The Horn of Africa, and specifically Ethiopia's Awash Basin, is experiencing accelerating hydroclimatic change, yet the mechanisms driving multi-decadal shifts in its regional water balance remain poorly understood. The terminal Afambo–Gemeri–Abhe lake system, an endorheic water body at the outlet of the Awash Basin, acts as a natural integrator of its catchment's net water balance, providing a sensitive record of regional atmospheric dynamics. This study reconstructs its hydrological history over 1985–2024 by coupling the GR2M hydrological model with a lake water-balance model, constrained by satellite observations and in situ data. The model reproduces observed lake surface area dynamics and reliably extends the record into the data-scarce pre-1998 period allowing us to relate recent lake surface area changes to large scale circulation variability and trends.</p> <p>The reconstructed history reveals two superimposed signals. The first is pronounced variability across interannual to interdecadal timescales, driven by the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Subtropical Indian Ocean Dipole (SIOD). The second is a non-oscillatory, multi-decadal expansion: the lake tripled in surface area from ∼150 km<sup>2</sup> in the 1980s to ∼450 km<sup>2</sup> by 2024, with growth accelerating markedly after 2020. This systematic expansion reflects a reorganization of the regional water balance driven by increased July–September rainfall and changes in potential evapotranspiration. We attribute these basin-scale changes to large-scale atmospheric circulation shifts, including a strengthening of the Somali Jet, the Tropical Easterly Jet, and the Indian monsoon circulation, and a northward shift of the Intertropical Convergence Zone and of the African Easterly Jet, likely linked to amplified regional warming over the Arabian Peninsula and the Tibetan Plateau. This modelling framework provides a transferable tool for evaluating past and future hydroclimatic variability, supporting more robust water resource assessments in data-scarce regions.