Water Vapor Increases after 2002 in the Stratosphere and Mesosphere Observed by SABER and MLS

Changes in middle-atmospheric water vapor, whether are caused by a climate feedback or internal variability, remain as a challenging problem for both observations and modeling. The SABER instrument has been observing the atmosphere nearly continuously since data collection began in January of 2002 using the technique of spectral broadband limb emission radiometry applied in 10 infrared spectral bands ranging from 1.27 to 17 μm. Water vapor is measured using the 6.8 μm band over the range of altitude from near the tropopause to 80 km. Measurements are made both night and day over the latitude range from 52 to 83 degrees with alternating hemisphere coverage every 60 days. This paper presents long-term globally and zonally averaged trends in SABER H2O over the 16-year period from 2002 to 2018 and in MLS from 2004 to 2018. During the periods examined, three altitude regimes emerge showing different levels of increasing water vapor. In the tropical lower stratosphere, both SABER and MLS H2O trends display similar increases up to 8% per decade at northern mid-latitudes. The H2O increases then reach a minimum near the stratopause, followed at higher altitudes by SABER H2O trend increases with altitude in the mesosphere, consistent with the methane trend and methane oxidation. The H2O increases in the mesosphere support the hypothesis that the observed positive trend in noctilucent cloud occurrence is mainly driven by increasing water vapor near the mesopause. SABER and MLS H2O in the stratosphere at 40N are in agreement with the NOAA Boulder frost point hygrometer measurement. Radiative forcing and surface temperature increase driven by the increasing lower stratosphere water vapor will be characterized. The SABER and MLS H2O data also show the expected increasing negative solar cycle correlation with increasing altitude in the mesosphere. Preliminary comparisons between observations and the NCAR WACCM water vapor trend will be carried out.