Elevated Stratopause Research Articles

Winds and temperatures of the Arctic middle atmosphere during January measured by Doppler lidar

Abstract. We present an extensive data set of simultaneous temperature and wind measurements in the Arctic middle atmosphere. It consists of more than 300 h of Doppler Rayleigh lidar observations obtained during three January seasons (2012, 2014, and 2015) and covers the altitude range from 30 km up to about 85 km. The data set reveals large year-to-year variations in monthly mean temperatures and winds, which in 2012 are affected by a sudden stratospheric warming. The temporal evolution of winds and temperatures after that warming are studied over a period of 2 weeks, showing an elevated stratopause and the reformation of the polar vortex. The monthly mean temperatures and winds are compared to data extracted from the Integrated Forecast System of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the Horizontal Wind Model (HWM07). Lidar and ECMWF data show good agreement of mean zonal and meridional winds below ≈ 55 km altitude, but we also find mean temperature, zonal wind, and meridional wind differences of up to 20 K, 20 m s−1, and 5 m s−1, respectively. Differences between lidar observations and HWM07 data are up to 30 m s−1. From the fluctuations of temperatures and winds within single nights we extract the potential and kinetic gravity wave energy density (GWED) per unit mass. It shows that the kinetic GWED is typically 5 to 10 times larger than the potential GWED, the total GWED increases with altitude with a scale height of ≈ 16 km. Since temporal fluctuations of winds and temperatures are underestimated in ECMWF, the total GWED is underestimated as well by a factor of 3–10 above 50 km altitude. Similarly, we estimate the energy density per unit mass for large-scale waves (LWED) from the fluctuations of nightly mean temperatures and winds. The total LWED is roughly constant with altitude. The ratio of kinetic to potential LWED varies with altitude over 2 orders of magnitude. LWEDs from ECMWF data show results similar to the lidar data. From the comparison of GWED and LWED, it follows that large-scale waves carry about 2 to 5 times more energy than gravity waves.

HEPPA-II model–measurement intercomparison project: EPP indirect effects during the dynamically perturbed NH winter 2008–2009

Abstract. We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80 km) atmospheric models with observations of odd nitrogen (NOx = NO + NO2), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) during the Northern Hemisphere (NH) polar winter 2008/2009. The models included in the comparison are the 3-D chemistry transport model 3dCTM, the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the modelling tools for SOlar Climate Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community Climate Model (WACCM4). The comparison focuses on the energetic particle precipitation (EPP) indirect effect, that is, the polar winter descent of NOx largely produced by EPP in the mesosphere and lower thermosphere. A particular emphasis is given to the impact of the sudden stratospheric warming (SSW) in January 2009 and the subsequent elevated stratopause (ES) event associated with enhanced descent of mesospheric air. The chemistry climate model simulations have been nudged toward reanalysis data in the troposphere and stratosphere while being unconstrained above. An odd nitrogen upper boundary condition obtained from MIPAS observations has further been applied to medium-top models. Most models provide a good representation of the mesospheric tracer descent in general, and the EPP indirect effect in particular, during the unperturbed (pre-SSW) period of the NH winter 2008/2009. The observed NOx descent into the lower mesosphere and stratosphere is generally reproduced within 20 %. Larger discrepancies of a few model simulations could be traced back either to the impact of the models' gravity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NOx mixing ratio at the uppermost model layer and low vertical resolution. In March–April, after the ES event, however, modelled mesospheric and stratospheric NOx distributions deviate significantly from the observations. The too-fast and early downward propagation of the NOx tongue, encountered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2–0.05 hPa), likely caused by an overestimation of descent velocities. In contrast, upper-mesospheric temperatures (at 0.05–0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too-slow descent and hence too-low NOx fluxes. As a consequence, the magnitude of the simulated NOx tongue is generally underestimated by these models. Descending NOx amounts simulated with medium-top models are on average closer to the observations but show a large spread of up to several hundred percent. This is primarily attributed to the different vertical model domains in which the NOx upper boundary condition is applied. In general, the intercomparison demonstrates the ability of state-of-the-art atmospheric models to reproduce the EPP indirect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between observed and simulated NOx, CO, and temperature distributions during the perturbed phase of the 2009 NH winter, however, emphasize the need for model improvements in the dynamical representation of elevated stratopause events in order to allow for a better description of the EPP indirect effect under these particular conditions.

