Two Decades of Aura MLS Observations of UTLS Water Vapor and Cloud Ice: ENSO Variability, SST Coupling, and Comparison with WACCM6

We present a comprehensive evaluation of upper-tropospheric and lower-stratospheric (UTLS) water vapor (H₂O) and ice water content (IWC) using two decades (2004–2024) of observations from the Aura Microwave Limb Sounder (MLS) and simulations from the Whole Atmosphere Community Climate Model version 6 (WACCM6). The analysis characterizes climatological structure, seasonal cycles, interannual variability, and extreme events governing UTLS hydration. MLS observations reveal the canonical stratospheric “tape recorder” signal and document the unprecedented stratospheric moistening following the January 2022 Hunga Tonga–Hunga Haʻapai eruption, which increased global stratospheric H₂O by ~10–15%. The WACCM6 specified-dynamics simulation, constrained by reanalysis through December 2023 and including volcanic forcing associated with the eruption, reproduces the large-scale spatial structure and seasonal–interannual variability of MLS H₂O, including ENSO-related phasing and the vertical transition from convective control in the upper troposphere to thermodynamic regulation near the tropopause. However, WACCM6 systematically underestimates the magnitude and vertical extent of convection-driven IWC, particularly over land, and exhibits reduced sensitivity of UTLS ice and moisture to tropical sea surface temperature variability. These results indicate that SST-driven variability is transmitted into the UTLS through vertically distinct pathways involving convection, cloud ice, and thermodynamic control, highlighting both strengths and persistent challenges in representing UTLS hydration in chemistry–climate models and underscoring the value of long-term MLS observations for constraining climate-relevant processes.