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

Abstract

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.

Seasonal habitat‐based density models for a marine top predator, the harbor porpoise, in a dynamic environment

“…= fall); this division also corresponds well to temperature cycles in the North Sea (Nauw et al. ). In addition, this division also captures sensitive periods in the annual reproduction cycle of harbor porpoises in the North Sea, where the birth and mating period occurs in summer (Lockyer ).…”

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

“…The thermocline breakdown at the 22/4b site in 2011 also was earlier than expected from climatology in this area (Berx and Hughes, 2009), which indicates a breakdown in the end of November/early December. In Figure 10 of Nauw et al, 2015), it can be seen that the difference in climatological temperature between the sea surface and near-bed layer is very similar at both sites throughout the year. It appears that the breakdown of the thermocline near the 22/4b site in 2011 occurs anomalously early in the season with respect to nearby locations.…”

“…Due to nonlinear interactions between the tidal components and topography, a residual circulation is driven in the same direction as the tidal wave, which enters the North Sea in the Northwest from the Atlantic and in the South through the English Channel and follows an anti-clockwise circulation (Nauw et al, 2015;Otto et al, 1990). At the 22/4b site, the M2 tidal current is (uM2, vM2)=(0.04, 0.23) m/s and the S2 tidal current is (uS2, vS2)=(0.02, 0.09) m/s, derived from results of the OSU Tidal Prediction Model Software applied to the European Shelf (Egbert et al, 2010).…”

“…The FIC strength displays seasonal variability and has an average velocity of about 0.1 m/s (ICES, 2005;Turrell et al, 1990) and is directed approximately to the north at the 22/4b site following the bathymetry. See Nauw and de Haas, 2013) and Nauw et al, 2015) for a complete review of North Sea circulation with an emphasis on the hydrographic conditions around site 22/4b.…”

“…It is at the transition between fully-mixed and stratified waters that mixing fronts occur. Climatological temperatures observations (Berx and Hughes, 2009) show that seasonal stratification starts in April/May in the central North Sea leading to mixing fronts just south of and/or around the Dogger Bank (Holt and Umlauf, 2008;Nauw et al, 2015;Pingree and Griffiths, 1978).Starting in September, it retreats to the north, passing across the 22/4b site sometime in November. Between the end of December and the end of April, the entire North Sea is wellmixed.…”