Decreased Western Tibet Snow Cover Amplifies the Increase in Summer Concurrent Compound Heatwaves in the Northern Hemisphere

Abstract The occurrence of summer concurrent compound heatwaves (CCHWs) across multiple regions in the Northern Hemisphere (NH) has substantially increased in recent decades, though the contributions made by external forcings remain uncertain. This work has revealed that decreased summer snow cover over the western Tibetan Plateau (SC_WTP) could have significantly amplified the increase in summer CCHWs in the NH during 1979–2021. These findings are supported by both observational data and Coupled Model Intercomparison Project Phase 6 model simulations. The decreased SC_WTP causes diabatic warming by modulating the surface energy budget and stimulates a tripolar Rossby wave source over the TP. The atmospheric response to the SC_WTP forcing-related disturbance generates a circumglobal circulation pattern, which is accompanied by a weakening of the meridional temperature and a “double jet stream” in the NH. These changes modulate the phase, amplitude, and proportion of quasis-tationary waves 6–8, which results in an increase in CCHWs in the NH.