Thermohaline forcing and interannual variability of northwestern inflows into the northern North Sea. (15th April 2017)

• Record Type: Journal Article
• Authors: Sheehan, Peter M.F.; Berx, Barbara; Gallego, Alejandro; Hall, Rob A.; Heywood, Karen J.; Hughes, Sarah L.
• Is Part Of: Continental shelf research. Volume 138(2017)
• Page Start: 120
• Page End: 131
• Publication Date: 2017-04-15
• Subjects: North Sea -- Fair Isle Current -- East Shetland Atlantic Inflow -- Thermohaline circulation -- Seasonal variability -- Interannual variability; Continental shelf -- Periodicals; Submarine geology -- Periodicals; 551.41
• DOI: 10.1016/j.csr.2017.01.016
• Languages: English
• ISSNs: 0278-4343

Abstract

Abstract: A long-established, 127 km-long hydrographic section in the northern North Sea at 59.28°N that runs from the eastern coast of Orkney (2.23°W) to the central North Sea (0°) crosses the path of the main inflows of Atlantic water. Data from 122 occupations between 1989 and 2015 are examined to determine the annual cycle and long-term trends of temperature, salinity and depth-varying geostrophic flow across the section. In an average year, the geostrophic flow referenced to the seafloor is at its narrowest (40 km) in winter, during which time it is driven by the strong horizontal salinity gradient; the horizontal temperature gradient is very weak. Velocity exceeds 4 cm s −1, but transport is at a minimum (0.11 Sv). In the deeper water in the east of the section, thermal stratification develops throughout summer and persists until October, whereas the west is tidally mixed all year. The bottom temperature gradient becomes the primary driver of the geostrophic flow, which is fastest (9 cm s −1 ) in September and broadest (100 km) in October. Maximum transport (0.36 Sv) occurs in October. Throughout the summer, the horizontal salinity gradient weakens, as does its contribution to the flow. However, it nevertheless acts to broaden the flow west of the location of the strongest horizontal temperature gradient. Section-mean de-seasoned temperature is found to be positively correlated to the Atlantic Multidecadal Oscillation and negatively correlated to the North Atlantic Oscillation. These results refine our understanding of the thermohaline forcing of Atlantic inflow into the northern North Sea, particularly in relation to the salinity distribution. Understanding the variability of this inflow is important for understanding the dynamics of the North Sea ecosystem. Abstract : Highlights: Robust seasonal-means and annual cycles are calculated for North Sea inflow. There is a strong annual cycle in density-driven transport. Salinity gradients maintain thermohaline flow in winter. De-seasoned temperature and salinity are related to the NAO and AMO.