Shelf sea tidal currents and mixing fronts determined from ocean glider observations.

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. [ABSTRACT FROM AUTHOR]