Tropospheric heating perturbations and topography are used to create Northern Hemisphere winter‐like stratospheric variability in an idealized atmospheric general circulation model. Wave 1 and wave 2 heating perturbations as well as wave 2 topography are used. With appropriate choices of amplitudes, the three forcings produce reasonable sudden stratospheric warming (SSW) frequencies. It is found that large numbers of both split and displacement sudden warmings occur when the model is forced by heating perturbations, regardless of the wave number of the forcing. This is different from the wave 2 topographic forcing, which produces almost only splits. We use the results of the three model runs to investigate the extent to which SSWs are caused by anomalous tropospheric wave fluxes. We find that SSWs in this model can form both as a direct result of anomalous tropospheric wave activity and due to internal stratospheric processes which alter the propagation of tropospheric wave flux into the stratosphere and that the fraction of the two mechanisms is similar to that of the observed atmosphere for all three forcings. We further investigate the circulation differences associated with splits and displacements and find that splits and displacements have different zonal mean surface signatures when the model is forced by wave 1 heating.

Superpressure balloon data of unprecedented coverage from Loon LLC is used to investigate the seasonal and latitudinal variability of lower stratospheric gravity waves over the entire intrinsic frequency spectrum. We show that seasonal variability in both gravity wave amplitudes and spectral slopes exist for a wide range of intrinsic frequencies and provide estimates of spectral slopes in five latitudinal regions for all four seasons, in five different frequency windows. The spectral slopes can be used to infer gravity wave amplitudes of intrinsic frequencies as high as 70 cycles/day from gravity waves resolved in model and reanalysis data. We also show that a robust relationship between the phase of the quasi-biennial oscillation and gravity wave amplitudes exists for intrinsic frequencies as high as the buoyancy frequency. These are the first estimates of seasonal and latitudinal variability of gravity wave spectral slopes and high-frequency amplitudes and constitute a significant step toward obtaining observationally constrained gravity wave parameterizations in climate models.

Stratosphere–troposphere interactions are conventionally characterized using the first empirical orthogonal function (EOF) of fields such as zonal-mean zonal wind. Perpetual-winter integrations of an idealized model are used to contrast the vertical structures of EOFs with those of principal oscillation patterns (POPs; the modes of a linearized system governing the evolution of zonal flow anomalies). POP structures are shown to be insensitive to pressure weighting of the time series of interest, a factor that is particularly important for a deep system such as the stratosphere and troposphere. In contrast, EOFs change from being dominated by tropospheric variability with pressure weighting to being dominated by stratospheric variability without it. The analysis reveals separate tropospheric and stratospheric modes in model integrations that are set up to resemble midwinter variability of the troposphere and stratosphere in both hemispheres. Movies illustrating the time evolution of POP structures show the existence of a fast, propagating tropospheric mode in both integrations, and a pulsing stratospheric mode with a tropospheric extension in the Northern Hemisphere–like integration.

Abstract. The effects of wave–wave interactions on sudden stratospheric warming formation are investigated using an idealized atmospheric general circulation model, in which tropospheric heating perturbations of zonal wave numbers 1 and 2 are used to produce planetary-scale wave activity. Zonal wave–wave interactions are removed at different vertical extents of the atmosphere in order to examine the sensitivity of stratospheric circulation to local changes in wave–wave interactions. We show that the effects of wave–wave interactions on sudden warming formation, including sudden warming frequencies, are strongly dependent on the wave number of the tropospheric forcing and the vertical levels where wave–wave interactions are removed. Significant changes in sudden warming frequencies are evident when wave–wave interactions are removed even when the lower-stratospheric wave forcing does not change, highlighting the fact that the upper stratosphere is not a passive recipient of wave forcing from below. We find that while wave–wave interactions are required in the troposphere and lower stratosphere to produce displacements when wave number 2 heating is used, both splits and displacements can be produced without wave–wave interactions in the troposphere and lower stratosphere when the model is forced by wave number 1 heating. We suggest that the relative strengths of wave number 1 and 2 vertical wave flux entering the stratosphere largely determine the split and displacement ratios when wave number 2 forcing is used but not wave number 1.

