Mesospheric Water Vapor From SABER as a Tracer for the Residual Mean Circulation During SSW Events

The sparse wind observations and reanalysis winds in the mesosphere make it challenging to accurately determine the residual mean meridional circulation (MMC). As winds distribute tracers, an alternative approach is to utilize long-lived trace species such as water vapor (H₂O). The recently released H₂O data from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) version 2.07, combined with simulations by the Whole Atmosphere Community Climate Model eXtension with specified dynamics (SD-WACCM-X) and with the Data Assimilation Research Testbed (WACCMX + DART), provide an opportunity to assess the accuracy of using H₂O isopleths to derive the vertical component of the residual MMC during the 2009 sudden stratospheric warming (SSW) event. In winter, the impact of photochemistry and diffusion on the distribution of mesospheric H₂O is negligible compared to advective processes. H₂O poleward of 70°N accurately captures the anomalous ascent that occurs a few days prior to the onset of the SSW and the subsequent descent. The derived vertical velocity in SD-WACCM-X and WACCMX + DART is qualitatively consistent with SABER observations. However, the derivation of vertical motion from isopleth analysis has limitations at the beginning of 2009 when the meridional transport is stronger than the vertical transport. While measuring winds in the mesosphere is challenging, satellite observations of mesospheric H₂O prove to be an effective dynamical tracer during time periods characterized by strong vertical coupling.