High salinity dominates the seasonal thermal stratification and its dynamics in Longmu Co: A hypersaline lake in western Tibetan Plateau

Study region Longmu Co, a deep hypersaline lake in the western Tibetan Plateau, located in a cold, arid region dominated by the westerlies. Study focus The annual mixing regime, under-ice thermal evolution, and relative roles of salinity, thermal forcing, and meteorological variability in deep hypersaline Tibetan Plateau lakes remain poorly understood. We investigated the thermodynamic regime of Longmu Co using 451 days of high-frequency water-temperature observations, a vertical salinity profile, concurrent meteorological measurements, bathymetric data, and satellite-derived ice phenology. TEOS-10-based density calculations and decomposition of squared buoyancy frequency showed that salinity provided the dominant background stability, separating a seasonally active mixolimnion from a persistently isolated monimolimnion. The mixolimnion exhibited cold monomictic circulation, mixing once annually from mid-November to early February, whereas the deep water column remained density-stratified. During complete ice cover, upper-water temperatures increased after early February, indicating a distinct under-ice thermal transition before ice breakup. Thermal forcing governed seasonal upper-layer stratification, while wind produced rapid but shallow perturbations. New hydrological insights for the region In deep hypersaline lakes of the high-altitude western Tibetan Plateau, persistent haline stratification can prevent full-depth overturn despite strong westerly winds and severe winter cooling. Meteorological forcing acts mainly within the mixolimnion. This buffering effect explains why such lakes may remain meromictic and should be represented explicitly in regional lake models and assessments of climate-driven hydrological change.