Evolution and decay of gravity wavefields in weak-rotating environments: a laboratory study

Gravity waves are prominent physical features that play a fundamental role in transport processes of stratified aquatic ecosystems. In a two-layer stratified basin, the equations of motion for the first vertical mode are equivalent to the linearised shallow water equations for a homogeneous fluid. We adopted this framework to examine the spatiotemporal structure of gravity wavefields weakly affected by the background rotation of a single-layer system of equivalent thickness (h_{ell }), via laboratory experiments performed in a cylindrical basin mounted on a turntable. The wavefield was generated by the release of a diametral linear tilt of the air–water interface, (eta _{ell }), inducing a basin-scale perturbation that evolved in response to the horizontal pressure gradient and the rotation-induced acceleration. The basin-scale wave response was controlled by an initial perturbation parameter, ({mathcal{A}}_{*} = eta _{0}/h_{ell }), where (eta _{0}) was the initial displacement of the air–water interface, and by the strength of the background rotation controlled by the Burger number, ({mathcal{S}}). We set the experiments to explore a transitional regime from moderate- to weak-rotational environments, (0.65le {mathcal{S}} le 2), for a wide range of initial perturbations, (0.05le {mathcal{A}}_{*}le 1.0). The evolution of (eta _{ell }) was registered over a diametral plane by recording a laser-induced optical fluorescence sheet and using a capacitive sensor located near the lateral boundary. The evolution of the gravity wavefields showed substantial variability as a function of the rotational regimes and the radial position. The results demonstrate that the strength of rotation and nonlinearities control the bulk decay rate of the basin-scale gravity waves. The ratio between the experimentally estimated damping timescale, (T_{d}), and the seiche period of the basin, (T_{g}), has a median value of (T_{d}/T_{g}approx 11), a maximum value of (T_{d}/T_{g}approx 10^{3}) and a minimum value of (T_{d}/T_{g}approx 5). The results of this study are significant for the understanding the dynamics of gravity waves in waterbodies weakly affected by Coriolis acceleration, such as mid- to small-size lakes.