Toby Sherwin

Reversal of the 1960s to 1990s freshening trend in the northeast North Atlantic and Nordic Seas

Hydrographic time series in the northeast North Atlantic and Nordic Seas show that the freshening trend of the 1960s–1990s has completely reversed in the upper ocean. Since the 1990s temperature and salinity have rapidly increased in the Atlantic Inflow from the eastern subpolar gyre to the Fram Strait. In 2003–2006 salinity values reached the previous maximum last observed around 1960, and temperature values exceeded records. The mean properties of the Atlantic Inflow decrease northwards, but variations seen in the eastern subpolar gyre at 57°N persist with the same amplitude and pattern along the pathways to Fram Strait. Time series correlations and extreme events suggest a time lag of 3–4 years over that distance. This estimate allows predictions to be made; the temperature of Atlantic water in the Fram Strait may start to decline in 2007 or 2008, salinity a year later, but both will remain high at least until 2010.

Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break

The vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the Malin Shelf has been determined using a combination of the free-falling light yo-yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large-amplitude high-frequency nonlinear internal waves (NIWs) occurred (around neap tides) and period II in which the internal wave spectral continuum was not dominated by any particular frequency band (around spring tides). Empirical orthogonal function analysis shows that for the low-frequency waves, 76% of the variance was described by mode 1, rising to 95% for the high-frequency waves. During period I the dissipation and vertical mixing were characterized by the NIWs, and 70% of the dissipation occurred in the bottom boundary layer. During period II the depth-integrated dissipation was more evenly distributed throughout the tidal cycle, whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification. During both periods I and II ∼30% of the total measured dissipation occurred within the thermocline when averaged over 12.4 hours; the remainder occurred within the bottom boundary layer(BBL). Tidal average values for depth-integrated dissipation and vertical eddy diffusivity for period I (II) were 1.1×10−2 W m−2 (4.0×10−2 W m−2) and 5 cm2 s−1 (12 cm2 s−1), respectively. Decay rates and internal damping are discussed, and vertical heat fluxes are estimated. Observed dissipation rates are compared with a simple model for BBL dissipation.

Coastal-Trapped Waves and Tides at Near-Inertial Frequencies

The nature of the transition in coastal-trapped wave behavior from trapped, subinertial modes to imperfectly trapped, superinertial waves (not modes), is investigated. When formulated purely in terms of pressure, the coastal-trapped wave eigenvalue problem admits a spurious inertial mode that distorts numerical calculations at nearby frequencies. By solving a pair of coupled equations, involving the component of velocity normal to the coastline as well as pressure, this spurious mode is removed. The transition through the inertial frequency is examined analytically by considering the effect on trapped inertial modes of a small frequency increment. It is shown that, to first order in this increment, modes remain trapped. At higher frequencies, the modal approach breaks down and a primitive equation model is used to represent the, now fully three-dimensional, situation. The scattering of energy from an oscillating barotropic alongshore flow by a topographic feature is considered. At superinertial frequencies, internal energy is scattered in all directions, although preferentially alongshore in the direction of coastal-trapped wave propagation. There is not a sudden change in behavior at the inertial frequency. As frequency becomes increasingly superinertial there is a gradual increase in the three-dimensionality of the response and a decrease in the proportion of energy represented by the trapped component. The work highlights the potential for spurs and canyons to generate alongslope-propagating internal tides.

Wind-driven monthly variations in transport and the flow field in the Faroe-Shetland Channel

The transport of water from the North Atlantic to the Nordic seas through the Faroe–Shetland Channel is analysed from a decade of conductivity, temperature and depth (CTD) and acoustic Doppler current profiler (ADCP) data. The long-term mean transport, integrated over the upper 500 m, is 3.5 ± 0.1 Sv (1 Sv =106m3s-1), of which 2.1 Sv is barotropic flow and 1.4 Sv is baroclinic flow. Short-term variability leads to a standard deviation of ca. 2.2 Sv in 3-day averages of the ADCP-measured transport. The barotropic transport is located over the upper part of the slope region of the Shetland Shelf, but sometimes broadens over deeper water. There is a peak surface baroclinic transport above the foot of the slope, and a weak recirculation of Modified North Atlantic Water (MNAW), which enters from the north, on the Faroese side. In September, when isobars downwell on the eastern side, the strong transport (ca. 4 Sv) is barotropic and evenly distributed across the Shetland slope, and both recirculation of MNAW from the Faroe side and mesoscale activity are weak. In spring, the net transport is small (ca. 2.5 Sv), the MNAW recirculation is strong and mesoscale activity is relatively large. These seasonal variations appear to correlate with the local south-west wind stress, which may contribute to nearly half of the long-term transport in the channel.

