Are Extreme Skew Surges Independent of High Water Levels in a Mixed Semidiurnal Tidal Regime?

Based on previous studies of tide-gauge records from locations with semidiurnal tidal regimes, extreme skew surges are always assumed independent of the high water (HW). However, differences in water depth between HW peaks of semidiurnal tidal regimes can be much lower than those in mixed semidiurnal regimes, where one daily HW is higher than the other. We statistically analyze tide-gauge records of 15 sites worldwide with a mixed semidiurnal regime and find that for approximately half of these sites extreme skew surges occur more often during smaller HWs. This dependence is not caused by seasonality effects, and the places that show dependence are all located in areas with a shallow continental shelf, thus indicating possible tide-skew surge interactions. In those cases where dependence exists, using the skew surge does not seem to offer any advantages over the nontidal residual. Plain Language Summary The water level during storms (i.e., storm surges) in coastal areas largely depends on the level of the tide. This effect can be more noticeable in places with a mixed semidiurnal regime, which is characterized by two high water peaks per day, with one high water peak being larger than the other. Knowledge of this effect can help forecasting and prevention of flooding events in these regions. We statistically analyze water level records of 15 sites worldwide with a mixed semidiurnal regime and find that in around half of these sites storm surges occur more often at lower high water levels. These places are all located in areas with a shallow continental shelf, such as estuaries or closed seas.