Research On Extreme Cold Wave Events In Northern Hemisphere Winter Based On Canonical Transport Theory

With a newly developed functional analysis tool,namely,multiscale window transform(MWT),and the MWT-based theory of canonical transfer and the localized multiscale energetics analysis(MS-EVA),the dynamics of several typical extreme cold wave events on the continents of the Northern Hemisphere are studied.The result shows that the dynamics behind the cold waves that occur in the boreal winter in North America,East Asia and Europe are completely different.In this study,variables in the cold wave processes are reconstructed onto two or three orthogonal subspaces,each of them with an exclusive range of scales(in time or in space,depending on the problem in question),while preserving its local properties.The reconstructed fields isolate the respective cold wave events from events on other subspaces,allowing for a faithful representation of the events in question for the study of the dynamics there-beneath.In late January 2019,a severe cold air outbreak brought the lowest temperatures in over20 years to Midwestern United States and Eastern Canada,incurring a heavy economic and life loss.In this study,we find that this cold surge is actually a phase sandwiched within two warming phases,and these phases make a whole for a quasi-stationary wave train.While it is believed that the southward migration of the Arctic air mass caused this event,we find that the Arctic air mass migration just initialized the cooling;it did not account for the extremely freezing weather and its persistence.It is found,based on the data from National Center for Environmental Prediction,that the cold surge is mainly caused by a synchronization between perturbation temperature and perturbation winds,which leads to a very strong baroclinic instability and hence an explosive growth of available potential energy on the cold surge scale window.The synchronization can serve as a precursor for this extremely severe cold surge.In East Asia,the 2016 severe cold wave event is found to be related to the blocking in the Ural Mountains.The upside-down ? flow field over Eurasia in the Northern Hemisphere provides an environment good for the accumulation of cold air masses,while the driving force behind the southward movement of the air masses is a strong mixed instability.In other words,during this period,the local atmospheric system over the East Asia experiences an instability which is both baroclinic and barotropic.Different from the above two,the cause of a typical extreme cold wave in Europe at the end of February 2018 is related to the phase transition of the North Atlantic Oscillation(NAO).Before the event,the NAO is in the positive phase;when the cold wave breaks out,the NAO transits from the positive phase to the negative phase.Using a systematically developed rigorous causality analysis,it is found that the negative NAO phase is a cause of the temperature change over Europe,and the causal relation between the two is significant at a 90% confidence level both at the 500-h Pa and surface levels.That is to say,this cold wave event is actually caused by a transition from an NAO positive phase to a negative phase.In short,three extreme cold wave events respectively on three continents in the middle and high latitudes of the Northern Hemisphere have been studied,revealing completely different dynamic mechanisms lying beneath.The one in North America is essentially a quasi-stationary wave train,caused by a remarkable synchronization between the zonal jet stream and the temperature anomaly that maximizes the baroclinic canonical transfer and subsequently leads to the plummeting of temperature.The event in East Asia is dominated by the deformation of the atmospheric blocking in Ural Mountains,which is caused by a strong mixed instability.The formation of the cold wave in Europe is related to the phase transition(from positive to negative)of the North Atlantic Oscillation.