A workshop in Whistler, British Columbia, Canada
29 April to 2 May, 2003
'Some observed features of stratosphere-troposphere coupling'
A complete understanding of how stratospheric anomalies affect the troposphere must include explanations of several observations: 1) The timescale of tropospheric circulation anomalies, particularly as measured by the annular modes, is longest when stratospheric circulation anomalies just above the tropopause are largest. In the NH, coupling seems to occur only during winter, while in the SH it occurs in November-December. 2) Circulation anomalies in the lowermost stratosphere are useful for predicting long-lived anomalies to the troposphere. The relationship is fairy linear, with both strong and weak stratospheric polar vortex conditions appearing to affect the troposphere. A similar effect occurs in the SH during November-December. 3) Planetary and synoptic scale waves in the upper troposphere appear to be affected by the vertical shear of the zonal wind (or the NAM) across the tropopause region. 4) The coupling between the stratosphere and troposphere projects strongly onto the annular mode patterns in both hemispheres. Why does the troposphere tend to respond in an annular pattern?
A related question involves the cause and effect of stratosphere-troposphere coupling. Can the observations be explained with a completely passive stratosphere? What constitutes stratospheric "influence" on the troposphere?
'Intraseasonal Case Studies of the Northern Annular Mode'
Recent observational and numerical modeling studies posit that the stratosphere may provide direct and/or indirect influences upon tropospheric climate. In particular, the tropospheric circulation is strongly coupled to variations in the strength of the stratospheric polar vortex, especially during extreme phases of the Northern Annular Mode (NAM). The hypothesized indirect influence is linked to stratospheric modulation of tropospheric planetary wave propagation whereas in the direct forcing mechanism stratospheric potential vorticity anomalies induce annular circulation anomalies in the troposphere. We perform diagnostic case studies of the NAM to test the extent to which the proposed mechanisms can account for idiosyncracies among different cases.
Baldwin and Dunkerton's 1999 observational study indicates that not all NAM cases follow the composite model of mid-stratospheric initiation followed by downward signal movement into the troposphere. In particular, certain stratospheric NAM events are not linked to tropospheric events and vice-versa. We test the role of the direct and indirect forcings in explaining such case to case variability. Focusing on strong stratospheric NAM events, we contrast those with and without strong succeeding tropospheric signals in order to identify dynamical reasons for the observed differences. We first apply potential vorticity (PV) methods to daily observational analyses to diagnose the direct dynamic interaction between the troposphere and stratosphere during NAM cases of interest. In this approach, circulation anomalies are decomposed into separate parts related to distinct PV anomaly features, permitting a diagnosis of far-field circulations associated with local PV structures. The PV analyses are complemented with parallel eddy-flux diagnostics to study the role of wave driving in locally forcing the anomalous zonal winds characteristic of the NAM. Our results indicate that, for individual cases, pre-existing tropospheric PV anomaly features can mask the downward penetration of an initial stratospheric NAM signal to tropospheric levels.
Byron Boville and Rolando Garcia
'Interaction between the winter stratosphere and the tropospheric Arctic Oscillation in NCAR's Whole Atmosphere Community Climate Model'
'The role of the Stratosphere in Medium-Range Weather forecasts'
In recent years a number of influential papers have suggested that the stratosphere and troposphere may be dynamically linked through the Arctic Oscillation (AO). Using observational data it is difficult to determine if such a link exists and its quantitative importance.
Data analysis of Arctic Oscillation index data shows that a statistically significant link between the Stratosphere and Troposphere exists on a 10- 60 day timescale. However tests with a simple statistical forecasting model show that the extra statistical skill derived from using this link is small.
To investigate this problem further we perform a number of experiments with the ECMWF IFS medium-range forecasting model. The model has high horizontal and vertical resolution which enables accurate simulation of stratospheric sudden warming events. It also has an operational system to generate large ensembles which we use to examine the statistical properties of our results.
