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Based on the data of ERA5 reanalysis, the dates of spring and autumn changes in the stratospheric circulation on isobaric surfaces of 30, 20, and 10 hPa in the latitude zone of 30°–90° N in the period 1979–2020 have been obtained. Of the 42 cases of spring changes, 10 are early, 15 are middle, and 17 are late. The spread in the dates of spring changes on the surface of 10 hPa is 69 days. Most often, the spring changes of the circulation occurs from top to bottom; in some years, the delay of spring changes on the surface of 30 hPa relative to the surface of 10 hPa reaches 22–25 days. Autumn change takes place from the bottom up, and their terms at the 3 levels under consideration are close to each other. The relationship between the timing of the spring changes of the stratospheric circulation with solar activity and large sudden winter stratospheric warming is shown. An analysis of the fields of anomalies of daily temperature values and zonal wind velocity in the 1000-1 hPa layer in the period of January–May showed their significant spatiotemporal difference in the case of early and late spring changes. Thus, foci of positive anomalies of temperature and wind speed are formed initially in the upper stratosphere and then shifted from top to bottom. The interrelations between the layers of the atmosphere in different seasons are considered. [ABSTRACT FROM AUTHOR]

The peculiarities of the troposphere and stratosphere dynamic processes that accompanied cooling events (cold waves) with a duration of more than a week in the northwest of European Russia (on the example of St. Petersburg) and in its north and center (on the example of Moscow) in early and late December 2021, respectively, are analyzed using reanalysis data, station observations, and trajectory modeling. In both cases the daily temperatures dropped below −10°C, and cold waves were accompanied by the air mass transport from the northeast [ABSTRACT FROM AUTHOR]

Historically, observational information about atmospheric temperature has been limited due to a lack of suitable measurements. Recent advances in satellite observations provide new insight into the fine structure of the free atmosphere, with the upper troposphere and lower stratosphere comprising essential components of the climate system. This is a prerequisite for understanding the complex processes of this part of the atmosphere, which is also known to have a large impact on surface climate. With unprecedented resolution, latest climate observations reveal a dramatic warming of the atmosphere. The tropical upper troposphere has already warmed about 1 K during the first two decades of the 21st century. The tropospheric warming extends into the lower stratosphere in the tropics and southern hemisphere mid-latitudes, forming a prominent hemispheric asymmetry in the temperature trend structure. Together with seasonal trend patterns in the stratosphere, this indicates a possible change in stratospheric circulation. [ABSTRACT FROM AUTHOR]

Impacts of stratospheric polar vortex shift on the wintertime East Asian trough (EAT) on intraseasonal time scales are investigated using a reanalysis dataset and a climate model. The result based on composite analysis shows that the shift of the stratospheric polar vortex toward eastern Siberia (ES-shift event) is associated with higher geopotential height at 500 hPa than normal over East Asia, corresponding to the weakened EAT. Furthermore, the simulated EAT is also weakened when nudging the stratospheric state toward that during the ES-shift events. This study further found that there is no significant difference in the stratospheric polar vortex intensity between the ES-shift events and nonshift events, implying that the polar vortex strength change may have little influence on the possible connection between the polar vortex shift and the EAT change. The underlying mechanisms are listed as follows: First, the positive potential vorticity (PV) anomalies in the lower stratosphere associated with ES-shift events could explain approximately 40% of the local westerly anomalies in the upper troposphere to the north of East Asia via PV conservation, leading to the rise of the geopotential height over East Asia and the weakening of the EAT. Second, the shift of stratospheric polar vortex could modulate the synoptic-scale Rossby wave activity in the upper troposphere, favorable for the southward propagation of synoptic-scale waves and divergence of extended Eliassen-Palm flux in the upper troposphere. Finally, the transient wave feedback could enhance the tropospheric westerly anomalies in the north of East Asia and induce positive height anomalies to its south, further weakening the EAT. Our results revealed that the stratospheric polar vortex shift leads the EAT intensity variation by around 2-5 days, implying that the stratospheric polar vortex shift could be applicable to the prediction of the EAT intensity. [ABSTRACT FROM AUTHOR]

This study investigates the stratosphere–troposphere coupling associated with the Scandinavian (SCA) pattern in boreal winter. The results indicate that the SCA impacts stratospheric circulation but that its positive and negative phases have different effects. The positive phase of the SCA (SCA+) pattern is restricted to the troposphere, but the negative phase (SCA−) extends to the upper stratosphere. The asymmetry between phases is also visible in the lead–lag evolution of the stratosphere and troposphere. Prominent stratospheric anomalies are found to be intensified following SCA+ events, but prior to SCA− events. Further analysis reveals that the responses are associated with upward propagation of planetary waves, especially wavenumber 1, which is asymmetric between SCA phases. The wave amplitudes in the stratosphere, originating from the troposphere, are enhanced after the SCA+ events and before the SCA− events. Furthermore, the anomalous planetary wave activity can be understood through its interference with climatological stationary waves. Constructive wave interference is accompanied by clear upward propagation in the SCA+ events, while destructive interference suppresses stratospheric waves in the SCA− events. Our results also reveal that the SCA+ events are more likely to be followed by sudden stratospheric warming (SSW) events, because of the deceleration of stratospheric westerlies following the SCA+ events. [ABSTRACT FROM AUTHOR]

A new regime index is constructed to capture the seasonal development of the stratospheric polar vortex in the northern winter, based on the standard deviation of Ertel's potential vorticity in the upper stratosphere in November–December. The narrow-jet flow regime is characterized by a polar vortex that is more confined to high latitudes in the early winter upper stratosphere. This upper-level vortex configuration is more susceptible to the disturbances of upward propagating planetary-scale Rossby waves; the stratospheric polar vortex thus weakens earlier and is vertically shallower. The wide-jet flow regime is characterized by a broader-than-average polar vortex that extends further into the subtropics in the early winter upper stratosphere. The polar night jet then gradually strengthens, moves poleward, and penetrates deep into the lowermost stratosphere, with a sharply defined polar vortex edge due to more frequent Rossby wave breaking. Composite difference analyses show that the wide- and narrow-jet regimes, defined in the uppermost stratosphere in November–December, lead to different circulation anomalies of the lower stratosphere and the troposphere in January–February, offering the potential for improved predictability of subseasonal to seasonal forecasts up to two months ahead. The lower-tropospheric responses in January–February are zonally asymmetric. The narrow-jet regime projects most strongly over the North Atlantic, with an equatorward-shifted and/or broader midlatitude westerly jet. The wide-jet-regime response is characterized by a weakened midlatitude westerly jet over the North Pacific. The two flow regimes also differ in their impacts on the storm track over western Europe and the east coast of North America, which may have implications for extreme weather events in those regions. [ABSTRACT FROM AUTHOR]

Observational and modeling evidence suggest a recent acceleration of the stratospheric Brewer-Dobson circulation (BDC), driven by climate change and stratospheric ozone depletion. However, slowly varying natural variability can compromise our ability to detect such forced changes over the relatively short observational record. Using observations and chemistry-climate model simulations, we demonstrate a link between multi-decadal variability in the strength of the BDC and the Interdecadal Pacific Oscillation (IPO), with knock-on impacts for composition in the stratosphere. After accounting for the IPO-like variability in the BDC, the modeled trend is approximately 7%-10% dec-1 over 1979-2010. Furthermore, we find that sea surface temperatures explain up to 50% of the simulated decadal variability in tropical mid-stratospheric ozone. Our findings demonstrate strong links between low-frequency variability in the oceans, troposphere and stratosphere, as well as their potential importance in detecting structural changes in the BDC and future ozone recovery. Plain Language Summary Natural variability is a key element of the climate system and can mask human-induced changes. Here, we are interested in the naturally varying strength of the stratospheric global circulation and how this impacts the composition of the stratosphere. Using observations and model simulations, we show that slowly changing (multi-decadal) natural variability in the Pacific Ocean is reflected in the stratospheric circulation. This link helps us to better understand structural changes in the stratospheric circulation arising due to human interferences. In turn, slow transport variability reconciles recent low levels of ozone in the middle tropical stratosphere, which otherwise are in disagreement with the expected ozone recovery. These results have implications for both reconciling theory and observed changes in the stratospheric circulation, as well as better understanding the rate of stratospheric ozone recovery. [ABSTRACT FROM AUTHOR]

