Transport of the High-Latitude Sodium Layer in the Mesosphere and Lower Thermosphere

The sodium (Na) layer is a valuable tracer for mesosphere and lower thermosphere (MLT) dynamics. Integrating the observations from the instrument Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin satellite with simulation from the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM), we quantify high-latitude Na transport within a transformed Eulerian-mean framework. The mean residual circulation drives a seasonally reversing transport poleward of 60°: winter downdrafts deplete Na, while summer upwelling enhances it. This transport is modulated by gravity wave–driven mixing and molecular diffusion, with rapid chemistry limiting Na residence time. These coupled processes collectively regulate the Na layer's column abundance, peak density, and vertical extent, explaining observed hemispheric asymmetries and establishing Na as a sensitive diagnostic for MLT circulation-chemistry coupling.