Cryosphere-hydrosphere interactions are rapidly intensifying in mid-latitude mountain regions, yet their combined impacts on lake water availability remain insufficiently understood. We quantified glacier area change, lake dynamics, hydrochemical characteristics, and isotopic tracers of six glacier-fed lakes connected to the upper Munkhkhairkhan Glacier in western Mongolia over the period 1990-2024. Results show substantial glacier loss (-38.1%) and persistent lake shrinkage (-2.54%), driven primarily by rising atmospheric dryness. Vapor pressure deficit (VPD) is identified as the dominant predictor of lake contraction, while hydrological moisture indicators (precipitation minus evapotranspiration, P-PET, and Palmer Drought Severity Index, PDSI) exert rapid but weaker positive influence. Principal Component Analysis (PCA) reveals two distinct climate modes: an atmospheric-drying mode explaining 79% of variability and a hydrological-moisture mode explaining 13%. Projections indicate continued lake losses by 2030-2040, with stronger declines in downstream basins. Hydrochemical enrichment and isotopically heavier signatures in lower-elevation lakes indicate enhanced evaporation and increasing dependence on seasonal rainfall. These findings highlight growing vulnerability of glacier-fed water systems and underscore the urgent need for adaptive water management in cold-arid mountain environments.