Modelling the descent of nitric oxide during the elevated stratopause event of January 2013

Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.

Sensitivity of simulated mesospheric transport of nitrogen oxides to parameterized gravity waves

AbstractGravity waves strongly influence the circulation and transport processes in the middle atmosphere. We analyze the sensitivity of the simulated mesospheric transport of nitrogen oxides (NOx) to differences in a parameterization of nonorographic gravity waves. After particularly strong sudden stratospheric warming (SSW) events as in January 2009, satellite instruments measured a strong mesospheric descent of NOx. However, this downward transport is in general underestimated in models covering this altitude range. We use simulations of the atmospheric general circulation and chemistry model HAMMONIA (Hamburg Model of Neutral and Ionized Atmosphere) to discuss both differences in a homogeneous background gravity wave source and a source related to frontal activity. The results show that the transport of NOx is highly sensitive to such differences. With a stronger gravity wave source, less NOx is transported after the SSW to the mesosphere and the elevated stratopause descends more rapidly to its climatological altitude. We observe the opposite by weakening the gravity wave sources yielding a better agreement with the observations. The amount of the transported NOx is controlled by the altitude at which momentum is deposited in the atmosphere. The higher the altitude where the momentum is deposited in the upper mesosphere, the stronger is the descent of NOx. A small wave amplitude favors the transition to turbulence at a higher altitude due to the exponential increase of the amplitude with height.

Satellite observations of middle atmosphere gravity wave absolute momentum flux and of its vertical gradient during recent stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) are circulation anomalies in the polar region during winter. They mostly occur in the Northern Hemisphere and affect also surface weather and climate. Both planetary waves and gravity waves contribute to the onset and evolution of SSWs. While the role of planetary waves for SSW evolution has been recognized, the effect of gravity waves is still not fully understood, and has not been comprehensively analyzed based on global observations. In particular, information on the gravity wave driving of the background winds during SSWs is still missing.We investigate the boreal winters from 2001/2002 until 2013/2014. Absolute gravity wave momentum fluxes and gravity wave dissipation (potential drag) are estimated from temperature observations of the satellite instruments HIRDLS and SABER. In agreement with previous work, we find that sometimes gravity wave activity is enhanced before or around the central date of major SSWs, particularly during vortex-split events. Often, SSWs are associated with polar-night jet oscillation (PJO) events. For these events, we find that gravity wave activity is strongly suppressed when the wind has reversed from eastward to westward (usually after the central date of a major SSW). In addition, gravity wave potential drag at the bottom of the newly forming eastward-directed jet is remarkably weak, while considerable potential drag at the top of the jet likely contributes to the downward propagation of both the jet and the new elevated stratopause. During PJO events, we also find some indication for poleward propagation of gravity waves. Another striking finding is that obviously localized gravity wave sources, likely mountain waves and jet-generated gravity waves, play an important role during the evolution of SSWs and potentially contribute to the triggering of SSWs by preconditioning the shape of the polar vortex. The distribution of these hot spots is highly variable and strongly depends on the zonal and meridional shape of the background wind field, indicating that a pure zonal average view sometimes is a too strong simplification for the strongly perturbed conditions during the evolution of SSWs.

