How European Climate Changes When You Include Ocean Tides in Regionally Coupled Ocean-Atmosphere Model Simulations.

We use the regionally coupled atmosphere-ocean-sea ice-marine biogeochemistry model ROM in order to investigate effects of the ocean tides on the European climate. The ocean part of the model is global with increased resolution over the North Atlantic. Coupling with the atmosphere is limited to the domain that covers North Atlantic, Europe and Eurasian Basin of the Arctic Ocean. We use AOGCM (ECHAM5-MPIOM) simulation as atmospheric boundary conditions Two sets of experiments were performed: with the ocean tidal forcing derived from the full ephemeridic lunisolar tidal potential and without ocean tides.

The agreement between observed and simulated tides in the North Atlantic is reasonable for a climate model, the amphidromic points are well captured. The simulation with tides shows pronounced differences in the structure of the surface circulation as compared to the simulation without tides. The Labrador current penetrates further to the south and “pushes” Gulfstream south-westward, which makes its simulated position closer to the observed one. The consequent shift of the North Atlantic current and the tidal circulation around the North Atlantic amphidromy supply more salty Atlantic water in to the Subpolar Gyre, thus increasing its strength and convection activity in the Labrador Sea.

Inclusion of tides leads to considerable changes in the simulated European climate. The main reason is increased mixing in the North Atlantic. We show that for mean 2 m temperatures over the Western Europe simulations with tides result in 0.2-0.4 K positive biases in winter and spring and 0.2-0.4 negative biases in summer and autumn.