Polar Stratopause Research Articles

Multi‐Step Vertical Coupling During the January 2017 Sudden Stratospheric Warming

AbstractThis study analyzes a simulation of the Arctic winter 2016–2017 with focus on multi‐step vertical coupling (MSVC) by primary, secondary, and higher‐order gravity waves (GWs). We employ the HIgh Altitude Mechanistic general Circulation Model with nudging of the large scales to MERRA‐2 reanalysis. Simulation results confirm the well‐known effects from primary GWs in the winter middle atmosphere regarding strong westward GW drag and a warm winter polar stratopause during the strong‐vortex period in late December 2016, as well as weak eastward GW drag and mesospheric cooling during the sudden stratospheric warming (SSW) in late January and early February 2017. Since the amplitudes of the primary GWs that dissipate in the middle atmosphere are weaker for a reversed or weakened polar vortex, the theory for secondary GW generation predicts reduced MSVC in this case. This is confirmed by strongly reduced secondary and higher‐order GW amplitudes during the SSW and the weak‐vortex period in February. The wintertime higher‐order GWs show partial concentric ring structures above their sources in the lower thermosphere. We find that mostly those higher‐order GWs propagate to higher altitudes that have horizontal propagation directions against the tidal winds. The simulated GWs at 300 km height and observed perturbations of total electron content over Europe and North America during selected days of low geomagnetic activity show very good agreement regarding (a) the wave characteristics and (b) the reduction of amplitudes during the SSW and early February as compared to late December.

Influence of observed seasonally varying composition on Titan’s stratospheric circulation

Titan’s atmosphere exhibits variations in composition as it progresses through its seasons. Past observations have shown a substantial enrichment in short-lived molecules in the winter stratosphere above 100 km. Seasonal variations in Titan’s stratospheric dynamics also lead to a transient detached haze layer (DHL) above 400 km. The seasonal variations in aerosol opacity and molecular abundance lead to varying radiative heating rates in both the shortwave and longwave spectral regions. In this paper, we report on the effects of a new dataset for aerosol opacity and trace gas abundance derived from Cassini observations on simulations of Titan’s stratospheric dynamics with the Titan Atmospheric Model (TAM). We find that including seasonally varying radiative species (SVRS) decreases the autumn and winter polar stratopause temperature by up to 10 K poleward of 60°. We also find a similar increased seasonality in the zonal winds with the early autumn polar jet strengthening by 60 m s−1, while the mid winter jet is unaffected. While including the observationally derived SVRS dataset increases the strength of the seasonal variations of the polar jet, it does not substantially affect the timing of the onset or dissipation of the jet or other seasonal phenomena.

A climatology of Rossby wave generation in the middle atmosphere of the Southern Hemisphere from MERRA reanalysis

AbstractA climatological characterization of Rossby wave generation events in the middle atmosphere of the Southern Hemisphere is conducted using 20 years of Modern‐Era Retrospective Analysis for Research and Applications (MERRA) reanalysis. An automatic detection technique of wave generation events is developed and applied to MERRA reanalysis. The Rossby wave generation events with wave period of 1.25 to 5.5 days and zonal wave number from one to three dominate the Eliassen‐Palm flux divergence around the stratopause at high latitudes in the examined 20 year period. These produce an eastward forcing of the general circulation between May and mid‐August in that region. Afterward from mid‐August to the final warming date, Rossby wave generation events are still present but the Eliassen‐Palm flux divergence in the polar stratopause is dominated by low‐frequency Rossby waves that propagate from the troposphere. The Rossby wave generation events are associated with potential vorticity gradient inversion, and so they are a manifestation of the dominant barotropic/baroclinic unstable modes that grow at the cost of smearing the negative meridional gradient of potential vorticity. The most likely region of wave generation is found between 60° and 80°S and at a height of 0.7 hPa, but events were detected from 40 hPa to 0.3 hPa (which is the top of the examined region). The mean number of events per year is 24, and its mean duration is 3.35 days. The event duration follows an exponential distribution.