A semi-empirical model for mesospheric and stratospheric NO<sub><i>y</i></sub> produced by energetic particle precipitation

Abstract. The MIPAS Fourier transform spectrometer on board Envisat has measured global distributions of the six principal reactive nitrogen (NOy) compounds (HNO3, NO2, NO, N2O5, ClONO2, and HNO4) during 2002–2012. These observations were used previously to detect regular polar winter descent of reactive nitrogen produced by energetic particle precipitation (EPP) down to the lower stratosphere, often called the EPP indirect effect. It has further been shown that the observed fraction of NOy produced by EPP (EPP-NOy) has a nearly linear relationship with the geomagnetic Ap index when taking into account the time lag introduced by transport. Here we exploit these results in a semi-empirical model for computation of EPP-modulated NOy densities and wintertime downward fluxes through stratospheric and mesospheric pressure levels. Since the Ap dependence of EPP-NOy is distorted during episodes of strong descent in Arctic winters associated with elevated stratopause events, a specific parameterization has been developed for these episodes. This model accurately reproduces the observations from MIPAS and is also consistent with estimates from other satellite instruments. Since stratospheric EPP-NOy depositions lead to changes in stratospheric ozone with possible implications for climate, the model presented here can be utilized in climate simulations without the need to incorporate many thermospheric and upper mesospheric processes. By employing historical geomagnetic indices, the model also allows for reconstruction of the EPP indirect effect since 1850. We found secular variations of solar cycle-averaged stratospheric EPP-NOy depositions on the order of 1 GM. In particular, we model a reduction of the EPP-NOy deposition rate during the last 3 decades, related to the coincident decline of geomagnetic activity that corresponds to 1.8 % of the NOy production rate by N2O oxidation. As the decline of the geomagnetic activity level is expected to continue in the coming decades, this is likely to affect the long-term NOy trend by counteracting the expected increase caused by growing N2O emissions.

On the composite response of the MLT to major sudden stratospheric warming events with elevated stratopause

AbstractBased on a climate‐chemistry model (constrained by reanalyses below ~50 km), the zonal‐mean composite response of the mesosphere and lower thermosphere (MLT) to major sudden stratospheric warming events with elevated stratopauses demonstrates the role of planetary waves (PWs) in driving the mean circulation in the presence of gravity waves (GWs), helping the polar vortex recover and communicating the sudden stratospheric warming (SSW) impact across the equator. With the SSW onset, strong westward PW drag appears above 80 km primarily from the dissipation of wave number 1 perturbations with westward period of 5–12 days, generated from below by the unstable westward polar stratospheric jet that develops as a result of the SSW. The filtering effect of this jet also allows eastward propagating GWs to saturate in the winter MLT, providing eastward drag that promotes winter polar mesospheric cooling. The dominant PW forcing translates to a net westward drag above the eastward mesospheric jet, which initiates downwelling over the winter pole. As the eastward polar stratospheric jet returns, this westward PW drag persists above 80 km and acts synergistically with the return of westward GW drag to drive a stronger polar downwelling that warms the pole adiabatically and helps reform the stratopause at an elevated altitude. With the polar wind reversal during the SSW onset, the westward drag by the quasi‐stationary PW in the winter stratosphere drives an anomalous equatorial upwelling and cooling that enhance tropical stratospheric ozone. Along with equatorial wind anomalies, this ozone enhancement subsequently amplifies the migrating semidiurnal tide amplitude in the winter midlatitudes.

Observations of planetary waves in the mesosphere-lower thermosphere during stratospheric warming events

Abstract. This study investigates the effect of stratospheric sudden warmings (SSWs) on planetary wave (PW) activity in the mesosphere–lower thermosphere (MLT). PW activity near 95 km is derived from meteor wind data using a chain of eight SuperDARN radars at high northern latitudes that span longitudes from 150° W to 25° E and latitudes from 51 to 66° N. Zonal wave number 1 and 2 components were extracted from the meridional wind for the years 2000–2008. The observed wintertime PW activity shows common features associated with the stratospheric wind reversals and the accompanying stratospheric warming events. Onset dates for seven SSW events accompanied by an elevated stratopause (ES) were identified during this time period using the Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM). For the seven events, a significant enhancement in wave number 1 and 2 PW amplitudes near 95 km was found to occur after the wind reversed at 50 km, with amplitudes maximizing approximately 5 days after the onset of the wind reversal. This PW enhancement in the MLT after the event was confirmed using SD-WACCM. When all cases of polar cap wind reversals at 50 km were considered, a significant, albeit moderate, correlation of 0.4 was found between PW amplitudes near 95 km and westward polar-cap stratospheric winds at 50 km, with the maximum correlation occurring ∼ 3 days after the maximum westward wind. These results indicate that the enhancement of PW amplitudes near 95 km is a common feature of SSWs irrespective of the strength of the wind reversal.

