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We present ground‐based microwave measurements of mesospheric water vapor made by the Water Vapor Millimeter‐wave Spectrometer (WVMS) instruments since the early 1990s from sites in California, Hawaii, and New Zealand. These measurements are compared with coincident measurements from the Halogen Occultation Experiment, the Aura Microwave Limb Sounder, and Sounding of the Atmosphere using Broadband Emission Radiometry; all of which combine to cover the entire time period of the ground‐based measurements. Comparisons are presented both on ∼weekly timescales in order to better identify discontinuities in the relative differences and on annual timescales in order to better study geophysical variations. The WVMS retrievals shown here are available on the Network for the Detection of Atmospheric Composition Change database. The range of WVMS trends and the differences from the satellite trends, with the latter varying over a range of ∼3%/decade, provide an estimate of how accurately it would be possible to determine multidecadal trends using ground‐based microwave instruments in a postsatellite era. This uncertainty is comparable to the trend in mesospheric water vapor that is expected to have occurred since the early 1990s.

Immediately following the breakup of the 2005 Arctic spring stratospheric vortex, a tropical air mass, characterized by low potential vorticity (PV) and high nitrous oxide (N₂O), was advected poleward and became trapped in the easterly summer polar vortex. This feature, known as a "Frozen-In Anticyclone (FrIAC)", was observed in Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) data to span the potential temperature range from ~580 to 1100 K (~25 to 40 km altitude) and to persist from late March to late August 2005. This study compares MLS N₂O observations with simulations from the Global Modeling Initiative (GMI) chemistry and transport model, the GEOS-5/MERRA Replay model, and the Van Leer Icosahedral Triangular Advection (VITA) isentropic transport model to elucidate the processes involved in the lifecycle of the FrIAC, which is here divided into three distinct phases. During the "spin-up phase" (March to early April), strong poleward flow resulted in a tight isolated anticyclonic vortex at ~70–90° N, marked with elevated N₂O. GMI, Replay, and VITA all reliably simulated the spin-up of the FrIAC, although the GMI and Replay peak N₂O values were too low. The FrIAC became trapped in the developing summer easterly flow and circulated around the polar region during the "anticyclonic phase" (early April to the end of May). During this phase, the FrIAC crossed directly over the pole between the 7 and 14 April. The VITA and Replay simulations transported the N₂O anomaly intact during this crossing, in agreement with MLS, but unrealistic dispersion of the anomaly occurred in the GMI simulation due to excessive numerical mixing of the polar cap. The vortex associated with the FrIAC was apparently resistant to the weak vertical shear during the anticyclonic phase, and it thereby protected the embedded N₂O anomaly from stretching. The vortex decayed in late May due to diabatic processes, leaving the N₂O anomaly exposed to horizontal and vertical wind shears during the "shearing phase" (June to August). The observed lifetime of the FrIAC during this phase is consistent with timescales calculated from the ambient horizontal and vertical wind shear. Replay maintained the horizontal structure of the N₂O anomaly similar to MLS well into August. The VITA simulation also captured the horizontal structure of the FrIAC during this phase, but VITA eventually developed fine-scale N₂O structure not observed in MLS data