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.

Mesospheric intrusion and anomalous chemistry during and after a major stratospheric sudden warming

Several major stratospheric sudden warmings (SSWs) characterized by a rapid vertical displacement of the winter polar stratopause are simulated in the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model. The stratopause descends into the mid-stratosphere at the onset of the SSW, and then abruptly reforms near 70km. The SSWs are accompanied by a strong equatorward and downward residual circulation between 40 and 60km. The descent occurs mainly through the core of the highly displaced vortex, and is accompanied by an intrusion of air rich in carbon monoxide (CO) from the mesosphere into the mid-stratosphere. Around the periods when the stratopause undergoes rapid vertical displacement, the simulation shows that the air of mesospheric origin is being cut off and remains distinct from surrounding stratospheric air masses for at least a month after SSW onset. Such mesospheric cut-off intrusion of CO-rich appears to be a defining signature of winters with major SSWs. Due to its strong temperature dependency, the secondary ozone maximum (between 90 and 110km) abruptly decreases in amplitude at the time of the high-altitude stratopause reformation that influences the thermal structure at these altitudes. The vertical location of the tertiary ozone maximum (between 70 and 75km) shows significant variations in response to the changing vertical motion during SSW.

The roles of planetary and gravity waves during a major stratospheric sudden warming as characterized in WACCM

The roles of planetary waves (PWs) and gravity waves (GWs) are examined during a realistic major stratospheric sudden warming (SSW), simulated in the National Center for Atmospheric Research Whole Atmosphere Community Climate Model (WACCM). This major SSW event is characterized by a well-separated polar stratopause during a wind-reversal period. Formed by adiabatic warming induced by westward GW drag, the early-winter stratopause layer appears at its climatological level. With the incipient wind reversal and SSW onset, this layer plunges ∼20km in time, as the amplified PW interacts with the mean flow. The SSW recovery starts in the upper mesosphere as GW drag becomes eastward due to the filtering effects of the underlying wind. During this recovery, the stratopause reforms at an elevated altitude due to adiabatic warming induced by strong upper mesospheric PW forcing. Intensified downward motion from the mesosphere then ensues as the stratopause descends toward its climatological position.

Temporal variations of Titan’s middle-atmospheric temperatures from 2004 to 2009 observed by Cassini/CIRS

We use five and one-half years of limb- and nadir-viewing temperature mapping observations by the Composite Infrared Radiometer-Spectrometer (CIRS) on the Cassini Saturn orbiter, taken between July 2004 and December 2009 ( L S from 293° to 4°; northern mid-winter to just after northern spring equinox), to monitor temperature changes in the upper stratosphere and lower mesosphere of Titan. The largest changes are in the northern (winter) polar stratopause, which has declined in temperature by over 20 K between 2005 and 2009. Throughout the rest of the mid to upper stratosphere and lower mesosphere, temperature changes are less than 5 K. In the southern hemisphere, temperatures in the middle stratosphere near 1 mbar increased by 1–2 K from 2004 through early 2007, then declined by 2–4 K throughout 2008 and 2009, with the changes being larger at more polar latitudes. Middle stratospheric temperatures at mid-northern latitudes show a small 1–2 K increase from 2005 through 2009. At north polar latitudes within the polar vortex, temperatures in the middle stratosphere show a ∼4 K increase during 2007, followed by a comparable decrease in temperatures in 2008 and into early 2009. The observed temperature changes in the north polar region are consistent with a weakening of the subsidence within the descending branch of the middle atmosphere meridional circulation.

