Diffusive-type of double diffusion in lakes - a review

This chapter is a contribution in honour of Gerhard H Jirka, who has been fascinated by the amazing variety of small-scale structures that nature surprises us with, particularly in stratified natural waters. Here, we focus on the diffusive regime of double-diffusive convection that occasionally occurs in lakes. Preconditions are a permanent stratification by dissolved constituents - such as salinity and carbon-dioxide – and convective forcing by deep sources of heat. After setting the stage for double diffusion to occur, possible genesis processes of the diffusive regime are reviewed by explaining specific examples of this unusual stratification such as (i) the flushing of fossil seawater by freshwater, (ii) the solar-pond phenomenon in ice-covered lakes in Antarctica, and (iii) the deep release of salt and gases in volcanic regions. In particular, the two most prominent examples of natural waters in which double diffusion occurs, Lakes Nyos and Kivu, are reviewed in more detail. The generation and evolution of staircase layering are discussed in relation to experiences gained from laboratory experiments, DNS modelling, and analysis of data from natural waters.