The influence of tides on the winter residual circulation of the North West European Shelf

<p>The Northwest European Shelf Seas (NWS) are geographically complex and are dominated by tidal processes. Tides drive vertical mixing, stratification, and contribute to the residual circulation, which influences the spatial patterns of temperature and salinity. Tides are not modelled in the current generation of Global Climate Models (GCMs), which is one of the reasons that the NWS is typically poorly simulated in most GCMs. Marine climate impact studies usually require dynamic downscaling with a tidal coastal ocean model to make use of GCM model data. However, this is expensive &#8211; the direct use of GCM data (for the NWS) would be preferable. The impact of tides on the vertical mixing and stratification of the NWS has been extensively studied. Here we focus on the impact of tides on the residual circulation, and whether this can be parameterised in non-tidal models.</p><p>We run a pair of NWS simulations with and without tides, to show the impact of tides on the NWS winter residual circulation. We show regional differences in temperature and salinity of up to ~0.5&#176;C, and 0.5 psu, which can be explained by the changes in advective pathways associated with the differences in the residual circulation.</p><p>We explore two processes by which the tides impact the residual circulation and investigate whether these could be parameterised within non-tidal GCMs: (1) Enhancing the seabed friction to mimic the equivalent energy loss from an oscillating tidal flow; (2) Tidal Phase-driven Transport (TPT), whereby tidal asymmetry drives a net transport due to the phase between tidal-elevation and velocities (equivalent to the bolus term in oceanographic literature).</p><p>To parameterise the TPT, we calculate a climatology of the transport from a harmonic tidal analysis from the tidal model and add it into the Navier Stokes equations of the non-tidal model as an additional force. Additionally, we modify the bed drag coefficient to balance the bed stress between the simulations &#8211; hypothesising that using this modified bed drag coefficient will simulate the effect of the tides. The modified drag coefficient tends to improve the mean and variability of the residual circulation, while the TPT improves the spatial distribution and temporal variability of the temperature and salinity. We show that our proof-of-concept parameterisation can replicate the tidally-driven impact on the residual circulation without direct simulation, thus reducing computational effort.</p><p>If our approach can be included in global ocean and climate models, we may be able to improve the representation of residual circulation in tidal regions, and their distribution and variability of tracers (including temperature, salinity and nutrients). This could allow some NWS impact studies to be based directly on GCM data, without the need for costly dynamic downscaling, making GCM data much more useful to end users in many regions around the world.</p><p>This work has been submitted to Frontiers of Marine Science.</p>