Multidecadal variability of potential temperature, salinity, and transport in the eastern subpolar North Atlantic

The Extended Ellett Line (EEL) hydrographic section extends from Scotland to Iceland crossing the Rockall Trough, Hatton-Rockall Basin and Iceland Basin. With 61 full-depth stations at a horizontal resolution of 10 to 50 km, the EEL samples the upper limb of the Atlantic Meridional Overturning Circulation flowing across the Iceland-Scotland Ridge into the Nordic Seas. The Rockall Trough has been sampled nearly four times per year from 1975 to 1996, and the full section annually since 1996. The EEL is an exceptionally long timeseries of deep-ocean temperatures and salinities. This study extends prior work in the Rockall Trough, and examines for the first time 18 year records in the Iceland and Hatton-Rockall Basins. We quantify errors in the timeseries from two sources: observational errors and aliasing. The data quality and annual sampling are suitable for observing interannual to decadal variability because the variability exceeds our error estimates. The upper waters of all 3 basins are cooler/fresher from 1997 to 2001, warmer/more saline 2001 to 2006, and cooler/fresher from 2006 to 2014. A reference level for geostrophic shear is developed heuristically and by comparison with sea-surface altimetry. The mean northward transport in the upper waters is 6.7±3.7 Sv and there is a 6.1±2.5 Sv southward flow below the thermocline. Although the magnitude of the Iceland Basin overturning circulation (4.3±1.9 Sv) is greater than in the Rockall Trough (3.0±3.7 Sv), the variability is greater in the Rockall Trough. We discuss the results in the context of our understanding of drivers of variability. This article is protected by copyright. All rights reserved.

The Extension of Baroclinic Coastal-Trapped Wave Theory to Superinertial Frequencies

Abstract The problem of finding baroclinic coastal-trapped wave modes is generalized from subinertial to superinertial frequencies at which complete trapping can only occur in special cases. Modes are found by a numerical resonance searching method in which forcing is applied to a vertical slice normal to the shelf, and resonant responses identified. An example is considered with topography approximating the Iberian shelf and uniform stratification. The buoyancy frequency is chosen such that the lowest subinertial trapped wave modes combine buoyancy and vorticity effects, and their dispersion curves approach the inertial frequency from below. Superinertial analogs of the first three subinertial modes are identified and have a small imaginary component of wavenumber corresponding to alongshelf decay due to leakage of energy to the ocean. These superinertial modes are apparently not physically realizable, however, since they contain components that do not decay into the ocean. Nevertheless, they can be inte...

Amplification of Tidal Currents by Overflow on the Iceland-Faeroe Ridge

Abstract During several days in July 1990, there was an abrupt increase in semidiurnal tidal current in the upper part of the water column seaward of the steep continental rise southeast of Iceland. This tidal burst was coincident with an alongslope current pulse formed by water from the Nordic seas, which had overflowed the Iceland-Faeroe Ridge. The tidal increase is shown to be caused by enhanced generation of internal tide when the alongshore current elevated otherwise deep isopycnals to higher levels, thereby increasing conversion from barotropic to baroclinic tides. A two-dimensional model, used to illustrate the mechanism, underpredicts the increased tidal amplitude and suggests that three-dimensional effects play a role in the present case.

Sedimentation patterns caused by scallop dredging in a physically dynamic environment

Scallop dredging grounds in the Firth of Lorn, western Scotland, are juxtaposed with rocky reef habitats raising concerns that reef communities may be impacted by sediment disturbed by nearby scallop dredging. A particle-tracking model of sediment transport and settling is applied at two scales. In the near-field, a suspension of typical sand/gravel-dominated bed sediment is subjected to a steady current across the dredge track. In the far-field, silt particles, which may persist in suspension for multiple tidal cycles, are tracked in the context of a regional model of tidally-driven flow. The principal sedimentary risk to reef habitats is predicted to come from settling sand particles when dredge tracks approach within tens of metres of a reef. The cumulative effect of dredging at the relatively low intensities recorded in this region is not expected to have a significant long-term impact on suspended silt concentrations and settlement in this highly dispersive environment.

The impact of ambient stratification on marine outfall studies in British waters

Abstract Not all of Britains coastline has strong tidal currents with a turbulent and mixed water column (as is often supposed). In St Austell Bay, Cornwall, very weak currents result in significant surface stratification close inshore in summer. Brackish water density currents and wind driven flows dominate the surface circulation, which can be very different to that only a few metres below, so that stratification can have a significant effect on the ultimate fate of buoyant marine discharges. Usual outfall survey techniques such as drogue-float tracking, tidal analysis and two-dimensional numerical models have to be treated with great care since the dynamics of stratified waters are very different to those of mixed waters.

Current status of deepwater oil spill modelling in the Faroe-Shetland Channel, Northeast Atlantic, and future challenges

As oil reserves in established basins become depleted, exploration and production moves towards relatively unexploited areas, such as deep waters off the continental shelf. The Faroe-Shetland Channel (FSC, NE Atlantic) and adjacent areas have been subject to increased focus by the oil industry. In addition to extreme depths, metocean conditions in this region characterise an environment with high waves and strong winds, strong currents, complex circulation patterns, sharp density gradients, and large small- and mesoscale variability. These conditions pose operational challenges to oil spill response and question the suitability of current oil spill modelling frameworks (oil spill models and their forcing data) to adequately simulate the behaviour of a potential oil spill in the area. This article reviews the state of knowledge relevant to deepwater oil spill modelling for the FSC area and identifies knowledge gaps and research priorities. Our analysis should be relevant to other areas of complex oceanography.