Anomalously High Water Vapor in the Northern Hemisphere Middle Stratosphere During Early 2019

Balloon-borne frost point hygrometers launched from Boulder, Colorado (40°N) and Hilo, Hawaii (20°N) measured water vapor in excess of 6.0 ppm and 5.5 ppm, respectively, at 26-27 km altitude (21-17 hPa) during March-April 2019. These translate to de-seasonalized mixing ratio anomalies of 0.6 to 1.2 ppm, among the largest anomalies ever observed in the middle stratosphere of the northern hemisphere. We examine near-global Aura Microwave Limb Sounder (MLS) water vapor retrievals to study the spatiotemporal development of these high positive anomalies. Our results show that a small region of wet anomalies developed during late 2018 at ~34 km (~6 hPa) above 20°N-30°N, then spread meridionally to reach both tropical and polar latitudes by mid-February 2019. During that period, the center of the wettest region also descended ~6 km to ~28 km (15 hPa). At first view, the wet anomalies in early 2019 appear similar to (but slightly weaker than) those in early 2013, suggesting that both high water events may be connected to processes associated with a northern hemisphere stratospheric sudden warming. However, in both cases, the wet anomalies developed in the middle latitudes and progressed poleward. Temperature and ozone anomalies will also be analyzed in an attempt to elucidate the mechanism(s) behind these wet anomalies in the northern hemisphere middle stratosphere.