Descent from the polar mesosphere and anomalously high stratopause observed in 8 years of water vapor and temperature satellite observations by the Odin Sub‐Millimeter Radiometer

Using newly analyzed mesospheric water vapor and temperature observations from the Sub‐Millimeter Radiometer instrument aboard the Odin research satellite over the period 2001–2009, we present evidence for an anomalously strong descent of dry mesospheric air from the lower mesosphere into the upper stratosphere in the winters of 2004, 2006, and 2009. In the three cases, the descent follows the recovery of the upper stratospheric polar vortex from a major midwinter stratospheric sudden warming. It is also accompanied by the rapid formation of an anomalously warm polar mesospheric layer, i.e., an elevated polar stratopause, near 75 km, and its slower descent to prewarming level (near 1 hPa) over 1.5–2 months. These three winters stand out in the current record of Odin/Sub‐Millimeter Radiometer observations started in July 2001.

High Resolution Dynamics Limb Sounder measurements of gravity wave activity in the 2006 Arctic stratosphere

Observations from the High Resolution Dynamics Limb Sounder (HIRDLS) instrument on NASA's Aura satellite are used to quantify gravity wave momentum fluxes in the middle atmosphere. The period around the 2006 Arctic sudden stratospheric warming (SSW) is investigated, during which a substantial elevation of the stratopause occurred. Analysis of the HIRDLS results, together with analysis of European Centre for Medium‐Range Weather Forecasting zonal winds, provide direct evidence of wind filtering of the gravity wave spectrum during this period. This confirms previous hypotheses from model studies and further contributes to our understanding of the effects of gravity wave driving on the winter polar stratopause.

The evolution of the stratopause during the 2006 major warming: Satellite data and assimilated meteorological analyses

Microwave Limb Sounder and Sounding of the Atmosphere with Broadband Emission Radiometry data provide the first opportunity to characterize the four‐dimensional stratopause evolution throughout the life‐cycle of a major stratospheric sudden warming (SSW). The polar stratopause, usually higher than that at midlatitudes, dropped by ∼30 km and warmed during development of a major “wave 1” SSW in January 2006, with accompanying mesospheric cooling. When the polar vortex broke down, the stratopause cooled and became ill‐defined, with a nearly isothermal stratosphere. After the polar vortex started to recover in the upper stratosphere/lower mesosphere (USLM), a cool stratopause reformed above 75 km, then dropped and warmed; both the mesosphere above and the stratosphere below cooled at this time. The polar stratopause remained separated from that at midlatitudes across the core of the polar night jet. In the early stages of the SSW, the strongly tilted (westward with increasing altitude) polar vortex extended into the mesosphere, and enclosed a secondary temperature maximum extending westward and slightly equatorward from the highest altitude part of the polar stratopause over the cool stratopause near the vortex edge. The temperature evolution in the USLM resulted in strongly enhanced radiative cooling in the mesosphere during the recovery from the SSW, but significantly reduced radiative cooling in the upper stratosphere. Assimilated meteorological analyses from the European Centre for Medium‐Range weather Forecasts (ECMWF) and Goddard Earth Observing System Version 5.0.1 (GEOS‐5), which are not constrained by data at polar stratopause altitudes and have model tops near 80 km, could not capture the secondary temperature maximum or the high stratopause after the SSW; they also misrepresent polar temperature structure during and after the stratopause breakdown, leading to large biases in their radiative heating rates. ECMWF analyses represent the stratospheric temperature structure more accurately, suggesting a better representation of vertical motion; GEOS‐5 analyses more faithfully describe stratopause level wind and wave amplitudes. The high‐quality satellite temperature data used here provide the first daily, global, multiannual data sets suitable for assessing and, eventually, improving representation of the USLM in models and assimilation systems.

A note on record‐high temperatures at the northern polar stratopause in winter 1997/98

A series of lidar temperature soundings from the ALOMAR observatory in northern Norway indicated an extreme warming of a descending stratopause in February 1998. The maximum temperature recorded during this event was +49°C at 40 km altitude. This stratospheric warming is described by means of SSU satellite radiance data and of stratospheric analyses from the Free University Berlin. Comparisons are made to a number of historical events with similar temperature observations from rocket soundings and to results from the Berlin general circulation model. It turns out that in all cases the highest stratopause temperatures occur close to the 40 km altitude level.

