Inertial currents in the northern North sea

Knight, P.; Howarth, M.J.; Rippeth, Tom P.

doi:10.1016/s1385-1101(02)00122-3

Cited by 37 publications

(24 citation statements)

References 11 publications

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“…A mid-depth local maximum in the absolute shear, |S|, is observed around the mean location of the MLD using the TDA or derived from the temperature profiles (Figure 9c). The local maximum in the shear at the location of the pycnocline is between 0.03 and 0.04 s -1 and is comparable to the vertical shear observed during the PROVESS experiment as shown by Knight et al, 2002). In the latter paper, these shears were found to be associated with inertial oscillations.…”

Section: Thermocline Detection Algorithmsupporting

confidence: 80%

“…The dominant processes in de-stratification are the (turbulent) heat and momentum flux by wind and (breaking) waves at the seas surface and shear-induced turbulence by bottom friction at the seabed. In general, turbulent fluxes at the thermocline are small except under certain conditions, such as breaking of internal waves which are generated by tide-topography interaction (Rippeth, 2005;van Haren et al, 1999) and shear generated by inertial currents following strong wind-forcing events (Knight et al, 2002;MacKinnon and Gregg, 2005;Rippeth, 2005;van Haren, 2000). Enhanced shear and mixing occurs when wind-stress, the bottom shear-stress (which is mostly tidally-driven), and the inertial shear-vector all are aligned (Burchard and Rippeth, 2009).…”

Section: Seasonal Thermoclinementioning

confidence: 99%

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Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

Nauw

Линке

Leifer

2015

Marine and Petroleum Geology

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The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57 o 55'N, 1 o 38'E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the near-bottom temperature. The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front. 2

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Section: Thermocline Detection Algorithmsupporting

confidence: 80%

Section: Seasonal Thermoclinementioning

confidence: 99%

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

Nauw

Линке

Leifer

2015

Marine and Petroleum Geology

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“…At the interface between the surface and bottom mixed layer, turbulence levels are reduced and vertical fluxes are small, except under certain conditions. For instance, vertical fluxes can be generated by internal waves breaking at the interface, when generated by tidal flows forced over topography (Rippeth, 2005;van Haren et al, 1999) or shear generation across the interface by inertial currents following a strong wind forcing event (Knight et al, 2002;MacKinnon and Gregg, 2005;Rippeth et al, 2005;van Haren, 2000). Periods of enhanced shear can lead to shear instability and diapycnal mixing as the system appears to be only marginally stable (van Haren et al, 1999).…”

Section: Climatology and Seasonal Stratificationmentioning

confidence: 99%

A review of oceanographic and meteorological controls on the North Sea circulation and hydrodynamics with a view to the fate of North Sea methane from well site 22/4b and other seabed sources

Nauw

Haas

Rehder

2015

Marine and Petroleum Geology

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The North Sea hydrodynamics are key to the redistribution of methane released at site 22/4b, located at (57 o 55'N, 1 o 38'E) in the UK Central North Sea, 200 km east of the Scottish mainland. This review summarizes the current state of knowledge on the North Sea circulation, stratification and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22/4b or other seabed sources. Astronomical tidal waves follow an anti-clockwise path and tide-topography interaction generates a residual circulation in the same direction. Wind stress forcing can enhance, reduce or even reverse this circulation. Variations in the strength of the Fair Isle Current (FIC) are important. The FIC enters the North Sea between The Orkneys and Shetland, follows approximately the 100-m isobath, passes along site 22/4b, and ends up in the Norwegian Trench. The North Atlantic Oscillation (NAO) also causes variability. A positive (negative) NAO index is associated with stronger (weaker) than normal westerly winds. NAO+ situations strengthen the circulation in the North Sea, whereas it weakens during NAOconditions and is directed northeastward. High positive correlations exist between the SST at site 22/4b and the NAO index. Climate change can have a long-term effect on the hydrodynamics of the North Sea. Seasonal stratification has potentially the most important imprint on methane derived from well site 22/4b. Summertime heating stratifies the northern part of the North Sea. In autumn, loss of heat to the atmosphere causes the stratification to break down until tides and storms mix the entire water column. During the period of stratification, the bulk of (dissolved) methane released from site 22/4b gets trapped below the thermocline. The loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation (oxidation) in the water column.

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“…Although measurements of inertial currents in deep water [ Weller , 1982] have demonstrated that they are generally surface trapped and can be represented by single point models containing specified dissipation and horizontal shear terms, in coastal regions a number of authors [ Rippeth et al , 2002; Knight et al , 2002; Orlic , 1987; de Young and Tang , 1990; Font , 1990] showed that there were near‐inertial currents throughout the water column with a 180° phase change in the current through depth. The inertial oscillations at depth arose through the presence of a coastline which inhibited the presence of inertial oscillations at the coastal boundary, and the inertial energy propagated downward and offshore from the coastline.…”

Section: Introductionmentioning

confidence: 99%

Influence of coastal effects, bottom topography, stratification, wind period, and mixing upon the propagation of internal waves and the profile of wind‐induced currents

Xing

Davies

2003

J. Geophys. Res.

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[1] A three-dimensional model in cross-sectional form is used to examine the influence of wind period, stratification and bottom topography upon internal wave generation and wind-induced current profiles in the coastal ocean. Calculations with fixed vertical diffusion coefficients and with a turbulence energy model are performed. These show that besides the wind period influencing the depth of penetration of the wind's momentum, as in a point model, it determines the offshore extent of internal waves generated at the coast. These waves together with elevation gradients modify the directly wind-driven current profile found in a point model. For forcing at the subinertial frequency the internal wave field is trapped in the coastal region. Offshore the current can be derived from a point model incorporating wind forcing and an elevation gradient related to it. For wind forcing at the superinertial frequency the offshore extent of internal wave propagation increases with frequency. The local topography influences the offshore propagation of internal waves, leading at superinertial frequencies to regions of enhanced surface current magnitude in a similar manner to that found with internal tides. Calculations with the turbulence energy model show increased surface diffusion in these regions, which allows wind energy at both superinertial and subinertial frequencies to penetrate to depth in offshore areas. This process does not involve subinertial internal wave propagation. Conclusions are drawn as to the conditions under which a single point model can reproduce wind induced current profiles at an offshore location.INDEX TERMS: 4219 Oceanography: General: Continental shelf processes; 4512 Oceanography: Physical: Currents; 4544 Oceanography: Physical: Internal and inertial waves; KEYWORDS: internal waves, bottom topography, coastal processes, wind period, mixing, turbulence energy Citation: Xing, J., and A. M. Davies, Influence of coastal effects, bottom topography, stratification, wind period, and mixing upon the propagation of internal waves and the profile of wind-induced currents,

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Inertial currents in the northern North sea

Cited by 37 publications

References 11 publications

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

Bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern North Sea

A review of oceanographic and meteorological controls on the North Sea circulation and hydrodynamics with a view to the fate of North Sea methane from well site 22/4b and other seabed sources

Influence of coastal effects, bottom topography, stratification, wind period, and mixing upon the propagation of internal waves and the profile of wind‐induced currents

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