Ertel potential vorticity versus Bernoulli streamfunction in earth's extratropical atmosphere

AbstractWe investigate the correlation of Ertel potential vorticity, Q, with Bernoulli streamfunction, B, on potential‐temperature surfaces, θ, in Earth's atmosphere poleward of latitude. For input, we use the 1979–2010 CMAM30‐SD specified‐dynamics data set. We fit the model separately in the midlatitude and polar regions, and nondimensionalize the slope parameter as . In the troposphere, the peak of is approximately 2–3 in winter and 3–4 in summer in both regions. In the stratosphere during winter, both and hover around unity. In spring and fall, continues to hover near unity, but rises by a factor of two, with its maximum located at to . In summer, the polar value drops to , while the midlatitude value swings negative to to −3. In the mesosphere, exhibits incessant fluctuations with values that usually fall within the range 0 to 1. The results support the paradigm for dynamically active regions primarily in the vicinity of – ( – ) in the mesosphere, whereas except in summer, we get the simple result that the monthly climate‐averaged profile, , in the polar regions is nearly unity across the entire stratosphere. During extended recoveries following SSWs, the midlatitude vertical profile develops a peak near 2000 K that descends over the course of a few weeks, alongside the descent of the elevated stratopause in the polar region.

Changes in the composition of the northern polar upper stratosphere in February 2009 after a sudden stratospheric warming

AbstractVariability in the chemistry of the upper stratosphere/lower mesosphere region has been analyzed focusing on high latitudes during the boreal winter in 2009 characterized by the strong sudden stratospheric warming (SSW) on 24 January. Data from Michelson Interferometer for Passive Atmospheric Sounding aboard Envisat and the Microwave Limb Sounder on Aura have been used to exemplify these changes. Record high (low) values of O3 and ClO (temperature and HCl) for the winters of 2005–2012, coupled with a simultaneous enhancement of ClONO2, have been observed in February 2009. This suggests that the very low temperatures favor a more effective ozone production and a greater O3/O ratio. The latter is the main factor controlling active chlorine partitioning. Increases of ClO lead to high ClONO2 concentrations in the upper stratosphere at high latitudes, where its photodissociation rate is smaller. Since this increase of ClONO2 happens at the expense of HCl, the region of high ClONO2 roughly coincides with the region of low HCl. Although this period was characterized by an elevated stratopause event, the investigated region was not influenced by the descent of mesospheric air rich in NOx. Some limited enhancements in NOx at ~1 hPa occurred at latitudes greater than 80°N after about 20 February, but they became consistent only in March. Intrusion of midlatitude air mostly occurred between the SSW and early February. Then, the sum of volume mixing ratios of ClONO2 + ClO + HCl remained approximately constant and close to the values of the other years. In contrast, it was up to 0.2 ppbv lower during the SSW period. These atypical chemical conditions occurred also in February 2006, but 2009 stands out for its long‐lasting effects, which persisted until late March.

Gravity wave activity during recent stratospheric sudden warming events from SOFIE temperature measurements

Stratospheric Sudden Warmings (SSWs) followed by the formation of an elevated stratopause at ~70–80 km occurred in four of the five recent Arctic winters (2009–2013). We use global high-latitude temperature measurements from the Solar Occultation for Ice Experiment (SOFIE) to analyze the gravity wave (GW) activity in the upper stratosphere and mesosphere (30–90 km) during different phases of the SSW events. We characterize GW activity in terms of temperature fluctuations and the growth of GW potential energy with altitude. At both 40 and 60 km, compared to the non-SSW year of 2011, the GW activity in the SSW years of 2009, 2010, 2012, and 2013 was reduced after the warming, during the occurrence of an isothermal atmosphere and an elevated stratopause. In contrast, at 80 km the GW activity was highly variable between the individual stratospheric warming events. A case study of GW activity during the 2013 warming event and coincident SOFIE observations of water vapor (H2O) from ~40 to 90 km indicate a correlation between increase in wave activity at each altitude and the time of descent of dry air. This study supports previous modeling studies' findings that enhanced GW activity is responsible for the downward transport of trace species from the mesosphere to the stratosphere following an SSW event.

