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Analyses of OH zonal means, recorded at boreal high latitudes by the AuraMicrowave Limb Sounder (MLS) in winters of2005–2009, have shown medium- (weeks)and short- (days) term variability of the nighttime OH layer.  Because of the exceptional descent of air from the mesosphere-lowerthermosphere (MLT) region, medium-term variability occurred during February 2006and February/March 2009. The layer normally situated at about 82 kmdescended by about 5–7 km, and its density increased to more than twiceJanuary values. In these periods and location the abundance of the lowered OHlayer is comparable to the OH values induced by Solar Energetic Particle(SEP) forcing (e.g., SEP events of January 2005) at the same altitudes. Inboth years, the descent of the OH layer was coupled withincreased mesospheric temperatures, elevated carbon monoxide and an almostcomplete disappearance of ozone at the altitude of the descended layer(which was not observed in other years). Moreover, under these exceptionalatmospheric conditions, the third ozone peak, normallyat about 72 km, is shown to descend about 5 km to lower altitudeand increase in magnitude, with maximum values recorded during February 2009.  Short-term variability occurred during Sudden Stratospheric Warming(SSW) events, in particular in January 2006, February 2008 and January2009, when dynamics led to a smaller abundance of the OH layer at itstypical altitude. During these periods, there wasan upward displacement of the OH layer coupled to changes in ozone and carbonmonoxide. These perturbations were the strongest during the SSW of January 2009;coincident upper mesospheric temperatures were the lowest recorded over thelate winters of 2005–2009. Finally, the series of SSW events that occurredin late January/February 2008 induced noticeable short-term variability inozone at altitudes of both the ozone minimum and the third ozone peak.  These phenomena, confined inside the polar vortex, are an additionaltool that can be used to investigate mesospheric vortex dynamics

Variability of the nighttime OH layer and mesospheric ozone at high latitudes during northern winter: influence of meteorology