Using the ERA-Interim reanalysis dataset for the time period 1979–2016, we analyzed the influence of the stratospheric polar vortex shift on the Arctic Oscillation (AO) in winter (December–March). The results show that a shift in the stratospheric polar vortex toward the Eurasian continent is favorable for the occurrence of the negative phase of the AO. The duration of the AO events accompanied by the stratospheric polar vortex shift toward the Eurasian continent (AO-shift events) is longer than that of the remaining negative AO events (AO-noshift events), and the intensity of AO-shift events is greater than that of AO-noshift events from day 4 to day 15 of the life cycle of the events. The enhancement in the AO intensity during AO-shift events is likely due to downward extension of the stratospheric northern annular mode (NAM) signals and more poleward-propagating planetary waves in the troposphere and lower stratosphere and their convergence in the mid-high latitudes. Furthermore, the polar vortex shift can lead to changes in the intensity of the three action centers in the AO spatial pattern at 500 hPa. In general, during AO-shift events, the three action centers are stronger than those during AO-noshift events. There is an overall westward shift of the Arctic action center during AO-shift events, which may be closely related to the changes of Greenland blocking frequency. [ABSTRACT FROM AUTHOR]

This paper reports a comprehensive instability analysis of a 3D Charney-like model with an observationally compatible generic stratosphere. It is found that the values of a single nondimensional parameter Γ =   λ f o 2 / (β D N 1 2 ) (detailed definition in text), in conjunction with representative values of four other nondimensional parameters, would dictate the existence of multiple branches of unstable modes as a function of the zonal wavenumber. Prototype Charney mode, Green mode, and two additional structurally distinct modes (Charney+ mode and tropopause mode) are identified. The latter result from the additional strong influence of the tropopause. The dynamical nature of all modes is delineated in terms of their meridional fluxes of heat and potential vorticity. The three-dimensional structure of the ageostrophic velocity field in each mode is presented to identify its potential of inducing frontogenesis. Optimal-mode analyses are also performed to ascertain how a disturbance with a predisposed structure would explosively develop toward each type of normal mode. In view of the pivotal role of Γ in baroclinic instability and for historical reason, we name it the Charney number. It is most instructive to think of Γ as a ratio of the meridional gradient of PV associated with the basic flow in the troposphere,   λ f o 2 / (D N 1 2) , to that associated with Earth's rotation, β. [ABSTRACT FROM AUTHOR]

Utilizing three different sets of reanalysis data, this study examines the long- and short-lived observed positive North Atlantic Oscillation (NAO) events (referred to as NAO+_LE and NAO+_SE) and long- and short-lived observed negative NAO events (referred to as NAO−_LE and NAO−_SE). Composite results indicate that the NAO-like circulation anomalies associated with the long-lived NAO events can reach the stratosphere, while they are primarily confined to the troposphere in the short-lived NAO events. Thus, the coupling/connection of stratospheric and tropospheric circulation anomalies is much better (worse) in the long-lived (short-lived) NAO events. A series of modified stratospheric initial-value experiments conducted with a simplified model indicate that a better (worse) connection between stratospheric and tropospheric circulation anomalies in the initial-value fields tend to gradually induce the NAO-like tropospheric circulation anomalies in the troposphere on the subsequent days, and thus naturally elongate (reduce) the lifetimes of the original NAO events by altering the tropospheric synoptic eddy vorticity flux over the North Atlantic region. [ABSTRACT FROM AUTHOR]

The stratospheric water vapor feedbacks on climate for abrupt CO2 quadrupling are investigated with the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate model. A feedback suppression method is used to quantify the stratospheric water vapor climate feedback parameter and the impacts of stratospheric water vapor increases on temperature and circulation. It is found that increases in stratospheric water vapor change the model's net climate feedback parameter by 0.11 W m−2 K−1, contributing to 0.5 K, or 10%, of the global-mean surface warming under abrupt CO2 quadrupling. Stratospheric water vapor increases lead to significant impacts on stratospheric temperature and dynamics. The increases induce stratospheric dynamical changes that strongly modify stratospheric cooling patterns. About 30% of the acceleration of the stratospheric Brewer-Dobson circulation under 4 × CO2 is attributed to the stratospheric water vapor increases. In the troposphere, the stratospheric water vapor feedback plays a role in Arctic amplification and is responsible for 14% of the Arctic warming. It also affects tropospheric circulation, causing greater poleward shift of the northern hemisphere tropospheric midlatitude jet. [ABSTRACT FROM AUTHOR]

Tropical tropospheric responses to a sudden stratospheric warming (SSW) event that occurred over Antarctica in September 2019 have been investigated by conducting a series of ensemble forecast experiments. Comparative examinations between the normal forecast and the partially nudged forecast, whose stratospheric circulation is constrained to a reanalysis, reveal a significant enhancement/suppression in the convective activity (upwelling/downwelling) over the northern/southern part of the tropics. Such an acceleration of the Hadley circulation is set up by thermal structural changes in the upper troposphere and lower stratosphere. The tropical cooling caused by the enhanced Brewer‐Dobson circulation destabilizes the environment there and stimulates deep cumulus convections. In this case, the southern area of the Asian monsoon region is particularly sensitive. Although details of the triggered response depend on the adopted cumulus parameterization scheme, a boosting tendency of the convective upwelling via the SSW is robust. Plain Language Summary: A rare Antarctic sudden stratospheric warming (SSW) event that occurred in September 2019 provides an opportunity to investigate its impact on convective activity in the tropics. By comparing the normal forecast and a "perfect stratosphere" forecast, this study confirmed the robust influence of the SSW on the cumulus convection over the Northern Hemisphere tropics. In other words, the Antarctic SSW could enhance the probability of extreme weather in boreal summer to early autumn, including the genesis and development of tropical cyclones. Therefore, this process of the teleconnection between the Antarctic stratosphere and the tropical troposphere has the potential to be a valuable source of extended‐range predictability in practical seasonal forecasts. Key Points: Enhancement in the tropical convective activity during the 2019 Antarctic sudden warming is confirmed by ensemble nudging integrationsEnhanced Brewer‐Dobson circulation is linked to Hadley cells via thermal structure changes in the upper troposphere and lower stratosphereFor certain parameterizations, the large stratospheric effect is observed in cumulus convections at the south of the Asian monsoon zone [ABSTRACT FROM AUTHOR]

Based on ERA-Interim and JRA-55 daily reanalysis datasets, connections among the variations in the Brewer–Dobson circulation (BDC) intensity, stratospheric air mass, surface pressure, and the tropospheric meridional circulation during the period from 1979 to 2015 are analyzed. The results show that the variations in the surface pressure, particularly at middle and high latitudes, have a close correlation with the variations in the stratospheric BDC intensity. When the upwelling at 450 K intensifies, the surface pressure increases in the high latitudes (poleward of 60°) and decreases in the adjacent mid-latitudes in both hemispheres. And these correlations are most significant during boreal winter in the Northern Hemisphere and during austral autumn in the Southern Hemisphere. It is found that the high latitude surface pressure changes follow the anomalous BDC by about 30 days. The increase in polar surface pressure associated with the stronger BDC is due to an increase in stratospheric air mass, while the decrease in mid-latitude surface pressure is related to a decrease in tropospheric air mass. These air mass changes are caused by the anomalous meridional transport of air mass by the residual mean circulation in the stratosphere and troposphere. In addition, we found a significant connection between the intensity of the BDC and the Ferrel cell, i.e., when the BDC strengthens (weakens), there is a weakened (strengthened) Ferrel cell with its position shifting equatorward (poleward). [ABSTRACT FROM AUTHOR]

