ALBERT

Description: Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. We use dive-average currents from a two-month (12th October–2nd December 2013) glider deployment along a zonal hydrographic section in the northern North Sea to determine M2 and S2 tidal velocities, which agree well with tidal velocities measured by current meters and extracted from a tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We use the glider-derived tidal velocities to investigate tidal controls on the location of a tidal mixing front. During the deployment, the front moves offshore at a rate of 0.51 km day−1. During the first period of the deployment (i.e. until mid November), the front's position is explained by the local balance between tidal mixing and surface heat fluxes: as heat is lost to the atmosphere, full-depth tidal mixing is able to occur in progressively deeper water. In the latter half of the deployment, the output of a simple one-dimensional model suggests that the front should have decayed. By comparing this model output to hydrographic observations from the glider, we attribute the persistence of the front beyond this period to the advection of cold, saline Atlantic-origin water across the deeper portion of the section. The glider captures the transition of the front from being one controlled by the balance between tidal mixing and surface heating, to being one controlled by advection of buoyancy. Fronts in shelf regions with oceanic influence may be geographically fixed and persist during periods of little to no thermal stratification, with implications for the thermohaline circulation of shelf seas.

Description: Tides and tidal mixing fronts are of fundamental importance to understanding shelf sea dynamics and ecosystems. Ocean gliders enable the observation of fronts and tide-dominated flows at high resolution. We use dive-average currents from a 2-month (12 October–2 December 2013) glider deployment along a zonal hydrographic section in the north-western North Sea to accurately determine M2 and S2 tidal velocities. The results of the glider-based method agree well with tidal velocities measured by current meters and with velocities extracted from the TPXO tide model. The method enhances the utility of gliders as an ocean-observing platform, particularly in regions where tide models are known to be limited. We then use the glider-derived tidal velocities to investigate tidal controls on the location of a front repeatedly observed by the glider. The front moves offshore at a rate of 0.51 km day−1. During the first part of the deployment (from mid-October until mid-November), results of a one-dimensional model suggest that the balance between surface heat fluxes and tidal stirring is the primary control on frontal location: as heat is lost to the atmosphere, full-depth mixing is able to occur in progressively deeper water. In the latter half of the deployment (mid-November to early December), a front controlled solely by heat fluxes and tidal stirring is not predicted to exist, yet a front persists in the observations. We analyse hydrographic observations collected by the glider to attribute the persistence of the front to the boundary between different water masses, in particular to the presence of cold, saline, Atlantic-origin water in the deeper portion of the section. We combine these results to propose that the front is a hybrid front: one controlled in summer by the local balance between heat fluxes and mixing and which in winter exists as the boundary between water masses advected to the north-western North Sea from diverse source regions. The glider observations capture the period when the front makes the transition from its summertime to wintertime state. Fronts in other shelf sea regions with oceanic influence may exhibit similar behaviour, with controlling processes and locations changing over an annual cycle. These results have implications for the thermohaline circulation of shelf seas.