Internal seiches in the strongly stratified Dead Sea

During two cruises, 8-10 June 1993 and 3-4 October 1994, a total of 60 temperature profiles and 8 salinity profiles were taken at 4 sites in the Dead Sea, Israel. The data collected show that a shallow (10-20 m deep) and sharp pycnocline (about 30 σ-units in 1993 and over 6 σ units in 1994 over a thickness of a few meters at most) existed on both occasions. Our data do not indicate horizontal heterogeneities in the thermal or saline structure, so that any station within the lake, far enough from the coast, is representative of the entire lake. An analysis of the data shows the propagation of internal seiches through the sites. The waves' amplitude, as determined form the change in the depth of maximal vertical gradient of temperature, is about 3 m at the four sites. The phase speed and direction of propagation of these waves are estimated to be close to 0.5 m.s^-1 and 15° due West of South, respectively. The waves propagate southward along the lake's western shore, and this cyclonic propagation sense, as well as their dispersion relation, are reminiscent of internal Kelvin waves. The extremely large density jump at the pycnocline and the accurate determination of its depth yield a very precise fit of the measured period of the internal seiches to theoretical predictions. These data show, for the first time, that the widely used linear dispersion relation of internal waves is valid even at large reduced gravity of the order of 10^-1 m^s-2.