Stratification potential-energy anomaly index standardized by external tide level

Simpson's stratification index φ is widely used in oceanography as an index of vertical stratification. However, the use of φ can be problematic when the tidal amplitude is significant compared to the water depth, since φ fluctuates periodically with the external tide even if the vertical density structure does not change. The stratification potential-energy anomaly V is proportional to the square of the water column thickness D ; therefore, V ∗ = ( H / D ) 2 V (where H is the mean water depth) is an appropriate standardized stratification index for estuaries and regions of freshwater influence. The usefulness of this index is demonstrated using time series f k = H / D k V k = 0 , 1 , 2 , 3 ( f 0 = V , f 1 = H φ , f 2 = V * ) and their power spectra as obtained from tower B3 in Isahaya Bay in Japan. Tidal cycle fluctuation is removed most effectively for f 2 ( = V ∗ ) .