A combination of 240 years of output from a state-of-the-art chemistry–climate model and a twentieth-century reanalysis product is used to investigate to what extent sudden stratospheric warmings are preceded by anomalous tropospheric wave activity. To this end we study the fate of lower tropospheric wave events (LTWEs) and their interaction with the stratospheric mean flow. These LTWEs are contrasted with sudden stratospheric deceleration events (SSDs), which are similar to sudden stratospheric warmings but place more emphasis on the explosive dynamical nature of such events. Reanalysis and model output provide very similar statistics: Around one-third of the identified SSDs are preceded by wave events in the lower troposphere, while two-thirds of the SSDs are not preceded by a tropospheric wave event. In addition, only 20% of all anomalous tropospheric wave events are followed by an SSD in the stratosphere. This constitutes statistically robust evidence that the anomalous amplification of wave activity in the stratosphere that drives SSDs is not necessarily due to an anomalous amplification of the waves in the source region (i.e., the lower troposphere). The results suggest that the dynamics in the lowermost stratosphere and the vortex geometry are essential, and should be carefully analyzed in the search for precursors of SSDs. [ABSTRACT FROM AUTHOR]

We revisit the various stratosphere–troposphere coupling relation from the perspective of the meridional mass circulation. We constructed 10-hPa northern annular mode (NAM) phase composites to show the typical spatiotemporal evolution of circulation anomalies during the NAM's life cycle. Our results indicate that there is large case-to-case difference in the temporal evolution and vertical profile of polar temperature anomalies during NAM events, which shows no strong dependence on the intensity and duration of NAM events, but agrees well with the variations of the three branches of mass circulation at 60°N: the stratospheric poleward warm air branch (ST), the poleward warm air branch in the upper troposphere (WB), and the equatorward cold air branch in the lower troposphere (CB). Such correspondence is due to the dynamic heating and cooling anomalies associated with the redistribution of air masses by the anomalous meridional mass circulation in different isentropic layers. The various relationship among the three mass circulation branches is attributed to anomalous wave activities. The amplitude and westward tilt of waves are always stronger (weaker) throughout the stratosphere before (after) the peak time of negative NAM events, leading to a stronger (weaker) ST before (after) the peak time. Variations in WB and CB are mostly dependent on wave variabilities in the mid- to lower troposphere, leading to variations in the timing of in- or out-of-phase coupling of the ST with the WB and CB, and thus various thermostructure during NAM events. At a later stage of the negative NAM events when the polar temperature becomes colder and the polar jet recovers, the weakened baroclinic instability in the lower stratosphere provides favorable conditions for the strengthening of the WB and CB if wave activities strengthen in the troposphere during that period. [ABSTRACT FROM AUTHOR]

A climatology of the 100- and 250-hPa 45°–75°N zonal-mean meridional eddy heat flux anomaly, hereafter heat flux anomaly, was created to examine its variability following cool-season (i.e., October–April) blocks and extratropical cyclones. The goal is to elucidate the dynamical and environmental differences between synoptic events followed by the most extreme heat flux anomalies. The analysis was conducted with the National Aeronautics and Space Administration's Modern-Era Retrospective Analysis for Research and Applications, version 2 reanalysis. The results show that, on average, European blocks and west Pacific cyclones are followed by positive heat flux anomalies while west Pacific blocks and Atlantic extratropical cyclones are followed by negative heat flux anomalies. However, there was a large range of the 11-day-average heat flux anomaly following the events. Events in each region were further partitioned by their 100-hPa heat flux anomaly for a temporal and spatial analysis of the top and bottom quartile of events. Top-quartile events exhibited a baroclinic wave structure with height from the troposphere through the stratosphere, whereas bottom-quartile events were associated with a barotropic wave structure with height; these structures are significant at the 5% level. The results suggest that the sign of the heat flux anomaly is not dependent on the location of the synoptic event alone, but that there are common climatological and anomalous wave patterns surrounding the synoptic events that result in positive or negative heat flux anomaly. Regardless of event region, the precursor stratospheric structure is a key indicator in whether an event is followed by positive or negative 100-hPa heat flux anomalies. [ABSTRACT FROM AUTHOR]

Using the NCEP–NCAR reanalysis dataset, this study classifies stratospheric northern annular mode (NAM) anomalies during the negative or positive phase into two categories—anomalies extending into the troposphere [trop event (TE); referred to as negative or positive TEs] and those not extending into the troposphere [nontrop event (NTE); referred to as negative or positive NTEs], and the corresponding tropospheric environments during the TEs and NTEs are identified. Compared with that for the negative NTEs, the upward wave fluxes entering the stratosphere are stronger and more persistent during the negative TEs. Furthermore, the stronger and more persistent upward wave fluxes during the negative TEs are due to more favorable conditions for upward wave propagation, which is manifested by fewer occurrences of negative refractive index squared in the mid- to high-latitude troposphere and stronger wave intensity in the mid- to high-latitude troposphere. However, the tropospheric wave intensity plays a more important role than the tropospheric conditions of planetary wave propagation in modulating the upward wave fluxes into the stratosphere. Stronger and more persistent upward wave fluxes in the negative TEs, particularly wave-1 fluxes, are closely related to the negative geopotential height anomalies over the North Pacific and positive geopotential height anomalies over the Euro-Atlantic sectors. These negative (positive) geopotential height anomalies over the North Pacific (Euro-Atlantic) are related to the positive (negative) diabatic heating anomalies and the decreased (increased) blocking activities in the mid- to high latitudes. The subtropical diabatic heating could also impact the strength of the mid- to high-latitude geopotential height anomalies through modulating horizontal wave fluxes. For positive NAM events, the results are roughly similar to those for negative NAM events, but with opposite signal. [ABSTRACT FROM AUTHOR]

Dynamical processes in the Arctic stratosphere in the winter of 2017/2018 are analyzed using data of the NCEP reanalysis and SABER and MLS satellite measurements. The following features are revealed: the downward propagation of wave activity from the stratosphere to troposphere over Canada in late December and early January, the major sudden stratospheric warming with the zonal wind reversal in February 2018 accompanied by the stratospheric polar vortex splitting, the propagation of stratospheric circulation anomalies to the lower troposphere, the mesospheric cooling, the stratopause height change, and the cooling in the tropical lower stratosphere. [ABSTRACT FROM AUTHOR]

Despite the advances in theories and data availability since the first observation of stratospheric sudden warmings (SSWs) in the 1950s, some dynamical aspects of SSWs remain elusive, including the roles of wave transience at finite amplitude and irreversible wave dissipation due to mixing. This is likely due to a limitation of the traditional theory for SSWs that is tailored to small-amplitude waves and is unsuitable for large-scale wave events. To circumvent these difficulties, the authors utilized a novel approach based on finite-amplitude wave activity theory to quantify the roles of finite-amplitude wave transience and mixing in the life cycle of SSWs. In this framework, a departure from the exact nonacceleration relation can be directly attributed to irreversible mixing and diabatic forcings. The results show that prior to the warming event, an increase in pseudomomentum/wave activity largely compensates for the anomalous Eliassen–Palm flux convergence, while the total wave dissipation due to mixing (enstrophy dissipation) and radiative forcing only plays a secondary role. After the vortex breaks down, enhanced mixing increases irreversible wave dissipation and in turn slows down vortex recovery. It is shown that (i) a rapid recovery of the polar vortex is characterized by weak wave transience that follows a nonacceleration relation reversibly and (ii) a delayed recovery is attributed to stronger and more persistent irreversible wave dissipation due to mixing, a deviation from the classical nonacceleration relation. The results highlight the importance of mixing in the asymmetry between breakdown and recovery of the polar vortex during SSWs. [ABSTRACT FROM AUTHOR]

