Canadian Middle Atmosphere Model Research Articles

On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices

Abstract During high solar activity, the atmosphere receives more energy from the sun, particularly in the form of shortwave radiation. Most notable is the effect in the middle and upper atmosphere, which in general shows a positive temperature response due to physical and chemical processes that are intensified at high solar activity. It is thus surprising that a clear solar cycle signal is absent in the summer polar mesosphere region in spite of it being illuminated around the clock. In this study, it is investigated how the circulation in the summer mesosphere is affected by changes in the solar flux using a 30-yr run from the nudged version of the Canadian Middle Atmosphere Model (CMAM30). It is found that—in July—the solar cycle signal from direct solar heating is counteracted by an enhanced residual circulation, which adiabatically cools the region at a higher rate when the solar activity is above average. The dynamical cooling is partly initiated in the Southern Hemisphere winter stratosphere.

Exploring noctilucent cloud variability using the nudged and extended version of the Canadian Middle Atmosphere Model

Ice particles in the summer mesosphere – such as those connected to noctilucent clouds and polar mesospheric summer echoes - have since their discovery contributed to the uncovering of atmospheric processes on various scales ranging from interactions on molecular levels to global scale circulation patterns. While there are numerous model studies on mesospheric ice microphysics and how the clouds relate to the background atmosphere, there are at this point few studies using comprehensive global climate models to investigate observed variability and climatology of noctilucent clouds. In this study it is explored to what extent the large-scale inter-annual characteristics of noctilucent clouds are captured in a 30-year run - extending from 1979 to 2009 - of the nudged and extended version of the Canadian Middle Atmosphere Model (CMAM30). To construct and investigate zonal mean inter-seasonal variability in noctilucent cloud occurrence frequency and ice mass density in both hemispheres, a simple cloud model is applied in which it is assumed that the ice content is solely controlled by the local temperature and water vapor volume mixing ratio. The model results are compared to satellite observations, each having an instrument-specific sensitivity when it comes to detecting noctilucent clouds. It is found that the model is able to capture the onset dates of the NLC seasons in both hemispheres as well as the hemispheric differences in NLCs, such as weaker NLCs in the SH than in the NH and differences in cloud height. We conclude that the observed cloud climatology and zonal mean variability are well captured by the model.

Temperature responses to the 11 year solar cycle in the mesosphere from the 31 year (1979–2010) extended Canadian Middle Atmosphere Model simulations and a comparison with the 14 year (2002–2015) TIMED/SABER observations

AbstractA recent 31 year simulation (1979–2010) by extended Canadian Middle Atmosphere Model (eCMAM30) and the 14 year (2002–2015) observation by the Thermosphere Ionosphere Mesosphere and Dynamics/Sounding of the Atmosphere using Broadband Emssion Radiometry (TIMED/SABER) are utilized to investigate the temperature response to the 11 year solar cycle on the mesosphere. Overall, the zonal mean responses tend to increase with height, and the amplitudes are on the order of 1–2 K/100 solar flux unit (1 sfu = 10−22 W m−2 Hz−1) below 80 km and 2–4 K/100 sfu in the mesopause region (80–100 km) from the eCMAM30, comparatively weaker than those from the SABER except in the midlatitude lower mesosphere. A pretty good consistence takes place at around 75–80 km with a response of ~1.5 K/100 sfu within 10°S/N. Also, a symmetric pattern of the responses about the equator agrees reasonably well between the two. It is noteworthy that the eCMAM30 displays an alternate structure with the upper stratospheric cooling and the lower mesospheric warming at midlatitudes of the winter hemisphere, in favor of the long‐term Rayleigh lidar observation reported by the previous studies. Through diagnosing multiple dynamical parameters, it is manifested that this localized feature is induced by the anomalous residual circulation as a consequence of the wave‐mean flow interaction during the solar maximum year.