The problem is formulated as an initial condition problem. A 30 member control ensemble forecast is run to simulate events which show downward phase progression in the AO index. A 30 member perturbed ensemble forecast is then run in which the initial conditions in the stratosphere have the opposite polarity of the AO index.
Differences in the initial state of the stratosphere are shown to impact on the tropospheric flow on a time scale of 15-20 days. These differences are largest in the lower troposphere and are typically of size 3-4 K in surface temperature and 20-40m in geopotential height. Differences occur over both storm track regions and are statistically significant at the 95 % confidence level.
We conclude that tropospheric evolution can be influenced by stratospheric conditions as suggested by diagnostics of the AO on medium-range timescales. However the quantitative size of this link and its benefit for medium-range weather prediction remains an outstanding issue.
'Stratospheric regimes, a regime shift, and the connection to the troposphere'
In this paper we identify two regimes in the inter-annual variability of the large scale stratospheric flow in the northern hemisphere cold season. The regimes are identified by studying the probability distribution of the leading principal component of the geopotential height which explains approximately 50ީ of the variance. The two regimes are characterized by strong and weak vortex, respectively, and they therefore resemble the stratospheric part of the Arctic Oscillation.
An abrupt regime shift is identified in the last half of the 70ies. Both regimes are visited before and after the shift, but the frequency of the regimes changes drastically around 1978 in favor of the strong vortex regime. The regime shift describes the data better than the linear trend in the PC of 1.5 m/year.
Strong statistical significance for two separate regimes is obtained by a Monte Carlo approach. The regimes and the regime shift are found in two different data sets reducing the possibility that the results are due to inhomogeneities in the data.
The regimes have significant imprints in the troposphere although no bimodality is found in probability distributions below 100 hPa. The difference of the zonal mean zonal wind between the strong vortex regime and the weak vortex regime shows a deep vertical structure. Studies of the 500- 1000 hPa thickness and the zonal mean temperature show that the lower tropospheric air temperature is warmer in the strong vortex regime than in the weak vortex regime while the upper troposphere and the lower stratosphere are colder. In the 500-1000 hPa thickness the largest signals are found over the continents just as in the cold ocean/warm land pattern. The regime shift in the last half of the 70ies is found to explain a large fraction of the recent tropospheric climate change.
Katie Coughlin and K.-K. Tung
'Tropospheric Reaction to the Descent of Stratospheric Anomalies - A Simple Model'
Theoretically, Rossby wave drag on the mean flow and the descent of critical surfaces seems to account for the occurrence of stratospheric warming events. And the use of EOF analysis in the upper atmosphere seems to support the idea that decelerating wind anomalies in the stratosphere will tend to move towards the surface (eg. Baldwin and Dunkerton  ). The question is, what happens when the signal reaches the troposphere? How can stratospheric anomalies influence the denser troposphere? In this analysis, the descent of stratospheric anomalies is assumed and linear quasi-geostrophic theory is used to describe the wave reaction in the troposphere. These simple, analytical results are then compared with observed tropospheric changes.
Timothy J. Dunkerton, Mark P. Baldwin and David A. Ortland
'Vertical structures of annular-mode variability and the contrasting roles of eddy momentum and heat fluxes'
PDF (3 MB)
Lag-zero regression of Eulerian mean meridional circulation with respect to the annular-mode index of surface pressure demonstrates that this circulation opposes the mean-flow anomaly in the upper troposphere and stratosphere, implying that eddy fluxes are responsible for maintenance of the annular mode, as first suggested by Thompson and Wallace. The sense of induced circulation responsible for the transport of mass is nevertheless consistent with the surface pressure anomaly over the polar cap. This 'static' picture of annular-mode anomalies of mean flow, induced circulation and surface pressure -- with troposphere and stratosphere apparently acting in concert -- can be uniquely explained by eddy momentum fluxes. More detailed examination of observations and model simulations demonstrates a richer spectrum of annular-mode behavior variously characterized by in-phase, delayed, and out-of-phase relationships between tropospheric and stratospheric anomalies. An out-of-phase relationship is uniquely explained by eddy heat fluxes, representing a flux of wave activity from troposphere to stratosphere, or vice versa. According to the theorem of Charney and Drazin, vertical fluxes of wave activity into the stratospheric polar vortex are limited to the gravest zonal wavenumbers of planetary scale. On the other hand, synoptic-scale waves contribute significantly to the horizontal flux of wave activity in the troposphere. The role of synoptic-scale waves in amplifying and maintaining the tropospheric annular-mode anomaly is discussed. At times when anomalies in the troposphere and stratosphere are coupled via planetary-wave propagation, as in winter, the synoptic-scale waves may play an important, if not essential, role in producing the predominant 'in-phase' relationship of anomalies in the two layers.