Electromagnetic ion cyclotron (EMIC) waves have been studied for decades, though remain a fundamentally important topic in heliospheric physics. The connection of EMIC waves to the scattering of energetic particles from Earth's radiation belts is one of many topics that motivate the need for a deeper understanding of characteristics and occurrence distributions of the waves. In this study, we show that EMIC wave frequencies, as observed at Halley Station in Antarctica from 2008 through 2012, increase by approximately 60% from a minimum in 2009 to the end of 2012. Assuming that these waves are excited in the vicinity of the plasmapause, the change in Kp in going from solar minimum to near solar maximum would drive increased plasmapause erosion, potentially shifting the generation region of the EMIC to lower L and resulting in the higher frequencies. A numerical estimate of the change in plasmapause location, however, implies that it is not enough to account for the shift in EMIC frequencies that are observed at Halley Station. Another possible explanation for the frequency shift, however, is that the relative density of heavier ions in the magnetosphere (that would be associated with increased solar activity) could account for the change in frequencies. In terms of effects on radiation belt dynamics, the shift to higher frequencies tends to mean that these waves will interact with less energetic electrons, although the details involved in this process are complex and depend on the specific plasma and gyrofrequencies of all populations, including electrons. In addition, the change in location of the generation region to lower L shells means that the waves will have access to higher number fluxes of resonant electrons. Finally, we show that a sunlit ionosphere can inhibit ground observations of EMIC waves with frequencies higher than ∼0.5 Hz and note that the effect likely has resulted in an underestimate of the solar-cycle-driven frequency changes described here.

The tropospheric response to Sudden Stratospheric Warmings (SSWs) is associated with an equatorward shift in the midlatitude jet and associated storm tracks, while Strong Polar Vortex (SPV) events elicit a contrasting response. Recent analyses of the North Atlantic jet using probability density functions of a jet latitude index have identified three preferred jet latitudes, raising the question of whether the tropospheric response to SSWs and SPVs results from a change in relative frequencies of these preferred jet regimes rather than a systematic jet shift. We explore this question using atmospheric reanalysis data from 1979 to 2018 (26 SSWs and 33 SPVs), and a 202‐years integration of the Whole Atmosphere Community Climate Model (92 SSWs and 68 SPVs). Following SSWs, the northern jet regime becomes less common and the central and southern regimes become more common. These changes occur almost immediately following “split” vortex events, but are more delayed following “displacement” events. In contrast, the northern regime becomes more frequent and the southern regime less frequent following SPV events. Following SSWs, composites of 500‐hPa geopotential heights, surface air temperatures, and precipitation most closely resemble composites of the southern jet regime, and are generally opposite in sign to the composites of the northern jet regime. These comparisons are reversed following SPVs. Thus, one possible interpretation is that the two southernmost regimes appear to be favored following SSWs, while the southernmost regime becomes less common following SPVs.

Recent work suggests that storm track diagnostics such as eddy heat fluxes and eddy kinetic energies have very small signatures in the first annular mode of zonal mean zonal wind, suggesting a lack of co‐variability between the locations of the extratropical jet and storm tracks. The frequency‐dependence of this apparent decoupling is explored in ERA‐Interim reanalysis data. The annular modes show similar spatial characteristics in the different frequency ranges considered. Cancellation between the signatures of storm track diagnostics in the leading low‐pass and high‐pass filtered annular modes is evident, partly explaining their small signature in the total. It is shown that at timescales greater than 30 days, the first zonal wind mode describes latitudinal shifts of both the midlatitude jet and its associated storm tracks, and it appears that the persistence of zonal wind anomalies is sustained primarily by a baroclinic feedback.

<p>The winter jet stream in the North Atlantic has been shown to preferentially occur at three distinct latitudes [Woolings et al., 2010; Woolings et al., 2018], which we will call the three Atlantic “jet regimes.” Distinct physical mechanisms may be responsible for each of the three jet regimes—for example, the northernmost jet regime is strongly linked to the Greenland tip jet [White et al., 2019]. We seek to investigate the role of stratospheric and CO2 forcing, such as from sudden stratospheric warmings (SSWs), strong polar vortex events (SPVs), and anthropogenic global warming, on the Atlantic jet in the context of these jet regimes.</p><p>To do so, we use a “jet latitude index” (JLI), which is determined by finding the latitude of the peak zonal winds over some latitude range, averaged over some longitude range, to show that sudden stratospheric warmings (SSWs) impact the likelihood that the Atlantic jet will be in any particular jet regime. These calculations are performed in the ECMWF Interim Reanalysis (ERAI) data set, an in-house 200-year Whole Atmosphere Community Climate Model (WACCM) run, and in a subset of CMIP6 models. We seek to investigate how changes in the composite response of the jet over the Atlantic associated with SSWs, SPVs, and greenhouse gas forcing, are borne out in the context of the three Atlantic jet regimes. We find that, following SSWs, the northern regime becomes less frequent, and the southern regime becomes more frequent, while the jet latitude peaks of the regimes do not notably shift. Following SPVs, the northern regime becomes more frequent, the southern regime becomes less frequent, and again, the peak latitudes do not shift. Under CO2 forcing, we do not find a consistent signal from model to model, and we test whether these differences may be related to model differences in local meridional temperature gradients over the Atlantic.</p>