An idealized stratospheric model useful for understanding differences between long‐lived trace gas measurements and global chemistry‐climate model output

AbstractWe use a modified version of the tropical leaky pipe (TLP) model of the stratosphere to explore how well an idealized model can (1) reproduce global chemistry‐climate model (CCM) output and (2) constrain transport characteristics necessary to replicate measurements of long‐lived trace gases. The version of the TLP model we use includes the simulation of long‐lived trace gases, such as SF6 and CO2, as well as photochemically active trace gases such as CFC‐11, CFC‐12, and N2O. The TLP model was found to accurately replicate trace gas output from the Canadian Middle Atmosphere Model (CMAM) for time‐averaged profiles in the tropics and each extratropical region. Given confidence that the TLP model represents the basic transport features in CMAM we then used the TLP model to interpret differences between CMAM output and measurements from the Atmospheric Chemistry Experiment and balloons. The TLP model is shown to uniquely determine residual mean circulation and recirculation (mixing between the extratropics and tropics) changes necessary for CMAM to more accurately simulate the measurements. Such guidance on these transport parameters is novel due to the relatively high precision and the simultaneous derivation of important parameters, as compared to previous studies. The TLP model can ideally be used as a bridge between measurements and CCMs to potentially allow more targeted modification of the CCMs than would otherwise be possible.

Radiative analysis of global mean temperature trends in the middle atmosphere: Effects of non-locality and secondary absorption bands

In this paper, we provide a refined and extended assignment of past and future temperature changes relative to previous analyses and describe and evaluate the relevance of vertical coupling and non-linear and secondary radiative mechanisms for the interpretation of climatic temperature variations in the middle atmosphere. Because of their nature, the latter mechanisms are not adequately accounted for in most regression analyses of temperature trends as a function of local constituent variations. These mechanisms are examined using (1) globally averaged profiles from transient simulations with the Canadian Middle Atmosphere Model (CMAM) forced by changes in greenhouse gases and ozone depleting substances and (2) a one-dimensional radiative-equilibrium model forced using the diagnosed global mean changes in radiatively active constituents as derived from the CMAM model runs. The conditions during the periods 1975 to 1995 and 2010 to 2040 (during which the rates of change in ozone and CO2 differ) provide a suitable contrast for the role of the non-linear and non-local mechanisms being evaluated in this paper to be clearly differentiated and evaluated. Vertical coupling of radiative transfer effects and the influence of secondary absorption bands are important enough to render the results of multiple linear regression analyses between the temperature response and constituent changes misleading. These effects are evaluated in detail using the 1D radiative-equilibrium model using profiles from the CMAM runs as inputs. In order to explain the differences in the CMAM temperature trends prior to and after 2000 these other radiative effects must be considered in addition to local changes in the radiatively active species. The middle atmosphere temperature cools in response to CO2 and water vapor increases, but past and future trends are modulated by ozone changes.

Comparison of the CMAM30 data set with ACE-FTS and OSIRIS: polar regions

Abstract. CMAM30 is a 30-year data set extending from 1979 to 2010 that is generated using a version of the Canadian Middle Atmosphere Model (CMAM) in which the winds and temperatures are relaxed to the Interim Reanalysis product from the European Centre for Medium-Range Weather Forecasts (ERA-Interim). The data set has dynamical fields that are very close to the reanalysis below 1 hPa and chemical tracers that are self-consistent with respect to the model winds and temperature. The chemical tracers are expected to be close to actual observations. The data set is here compared to two satellite records – the Atmospheric Chemistry Experiment Fourier transform spectrometer and the Odin Optical Spectrograph and Infrared Imaging System – for the purpose of validating the temperature, ozone, water vapour and methane fields. Data from the Aura microwave limb sounder are also used for validation of the chemical processing in the polar vortex. It is found that the CMAM30 temperature is warmer by up to 5 K in the stratosphere, with a low bias in the mesosphere of ~ 5–15 K. Ozone is reasonable (±15 %), except near the tropopause globally and in the Southern Hemisphere winter polar vortex. Water vapour is consistently low by 10–20 %, with corresponding high methane of 10–20 %, except in the Southern Hemisphere polar vortex. Discrepancies in this region are shown to stem from the treatment of polar stratospheric cloud formation in the model.

