Forcing and variability of northwestern inflows into the northern North Sea

Quantifying the volume of water flowing into the North Sea and understanding its temporal and spatial variability is essential for understanding, modelling and managing North Sea ecosystems. Here, northwestern inflows are studied using traditional and cutting-edge instrumentation from a hydrographic section. Long-term variability is assessed using a 27-year time series of temperature and salinity observations from hydrographic cruises. There is a pronounced annual cycle of temperature-driven thermohaline flow that is tied the growth and decay of stratification. When present, this flow is concentrated above a strong bottom front that separates mixed waters in the west from seasonally stratified waters in the east. The front is maintained in summer by the balance between tidal stirring and surface heating. Salinity-driven flow, not identified in previous studies, maintains appreciable thermohaline flow throughout the winter when thermal stratification is absent and when salinity gradients maintain the frontal boundary. Short-term variability is assessed using observations from a two-month ocean glider deployment in autumn of 2013. The properties of inflowing water masses are observed at high resolution. The distribution of fresh coastal water and saline oceanic water are more spatially variable than previously thought, and the decay of thermal stratification during the deployment permits a greater volume of oceanic water to enter the North Sea. Absolute transport (mean 0.44 Sv) calculated using glider dive-average currents indicate that the flow is predominantly barotropic, with a small contribution from thermohaline transport (mean 0.14 Sv). However, the limited width of the glider-occupied section hinders separation of spatial and temporal changes. Wind forcing, which acts to “push” water into the North Sea, has a moderate influence on daily current variability, likely because the presence of stratification and shallow Ekman layers promotes a two-layer flow structure. The advantages and disadvantages of ocean gliders as a shelf sea observing platform are considered