John C. Fyfe
'Linked stratosphere-troposphere variability as revealed through the leading modes of zonal wind'
In the Northern Hemisphere, the first and second modes of annual and zonal mean zonal wind are primarily stratospheric and tropospheric in character, respectively. The surface pressure manifestations of these modes are intimately linked to the Arctic Oscillation (AO), and together suggest separate stratospheric and tropospheric origins for the AO. Here we consider the dynamical mechanisms responsible for these modes, as well as their connections to the patterns of long term (anthropogenic?) climate change.
Rolando Garcia and Byron Boville
'Interaction between the winter stratosphere and the tropospheric Arctic Oscillation in NCAR's Whole Atmosphere Community Climate Model'
Nathan Gillett, David Thompson
'Simulating the effects of stratospheric ozone depletion on Antarctic climate'
Recent observations indicate that climate change over the high latitudes of the Southern Hemisphere is dominated by a strengthening of the circumpolar westerly flow that extends from the surface to the stratosphere. Here we demonstrate that both the seasonality and structure of the observed climate trends are simulated in a state-of-the-art climate model run with high vertical resolution that is forced solely with prescribed stratospheric ozone depletion. As in observations, the modelled tropospheric response reflects a shift in the Southern Hemisphere annular mode towards stronger circumpolar westerly flow that peaks several months after the most pronounced ozone depletion. As in observations, the simulations show a surface cooling of the Antarctic interior and a warming of the Antarctic Peninsula.
'Is there a feedback of the QBO to the tropical meteorology?'
PDF (2 MB)
'Strat-Trop coupling: Some 'new' aspects from models and observations'
PDF (4 MB)
Using monthly NCEP-NCAR reanalysis data from 1958 to 1998 two interannual oscillations in the eddy forcing (EP flux divergence) due to planetary waves (zonal wave numbers 1-3) were found in the Northern Hemisphere winter, one in the polar upper stratosphere (SIO) and one in the midlatitude troposphere (TIO). The index time series SIO and TIO are statistically independent. Planetary waves are bent equatorward (poleward) in the troposphere during a positive (negative) TIO phase, and in the mid- and upper stratosphere during a positive (negative) SIO phase. The upward propagation of planetary waves into the polar wave guide across the tropopause is closely related to the anomalous meridional wave propagation in the troposphere. However, the anomalous meridional wave propagation in the mid- and upper stratosphere is independent of the tropospheric wave activity. Regression and correlation analysis between zonal mean wind and planetary wave forcing suggest that the TIO is closely related to a Northern Annular Mode, while SIO is not. The TIO index is significantly correlated to midlatitude sea surface temperatures (SST) without any time lag on a monthly to interannual time scale. The significant correlation of SIO with tropical SST (basically with an El Nino pattern), however, reveals that tropical SST is leading the SIO by up to 9 months. Our results suggest that the TIO and the NAM are integral parts of an internal atmospheric mode. They may exert strong impact on the mid-latitude SST in the winter, whereas the SIO is significantly influenced by the ENSO.
The structure of coupling of EP flux divergence with TIO shows a gap in correlation near the tropopause, hinting on an effect of this area on wave propagation. An investigation of the index of refractivity shows clear differences between the frequency of negative Ri in reanalysis data and in models. These differences may be used to explain the cold polar bias of models in the lower stratosphere and sensitivity to low latitude SST anomalies.