A numerical study of the effects of migrating tides on thermosphere midnight density maximum

AbstractIn this study, we employed the National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) and the extended Canadian Middle Atmosphere Model (eCMAM) to investigate the role of the migrating terdiurnal tide on the formation and variation of the thermosphere midnight temperature maximum (MTM) and midnight mass density maximum (MDM). The migrating terdiurnal tide from the eCMAM was applied at the TIEGCM's lower boundary, along with the migrating diurnal and semidiurnal tides from the Global‐Scale Wave Model. Several numerical experiments with different combinations of tidal forcing at the TIEGCM's lower boundary were carried out to determine the contribution of each tide to MTM/MDM. We found that the interplay between diurnal, semidiurnal, and terdiurnal tides determines the formation of MTM/MDM and their structure in the upper thermosphere. The decrease of thermospheric mass density after MDM reaches its maximum at ~02:00 local time is mainly controlled by the terdiurnal tide. Furthermore, we examined the generation mechanisms of the migrating terdiurnal tide in the upper thermosphere and found that they come from three sources: upward propagation from the lower thermosphere, in situ generation via nonlinear interaction, and thermal excitation.

The Hiccup: a dynamical coupling process during the autumn transition in the Northern Hemisphere – similarities and differences to sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) are the most prominent vertical coupling process in the middle atmosphere, which occur during winter and are caused by the interaction of planetary waves (PWs) with the zonal mean flow. Vertical coupling has also been identified during the equinox transitions, and is similarly associated with PWs. We argue that there is a characteristic aspect of the autumn transition in northern high latitudes, which we call the "hiccup", and which acts like a "mini SSW", i.e. like a small minor warming. We study the average characteristics of the hiccup based on a superimposed epoch analysis using a nudged version of the Canadian Middle Atmosphere Model, representing 30 years of historical data. Hiccups can be identified in about half the years studied. The mesospheric zonal wind results are compared to radar observations over Andenes (69° N, 16° E) for the years 2000–2013. A comparison of the average characteristics of hiccups and SSWs shows both similarities and differences between the two vertical coupling processes.

The Downward Influence of Stratospheric Sudden Warmings*

Abstract The coupling between the stratosphere and the troposphere following two major stratospheric sudden warmings is studied in the Canadian Middle Atmosphere Model using a nudging technique by which the zonal-mean evolution of the reference sudden warmings are artificially induced in an ~100-member ensemble spun off from a control simulation. Both reference warmings are taken from a freely running integration of the model. One event is a displacement, the other is a split, and both are followed by extended recoveries in the lower stratosphere. The methodology permits a statistically robust study of their influence on the troposphere below. The nudged ensembles exhibit a tropospheric annular mode response closely analogous to that seen in observations, confirming the downward influence of sudden warmings on the troposphere in a comprehensive model. This tropospheric response coincides more closely with the lower-stratospheric annular mode anomalies than with the midstratospheric wind reversal. In addition to the expected synoptic-scale eddy feedback, the planetary-scale eddies also reinforce the tropospheric wind changes, apparently responding directly to the stratospheric anomalies. Furthermore, despite the zonal symmetry of the stratospheric perturbation, a highly zonally asymmetric near-surface response is produced, corresponding to a strongly negative phase of the North Atlantic Oscillation with a much weaker response over the Pacific basin that matches composites of sudden warmings from the Interim ECMWF Re-Analysis (ERA-Interim). Phase 5 of the Coupled Model Intercomparison Project models exhibit a similar response, though in most models the response’s magnitude is underrepresented.

