A Triple-Structured Mesospheric Climate Pattern Driven by a Double-Celled Meridional Circulation during the Equinox-Solstice Transition Months of November and May

Abstract. The upper mesosphere is a dynamically and chemically complex region where interannual climate variability remains incompletely understood, particularly during the transition months between the equinox and solstice circulation regimes. Using multi-satellite observations from MLS and SABER, we investigate the coupled dynamical-microphysical-chemical-thermal structures during November and May, building on the bottom-up mechanism of “upwelling—water vapor (H₂O)—ozone (O₃)—temperature”. We employ temperature near 80 km (the T80 index) as a proxy for upwelling intensity and identify two distinct centers: a summer polar upwelling and a tropical upwelling. Together they drive a double-celled anomalous meridional circulation that organizes the global climate into a novel triple-structured pattern, with coherent signatures in the summer high-latitude, equatorial, and winter high-latitude regions. A key finding is that hydration occurs below polar mesospheric clouds (PMCs) without pronounced dehydration above them. This “hydration-without-dehydration” configuration, made possible by the weak PMCs typical of November and May, indicates the dominance of the cold-trap effect over the conventional freeze-drying effect. The absence of dehydration further isolates the temperature-dependent ozone kinetic pathway for polar ozone enhancement, a pathway that is otherwise convolved with dehydration effects in stronger PMC seasons. Ozone and atomic oxygen (O) respond to the combined influences of meridional H₂O transport and local thermal forcing, and the resulting radiative and chemical heating governs temperatures near 90 km (T90). These results establish the structure of the transitional climate regime, demonstrating that the shift from symmetric (equinox) to antisymmetric (solstice) variability is mediated by a well-organized, upwelling-driven double-celled circulation.