Seasonal stratification of a deep, high-altitude, dimictic lake: Nam Co, Tibetan Plateau

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 (30 degrees N) where it receives high solar radiative forcing (observed maximum daily average 400 W m(-2)), yet the annual average air temperature is close to 0 degrees C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co's cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 degrees C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the > 1100, largely unstudied, lakes on the Tibetan Plateau.