Stratospheric warming influence on the mesosphere/lower thermosphere as seen by the extended CMAM

Abstract. The response of the upper mesosphere/lower thermosphere region to major sudden stratospheric warming (SSW) is examined employing temperature, winds, NOX and CO constituents from the extended Canadian Middle Atmosphere Model (CMAM) with continuous incremental nudging below 10 hPa (~ 30 km). The model results considered cover high latitudes (60–85° N) from 10 to 150 km height for the December–March period of 2003/2004, 2005/2006 and 2008/2009, when some of the strongest SSWs in recent years were observed. NOX and CO are used as proxies for examining transport. Comparisons with ACE-FTS (Atmospheric Chemistry Experiment–Fourier Transform Spectrometer) satellite observations show that the model represents well the dynamics of the upper mesosphere/lower thermosphere region, the coupling of the stratosphere–mesosphere, and the NOX and CO transport. New information is obtained on the upper mesosphere/lower thermosphere up to 150 km showing that the NOX volume mixing ratio in the 2003/2004 winter was very perturbed indicating transport from the lower atmosphere and intense mixing with large NOX influx from the thermosphere compared to 2006 and 2009. These results, together with those from other models and observations, clearly show the impact of stratospheric warmings on the thermosphere.

The character of polar tidal signatures in the extended Canadian Middle Atmosphere Model

AbstractThe characteristics of the diurnal, semidiurnal, and terdiurnal tides (zonal wave numbers −5 to +5 in temperature and zonal wind) in the polar mesosphere and lower thermosphere region as simulated by the extended Canadian Middle Atmosphere Model are examined. The most significant diurnal, semidiurnal, and terdiurnal tides in the polar regions are Ds0, Dw1, and De1; Sw3, Sw2, Sw1, Ss0, Se1, and Se2; and Tw3, Ts0, and Tw1, respectively, and their latitudinal structures, seasonal variations, and hemispheric asymmetries noted. Of these components, Ds0, Tw1, Ts0, and Tw3 exhibit a seasonally symmetric variation with both hemispheres strengthening simultaneously. On the other hand, Dw1 strengthens asymmetrically so that when one hemisphere is strong, the other is weak. The remainder show no seasonal tendency but vacillate on shorter than seasonal time scales in a symmetric or antisymmetric manner at different times of the year. Global‐scale correlations of the amplitudes of the migrating tides Dw1/Sw2 and the stationary planetary wave 1 and the assumed “child” nonmigrating tides are also examined. The results indicate that the correlations are highly time scale‐dependent and the significant correlations seen with the original time series are mainly due to longer‐term variations (>18 days). There are no consistent global correlations associated with the short‐term variations (<18 days) among these waves.

Technical Note: A simple procedure for removing temporal discontinuities in ERA-Interim upper stratospheric temperatures for use in nudged chemistry-climate model simulations

Abstract. This note describes a simple procedure for removing unphysical temporal discontinuities in ERA-Interim upper stratospheric global mean temperatures in March 1985 and August 1998 that have arisen due to changes in satellite radiance data used in the assimilation. The derived temperature adjustments (offsets) are suitable for use in stratosphere-resolving chemistry-climate models that are nudged (relaxed) to ERA-Interim winds and temperatures. Simulations using a nudged version of the Canadian Middle Atmosphere Model (CMAM) show that the inclusion of the temperature adjustments produces temperature time series that are devoid of the large jumps in 1985 and 1998. Due to its strong temperature dependence, the simulated upper stratospheric ozone is also shown to vary smoothly in time, unlike in a nudged simulation without the adjustments where abrupt changes in ozone occur at the times of the temperature jumps. While the adjustments to the ERA-Interim temperatures remove significant artefacts in the nudged CMAM simulation, spurious transient effects that arise due to water vapour and persist for about 5 yr after the 1979 switch to ERA-Interim data are identified, underlining the need for caution when analysing trends in runs nudged to reanalyses.

Climatology of the diurnal tides from eCMAM30 (1979 to 2010) and its comparison with SABER

