Abstract. The upper mesosphere, a transition region between Earth’s atmosphere and space, is characterized by complex interactions among water vapor (H₂O), atomic hydrogen (H), ozone (O₃), atomic oxygen (O), and temperatures. Using the MLS, SABER, and SOFIE satellite data, we explore the upwelling-driven interannual variability of temperatures above 90 km (T90) and atmospheric constituents during solstice months, revealing a bottom-up control mechanism of “upwelling—H₂O(H)—O₃(O)—T90” in the two hemispheres. First, summer polar upwelling transports H₂O upward, which is then transported toward winter hemisphere by meridional winds. Subsequently, the hydration increases H via photolysis and depletes O₃ in the winter hemisphere through H-driven catalytic loss. The O varies in pace with O₃ due to ozone chemical equilibrium assumption, and the radiative and chemical heating of O/O₃ reduces the T90 in winter hemisphere (T90_W). Second, upwelling-induced cooling promotes polar mesospheric cloud (PMC) formation, with ice particle growth blocking H₂O transport and dehydrating heights above PMCs. This dehydration reduces H abundance, thereby decreasing H-driven O₃ loss. Meanwhile, the colder temperatures directly increase O₃ through ozone kinetics. The enhanced O₃, together with the coupled O, collectively increase the summer polar temperatures above 90 km (T90_S). This anti-phase interannual variability between hemispheres, mediated by PMC microphysics and H₂O-O₃ chemistry, establishes summer polar upwelling as a fundamental driver of mesospheric climate and highlights the importance of dynamical-chemical coupling in the upper mesosphere.