New data on vertical structure and variability of Lake Issyk-Kul

Lake Issyk-Kul is the World's second deepest mountain lake (depth 668 m), containing over 1700 km³ of brackish (about 6 g kg^-1) water. It has been demonstrated by analyses of chemical tracers that the lake mixes very intensively, with the bottom water residence time of only about 10 years [Hofer et al., 2002], although the mechanisms resonsible for such a rapid renewal remain unknown. Some previous studies also suggested that the deep layers of the Issyk-Kul were subject to significant warming at decadal scales in response to climate forcing, however, direct measurements of the lake's thermohaline structure are very sparse.

Field measurements carried out in 5 consecutive expeditions (2015-2019) made it possible to establish previously unknown features of thermohaline fields and circulation of Lake Issyk-Kul. The most detailed ever salinity distribution maps 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, sub-basin scale (which is partly confirmed by direct measurements of the current velocity). It has been established that, generally speaking, salinity fields in Issyk-Kul are extremely conservative - their 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 maximum of salinity 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 measurements of current velocities, the autumn-winter differential cooling leads to the fact that in canyons (i.e., the ancient river 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 more saline waters lying above, which, in a situation where the upper layer of the lake is also desalinated by river runoff, leads to the appearance of a salinity maximum at intermediate depths. Our measurements do not confirm the manifestations of global warming in the form of an inter-decadal 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^oC. 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. Based on the analysis of water samples taken from the lake and from the 12 main tributary rivers, improved estimates of the nutrient budget were also obtained.