Horizontal transport and dispersion in the surface layer of a medium-sized lake

Lagrangian GPS drifter experiments, carried out in the surface layer of stratified Lake Kinneret (Israel), are presented. Differential kinematic properties and Lagrangian statistics were calculated and used to estimate the dominant mechanisms for horizontal dispersion. On time scales smaller than a few internal wave periods, internal waves lead to strong divergence and convergence events, causing instantaneous apparent horizontal growth rates that were larger, by up to an order of magnitude, than the actual mean dispersion coefficient. It is shown that the internal wave field modulated the vorticity field so as to satisfy conservation of potential vorticity. On time scales larger than a few internal wave periods, unbounded horizontal shear dispersion was of the same order as the actual mean observed dispersion coefficient (Kxy 5 17.1 m2 s21), while vertical shear dispersion was negligible. In a stratified lake, the surface layer is where most primary production takes place. Knowledge of the processes responsible for horizontal dispersion is therefore of key importance in understanding the biological dynamics of a lake. Smallto medium-sized lakes, in particular, are of ubiquitous nature and importance. Despite this, very few horizontal dispersion studies exist for lakes of these sizes, and those that do exist are often limited to the hypolimnion (Quay et al. 1979; Peeters 1994; Lawrence et al. 1995; Peeters et al. 1996). The size of a lake is interpreted here in terms of the effect of the Earth’s rotation on the lake’s response to external disturbances, summarized by the Burger number S 5 c/Lf (Antenucci and Imberger 2001), where c is the nonrotating internal wave phase speed, f the Coriolis frequency, and L the horizontal dimension of the lake. In large lakes and in the ocean, rotation confines the motion to the boundaries (S → 0), in medium lakes it influences the basin-scale response through a balance with stratification (S is O(1)), while in small lakes rotational effects are negligible (S → `). While so far no evidence exists that the Burger number directly affects dispersion processes, it will be shown that large-scale divergence and convergence events are associated with the internal wave field, whose shape and amplitude are in turn strongly dependent on the Burger number. For example, Antenucci et al. (2000) presented the seasonal evolution of the internal wave field in terms of the Burger number, finding that resonance conditions can be established as the stratification changes, while Stocker and Imberger (in