Downward wave coupling (DWC) is an important process that characterizes the dynamical coupling between the stratosphere and troposphere via planetary wave reflection. A recent modeling study has indicated that natural forcing factors, including sea surface temperature (SST) variability and the quasi-biennial oscillation (QBO), influenceDWCand the associated surface impact in the Northern Hemisphere (NH). In light of this, the authors further investigate how DWC in the NH is affected by anthropogenic forcings, using a fully coupled chemistry--climate model CESM1(WACCM). The results indicate that the occurrence of DWC is significantly suppressed in the future, starting later in the seasonal cycle, with more events concentrated in late winter (February and March). The future decrease in DWC events is associated with enhanced wave absorption in the stratosphere due to increased greenhouse gases (GHGs), which is manifest as more absorbing types of stratospheric sudden warmings (SSWs) in early winter. This early winter condition leads to a delay in the development of the upper-stratospheric reflecting surface, resulting in a shift in the seasonal cycle of DWC toward late winter in the future. The tropospheric responses to DWC events in the future exhibit different spatial patterns, compared to those of the past. In the North Atlantic sector, DWC-induced circulation changes are characterized by a poleward shift and an eastward extension of the tropospheric jet, while in the North Pacific sector, the circulation changes are characterized by a weakening of the tropospheric jet. These responses are consistentwith a change in the pattern ofDWC-induced synoptic-scale eddy--mean flow interaction in the future. [ABSTRACT FROM AUTHOR]

Using reanalysis datasets and numerical simulations, the relationship between the stratospheric Arctic vortex (SAV) and the Pacific decadal oscillation (PDO) on decadal time scales was investigated. A significant in-phase relationship between the PDO and SAV on decadal time scales during 1950-2014 is found, that is, the North Pacific sea surface temperature (SST) cooling (warming) associated with the positive (negative) PDO phases is closely related to the strengthening (weakening) of the SAV. This decadal relationship between the North Pacific SST and SAV is different from their relationship on subdecadal time scales. Observational and modeling results both demonstrate that the decadal variation in the SAV is strongly affected by the North Pacific SSTs related to the PDO via modifying the upward propagation of planetary wavenumber-1 waves from the troposphere to the stratosphere. The decreased SSTs over the North Pacific tend to result in a deepened Aleutian low along with a strengthened jet stream over the North Pacific, which excites a weakened western Pacific pattern and a strengthened Pacific-North American pattern. These tropospheric circulation anomalies are in accordance with the decreased refractive index (RI) at middle and high latitudes in the northern stratosphere during the positive PDO phase. The increased RI at high latitudes in the upper troposphere impedes the planetary wavenumber-1 wave from propagating into the stratosphere, and in turn strengthens the SAV. The responses of the RI to the PDO are mainly contributions of the changes in the meridional gradient of the zonal-mean potential vorticity via alteration of the baroclinic term - f²2/ρ0(ρ0...z/N²)z. [ABSTRACT FROM AUTHOR]

The skill of the Arctic stratospheric retrospective ensemble forecasts (hindcasts) of the European Centre for Medium-Range Weather Forecasts extended-range system is analyzed with a focus on the predictability of the major sudden stratospheric warmings (SSWs) during the period 1993-2016. Thirteen SSWs took place during this period. It is found that forecasts initialized 10-15 days before the SSWs show worse skill in the stratosphere than forecasts initialized during normal conditions in terms of root-mean-square errors but not in terms of anomaly correlation. Using the spread of ensemble members to estimate forecasted SSW probability, it is shown that some SSWs can be predicted with high (.0.9) probability at lead times of 12-13 days if a difference of 3 days between actual and forecasted SSW is allowed. Focusing on SSWs with significant impacts on the tropospheric circulation, on average, the forecasted SSW probability is found to increase from nearly 0 at 1-month lead time to 0.3 at day 13 before SSW, and then rapidly increases to nearly 1 at day 7. The period between days 8 and 12 is when most of the SSWs are predicted, with a probability of 0.5-0.9, which is considerably larger than the observed SSW occurrence frequency. Therefore, this period can be thought of as an estimate of the SSW predictability limit in this system. Indications that the predictability limit for some SSWs may be longer than 2 weeks are also found; however, this result is inconclusive and more studies are needed to understand when and why such long predictability is possible. [ABSTRACT FROM AUTHOR]

Tropospheric forcing of planetary wavenumber 2 is examined in the prephase of the major stratospheric sudden warming event in January 2009 (MSSW 2009). Because of a huge increase in Eliassen-Palm fluxes induced mainly by wavenumber 2, easterly angular momentum is transported into the Arctic stratosphere, deposited, and then decelerates the polar night jet. In agreement with earlier studies, the results reveal that the strongest eddy heat fluxes, associated with wavenumber 2, occur at 100 hPa during the prephase of MSSW 2009 in ERA-Interim. In addition, moderate conditions of the cold phase of ENSO (La Niña) contribute to the eddy heat flux anomaly. It is shown that enhanced tropospheric wave forcing over Alaska and Scandinavia is caused by tropical processes in two ways. First, in a climatological sense, La Niña contributes to an enhanced anticyclonic flow over both regions. Second, the Madden-Julian oscillation (MJO) has an indirect influence on the Alaskan ridge by enhancing eddy activity over the North Pacific. This is manifested in an increase in cyclone frequency and associated warm conveyor belt outflow, which contribute to the maintenance and amplification of the Alaskan anticyclone. The Scandinavian ridge is maintained by wave trains emanating from the Alaskan ridge propagating eastward, including an enhanced transport of eddy kinetic energy. The MSSW 2009 is an extraordinary case of how a beneficial phasing of La Niña and MJO conditions together with multiscale interactions enhances tropospheric forcing for wavenumber 2-induced zonal mean eddy heat flux in the lower stratosphere. [ABSTRACT FROM AUTHOR]

This work examines the life cycle of sudden stratospheric warmings (SSWs) from composites of a large number of events. The events are sampled from idealized general circulation model (GCM) integrations and form a database of several hundred major, displacement, splitting, and weak vortex events. It is shown that except for a few details, the generic zonal-mean evolution does not depend on the definition used to detect SSWs. In all cases, the composites show the stratosphere in a positive annular mode phase prior to the events and a barotropic response in the stratosphere at onset. There is a clear positive peak in upward Eliassen-Palm (EP) flux prior to the onset date in the stratosphere and a much weaker peak in the troposphere, making the evolution more consistent with the picture of the stratosphere acting as a variable filter of tropospheric EP flux, rather than SSWs being forced by a strong 'burst' in the troposphere. When comparing composites of SSWs from the database with apparent influence at the surface (downward 'propagating') to those without such influence, the only significant differences are a somewhat more barotropic response at the onset date and longer persistence in the lower stratosphere after the onset for propagating SSWs. There is no significant difference in EP flux between propagating and nonpropagating events, and none of the definitions considered here shows a particular skill in selecting propagating events. [ABSTRACT FROM AUTHOR]

