have the tendency to become asymmetric.Funding for this work was provided by the U.S. Geological Survey as part of the SC Coastal Erosion Study and by the South Carolina Sea Grant Consortium (grant V169). Additional support for one of the authors (G. Voulgaris) was provided by the Office of Naval Research (Southeast Coastal Ocean Observing Systems) and by the National Science Foundation (award OCE-0451989)

On internal waves propagating across a geostrophic front

Reflection and transmission of normally-incident internal waves propagating across a geostrophic front, like the Kuroshio or Gulf Stream, are investigated using a modified linear internal-wave equation. A transformation from depth to buoyancy coordinates converts the equation to a canonical partial differential equation, sharing properties with conventional internal-wave theory in the absence of a front. The equation type is determined by a parameter Δ, which is a function of horizontal and vertical gradients of buoyancy, the intrinsic frequency of the wave and the effective inertial frequency, which incorporates the horizontal shear of background geostrophic flow. In the northern hemisphere, positive vorticity of the front may produce Δ≤0, i.e., a “forbidden zone”, in which wave solutions are not permitted. Thus, Δ=0 is a virtual boundary that causes wave reflection and refraction, although waves may tunnel through forbidden zones that are weak or narrow. The slope of the surface and bottom boundaries in buoyancy coordinates (or the slope of the virtual boundary if a forbidden zone is present) determine wave reflection and transmission. The reflection coefficient for normally-incident internal waves depends on rotation, isopycnal slope, topographic slope and incident mode number. The scattering rate to high vertical modes allows a bulk estimate of the mixing rate, although the impact of internal-waves driven mixing on the geostrophic front is neglected