Lesley Gray, Sarah Sparrow
'The influence of equatorial / subtropical winds on sudden warming events and stratosphere - troposphere interactions'
A short summary will be given of a recent study of rocketsonde data that suggests a dependence of Northern Hemisphere sudden warmings on equatorial winds in the upper stratosphere. This influence is in addition to the well known Holton-Tan relationship with winds in the equatorial lower stratosphere. Studies using a stratosphere mesosphere model will be described that test the response of the modelled warmings to various equatorial wind distributions and also to different levels of tropospheric wave forcing at the lower boundary (100 hPa). The model results confirm that flow modification in the upper subtropical stratosphere as the Aleutian High is developing is a key aspect of the early stages of the stratospheric warming. We use selected model experiments to suggest that the strength and timing of sudden warmings are sensitive to the shape of the vortex in the upper stratosphere as the vortex develops in early winter. We note that the vertical extent of the sudden warmings i.e. whether they are major warmings or minor, is likely to be an important factor that determines the level of stratospheric influence on the underlying tropospheric circulation. We suggest that the shape of the vortex in the upper stratosphere may be modified by a variety of different influences; these include the strength of the tropospheric planetary wave forcing, the phase of the quasi biennial oscillation, the amplitude and vertical extent of the preceding westerly phase of the semi-annual oscillation and the phase of the 11-year solar cycle. If time permits, early results of some model simulations using a full GCM (the UK Met. Office Hadley Centre Model) that test some of these ideas will also be described.
Richard Greatbatch, Jian Lu and Andrew Peterson
'Tropical/extratropical forcing of the AO/NAO revisited'
Joanna Haigh, Michael Blackburn, Rebecca Day
'The response of tropospheric circulation to perturbations in lower stratospheric heating'
Previous studies with different GCMs have shown that increases in solar ultraviolet radiation can induce a small but identifiable pattern of response in tropospheric climate, notably a weakening and poleward shift of the sub-tropical jets and a weakening and expansion of the tropical Hadley cells. A very similar response, in both pattern and amplitude, is found in response to solar activity from multiple linear regression of NCEP/NCAR Reanalysis zonal wind data and is separable from any influence of the AO, ENSO or QBO. The mechanisms whereby these effects take place have been analysed using idealised climate experiments driven by Newtonian forcing. While lower stratospheric heating generally tends to result in weaker jets, and weaker EP-flux at the tropopause, the distribution of the heating perturbation determines any latitudinal shift.
Kevin Hamilton, Mark Baldwin, Georgiy Stenchikov
'Effects of Polar Vortex Perturbations on Tropospheric Winter Circulation'
Interest in the dynamical influence of the circulation in middle atmosphere on the troposphere has had a resurgence largely due to recent observational studies that find evidence for downward propagation of large-scale perturbations in the extratropical circulation from the stratosphere to the troposphere. In particular, anomalies in the Arctic Oscillation (AO) index tend to propagate downward so that, on average, a weak vortex (warm pole) condition in the stratosphere is followed later by a positive AO index in the tropospheric circulation. The same general tendency is also found in control integrations of at least some atmospheric GCMs. The existence of a significant downward dynamical link of this sort would have implications for both seasonal prediction and sustained climate change. However, there is no guarantee that the tendency for downward propagation of anomalies across the tropopause reflects an actual physical effect of the stratospheric flow on that in the troposphere. In geophysics there are numerous examples of phenomena showing phase progression actually opposite to the physical propagation of disturbances.
This paper will report first on results from a series of model integrations performed with initial conditions that are perturbed from those in a control run, but with the perturbation restricted to the extratropical stratosphere. In particular, the GFDL "SKYHI" model is applied in a series of winter seasonal integrations, each with initial condition taken from early winter in one year a long control run, but with a strong perturbation arbitrarily added to the stratospheric polar vortex. Another somewhat different set of experiments has been undertaken with a version of the model which includes an extra zonally-symmetric momentum source in the tropical stratosphere. The momentum source is designed to force the tropical stratospheric circulation to resemble that actually observed for the period 1978-1999. The results of these experiments will be examined to see if the imposed QBO in the tropics has a detectable effect on the extratropical tropospheric circulation.