Measurements of artificial periodic inhomogeneities at HIPAS Observatory

Results are presented from recent experiments that employ high‐power, high‐frequency (HF) radio waves to probe the mesosphere and lower thermosphere. The measurements were made at the High‐Power Auroral Stimulation (HIPAS) Observatory located near Fairbanks, Alaska. One objective of the study was to determine the feasibility of using artificial electron density perturbations created in the auroral environment to measure the properties of the background neutral gas between ∼50 to ∼120 km altitude. The observing technique relies on the production of so‐called “artificial periodic inhomogeneities” (API) in the altitude region(s) of interest. These induced irregularities are believed to be horizontally stratified and conform to the standing wave pattern produced by the reflection of the powerful HF wave in the ionosphere. In the D region above HIPAS, API decay curves are strictly exponential and the phase histories are strictly linear. On occasion, echoes are detected at very low altitudes (∼45 km) in the vicinity of the polar stratopause. The API backscatter at HIPAS is often superimposed on regions of partial reflection, auroral E, and sporadic E. Information about ambipolar diffusion rates and electron attachment to O2 is obtained by measuring the relaxation time of the induced irregularities. In general, API phase velocities below ∼95 km altitude appear to be related to vertical neutral motions. However, detailed validation studies throughout the mesosphere have not yet been performed at HIPAS or any other API facility. At altitudes between ∼45 and ∼80 km, high‐resolution observations at HIPAS reveal the presence of sharply defined bands of API scatter 15–20 km in altitude extent. The existence of such bands and their fluctuation in altitude cannot be explained within the context of existing theory. Large variations in API backscatter power (10–20 dB) are typically observed in the D region over timescales of 30 s or less. Most likely this is caused by fading in the ionospherically reflected component of the standing wave pattern. Finally, power stepping studies reveal a roughly linear relationship between D region backscatter power and HF power. Fundamental questions related to the horizontal dimension of the API patch and its spatial structure remain to be addressed in future experiments.

A globally balanced two‐dimensional middle atmosphere model: Dynamical studies of mesopause meridional circulation and stratosphere‐mesosphere exchange

A globally balanced two‐dimensional middle atmosphere model has been developed. The dynamical fields are obtained by numerically solving two elliptic equations for the mass stream function of the residual circulation and the geopotential field. By comparison with exact solutions, it is demonstrated that this model is more accurate than two‐dimensional models in which the nonlinear advection equations are solved numerically. General features of the dynamical fields from the model and a focused study of meridional circulation in the mesopause region are presented. The simulated zonal mean temperature and zonal wind are consistent with the CIRA (1986) model atmosphere and the direct wind measurements from the high‐resolution Doppler imager (HRDI) and wind imaging interferometer (WINDII) instruments on board the Upper Atmosphere Research Satellite (UARS). The predicted meridional component of the residual circulation around 92 km is approximately 25 m s−1 with maximum near the summer mesopause. In the mesopause region the salient features in the UARS data are peak zonal mean meridional velocities of 20–30 m s−1 and substantial latitudinal gradients from equator to midlatitude maxima. The consistency between the model and the direct measurements supports the hypothesis that the mesospheric circulation is forced by gravity wave induced drag. The model tracer fields indicate the existence of a mesospheric “depletion zone” and “tracer front” similar to the “surf zone” and “barrier” in the lower stratosphere. The depletion zone and tracer front are generated by divergent and convergent fields analogous to tropospheric frontogenesis by a deformation field. In addition, the model shows that the most active region for the stratosphere‐mesosphere mass exchange is at the summer polar stratopause. The active exchange is driven by a strong summer mesospheric drag force through the “downward control” principle.

