A criterion to locate regions with anticyclonic tidal current rotation

The equations of motion written in rotational form, with the rotary components of the velocity R+ (cyclonic) and R− (anticyclonic) and the rotary components of sea surface elevation gradients S+ (cyclonic) and S− (anticyclonic), are applied to investigate a link between sea surface amphidromic system and sense of rotation of tidal currents. It is shown that the assumptions S+=S− or |S+|=|S−| lead to a criterion to locate regions with anticyclonic tidal current rotation from co-tidal and co-range charts. Anticyclonic rotation seems to occur in regions where the gradients of sea surface elevation ζ and phase φζ tend to be parallel, i.e. where ∇ζ×∇φζ=0. It is also shown that this situation takes place in areas where the limits ∇ζ→0 and ∇φζ→0 exist, i.e. where co-tidal and co-range lines are quite separated. Results of numerical modelling of the M2 tide in the North Sea and the Bohai Sea validate this prediction.