The extended Canadian Middle Atmosphere Model (eCMAM) was recently run in a nudged mode using reanalysis data from the ground to 1 hPa for the period of January 1979 to June 2010 (hence the name eCMAM30). In this paper, eCMAM30 temperature is used to examine the background mean temperature, the spectrum of the diurnal tides, and the climatology of the migrating diurnal tide Dw1 and three nonmigrating diurnal tides De3, Dw2, and Ds0 in the stratosphere, mesosphere, and lower thermosphere. The model results are then compared to the diurnal tidal climatology derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations between 40 to 110 km and 50° S to 50° N from January 2002 to December 2013. The model reproduces the latitudinal background mean temperature gradients well except that the cold mesopause temperature in eCMAM30 is 10 to 20 K colder than SABER. The diurnal tidal spectra and their relative strengths compare very well between eCMAM30 and SABER. The altitude-latitude structures for the four diurnal tidal components (Dw1, De3, Dw2, and Ds0) from the two datasets are also in very good agreement even for structures in the stratosphere with a weaker amplitude. The largest discrepancy between the model and SABER is associated with the seasonal variation of De3. In addition to the Northern Hemisphere (NH) summer maximum, a secondary maximum occurs during NH winter (December-February) in the model but is absent in SABER. The seasonal variations of the other three diurnal tidal components are in good agreement. Interannual time series of Dw1 and De3 from both eCMAM30 and SABER reveal variability with a period of 25 to 26 months, which indicates the modulation of the diurnal tides by the stratospheric quasi-biennial oscillation (QBO).

The diurnal and semidiurnal tides over Ascension Island (° S, 14° W) and their interaction with the stratospheric quasi-biennial oscillation: studies with meteor radar, eCMAM and WACCM

Abstract. Horizontal winds in the mesosphere have been measured over Ascension Island (8° S, 14° W) in the tropical mid-Atlantic region throughout the years 2002–2011. The observations were made by a very high frequency (VHF) meteor radar. The observations reveal the presence of atmospheric tides of large amplitude. The observations are analysed to characterise the seasonal and interannual variability of the diurnal and semidiurnal tides. Monthly-mean diurnal tidal amplitudes are found to reach values as large as 48 m s−1 in the meridional component and 41 m s−1 in the zonal. A semiannual seasonal variation is found in diurnal tidal amplitudes with amplitude maxima at the equinoxes and amplitude minima at the solstices. Diurnal tidal meridional vertical wavelengths are generally in the range 24–30 km. The diurnal zonal vertical wavelengths are similar to the meridional, except for the winter months when the zonal vertical wavelengths are much longer, occasionally exceeding 100 km. Semidiurnal amplitudes are observed to be significantly smaller than diurnal amplitudes. Semidiurnal vertical wavelengths range from 20 to more than 100 km. Our observations of tidal amplitudes and phases are compared with the predictions of the extended Canadian Middle Atmosphere Model (eCMAM) and the Whole Atmosphere Community Climate Model (WACCM). Both eCMAM and WACCM reproduce the trend for greater diurnal amplitudes in the meridional component than the zonal. However, eCMAM tends to overestimate meridional amplitudes, while WACCM underestimates both zonal and meridional amplitudes. Vertical wavelength predictions are generally good for both models; however, eCMAM predicts shorter diurnal zonal vertical wavelengths than are observed in winter, while WACCM predicts longer zonal vertical wavelengths than observed for the semidiurnal tide for most months. Semidiurnal amplitude predictions are generally good for both models. It is found that larger-than-average diurnal and semidiurnal tidal amplitudes occur when the stratospheric quasi-biennial oscillation (QBO) at 10 hPa is eastwards, and smaller-than-average amplitudes occur when it is westwards. Correlations between the amplitude perturbations and the El Niño Southern Oscillation are also found. The precise mechanism for these correlations remains unclear.

Southern Annular Mode Dynamics in Observations and Models. Part II: Eddy Feedbacks