To highlight the details of stratosphere-troposphere dynamical coupling during the onset of strong polar vortex variability, this study identifies stratospheric vortex weakening (SVW) events by rapid deceleration of the polar vortex and performs composite budget analyses in the transformed Eulerian-mean (TEM) framework on daily time scales. Consistent with previous work, a rapid deceleration of the polar vortex, followed by a rather slow recovery, is largely explained by conservative dynamics with nonnegligible contribution by nonconservative sinks of wave activity. During the onset of such events, stratospheric zonal wind anomalies show a near-instantaneous vertical coupling to the troposphere, which results from an anomalous upward and poleward propagation of planetary-scale waves. In the troposphere, zonal wind anomalies are also influenced by synoptic-scale waves, confirming previous studies. The SVW events driven by wavenumber-1 disturbances show comparable circulation anomalies to those driven by wavenumber-2 disturbances both in the stratosphere and troposphere. The former, however, exhibits more persistent anomalies after the onset than the latter. During both events, tropospheric wavenumber-1 and 2 disturbances project strongly onto the climatological waves, indicating that vertical propagation of planetary-scale waves into the stratosphere is largely caused by constructive linear interference. It is also found that the SVW-related vertical coupling is somewhat sensitive to the stratospheric mean state. Although overall evolution of zonal-mean circulation anomalies are reasonably similar under an initially weak or strong polar vortex, the time-lagged downward coupling is evident only when the polar vortex is decelerated under a weak vortex state. These results are compared with other definitions of weak polar vortex events, such as stratospheric sudden warming events. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) are characterized by a pronounced increase of the stratospheric polar temperature during the winter season. Different definitions have been used in the literature to diagnose the occurrence of SSWs, yielding discrepancies in the detected events. The aim of this paper is to compare the SSW climatologies obtained by different methods using reanalysis data. The occurrences of Northern Hemisphere SSWs during the extended-winter season and the 1958-2014 period have been identified for a suite of eight representative definitions and three different reanalyses. Overall, and despite the differences in the number and exact dates of occurrence of SSWs, the main climatological signatures of SSWs are not sensitive to the considered reanalysis. The mean frequency of SSWs is 6.7 events decade−1, but it ranges from 4 to 10 events, depending on the method. The seasonal cycle of events is statistically indistinguishable across definitions, with a common peak in January. However, the multidecadal variability is method dependent, with only two definitions displaying minimum frequencies in the 1990s. An analysis of the mean signatures of SSWs in the stratosphere revealed negligible differences among methods compared to the large case-to-case variability within a given definition. The stronger and more coherent tropospheric signals before and after SSWs are associated with major events, which are detected by most methods. The tropospheric signals of minor SSWs are less robust, representing the largest source of discrepancy across definitions. Therefore, to obtain robust results, future studies on stratosphere-troposphere coupling should aim to minimize the detection of minor warmings. [ABSTRACT FROM AUTHOR]

The formation of the tropical cold-point tropopause (CPT) is examined using a dry primitive equation model driven by the Held-Suarez forcing. Without moist and realistic radiative processes, the dry model successfully reproduces the zonal-mean structure of the CPT. The modeled CPT is appreciably colder (~10 K) than the prescribed equilibrium temperature, and it is maintained by upwelling in the tropical upper troposphere and lower stratosphere (UTLS). A transient simulation starting from an axisymmetric steady state without the CPT shows that the evolution and maintenance of the CPT are closely related to the zonal-mean-flow response to wave driving in the stratosphere. The transformed Eulerian-mean analysis indicates that the wave driving is mostly due to convergence of synoptic-scale waves originating from the midlatitude troposphere and propagating into the subtropical UTLS in this model simulation. The modeled CPT also shows a large sensitivity to increased baroclinicity in the equilibrium temperature. Although planetary-scale waves are not considered in this simulation, the result confirms that wave-driven upwelling in the tropical UTLS is a crucial process for the formation and maintenance of the CPT. In addition, it also implies that synoptic-scale waves may play a nonnegligible role in this mechanism, particularly in the seasons when planetary-scale wave activity in the lower stratosphere is weak. [ABSTRACT FROM AUTHOR]

Using two generations of reanalysis datasets from the National Center for Atmospheric Research ( NCAR), European Centre for Medium-Range Weather Forecasts ( ECMWF), and Japan Meteorological Agency ( JMA), we showed that on the interannual timescale the two leading modes of the East Asian winter monsoon ( EAWM) are associated with tropospheric annular mode ( AM) and stratospheric polar vortex ( SPV), respectively. The relationship between AM and the first EAWM mode remained stable during 1958-2013, whereas that between SPV and the second EAWM mode increased suddenly since the late 1980s. The SPV-related circulation and planetary wave activities are intensified in the latter period. We suggested that this change might be caused by the global warming and ozone depletion. [ABSTRACT FROM AUTHOR]

Tropospheric blocking events over the Northern Hemisphere during 1980-2012 were composited and contrasted according to whether they coincided in time with a sudden stratospheric warming (SSW). Those that coincided with an SSW were associated with significantly greater poleward eddy heat fluxes in the upper troposphere near the block onset time than were those blocking events not coinciding with an SSW. Furthermore, the heat fluxes in the SSW-blocking composites were concentrated inside the stratospheric polar vortex (i.e., within an area enclosed by the outer edge of an objectively defined polar vortex). Thermally forced stratospheric geopotential height rises were also significantly larger near block onset time inside the stratospheric polar vortex in the SSW-blocking composites than in the non-SSW-blocking cases. Although all the SSW events during the investigated period coincided with tropospheric blocking, the reverse was not true since there were many more blocking events than SSWs. Therefore, blocking itself was not a sufficient condition for an SSW. It is conjectured that blocking may not be a necessary condition for an SSW if persistently anomalous tropospheric heat fluxes and thermally forced, stratospheric geopotential height rises, concentrated inside the stratospheric vortex, occur in the absence of blocking. [ABSTRACT FROM AUTHOR]

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) presents new opportunities for the retrieval of trace gases in the upper troposphere and lower stratosphere. The radiometric calibration of the measured signal is achieved using in-flight measurements of reference blackbody and upward-pointing "deep space" scenes. In this paper, we present techniques developed specifically to calibrate GLORIA data exploiting the instrument's imaging capability. The algorithms discussed here make use of the spatial correlation of parameters across GLORIA's detector pixels in order to mitigate the noise levels and artefacts in the calibration measurements. This is achieved by combining a priori and empirical knowledge about the instrument background radiation with noise-mitigating compression methods, specifically low-pass filtering and principal component analysis. In addition, a new software package for the processing of GLORIA data is introduced which allows us to generate calibrated spectra from raw measurements in a semi-automated data processing chain. [ABSTRACT FROM AUTHOR]

Numerical experiments, presented in a companion paper, have been performed in which the zonal-mean state of the stratosphere in a comprehensive, stratosphere-resolving, general circulation model is strongly relaxed (or 'nudged') toward the evolution of a reference sudden warming event in order to investigate its influence on the freely evolving troposphere below. Similar approaches have been used in a number of other studies. This raises the question of whether such an artificial relaxation induces the adiabatic and diabatic adjustments expected below the region of nudging, even in the absence of the stratospheric wave driving responsible for the reference event. Motivated by this question, the zonally symmetric quasigeostrophic diabatic response to zonal forces (representing wave driving) in a system nudged to a time-dependent reference state is studied. In the presence of wave driving in the nudging region that differs from the reference state, the meridional mass circulation of the reference state is reproduced only in the region below the nudging up to a correction that is inversely proportional to the strength of the nudging. The anomalous circulation is confined because of an effective boundary condition at the interface of the nudging layer. The nudging also produces an artificial 'sponge-layer feedback' immediately below the region of the nudging in response to differences in the tropospheric wave driving. The strength of this artificial feedback is closely related to the strength of the effective boundary condition; however, the time scale required for the sponge-layer feedback to be established is typically much longer than that required for the confinement. [ABSTRACT FROM AUTHOR]

