Density Currents in Tropical and Saline Lakes

In tropical lakes and also in saline lakes several of the processes mentioned above do not occur. Because the development of a therma bar requires a lateral transition of water temperature from above to below Tmd at the lake surface, a thermal bar never occurs in tropical lakes where water temperatures are above 4 °C all year round and thus always exceed Tmd. Because Tmd decreases with increasing salinity reaching freezing temperature at about 25gkg_1, thermal bar development and the associated density currents also do not play an important role for the vertical exchange in saline lakes. The same arguments exclude the thermobaric effect as a significant cause of density currents in tropical and saline lakes.

In tropical lakes temperature gradients play the dominant role in the generation of density currents because at high water temperatures, slight temperature gradients imply large differences in density. Therefore, salinity gradients play a smaller role for density plume generation in tropical lakes than for lakes of temperate regions during the cold season. Convection due to night-time cooling and density currents due to differential cooling and river inflows can be expected to be the most important processes for advective deep-water renewal in tropical lakes.

In saline lakes, however, river inflows usually cannot drive density currents because rivers typically introduce freshwater that in most cases has a much lower density than the saline lake water, even if the inflow has a low temperature. For example water with a salinity of 4gkg_1 and a temperature of 24 ° C has a greater density than freshwater with 4 °C. Hence, river inflows can only directly drive density currents in saline lakes, if the riverine water carries a substantial load of suspended particles. Nevertheless, in saline lakes river inflows can generate large scale differences in salinity and thus may indirectly lead to density currents at sills induced by inter basin exchange as is probably the case in the Caspian Sea. Another process that is likely to cause density currents in saline lakes is differential cooling. In contrast to freshwater lakes this process can trigger density currents with temperatures well below 4 °C and even down to freezing temperature, because Tmd can be substantially reduced or even does not exist depending on the salinity of the water.

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