Asymmetric Warming and Seasonal Reversal of the Mixed Layer in Plateau Lakes of the Northern Hemisphere

Plateau lakes serve as important indicators of climate warming; however, systematic comparisons of regional differences in thermodynamic evolution and their driving mechanisms across different plateaus are still lacking. Based on ERA5 reanalysis data from 1980 to 2023, this study employed the Theil–Sen trend estimation, the Morlet wavelet period extraction, and the XGBoost–SHAP attribution analysis to investigate the variation mechanisms of mixed layer temperature (LMLT) and mixed layer depth (LMLD) in 18 lakes across five major plateaus in the mid-latitudes of the Northern Hemisphere. The results show that summer LMLT increased significantly at rates of 0.22–0.39 °C per decade, with the fastest warming on the Qinghai–Tibetan Plateau, while winter LMLT remained nearly stagnant, forming an asymmetric pattern of “summer warming, winter non-warming”. LMLT responded significantly to global warming (summer r = 0.60–0.87). Volcanic eruptions caused summer LMLT to rise by 0.07–0.28 °C, and compound heatwaves led to increases of 0.38–1.04 °C. Lake mixed layer depth exhibited a seasonal reversal: summer evaporation (SHAP = 49.2%) indirectly influenced mixed layer development through temperature, whereas intense winter evaporation (SHAP = 62.0%) caused water level decline, making the mixed layer appear shallower. Both linear and nonlinear models consistently identified evaporation as the key factor controlling mixed layer depth. The LMLD patterns of the five plateaus can be summarised into three driving modes: temperature-evaporation-dominated (Qinghai–Tibetan and Iranian), temperature-wind-synergistic (Rocky and Pamir), and wind-dominated (Mongolian). These findings reveal the sensitive response of plateau lake thermodynamics to global change, highlight the key role of evaporation in modulating mixed layer depth and its seasonal reversal effect, and complement the traditional temperature-centred paradigm of lake mixing mechanisms.