Seasonal stratification of a deep, high-altitude, dimictic lake: Nam Co, Tibetan Plateau and its thermal structure variations response to climate change

We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.)dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30º N), but high altitude where it receives high solar radiative forcing (observed maximum daily average 400 W m^-2), yet annual average air temperature is close to 0 ºC. While other continental mountain regions of the world have similar forcing, Nam Co is distinct from lakes in most well-documented dimictic lake regions, which are at higher latitude and lower altitude. Although the lake is endorheic, with a relatively high salinity (1.5 g L^-1), temperature effects dominate density stratification except at spring turnover. These characteristics create two important winter stratification features. 1) During fall turnover, once Nam Co cools below the temperature of maximum density, persistent winds aided by radiatively driven convection maintain a mixing water column (i.e., turnover) to water temperatures as low as 1 ºC throughout water column, prior to complete ice-cover. 2) A weakly stratified water column persists under ice-cover, while warming continuously at a rate comparable to the estimated solar radiative flux. While protracted fall cooling depresses hypolimnetic temperature, continuous under-ice heating compensates for the extended fall cooling, resulting in hypolimnetic temperature at spring turnover similar to that observed in high latitude lakes. These observations quantify stratification dynamics of Nam Co, whose altitude and climatic conditions are representative of the >1100, largely unstudied, lakes on the Tibetan Plateau. A General Lake Model was used to simulate water temperature changes in Nam Co for the period 1979 - 2012 and the results indicate that Nam Co has responded to the recent warming climate. The average summer surface water temperature fl uctuated yearly, but its trend is positive at a rate of 0.52 ± 0.25°C per decade. At the same time, the onset of summer stratifi cation advanced by 4.20 ± 2.02 d per decade, and the duration increased at a rate of 6.00 ± 3.54 d per decade. It is suggested that both increased air temperature and downward longwave radiation are two driving factors responsible for the warming of Nam Co.