Seiche: the phenomenon that rocked the Earth in September 2023

Deleersnijder, Eric [UCL]

In September 2023 a rockslide generated a tsunami approximately 200 m in height in a Greenland fjord, which in turn triggered strong seiching motion (initial amplitude of order 7 m) lasting for about 9 days (Svennevig et al., Science, 2024). The amount of water involved in the latter phenomenon was so huge that it was detected by most of Earth's seismographs. A seiche is a large-scale oscillation of the surface (external seiche) or the pycnocline (internal seiche) of enclosed or nearly enclosed water bodies that is usually thought of as a standing wave. This is why eigenmodes of oscillation have been regarded as key in comprehending seiches, with generally a focus on the first oscillatory modes. We submit that there are several misconceptions about seiches. For simple initial conditions or forcings, the first mode of oscillation, a well-behaved function, is barely representative of the solution of the PDEs governing one-dimensional seiching motion. Presumably, continuous piecewise linear functions (e.g., triangle waves) should be resorted to for dealing with either free or forced oscillations. Accordingly, we believe that classical cartoons meant to illustrate the dynamics of external or internal seiches should be revised. Furthermore, some well-known oscillatory processes (e.g., thermocline oscillations in Lake Tanganyika) are perhaps wrongly interpreted as seiches: they are probably directly forced by wind stress variations, leading to near-resonance. Finally, we question the relevance of external and internal seiches as a test case of choice for assessing the mode splitting of three-dimensional numerical models of lakes and seas.