Elevated Stratopause Events During 2003–2011 Revealed by SABER/TIMED Temperature Observations

AbstractThe elevated stratopause (ES) events in the winter (January–March) of 2006, 2009 and 2010 at 70°N are studied based on the SABER/TIMED temperature measurements in the period from 2003 to 2011. In each of these three winters, there suddenly appeared the maximum atmospheric temperature around 80 km between 25 and 85 km in late January–early February, which meant that the stratopause jumped from the normal location (∼50 km) with temperature ∼260 K up to the higher location (∼80 km) with temperature ∼230 K, and this phenomenon was identified as the ES event. As time went on, the ES height decreased gradually and returned to its normal level. Meanwhile the ES temperature increased from ∼230 K to ∼260 K. It is noteworthy that the applied data set indicates that although stratospheric warming occurred in every boreal winter, only after the major sudden stratospheric warming (SSWs) accompanying with the polar vortex splitting, the ES events could occur. Furthermore, in the ES events winter, gravity wave (GW) activity enhancement could be observed at ∼80 km between late January and early February, corresponding to the ES occurrence height and time. After February GW activity became weaker above 75 km and stronger below 75 km when the ES decreased with time. The correlation analyses between GW activity and the ES event indicate the possible contribution of GW activity to the formation of the ES event.

The influence of major sudden stratospheric warming and elevated stratopause events on the effects of energetic particle precipitation in WACCM

We investigate the influence of major sudden stratospheric warming (SSW) and elevated stratopause (ES) events in the Northern Hemisphere winter on the transport of NOx produced by energetic particle precipitation (EPP) from the mesosphere–lower thermosphere to the stratosphere using the Whole Atmosphere Community Climate Model (WACCM). Increases in NOx following a major SSW and/or ES event are in excess of 100% compared to winters when no major SSW or ES event occurred. The increase in NOx is attributed to an increase in the descending branch of the residual circulation () following the event. The timing of the event strongly affects the amount of NOx that descends to the stratosphere: the earlier the event occurs, the more NOx descends to the stratosphere. We also quantify the amount of NOx produced by EPP descending to the stratosphere in each winter and find that the largest increases in NOx are in years that have a major SSW followed by an ES event early in the season (December or early January). The strength of following an event shows a very strong seasonal dependence and explains why the timing of the event affects the transport of NOx.

Nighttime secondary ozone layer during major stratospheric sudden warmings in specified‐dynamics WACCM

A major stratospheric sudden warming (SSW) strongly impacts the entire middle atmosphere up to the thermosphere. Currently, the role of atmospheric dynamics on polar ozone in the mesosphere‐lower thermosphere (MLT) during SSWs is not well understood. Here we investigate the SSW‐induced changes in the nighttime “secondary” (90–105 km) ozone maximum by examining the dynamics and distribution of key species (like H and O) important to ozone. We use output from the National Center for Atmospheric Research Whole Atmosphere Community Climate Model with “Specified Dynamics” (SD‐WACCM), in which the simulation is constrained by meteorological reanalyses below 1 hPa. Composites are made based on six major SSW events with elevated stratopause episodes. Individual SSW cases of temperature and MLT nighttime ozone from the model are compared against the Sounding of the Atmosphere using Broadband Emission Radiometry observations aboard the NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. The evolution of ozone and major chemical trace species is associated with the anomalous vertical residual motion during SSWs and consistent with photochemical equilibrium governing the MLT nighttime ozone. Just after SSW onset, enhanced upwelling adiabatically cools the polar region from 80 to 100 km and transports low H from below. These conditions promote a concentration increase in the secondary ozone layer. Subsequent downwelling from the lower thermosphere warms the MLT and enhances the descent of H from the thermospheric reservoir, thereby limiting the secondary ozone concentration increase. Negative correlation of secondary ozone with respect to temperature and H is more pronounced during winters with SSWs than during non‐SSW winters.

