Influence of stratification on decaying surface seiche modes

Abstract

A traditional understanding of external (surface) seiche modes in lakes, bays and coastal seas is that their period depends exclusively on the basin geometry (size, depth) and planetary constants (gravity, Coriolis parameter) and is therefore independent of the water stratification. We show here that this is not the case. In enclosed water bodies such as lakes, in which external seiches are standing waves, their period is weakly dependent on the stratification (on the order of Δ ρ / ρ ), lending support to the traditional understanding. But, in semi-enclosed systems such as bays and coastal seas, from which surface gravity waves can radiate into the open ocean, the external seiche period is a more sensitive function of stratification (on the order of Δ ρ / ρ ). Those seiches also decay over time, as wave radiation gradually takes their energy away. This dependency of the period of the external seiches on water stratification may explain why repeated observations of seiches in the Adriatic Sea have yielded no precise values for the periods of the first few seiche modes. This situation has implications in practical applications such as the forecasting of flooding at Venice.