Middle Atmosphere Instability Structure And Polar Atmosphere Anomalous Variability

Waves are an integrated part of the atmospheric circulation at different spatial and time scales.The atmospheric circulation is strongly affected by wave-induced momentum and energy transport and at the same time defines conditions for wave generation,propagation and dissipation.In this disseration,the dynamics of gravity waves and the instability structures associated with wave breaking,and roles of planetary(Rossby)waves during the anomalous lower atmosphere conditions in 2015/2016 winter are studied through observational analyses of a variety of data from the stratosphere up to the mesopause region.These results combined give a better picture of the effects of waves in the coupling of the whole atmosphere circulation.Gravity waves(GWs)play an important role in driving the general circulation in the mesosphere and lower thermosphere(MLT)region through deposition of their momentum and energy into the mean flow.For the momentum and energy transport,these motions must undergo dissipation in order to have a permanent effect on the background flow.The mechanisms that contribute most to the dissipation of the dominant gravity waves are thought to be the dynamic instability and convective instability.One identifiable feature of such instabilities is the appearance of ripple structures,which can be observed with airglow imagers.These structures are believed to be caused by either dynamic instability owing to large vertical wind shear,or by convective instability owing to super-adiabatic lapse rate.Based on the dataset from an OH all-sky imager at Yucca Ridge Field Station,Colorado(40.7°N,104.9°W)from September 2003 to December 2005,the statistical characteristics and seasonal variations of such ripple structures in the airglow altitude(?87 km)are studied.By further analyses of the simultaneous background wind and temperature measured by a nearby sodium temperature/wind lidar at Fort Collins,Colorado(40.6°N,105°W)and a nearby Medium Frequency(MF)radar at Platteville,Colorado(40.2°N,105.8°W),the variation of ripple statitics correlated with the background atmosphere conditions.It is found that the occurrence frequency of ripples exhibits clear seasonal variability,with peak in autumn and minimum in summer as well as local time dependence,which is most likely associated with the solar tides.The lifetime and spatial scale of these ripples are typically 5-20 minutes and 5-10 km respectively,and most of the ripples move preferentially either southward or northward.More than half of the observed ripples do not advect with background flow,which have higher Richardson numbers than those ripples that do advect with background flow.This suggests that ripples may also occur in stable atmosphere and perhaps more frequently than previously expected.This finding is inconsistent with some previous studies,we need supports additional high-resolution observations and numerical simulation to understand the detailed formation mechanism of such ripples.Interaction of Rossby waves with the atmospheric circulation influences the atmospheric circulation itself and the ozone distribution globally.During winter,wave forcing associated with Rossby waves plays a primary role in driving the Brewer-Dobson(BD)circulation,which is responsible for poleward ozone transport and adiabatic warming of the polar stratosphere.In the winter of 2015/2016,the strongest El Nino(EN)events on record occurred,which can increase the probability of stratospheric sudden warming(SSW).In the meantime an unexpected disruption of the atmospheric quasi-biennial oscillation(QBO)also happened,which can modulate the mean zonal wind and planetary wave activity in the northern winter stratosphere.Both EN and disruption of QBO may have great influences on the background flow in the northern winter.Using the datasets observed with the OH all-sky imager and Fabry-Perot Interferometer(FPI)at Resolute Bay,Nunavut,Canada(74.7°N,265.1°E),Microwave Limb Sounder(MLS)onboard Aura satellite and analysis data from European Centre for Medium-Range Weather Forecasts(ECMWF)from September 2012 to December 2018,we studied the complex response of stratospheric and mesospheric region to the anomalous events introduced above.By analyzing the OH airglow image,MLS and FPI data,we found a few unusual small-scale GWs,of which there are two clear events in November and December with wavelengths of?18km and lifetimes of?35min.The occurrence of these small-scale GWs may be related to the anomalous events in lower atmosphere,while the detailed mechanisms are unclear because of the lack of the wind data in mesosphere.Study during the whole winter in 2015/2016(Oct 2015-Mar 2016)in the North Hemisphere suggests that,the stratospheric wind turned to westward and the temperature increased earlier than in normal years in the Arctic.At the same time,the Eliassen-Palm(EP)flux divergence of this winter in the stratosphere is negative and also lower than in normal years,which means much more planetary wave breaking in the stratosphere.It's known that the planetary wave breaking in the winter stratosphere will transport westward momentum and induce a westward drag which in combination with the Coriolis force results in the net poleward enhancement of the atmospheric circulation and decrease in zonal wind.The BD circulation is responsible for driving the atmosphere away from radiative equilibrium and for poleward tracer transport,which causes adiabatic warming in middle and high latitudes.Although there are always planetary wave breaking and the above dynamic processes in the atmosphere,we find that there was much more planetary wave breaking in the 2015/2016 winter,which increased the poleward circulation and changed the zonal wind to westward earlier than in normal winters.In other word,it increased the BD circulation and caused stratospheric warming more quickly.In summary,these events above are both related to anomalous events at lower latitudes(including EN and disruption of QBO).