The 4-Day Wave as Observed from theUpper Atmosphere Research SatelliteMicrowave Limb Sounder

The is an eastward moving quasi-nondispersive feature with period near 4 days occurring near the winter polar stratopause. This paper presents evidence of the 4-day feature in Microwave Limb Sounder (MLS) temperature, geopotential height, and ozone data from the late southern winters of 1992 and 1993. Space-time spectral analyses reveal a double-peaked temperature structure consisting of one peak near the stratopause and another in the lower mesosphere, with an out-of-phase relationship between the two peaks. This double- peaked structure is reminiscent of recent three-dimensional barotropic/baroclinic instability model predictions and is observed here for the first time. The height variation of the 4-day ozone signal is shown to compare well with a linear advective-photochemical tracer model. Negative regions of quasigeostrophic potential vorticity (PV) gradient and positive Eliassen-Palm flux divergence are shown to occur, consistent with instability dynamics playing a role in wave forcing. Spectral analyses of PV derived from MLS geopotential height fields reveal a 4-day signal peaking near the polar stratopause. The three-dimensional structure of the 4-day wave resembles the potential vorticity concept, wherein a PV anomaly in the atmosphere (analogous to an electrical charge in a dielectric material) induces a geopotential field, a vertically oriented temperature dipole, and circulation about the vertical axis.

Instability at the Winter Stratopause: A Mechanism for the 4-Day Wave

Studies using climatological fields in a three-dimensional stability model show unstable modes near the polar winter stratopause with periods near 4 days. The calculated modes exhibit equatorward momentum and heat fluxes near the stratopause, similar to characteristics of observed 4-day wave events, demonstrating that both baroclinic and barotropic processes are important for this instability. The baroclinic and barotropic components are considered separately by selectively removing either horizontal or vertical shear from the background flow. Although both situations reveal instability, growth rates are very slow; realistic growth rates occur only for climatological flows including both horizontal and vertical shears. Similar unstable, fast-moving waves are found near the polar stratopause for several winter months in both hemispheres.

The Separated Polar Winter Stratopause: A Gravity Wave Driven Climatological Feature

An examination of satellite-derived temperatures reveals that the winter polar stratopause is usually elevated and warmer than the adjacent midlatitude stratopause. This separated stratopause occurs in both hemispheres, but is more pronounced and persistent in the southern winter. It descends with time towards spring and exhibits week-to-week variability. Observational diagnostics and results from a two-dimensional model suggest that gravity-wave driving can account for this separated polar stratopause by driving a meridional circulation, with downwelling over the winter pole. In the model, the solar heating pattern induces stronger winter westerlies than summer easterlies, which leads to a stronger gravity-wave-driven circulation in the winter hemisphere. Spherical geometry and the high latitude location of the winter westerly jet combine to yield a concentrated region of downwelling. Model results suggest that descent of the temperature maximum with time is probably caused by wave/mean-flow interaction.

The Middle Atmosphere In the Absence of Dynamical Heat Fluxes

AbstractA new radiative transfer scheme, developed for use in a three‐dimensional dynamical model of the stratosphere and mesosphere, is used to consider the hypothetical state of the middle atmosphere in the absence of dynamical heat fluxes. Most previous work in this area has considered radiative equilibrium temperatures using a fixed solar forcing. This equilibrium state is shown to be different from a state calculated using a seasonal cycle of solar forcing; in particular, the winter pole is found to be warmer and the latitudinal temperature gradient across the edge of the polar night is found to be smaller. This difference is due to two processes. Firstly, latitudes just polewards of the edge of the polar night at solstice are in darkness for only a short period. Secondly, radiative time scales lengthen greatly as temperatures decrease, so that equilibrium cannot be reached before the sun reappears.Recent ozone climatologies are used to impose a realistic annual cycle of ozone on the model. It is found that the simulation of the lower stratosphere is in considerably better agreement with observations than previous radiation‐only calculations. the better simulation is shown to be, in most cases, due to the use of the new ozone climatology. It is also shown that a realistic ozone distribution is necessary for a good simulation of the polar stratopause at the summer solstice.A comparison of the observed and calculated annual cycle of temperature in the polar regions of the southern hemisphere lower stratosphere indicates that this region is under strong radiative control. This contrasts with an implied strong dynamical control for the same region in the northern hemisphere.