Features of Thermohaline Structure and Circulation in Lake Issyk-Kul

Abstract

Field measurements carried out over 4 years made it possible to establish previously unknown features of the thermohaline fields and circulation of Lake Issyk-Kul. The most detailed salinity distribution maps for the entire history of observations were constructed. An area of slightly increased salinity was found in the central part of the lake, the specific “dipole” shape of which indicates the existence of not only a general cyclonic circulation, but also two separate gyres of a smaller, subbasin scale (which is partially confirmed by direct current velocity measurements). It has been established that, generally speaking, the salinity fields in Lake Issyk-Kul are extraordinarily conservative: its interannual and seasonal changes, as well as spatial variability throughout the lake (with the exception of estuarine regions), are usually measured only in hundredths of g kg–1. An important result of the project is the discovery of a subsurface salinity maximum persisting from year to year at depths from 70 to 130 m. As shown on the basis of balance estimates and then confirmed by analysis of direct current velocity measurements, autumn–winter differential cooling leads to the fact that in canyons (i.e., paleoriver channels) in the eastern littoral region, a significant amount (up to 1 km3) of cold coastal waters freshened by river runoff enters the bottom layers of the central part of the lake. These waters are then mixed with the overlying more saline waters, which, in a situation where the upper layer of the lake is also desalinated by river runoff, leads to a salinity maximum at intermediate depths. Our measurements do not confirm manifestations of global warming in the form of an interdecadal temperature increase in the deeper layers of Lake Issyk-Kul, which was previously reported: the current (in 2018) temperature at a depth of 500 m exactly coincided with that noted in the 2003 measurements, namely, about 4.44°C. However, one can point to a very weak (about 0.03 g kg–1) increase in the salinity of the bottom layer over the past 40 years.