The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes.

An exceptionally strong stationary planetary wave with Zonal Wavenumber 1 led to a sudden stratospheric warming (SSW) in the Southern Hemisphere in September 2019. Ionospheric data from European Space Agency's Swarm satellite constellation mission show prominent 6‐day variations in the dayside low‐latitude region at this time, which can be attributed to forcing from the middle atmosphere by the Rossby normal mode “quasi‐6‐day wave” (Q6DW). Geopotential height measurements by the Microwave Limb Sounder aboard National Aeronautics and Space Administration's Aura satellite reveal a burst of global Q6DW activity in the mesosphere and lower thermosphere during the SSW, which is one of the strongest in the record. The Q6DW is apparently generated in the polar stratosphere at 30–40 km, where the atmosphere is unstable due to strong vertical wind shear connected with planetary wave breaking. These results suggest that an Antarctic SSW can lead to ionospheric variability through wave forcing from the middle atmosphere.

The present study compares simulations of the 2009 sudden stratospheric warming (SSW) from four different whole atmosphere models. The models included in the comparison are the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy, Hamburg Model of the Neutral and Ionized Atmosphere, Whole Atmosphere Model, and Whole Atmosphere Community Climate Model Extended version (WACCM-X). The comparison focuses on the zonal mean, planetary wave, and tidal variability in the middle and upper atmosphere during the 2009 SSW. The model simulations are constrained in the lower atmosphere, and the simulated zonal mean and planetary wave variability is thus similar up to approximate to 1 hPa (50 km). With the exception of WACCM-X, which is constrained up to 0.002 hPa (92 km), the models are unconstrained at higher altitudes leading to considerable divergence among the model simulations in the mesosphere and thermosphere. We attribute the differences at higher altitudes to be primarily due to different gravity wave drag parameterizations. In the mesosphere and lower thermosphere, we find both similarities and differences among the model simulated migrating and nonmigrating tides. The migrating diurnal tide (DW1) is similar in all of the model simulations. The model simulations reveal similar temporal evolution of the amplitude and phase of the migrating semidiurnal tide (SW2); however, the absolute SW2 amplitudes are significantly different. Through comparison of the zonal mean, planetary wave, and tidal variability during the 2009 SSW, the results of the present study provide insight into aspects of the middle and upper atmosphere variability that are considered to be robust features, as well as aspects that should be considered with significant uncertainty

In recent years there have been a series of reported ground‐ and satellite‐based observations of lunar tide signatures in the equatorial and low latitude ionosphere/thermosphere around sudden stratospheric warming (SSW) events. This lower atmosphere/ionosphere coupling has been suggested to be via the E region dynamo. In this work we present the results of analyzing 6 years of hourly upper mesospheric winds from specular meteor radars over a midlatitude (54°N) station and a high latitude (69°N) station. Instead of correlating our results with typical definitions of SSWs, we use the definition of polar vortex weaking (PVW) used by Zhang and Forbes (). This definition provides a better representation of the strength in middle atmospheric dynamics that should be responsible for the waves propagating to the E region. We have performed a wave decomposition on hourly wind data in 21 day segments, shifted by 1 day. In addition to the radar wind data, the analysis has been applied to simulations from Whole Atmosphere Community Climate Model Extended version and the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model. Our results indicate that the semidiurnal lunar tide (M2) enhances in northern hemispheric winter months, over both middle and high latitudes. The time and magnitude of M2 are highly correlated with the time and associated zonal wind of PVW. At middle/high latitudes, M2 in the upper mesosphere occurs after/before the PVW. At both latitudes, the maximum amplitude of M2 is directly proportional to the strength of PVW westward wind. We have found that M2 amplitudes could be comparable to semidiurnal solar tide amplitudes, particularly around PVW and equinoxes. Besides these general results, we have also found peculiarities in some events, particularly at high latitudes. These peculiarities point to the need of considering the longitudinal features of the polar stratosphere and the upper mesosphere and lower thermosphere regions. For example, during SSW 2009, we found that M2 enhances many days before PVW which is not in agreement with most of our results.

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Geopotential height measurements from the Aura Microwave Limb Sounder between 9-and 97-km altitudes during 2004-2018 are used to examine long-period (3-20 days) wave activity during the Northern Hemisphere winter and spring, with the primary focus on the response of normal mode Rossby waves in the middle atmosphere to sudden stratospheric warmings (SSWs). Unusually large westward propagating waves with Zonal Wave Number 1 and period ∼10 days are observed at 55 • latitude at the stratopause height (∼48 km) and above following final warmings of 2016, 2015, and 2005. In each case, large-amplitude waves are observed for the duration of two to three wave cycles. Characteristics of the waves are in conformity with the second antisymmetric Rossby normal mode of Zonal Wave Number 1, or the quasi-10-day wave. The growth rate of the waves is significantly greater than the classical normal mode in the upper stratosphere (approximately 30-50 km) where instability conditions are met, indicating the amplification or excitation of the waves in that region. The response of the quasi-10-day wave during midwinter SSWs, and also during the spring transition without an SSW, is not as obvious as the wave response during final warmings. The results suggest that not only the occurrence of SSW but also the seasonal timing of SSW is an important factor for the transient variability of the quasi-10-day wave in the middle atmosphere.

Specular meteor radars have provided essential information about the mesospheric/lower thermosphere (MLT) dynamics. The Canadian Meteor Orbit Radar has been conducting continuous meteor echo observations in a fixed, stable configuration since 2002. Here we present estimates of the neutral air density variations derived from observations of the meteor peak flux altitude. Using a simple model assuming a linear trend and a sinusoidal solar cycle we derived a trend of a decreasing neutral density of 5.8 ± 1.1% per decade at approximately 91 km altitude and an amplitude across the most recent solar cycle, the solar cycle of 2.4 ± 0.7% for solar cycle 23/24. The long‐term trend of decreasing neutral air density in the MLT is in good agreement with the model results from Akmaev et al. (2006).