Past exam of the physics of atmospheres and oceans course of the University of Oxford / 2018 / 5 / c

For an eastward tidal signal entering the English Channel from the Atlantic, the northern coast of France lies on the right of propagation, with land to the south. It is therefore the trapping coast of the corresponding northern-hemisphere Kelvin mode. The south coast of England is across the ocean from that boundary and receives a smaller amplitude through the offshore exponential decay. For a uniform channel of width $W$, this ideal mode would have $A_{\mathrm{England}}/A_{\mathrm{France}}=e^{-W/L_R}$.

Thus the mode assumption predicts the greater tidal range on the French side. It does not mean all local tides are a single Kelvin wave: channel depth and shape, friction, reflections, resonances and interference also influence the actual tidal range. The explanation concerns the stated eastward incoming tidal-mode idealization, not a universal comparison at every location or tidal constituent.