'The relative roles of wave and zonal mean processes in the downward coupling of the stratosphere and troposphere'
Observations show evidence for a downward effect on the troposphere through planetary wave reflection, which is significant during about half of the winters of the observational record. In this talk we will discuss how these finding relate to the more commonly studied downward coupling through the annular modes. We suggest that downward reflection provides a distinct coupling mechanism from the annular mode, which is based on wave absorption. Moreover, the observations also suggest that there are two kinds of winter dynamics in the stratosphere, reflective and absorptive. While the annular mode dynamics dominates the coupling to the troposphere during the absorptive years, wave reflection dominates during reflective years. The dynamics which are behind the stratosphere assuming one state or the other will be discussed, as well as the possible implications for climate and weather forecasting.
'Possible dynamical mechanisms for connections between the stratosphere and troposphere'
This talk will aim to set out basic dynamical principles underlying possible mechanisms for connection between the stratospheric and tropospheric circulations and, in particular, for ways in which changes in the stratosphere might affect the troposphere.
There are at least four distinct mechanisms by which the stratosphere might affect the troposphere.
(i) Large-scale atmospheric dynamics is fundamentally non-local. Changes in the circulation at one level are almost instantaneously communicated to levels above and below. So changes in the lower stratosphere will inevitably imply changes in the troposphere.
(ii) Irrespective of the effect of waves, changes in the averaged circulation inevitably propagate downward through an interaction of the dynamics with radiative heating. This is an enhanced downward effect over that in (i). The major questions over (i) and (ii) are whether the strength of the effects is significant and this focusses attention on whether the tropospheric flow may be sensitive to small changes.
(iii) Two-way interactions between waves and averaged flow naturally lead to downward propagating anomalies and this propagation might continue from stratosphere to troposphere. A major question over (iii) is whether there is true downward propagation of information, or simply `phase propagation', with no real propagation of information. Certain simple models of wave mean-flow interaction (e.g. WKB-based models of the equatorial QBO) show downward propagation that is purely phase propagation, but the situation is not so clear for non-WKB waves in the extratropics.
(iv) There may be downward propagation of information due to planetary wave propagation -- for example resonant growth of planetary waves has at various times been suggested as a mechanism for sudden stratospheric warmings and for internal variability and this usually requires downward as well as upward propagation of waves.
The talk will discuss the viability of these different mechanisms and how one might distinguish between them.
Matthew H. Hitchman, Chris Collimore, Amihan Huesmann, David Martin, Duane Waliser
'On the Relationship Between the Stratospheric QBO and Tropical Deep Convection'
The hypothesis that the stratospheric quasibiennial oscillation (QBO) influences tropical deep convection is tested by analyzing 43 years (1958-2000) of National Centers for Environmental Prediction (NCEP) reanalyses, together with 23 years of outgoing longwave radiation (OLR; 1974-1998) and 17 years of highly reflective cloud data (HRC; 1971-1987) as measures of deep convection. Three mechanisms which can link the QBO to deep convection are investigated, where the QBO regulates convection by variations in 1) tropopause temperature and altitude, 2) cross-tropopause wind shear, and 3) inertial stability near the tropopause. Seasonal mean fields of NCEP tropopause temperature, tropopause pressure, 70-150 hPa wind shear, and 150 hPa relative vorticity exhibit highly significant QBO variations. During the QBO westerly phase the tropopause is 0.5-1 K warmer, 2-3 hPa higher pressure, and 200-300 m lower altitude throughout the tropics relative to the QBO easterly phase. The QBO in 70-150 hPa wind shear depends strongly on season and geographical distribution.