This study investigates the predictability of three major stratospheric sudden warmings (MSSWs) of the vortex split type: the Southern Hemisphere case in September 2002 and two Northern Hemisphere cases in January 2009 and February 1989. The author examines changes in the predictability of the MSSWs with lead time, as well as the connection of the predictability to lower-atmospheric features for pre- and post-MSSW periods. The Japan Meteorological Agency (JMA)'s 1-month ensemble hindcast (HC) experiment data are compared to the Japanese 25-year Reanalysis Project (JRA-25)/JMA Climate Data Assimilation System (JCDAS) data. For the pre-MSSW period, a strong predictability connection is observed among all three cases. Unsuccessful predictions of the MSSWs are characterized by an underestimation (or lack) of the enhanced wave activity in the lower stratosphere, which is further related to the strength and persistence of the upper-tropospheric ridge and trough. The mean zonal wind profile in the upper troposphere is also important for the 2009 case. These results confirm the role of tropospheric wave forcing of the MSSWs in the context of predictability. The characteristic time scale for successful predictions is approximately 10 days-2 weeks, which roughly corresponds to the time scale of the tropospheric wave forcing. No ensemble member successfully predicts the MSSWs with lead times longer than the time scale. The predictability connection between the stratospheric and tropospheric anomalies is more subtle for the post-MSSW period. In particular, the HC group initialized about 1 week before the MSSWs tends to reproduce the evolution of the stratosphere after the MSSWs well but not that of the troposphere in some cases. [ABSTRACT FROM AUTHOR]

A stratospheric trace gas measurement program using balloon- based sonde and AirCore sampler techniques is proposed as a way to monitor the strength of the stratospheric mean meridional or Brewer-Dobson circulation. Modeling work predicts a strengthening of the Brewer-Dobson circulation in response to increasing greenhouse gas concentrations; such a change will likely impact tropospheric climate. Because the strength of the Brewer-Dobson circulation is an unmeasureable quantity, trace gas measurements are used to infer characteristics of the circulation. At present, stratospheric trace gas measurements are sporadic in time and/or place, primarily associated with localized aircraft or balloon campaigns or relatively short-lived satellite instruments. This program would consist of regular trace gas profile measurements taken at multiple latitudes covering tropical, midlatitude, and polar regimes. The program would make use of the relatively low-cost AirCore and sonde techniques, allowing more frequent measurements given the significantly lower cost than with current techniques. The program will provide a means of monitoring changes in the strength and redistribution of the stratospheric circulation. The goals are to monitor the strength of the Brewer-Dobson circulation and to improve understanding of the reasons for stratospheric circulation changes, ultimately resulting in more realistic model predictions of climate change for the coming decades. [ABSTRACT FROM AUTHOR]

Presented is a review of the papers dealing with a problem of the tropopause definition as an interface between the troposphere and stratosphere. It is demonstrated that there are more than ten approaches to the tropopause identification based on the differences in thermal, chemical, dynamic, and radiation regimes of the troposphere and stratosphere. Given are the examples of the tropopause approximation with different surfaces or layers of specific properties. [ABSTRACT FROM AUTHOR]

This work explores the maintenance of the stratospheric structure in a primitive equation model that is forced by a Newtonian cooling with a prescribed radiative equilibrium temperature field. Models such as this are well suited to analyze and address questions regarding the nature of wave propagation and troposphere-stratosphere interactions. The focus lies on the lower to midstratosphere and the mean annual cycle, with its large interhemispheric variations in the radiative background state and forcing, is taken as a benchmark to be simulated with reasonable verisimilitude. A reasonably realistic basic stratospheric temperature structure is a necessary first step in understanding stratospheric dynamics. It is first shown that using a realistic radiative background temperature field based on radiative transfer calculations substantially improves the basic structure of the model stratosphere compared to previously used setups. Then, the physical processes that are needed to maintain the seasonal cycle of temperature in the lower stratosphere are explored. It is found that an improved stratosphere and seasonally varying topographically forced stationary waves are, in themselves, insufficient to produce a seasonal cycle of sufficient amplitude in the tropics, even if the topographic forcing is large. Upwelling associated with baroclinic wave activity is an important influence on the tropical lower stratosphere and the seasonal variation of tropospheric baroclinic activity contributes significantly to the seasonal cycle of the lower tropical stratosphere. Given a reasonably realistic basic stratospheric structure and a seasonal cycle in both stationary wave activity and tropospheric baroclinic instability, it is possible to obtain a seasonal cycle in the lower stratosphere of amplitude comparable to the observations. [ABSTRACT FROM AUTHOR]

Midtropospheric CO2 retrievals from the Atmospheric Infrared Sounder (AIRS) were used to explore the influence of stratospheric sudden warming (SSW) on CO2 in the middle to upper troposphere. To choose the SSW events that had strong coupling between the stratosphere and troposphere, the authors applied a principal component analysis to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2) geopotential height data at 17 pressure levels. Two events (April 2003 and March 2005) that have strong couplings between the stratosphere and troposphere were chosen to investigate the influence of SSW on AIRS midtropospheric CO2. The authors investigated the temporal and spatial variations of AIRS midtropospheric CO2 before and after the SSW events and found that the midtropospheric CO2 concentrations increased by 2-3 ppm within a few days after the SSW events. These results can be used to better understand how the chemical tracers respond to the large-scale dynamics in the high latitudes. [ABSTRACT FROM AUTHOR]

The summer (June-August) Asian-Pacific Oscillation (APO) measures the interannual variability of large-scale atmospheric circulation over the Asian-North Pacific Ocean sector. In this study, the authors assess the predictability of the summer APO index interannual variability and the associated atmospheric circulation anomalies using the 1959-2001 hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF), Centre National de Recherches Météorologiques (CNRM), and the Met Office (UKMO) general circulation models from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project. The results show that these models predict the summer APO index interannual variability well and have higher skill for the North Pacific than for the Asian upper-tropospheric temperature. Meanwhile, the observed APO-related atmospheric circulation anomalies in the South Asian high, the tropical easterly wind jet over the Asian monsoon region in the upper troposphere, the subtropical anticyclone over the North Pacific, and the summer southwest monsoon over Asia in the lower troposphere are reasonably well predicted in their spatial patterns and intensities. Compared with the observations, however, these models display low skill in predicting the long-term varying trends of the upper-tropospheric temperature over the Asian-North Pacific sector or the APO index during 1959-2001. [ABSTRACT FROM AUTHOR]

The effect of stratospheric radiative damping time scales on stratospheric variability and on stratosphere-troposphere coupling is investigated in a simplified global circulation model by modifying the vertical profile of radiative damping in the stratosphere while holding it fixed in the troposphere. Perpetual-January conditions are imposed, with sinusoidal topography of zonal wavenumber 1 or 2. The depth and duration of the simulated sudden stratospheric warmings closely track the lower-stratospheric radiative time scales. Simulations with the most realistic profiles of radiative damping exhibit extended time-scale recoveries analogous to polar-night jet oscillation (PJO) events, which are observed to follow sufficiently deep stratospheric warmings. These events are characterized by weak lower-stratospheric winds and enhanced stability near the tropopause, which persist for up to 3 months following the initial warming. They are obtained with both wave-1 and wave-2 topography. Planetary-scale Eliassen-Palm (EP) fluxes entering the vortex are also suppressed, which is in agreement with observed PJO events. Consistent with previous studies, the tropospheric jets shift equatorward in response to the warmings. The duration of the shift is closely correlated with the period of enhanced stability. The magnitude of the shift in these runs, however, is sensitive only to the zonal wavenumber of the topography. Although the shift is sustained primarily by synoptic-scale eddies, the net effect of the topographic form drag and the planetary-scale fluxes is not negligible; they damp the surface wind response but enhance the vertical shear. The tropospheric response may also reduce the generation of planetary waves, further extending the stratospheric dynamical time scales. [ABSTRACT FROM AUTHOR]

