Abstract
Abstract. Free sea level oscillations in barotropic and baroclinic conditions were examined using numerical experiments based on a 3-D hydrodynamic model of the Baltic Sea. In a barotropic environment, the highest amplitudes of free sea level oscillations are observed in the northern Gulf of Bothnia, eastern Gulf of Finland, and south-western Baltic Sea. In these areas, the maximum variance appears within the frequency range corresponding to periods of 13–44 h. In a stratified environment, after the cessation of meteorological forcing, water masses relax to the equilibrium state in the form of mesoscale oscillations at the same frequencies as well as in the form of rapidly decaying low-frequency (seasonal) oscillations. The total amplitudes of free baroclinic perturbations are significantly larger than those of barotropic perturbations, reaching 15–17 cm. Contrary to barotropic, oscillations in baroclinic conditions are strongly pronounced in the deep-water areas of the Baltic Sea proper. Specific spatial patterns of amplitudes and phases of free barotropic and baroclinic sea level oscillations identified them as progressive–standing waves representing barotropic or baroclinic modes of gravity waves and topographic Rossby waves.
Highlights
The Baltic Sea level perturbations represent a superposition of free and forced oscillations of different spatial and temporal scales
The present study investigates the difference between barotropic and baroclinic free sea level oscillations in the Baltic Sea using a three-dimensional hydrodynamic model
Our numerical experiments based on a three-dimensional hydrodynamic model demonstrated that after the cessation of meteorological forces, the return of the Baltic Sea water mass to equilibrium in barotropic and baroclinic conditions is different
Summary
The Baltic Sea level perturbations represent a superposition of free and forced oscillations of different spatial and temporal scales. The basin water masses return to equilibrium conditions after meteorological forcing (Fennel and Seifert, 2008; Leppäranta and Myrberg, 2009; Lisitzin, 1974) Within these frequencies, the free oscillations may resonate with wind forcing, resulting in an anomalous sea level rise followed by the inundation of coastal areas (Jönsson et al, 2008; Zakharchuk and Tikhonova, 2011). Jönsson et al (2008), based on the analysis of linear shallow-water model simulations, identified three different local oscillatory modes: in the Gulf of Finland (with two 23 and 27 h periods), Danish Great Belt (with periods of 23–27 h), and Gulf of Riga (with 17 h periods) The authors attributed these variations to seiches and noted that they were not connected to each other.
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