Gravity wave variations during elevated stratopause events using SABER observations

AbstractElevated stratopauses formed at ~80–90 km altitude during the recovery phase of stratospheric sudden warmings in February 2006 and 2009. These likely occurred in response to changes in the downward circulation due to gravity waves (GWs) and/or planetary waves in the mesosphere and the lower thermosphere (MLT). However, the physical mechanisms are not fully understood, due in part to the lack of global GW observations in the MLT. This study presents global‐scale GW observations in the MLT during elevated stratopause events using Thermosphere, Ionosphere, Mesosphere Energetics Dynamics (TIMED)‐Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature observation, which provide a better insight into the formation of an elevated stratopause. During the downward movement of elevated stratopause events in 2006 and 2009, GWs were suppressed below ~60 km and enhanced above ~60 km at high latitudes compared to non‐elevated stratopause years (2005 and 2007). Global SABER GW observations indicate that the regions of GW enhancement propagate from low‐mid latitudes to high latitudes in association with the equatorward shift of the polar night jet during elevated stratopause events. Ray‐tracing simulations show enhancements of the poleward propagation of GWs during elevated stratopause events as well as continuous propagation of non‐orographic GWs within high latitudes. Therefore, our results suggest that meridional propagation of GWs from lower to higher latitudes, which is typically not included in global‐scale models, plays an important role in determining GW variations and thus the downward movement of an elevated stratopause, in addition to non‐orographic GWs originating at high latitudes.

Secondary planetary waves in the middle and upper atmosphere following the stratospheric sudden warming event of January 2012

AbstractThe role of planetary waves in causing stratospheric sudden warmings (SSWs) is well understood and quantified. However, recent studies have indicated that secondary planetary waves are excited in the mesosphere and lower thermosphere following SSWs. We use a version of the Whole Atmosphere Community Climate Model constrained by reanalysis data below 50 km to simulate the SSW of January 2012, a minor warming followed by the formation of an elevated stratopause. We document the occurrence of enhanced Eliassen‐Palm flux divergence in the mesosphere and lower thermosphere associated with faster, secondary westward‐propagating planetary waves of wave number 1 and period <10 days. We confirm the presence of these secondary planetary waves using observations made by the Sounding of the Atmosphere using the Broadband Emission Radiometry instrument onboard NASA's Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite.

A climatology of the stratopause in WACCM and the zonally asymmetric elevated stratopause

A climatology of the stratopause is produced using a 40 year simulation of the Whole Atmosphere Community Climate Model (WACCM). Anomalies in polar winter stratopause temperature and height are interpreted with respect to the location of the polar vortices and anticyclones. The WACCM climatology is compared to an 8 year climatology based on Microwave Limb Sounder (MLS) observations and data from the Goddard Earth Observing System (GEOS) version 5 from August 2004 to July 2012. The WACCM climatology is in excellent agreement with observations, except in the Antarctic vortex where the WACCM stratopause is ~10K warmer and ~5 km higher than observations. WACCM diabatic heating rates support the hypothesis that ageostrophic vertical motions associated with baroclinic planetary waves are responsible for producing Arctic winter temperature anomalies. The area of the winter polar vortices in WACCM at the stratopause is 30% smaller in the NH and 45% smaller in the SH compared to GEOS. The long WACCM record allows us to explore the geographical distribution and temporal evolution of a composite of 15 elevated stratopause (ES) events. This composite is in good agreement with the 2012 ES event observed by MLS, though December ES events in WACCM are not observed by MLS. This is the first work to show that ES events are not zonally symmetric. In the 30 days following ES events, the ES composite shows that the stratopause altitude is highest over the Canadian Arctic, and the highest stratopause temperatures occur 90° to the east over the Norwegian Sea.