Abstract Many global climate models (GCMs) have trouble simulating southern annular mode (SAM) variability correctly, particularly in the Southern Hemisphere summer season where it tends to be too persistent. In this two-part study, a suite of experiments with the Canadian Middle Atmosphere Model (CMAM) is analyzed to improve the understanding of the dynamics of SAM variability and its deficiencies in GCMs. Here, an examination of the eddy–mean flow feedbacks is presented by quantification of the feedback strength as a function of zonal scale and season using a new methodology that accounts for intraseasonal forcing of the SAM. In the observed atmosphere, in the summer season, a strong negative feedback by planetary-scale waves, in particular zonal wavenumber 3, is found in a localized region in the southwest Pacific. It cancels a large proportion of the positive feedback by synoptic- and smaller-scale eddies in the zonal mean, resulting in a very weak overall eddy feedback on the SAM. CMAM is deficient in this negative feedback by planetary-scale waves, making a substantial contribution to its bias in summertime SAM persistence. Furthermore, this bias is not alleviated by artificially improving the climatological circulation, suggesting that climatological circulation biases are not the cause of the planetary wave feedback deficiency in the model. Analysis of the summertime eddy feedbacks in the models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) confirms that this is indeed a common problem among GCMs, suggesting that understanding this planetary wave feedback and the reason for its deficiency in GCMs is key to improving the fidelity of simulated SAM variability in the summer season.

Dynamical Control of the Mesosphere by Orographic and Nonorographic Gravity Wave Drag during the Extended Northern Winters of 2006 and 2009

Abstract A version of the Canadian Middle Atmosphere Model (CMAM) that is nudged toward reanalysis data up to 1 hPa is used to examine the impacts of parameterized orographic and nonorographic gravity wave drag (OGWD and NGWD) on the zonal-mean circulation of the mesosphere during the extended northern winters of 2006 and 2009 when there were two large stratospheric sudden warmings. The simulations are compared to Aura Microwave Limb Sounder (MLS) observations of mesospheric temperature and carbon monoxide (CO) and derived zonal winds. The control simulation, which uses both OGWD and NGWD, is shown to be in good agreement with MLS. The impacts of OGWD and NGWD are assessed using simulations in which those sources of wave drag are removed. In the absence of OGWD the mesospheric zonal winds in the months preceding the warmings are too strong, causing increased mesospheric NGWD, which drives excessive downwelling, resulting in overly large lower-mesospheric values of CO prior to the warming. NGWD is found to be most important following the warmings when the underlying westerlies are too weak to allow much vertical propagation of the orographic gravity waves to the mesosphere. NGWD is primarily responsible for driving the circulation that results in the descent of CO from the thermosphere following the warmings. Zonal-mean mesospheric winds and temperatures in all simulations are shown to be strongly constrained by (i.e., slaved to) the stratosphere. Finally, it is demonstrated that the responses to OGWD and NGWD are nonadditive because of their dependence and influence on the background winds and temperatures.

Southern Annular Mode Dynamics in Observations and Models. Part I: The Influence of Climatological Zonal Wind Biases in a Comprehensive GCM

Abstract A common bias among global climate models (GCMs) is that they exhibit tropospheric southern annular mode (SAM) variability that is much too persistent in the Southern Hemisphere (SH) summertime. This is of concern for the ability to accurately predict future SH circulation changes, so it is important that it be understood and alleviated. In this two-part study, specifically targeted experiments with the Canadian Middle Atmosphere Model (CMAM) are used to improve understanding of the enhanced summertime SAM persistence. Given the ubiquity of this bias among comprehensive GCMs, it is likely that the results will be relevant for other climate models. Here, in Part I, the influence of climatological circulation biases on SAM variability is assessed, with a particular focus on two common biases that could enhance summertime SAM persistence: the too-late breakdown of the Antarctic stratospheric vortex and the equatorward bias in the SH tropospheric midlatitude jet. Four simulations are used to investigate the role of each of these biases in CMAM. Nudging and bias correcting procedures are used to systematically remove zonal-mean stratospheric variability and/or remove climatological zonal wind biases. The SAM time-scale bias is not alleviated by improving either the timing of the stratospheric vortex breakdown or the climatological jet structure. Even in the absence of stratospheric variability and with an improved climatological circulation, the model time scales are biased long. This points toward a bias in internal tropospheric dynamics that is not caused by the tropospheric jet structure bias. The underlying cause of this is examined in more detail in Part II of this study.