A strong link exists between stratospheric variability and anomalous weather patterns at the earth's surface. Specifically, during extreme variability of the Arctic polar vortex termed a 'weak vortex event,' anomalies can descend from the upper stratosphere to the surface on time scales of weeks. Subsequently the outbreak of cold-air events have been noted in high northern latitudes, as well as a quadrupole pattern in surface temperature over the Atlantic and western European sectors, but it is currently not understood why certain events descend to the surface while others do not. This study compares a new classification technique of weak vortex events, based on the distribution of potential vorticity, with that of an existing technique and demonstrates that the subdivision of such events into vortex displacements and vortex splits has important implications for tropospheric weather patterns on weekly to monthly time scales. Using reanalysis data it is found that vortex splitting events are correlated with surface weather and lead to positive temperature anomalies over eastern North America of more than 1.5 K, and negative anomalies over Eurasia of up to −3 K. Associated with this is an increase in high-latitude blocking in both the Atlantic and Pacific sectors and a decrease in European blocking. The corresponding signals are weaker during displacement events, although ultimately they are shown to be related to cold-air outbreaks over North America. Because of the importance of stratosphere-troposphere coupling for seasonal climate predictability, identifying the type of stratospheric variability in order to capture the correct surface response will be necessary. [ABSTRACT FROM AUTHOR]

Temperatures in the tropical tropopause layer (TTL) play an important role in stratosphere-troposphere exchange and in the formation and maintenance of thin cirrus clouds. Many previous studies have examined the contributions of extratropical and equatorial waves to the TTL using coarse-vertical-resolution satellite and reanalysis data. In this study, the authors provide new insight into the role of extratropical and equatorial waves in the TTL using high-vertical-resolution GPS radio occultation data. The results examine the influence of four different wave forcings on the TTL: extratropical waves that propagate vertically into the stratosphere, extratropical waves that propagate meridionally into the subtropical stratosphere, extratropical waves that propagate meridionally into the subtropical troposphere, and the equatorial planetary waves. The vertically and meridionally propagating extratropical stratospheric waves are associated with deep, zonally symmetric temperature anomalies that extend and amplify with height throughout the lower-to-middle tropical stratosphere. In contrast, the extratropical tropospheric waves and the equatorial planetary waves are associated with tropical temperature anomalies that are confined below 20-km altitude. The equatorial planetary waves dominate the zonally asymmetric component of the TTL temperature field, and both the equatorial planetary waves and the extratropical tropospheric waves are linked to large temperature variability in a 1-2-km-deep layer near the tropical tropopause. The fine vertical scale of the TTL temperature features associated with the equatorial planetary waves and the extratropical tropospheric waves is only readily apparent in high-vertical-resolution data. [ABSTRACT FROM AUTHOR]

Changes of the upper-tropospheric temperature teleconnection and the associated atmospheric circulations in spring (Mar-April-May, MAM) during the mid-Holocene are preliminarily examined by using the CCSM3 simulation data archived in the Paleoclimate Modeling Intercomparison Project (PMIP) database. The simulation result shows that in the mid-Holocene the atmospheric circulation over the Asian-North Pacific region showed the variability of the teleconnection pattern resembling the present Asian-Pacific oscillation (APO), which is characterized by a warming (cooling) upper troposphere over Asia corresponding to a cooling (warming) upper troposphere over the North Pacific and indicates a thermal contrast between Asia and the North Pacific. In comparison with the present climate, the mid-Holocene upper-tropospheric temperature in spring was modelled to be lower over Asia and higher over the North Pacific, indicative of a weaker spring APO. Associated with such a condition, both the upper-tropospheric high (low) and the lower-tropospheric low (high) over Asia (central-eastern Pacific) were weakened in the mid-Holocene. Accordingly, anomalous descending (ascending) motion appeared over East Asia (central-eastern Pacific). This circulation background is unfavourable (favourable) for the occurrence of precipitation, and thus the mid-Holocene precipitation decreases (increases) in this region as compared with the present. Hence, the change of the spring thermal contrast between Asia and the North Pacific played a crucial role in the anomalies of the atmospheric circulations and precipitation over the Asian-Pacific region during the mid-Holocene. Copyright © 2012 Royal Meteorological Society [ABSTRACT FROM AUTHOR]

The life cycle of Northern Hemisphere downward wave coupling between the stratosphere and troposphere via wave reflection is analyzed. Downward wave coupling events are defined by extreme negative values of a wave coupling index based on the leading principal component of the daily wave-1 heat flux at 30 hPa. The life cycle occurs over a 28-day period. In the stratosphere there is a transition from positive to negative total wave-1 heat flux and westward to eastward phase tilt with height of the wave-1 geopotential height field. In addition, the zonal-mean zonal wind in the upper stratosphere weakens leading to negative vertical shear. Following the evolution in the stratosphere there is a shift toward the positive phase of the North Atlantic Oscillation (NAO) in the troposphere. The pattern develops from a large westward-propagating wave-1 anomaly in the high-latitude North Atlantic sector. The subsequent equatorward propagation leads to a positive anomaly in midlatitudes. The near-surface temperature and circulation anomalies are consistent with a positive NAO phase. The results suggest that wave reflection events can directly influence tropospheric weather. Finally, winter seasons dominated by extreme wave coupling and stratospheric vortex events are compared. The largest impacts in the troposphere occur during the extreme negative seasons for both indices, namely seasons with multiple wave reflection events leading to a positive NAO phase or seasons with major sudden stratospheric warmings (weak vortex) leading to a negative NAO phase. The results reveal that the dynamical coupling between the stratosphere and NAO involves distinct dynamical mechanisms that can only be characterized by separate wave coupling and vortex indices. [ABSTRACT FROM AUTHOR]

Global horizontal wavenumber kinetic energy spectra and spectral fluxes of rotational kinetic energy and enstrophy are computed for a range of vertical levels using a T799 ECMWF operational analysis. Above 250 hPa, the kinetic energy spectra exhibit a distinct break between steep and shallow spectral ranges, reminiscent of dual power-law spectra seen in aircraft data and high-resolution general circulation models. The break separates a large-scale 'balanced' regime in which rotational flow strongly dominates divergent flow and a mesoscale 'unbalanced' regime where divergent energy is comparable to or larger than rotational energy. Between 230 and 100 hPa, the spectral break shifts to larger scales (from n = 60 to n = 20, where n is spherical harmonic index) as the balanced component of the flow preferentially decays. The location of the break remains fairly stable throughout the stratosphere. The spectral break in the analysis occurs at somewhat larger scales than the break seen in aircraft data. Nonlinear spectral fluxes defined for the rotational component of the flow maximize between about 300 and 200 hPa. Large-scale turbulence thus centers on the extratropical tropopause region, within which there are two distinct mechanisms of upscale energy transfer: eddy-eddy interactions sourcing the transient energy peak in synoptic scales, and zonal mean-eddy interactions forcing the zonal flow. A well-defined downscale enstrophy flux is clearly evident at these altitudes. In the stratosphere, the transient energy peak moves to planetary scales and zonal mean-eddy interactions become dominant. [ABSTRACT FROM AUTHOR]

Kinetic energy spectra from the mesoscale numerical weather prediction (NWP) model ALADIN with horizontal resolution 4.4 km are split into divergent and rotational components which are then compared at horizontal scales below 300 km and various vertical levels. It is shown that about 50% of kinetic energy in the free troposphere in ALADIN is divergent energy. The percentage increases towards 70% near the surface and in the upper troposphere towards 100 hPa. The maximal percentage of divergent energy is found at stratospheric levels around 100 hPa and at scales below 100 km which are not represented by the global models. At all levels, the divergent energy spectra are characterised by shallower slopes than the rotational energy spectra, and the difference increases as horizontal scales become larger. A very similar vertical distribution of divergent energy is obtained by using the standard ALADIN approach for the computation of spectra based on the extension zone and by applying detrending approach commonly used in mesoscale NWP community. [ABSTRACT FROM AUTHOR]