A climatology of elevated stratopause events in the whole atmosphere community climate model

Elevated stratopause (ES) events occurring during Northern Hemisphere winter are identified in four climate simulations of the period 1953–2005 made with the Whole Atmosphere Community Climate Model (WACCM). We find 68 ES events in 212 winters. These events are found in winters when the middle atmosphere is disturbed and there are zonal wind reversals in the stratosphere at high latitudes. These disturbances can be associated with both major and minor stratospheric sudden warming events (SSWs). The ES events occur under conditions where the stratospheric jet, the gravity wave forcing, and the residual circulation remain reversed longer than in those winters where an SSW occurs without an ES. We compare ES events with the type of SSW (vortex splitting and vortex displacement) and find that 68% of ES events form after vortex splitting events. We also present a climatology of ES events based on NASA's Modern‐Era Retrospective Analysis for Research and Applications reanalysis data from 1979 to 2012 and compare it to the model results. WACCM composites of major SSW and ES also show enhanced Eliassen‐Palm flux divergences in the upper mesosphere after the stratospheric warming, immediately before the formation of an ES. However, the formation of an ES in WACCM is due primarily to adiabatic heating from gravity wave‐driven downwelling, which follows the reestablishment of the eastward jet in the upper stratosphere. We find nine winters where an ES forms in the absence of any significant planetary wave activity in the upper mesosphere and illustrate one such event.

High Resolution Dynamics Limb Sounder observations of the gravity wave‐driven elevated stratopause in 2006

Temperature observations during January and February 2006 from the High Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments are compared to illustrate the vertical range over which version 6 HIRDLS temperatures are scientifically useful. In order to determine the quality of HIRDLS temperatures in the middle atmosphere, we compare the height and temperature of the HIRDLS stratopause with MLS and SABER before, during, and after the 2006 major stratospheric sudden warming. Results show that HIRDLS observes the elevated stratopause at 78 km two days later than MLS and five days after SABER. We compare the geographical temperature structure of these data sets at 0.01 hPa during this period. Though HIRDLS temperatures are consistently 5–10 K lower in the mesosphere, this is the first study to show that the horizontal temperature distribution is in good spatial and temporal agreement with MLS and SABER up to ∼80 km. Gravity wave momentum flux and planetary wave 1 amplitudes are derived from HIRDLS and shown to be in agreement with previous studies. We use HIRDLS to show a ∼30 K increase in stratopause temperature following enhanced gravity wave momentum flux in the lower mesosphere.

Growth of planetary waves and the formation of an elevated stratopause after a major stratospheric sudden warming in a T213L256 GCM

Recovery processes after a major stratospheric sudden warming (SSW) with the formation of an elevated stratopause and a strong polar‐night jet are investigated using a gravity‐wave‐resolving GCM. The major SSW that occurred in the GCM bears a strong resemblance to observations in January 2006 and January 2009. The recovery phase of the SSW in the GCM is divided into two stages. In the first stage during about five days just after the SSW, a large positive Eliassen‐Palm (E‐P) flux divergence associated with the growth of planetary waves contributes to the quick recovery of eastward wind above 2 hPa (about 42 km), which is likely due to baroclinic and/or barotropic instabilities. In the second stage over the next three weeks, a prolonged westward wind in the lower stratosphere blocked upward propagation of gravity waves with westward intrinsic phase velocities. It reduces the deceleration of eastward wind in the upper mesosphere and raises the breaking height of gravity waves. Since the height of westward gravity wave forcing also rises, the polar stratopause created by the gravity‐wave‐driven meridional circulation is formed at an elevated height (about 75 km) compared to that before the SSW (55–65 km). In addition, the weaker westward gravity‐wave forcing in the upper mesosphere drives weaker downwelling around 1 hPa and forms a cold layer. Consequently, the strong polar‐night jet forms at a higher altitude than before the SSW as a result of adjustment toward the thermal wind balance. This indicates that these two stages provide eastward acceleration in different ways.