Statistical Characterization of Arctic Polar-Night Jet Oscillation Events

AbstractA novel diagnostic tool is presented, based on polar-cap temperature anomalies, for visualizing daily variability of the Arctic stratospheric polar vortex over multiple decades. This visualization illustrates the ubiquity of extended-time-scale recoveries from stratospheric sudden warmings, termed here polar-night jet oscillation (PJO) events. These are characterized by an anomalously warm polar lower stratosphere that persists for several months. Following the initial warming, a cold anomaly forms in the middle stratosphere, as does an anomalously high stratopause, both of which descend while the lower-stratospheric anomaly persists. These events are characterized in four datasets: Microwave Limb Sounder (MLS) temperature observations; the 40-yr ECMWF Re-Analysis (ERA-40) and Modern Era Retrospective Analysis for Research and Applications (MERRA) reanalyses; and an ensemble of three 150-yr simulations from the Canadian Middle Atmosphere Model. The statistics of PJO events in the model are found to agree very closely with those of the observations and reanalyses.The time scale for the recovery of the polar vortex following sudden warmings correlates strongly with the depth to which the warming initially descends. PJO events occur following roughly half of all major sudden warmings and are associated with an extended period of suppressed wave-activity fluxes entering the polar vortex. They follow vortex splits more frequently than they do vortex displacements. They are also related to weak vortex events as identified by the northern annular mode; in particular, those weak vortex events followed by a PJO event show a stronger tropospheric response. The long time scales, predominantly radiative dynamics, and tropospheric influence of PJO events suggest that they represent an important source of conditional skill in seasonal forecasting.

Zonal-Mean Dynamics of Extended Recoveries from Stratospheric Sudden Warmings

AbstractThe recovery of the Arctic polar vortex following stratospheric sudden warmings is found to take upward of 3 months in a particular subset of cases, termed here polar-night jet oscillation (PJO) events. The anomalous zonal-mean circulation above the pole during this recovery is characterized by a persistently warm lower stratosphere, and above this a cold midstratosphere and anomalously high stratopause, which descends as the event unfolds. Composites of these events in the Canadian Middle Atmosphere Model show the persistence of the lower-stratospheric anomaly is a result of strongly suppressed wave driving and weak radiative cooling at these heights. The upper-stratospheric and lower-mesospheric anomalies are driven immediately following the warming by anomalous planetary-scale eddies, following which, anomalous parameterized nonorographic and orographic gravity waves play an important role. These details are found to be robust for PJO events (as opposed to sudden warmings in general) in that many details of individual PJO events match the composite mean.A zonal-mean quasigeostrophic model on the sphere is shown to reproduce the response to the thermal and mechanical forcings produced during a PJO event. The former is well approximated by Newtonian cooling. The response can thus be considered as a transient approach to the steady-state, downward control limit. In this context, the time scale of the lower-stratospheric anomaly is determined by the transient, radiative response to the extended absence of wave driving. The extent to which the dynamics of the wave-driven descent of the stratopause can be considered analogous to the descending phases of the quasi-biennial oscillation (QBO) is also discussed.

Momentum budget of the migrating diurnal tide in the Whole Atmosphere Community Climate Model at vernal equinox

The momentum budget of the migrating diurnal tide (DW1) at the vernal equinox is studied using the Whole Atmosphere Community Climate Model, version 4 (WACCM4). Classical tidal theory provides an appropriate first‐order prediction of the DW1 structure, while gravity wave (GW) forcing and advection are the two most dominant terms in the momentum equation that account for the discrepancies between classical tidal theory and the calculation based on the full primitive equations. It differs from the conclusion by McLandress (2002a) that the parameterized GW effect is substantially weaker than advection terms based on the Canadian Middle Atmosphere Model (CMAM). In the region where DW1 maintains a large amplitude, GW forcing in the wave breaking region always damps DW1 and advances its phase. The linear advection largely determined by the latitudinal shear of the zonal mean zonal wind makes a dominant contribution to the phase change of DW1 in the zonal wind compared to the GW forcing and nonlinear advection. However, nonlinear advection is more important than GW forcing and linear advection in modulating the amplitude and phase of DW1 in the meridional wind. The DW1 amplitudes in temperature and winds are smaller than the TIMED observations, suggesting that GW forcing is overestimated in the WACCM4 and results in a large damping of DW1.