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 the 22/4b Site, located at (57°55′N, 1°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 a counter-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 NAO- conditions 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 the well 22/4b Site. Summertime heating stratifies the northern part of the North Sea. In autumn, loss of heat to the atmosphere causes the stratification to breakdown 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.