The stratospheric polar vortex breakup (SPVB) is an important phenomenon closely related to the seasonal transition of stratospheric circulation. In this paper, 62-year NCEP/NCAR reanalysis data were employed to investigate the distinction between early and late SPVB. The results showed that the anomalous circulation signals extending from the stratosphere to the troposphere were reversed before and after early SPVB, while the stratospheric signals were consistent before and after the onset of late SPVB. Arctic Oscillation (AO) evolution during the life cycle of SPVB also demonstrated that the negative AO signal can propagate downward after early SPVB. Such downward AO signals could be identified in both geopotential height and temperature anomalies. After the AO signal reached the lower troposphere, it influenced the Aleutian Low and Siberian High in the troposphere, leading to a weak winter monsoon and large-scale warming at mid latitudes in Asia. Compared to early SPVB, downward propagation was not evident in late SPVB. The high-latitude tropospheric circulation in the Northern Hemisphere was affected by early SPVB, causing it to enter a summer circulation pattern earlier than in late SPVB years. [ABSTRACT FROM AUTHOR]

The circulation response of the atmosphere to climate change-like thermal forcing is explored with a relatively simple, stratosphere-resolving general circulation model. The model is forced with highly idealized physics, but integrates the primitive equations at resolution comparable to comprehensive climate models. An imposed forcing mimics the warming induced by greenhouse gasses in the low-latitude upper troposphere. The forcing amplitude is progressively increased over a range comparable in magnitude to the warming projected by Intergovernmental Panel on Climate Change coupled climate model scenarios. For weak to moderate warming, the circulation response is remarkably similar to that found in comprehensive models: the Hadley cell widens and weakens, the tropospheric midlatitude jets shift poleward, and the Brewer-Dobson circulation (BDC) increases. However, when the warming of the tropical upper troposphere exceeds a critical threshold, ~5 K, an abrupt change of the atmospheric circulation is observed. In the troposphere the extratropical eddy-driven jet jumps poleward nearly 10°. In the stratosphere the polar vortex intensifies and the BDC weakens as the intraseasonal coupling between the troposphere and the stratosphere shuts down. The key result of this study is that an abrupt climate transition can be effected by changes in atmospheric dynamics alone, without need for the strong nonlinearities typically associated with physical parameterizations. It is verified that the abrupt climate shift reported here is not an artifact of the model's resolution or numerics. [ABSTRACT FROM AUTHOR]

This paper discusses the impact of changing the vertical coordinate from a hybrid pressure to a hybrid-isentropic coordinate within the finite-volume (FV) dynamical core of the Community Atmosphere Model (CAM). Results from a 20-yr climate simulation using the new model coordinate configuration are compared to control simulations produced by the Eulerian spectral and FV dynamical cores of CAM, which both use a pressure-based ( σ − P) coordinate. The same physical parameterization package is employed in all three dynamical cores. The isentropic modeling framework significantly alters the simulated climatology and has several desirable features. The revised model produces a better representation of heat transport processes in the atmosphere leading to much improved atmospheric temperatures. The authors show that the isentropic model is very effective in reducing the long-standing cold temperature bias in the upper troposphere and lower stratosphere, a deficiency shared among most climate models. The warmer upper troposphere and stratosphere seen in the isentropic model reduces the global coverage of high clouds, which is in better agreement with observations. The isentropic model also shows improvements in the simulated wintertime mean sea level pressure field in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

Using the Arctic Oscillation (AO) index, the exceptional winter (DJF) of 2009 has been analyzed. The middle-to-high latitudes of the Northern Hemisphere suffered from a nearly zonally symmetric anomaly of temperature and pressure. This situation revealed that two negative AO events occurred in the winter of 2009/2010, with unprecedented low values in January 2009 and February 2010. The negative AO event in January 2009 can be further divided into two stages: the first stage was mainly driven by enhanced upward-propagating planetary waves, which led to a weak stratospheric polar vortex associated with a downward-propagating negative AO signal; the second stage was caused by a lower tropospheric positive temperature anomaly in the high latitudes, which maintained the positive geopotential height anomaly of the first stage. The two successively occurring stages interacted and caused the lower troposphere to experience a strong and lengthy persistence of the negative AO event. We consider that the second event of negative AO in February 2010 is related to the downward-propagating negative AO after sudden stratospheric warming. Eleven long-persistence negative AO events were analyzed using reanalysis data. The results suggest that the negative AO in the troposphere might have been caused by stratospheric sudden warming, a downward-propagating weak stratospheric circulation anomaly or dynamic processes in the troposphere. Further study shows that the negative phase of the AO in the winter of 2009/2010 corresponded to a wide range of temperature and precipitation anomalies in the Northern Hemisphere. Therefore, to improve the accuracy of weather forecasting and climate prediction, more attention should be paid to the AO anomaly and its impact. [ABSTRACT FROM AUTHOR]

The interdecadal variation of the association of the stratospheric quasi-biennial oscillation (QBO) with tropical sea surface temperature (SST) anomalies (SSTA) and with the general circulation in the troposphere and lower stratosphere is examined using the ERA40 and NCEP/NCAR reanalyses, as well as other observation-based analyses. It is found that the relationship between the QBO and tropical SSTA changed once around 1978-1980, and again in 1993-1995. During 1966-1974, negative correlation between the QBO and NINO3.4 indices reached its maximum when the NINO3.4 index lagged the QBO by less than 6 months. Correspondingly, the positive correlations were observed when the NINO3.4 index led the QBO by about 11-13 months or lagged by about 12-18 months. However, maximum negative correlations were shifted from the NINO3.4 index lagging the QBO by about 0-6 months during 1966-1974 to about 3-12 months during 1985-1992. During 1975-1979, both the negative and positive correlations were relatively small and the QBO and ENSO were practically unrelated to each other. The phase-based QBO life cycle composites also confirm that, on average, there are two phase (6-7 months) delay in the evolution of the QBO-associated anomalous Walker circulation, tropical SST, atmospheric stability, and troposphere and lower stratosphere temperature anomalies during 1980-1994 in comparison with those in 1957-1978. The interdecadal variation of the association between the QBO and the troposphere variability may be largely due to the characteristic change of El Niño-Southern Oscillation. The irregularity of the QBO may play a secondary role in the interdecadal variation of the association. [ABSTRACT FROM AUTHOR]

Many tropospheric Arctic Oscillation (AO) events are preceded by stratospheric AO events and even earlier in time by anomalous upward energy flux associated with Rossby waves in the troposphere. This study identifies lower-tropospheric circulation anomalies that precede large AO events in both the troposphere and stratosphere and the anomalous upward energy flux. Compositing analysis of stratospheric warming events identifies regional tropospheric precursors, which precede stratospheric warmings. The tropospheric precursor is found to vary when compositing over polar vortex displacements and splits separately. Prior to vortex displacements the main anomaly sea level pressure center of the tropospheric precursor is located across northwest Eurasia and is associated with the Siberian high. Prior to vortex splits a similar anomaly center is identified in the tropospheric precursor but is weaker and appears to be more strongly related to a shift in the storm tracks. Differences in the sea level pressure anomalies in the North Atlantic and the North Pacific are also observed when comparing the precursors prior to vortex displacements and splits. Identification of a unique tropospheric precursor to stratospheric warming and subsequent tropospheric AO events can help to improve understanding troposphere-stratosphere coupling. Furthermore, the observational evidence presented here can be compared with model simulations of winter climate variability and lead to potential model improvements. [ABSTRACT FROM AUTHOR]