Stratification potential-energy anomaly index standardized by external tide level

Simpson's stratification index phi is widely used in oceanography as an index of vertical stratification. However, the use of phi can be problematic when the tidal amplitude is significant compared to the water depth, since phi 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 phi, 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*).

Automated summary

The Simpson's stratification index phi may have limitations in scenarios with significant tidal amplitudes, therefore the stratification potential-energy anomaly V* is proposed as a more suitable standardized stratification index in estuaries and regions of freshwater influence. Analysis of observational data from Isahaya Bay in Japan shows that the V* index is most effective in removing tidal cycle fluctuations.