QBO in HRC and OLR show that convection is inhibited during the QBO westerly phase, when the tropopause is low, and enhanced during the QBO easterly phase. The QBO in tropopause temperature is largest where convection is deepest, especially during boreal winter. The tropopause influence is the most dominant of the three mechanisms, but is least robust during boreal summer, when convection maximizes well off of the equator. Enhanced convection occurs in regions of reduced wind shear, most notably during boreal summer and QBO westerly phase in the northern subtropics. Inertial stability did not exhibit significant correlation with deep convection. Significant QBO signals in 150 hPa heights and winds are evident in the extratropical winter hemisphere, suggesting a possible modulation of the polar annular modes.
'The Southern Hemisphere: Another laboratory for testing ideas (on the role of the stratosphere in tropospheric climate)'
Much of the analysis and discussion of the role of the stratosphere on tropospheric climate has focussed on the Northern Hemisphere. However, the Southern Hemisphere (SH) provides an alternative and somewhat independent environment to test ideas about mechanisms linking the troposphere and stratosphere. Here, I will discuss variability in the SH high latitudes in the troposphere and stratosphere on interannual and longer timescales using two examples.
The first example considers the recent observed trends in the SH annular mode, including strengthening of the zonal flow in the troposphere and of the polar vortex in the lower stratosphere. Possible mechanisms for the trends in the lower stratosphere and troposphere will be described, including stratospheric ozone depletion and greenhouse climate change. The different mechanisms will be examined by comparing the observed changes to climate model simulations with different forcings.
The second example is the anomalous SH polar vortex in spring 2002, when the first SH major stratospheric warming was observed. This appears to directly contradict the long-term trends to a stronger polar vortex, as the SH polar vortex in spring 2002 was the weakest ever observed. Possible causes of these anomalies will be discussed in terms of links between the troposphere and the stratosphere.
Both these examples show strong coupling between the troposphere and the stratosphere and that there may not be simple answers.
'Regional and hemispheric seesaw modes in the northern hemisphere winter: the NAO, Annular mode, and coupled stratosphere-troposphere mode'
According to the Annular mode (AM) paradigm, the leading empirical orthogonal function (EOF) of the sea-level pressure is a fundamental mode of variability of the atmosphere. In particular, during the northern hemisphere winter the AM extends from the surface of the Earth to the stratosphere including the North Atlantic Oscillation (NAO) as a subset over the Atlantic sector. Some other people consider, however, that the hemispherical aspect of the AM is not real, but the NAO is a physical mode, which extends from the Atlantic region over the wide ranges of the northern hemisphere from the troposphere and stratosphere during the winter. It should be noted, however, that the AM in the surface pressure is not always related to the stratospheric variability, and also a change in the NAO index does not necessary related to a hemispheric change. There is a regional seesaw mode over the North Atlantic sector as the NAO, but there exists also a hemispherical seesaw mode between higher- and lower latitudes which includes an action center over the Pacific sector as the AM. However, these modes are not strongly related to the stratospheric polar vortex. The coupled stratosphere-troposphere mode can be found as a separate mode of variability. Distinction of the above three modes are important to understand the mechanism of the stratosphere- troposphere coupling.
Paul Kushner and Lorenzo Polvani
'Which Time Scales Matter for Stratosphere-Troposphere Coupling? Lessons from a Relatively Simple AGCM'
An open question in the dynamics of stratosphere-troposphere (ST) coupling is whether there is a connection between "Baldwin-Dunkerton" annular mode signals that propagate from the stratosphere into the troposphere and observed climate-timescale annular-mode trends that appear to have a stratospheric origin. We investigate this question using a simple, dry, hydrostatic, primitive-equation GCM with a relatively well resolved stratosphere and a reasonable stratospheric circulation. We find that the model troposphere and stratosphere are only weakly correlated. The model's annular mode, when defined in terms of the surface pressure, is confined to the troposphere and appears similar to the Southern Annular Mode in the "inactive" seasons. Furthermore, the model is found to have relatively weak Baldwin-Dunkerton type signals that only rarely penetrate into the troposphere. But despite these suggestions that the model's ST coupling is weak in terms of its internal variability, the model exhibits an unambiguous ST coupling when perturbed externally. In particular, when the polar winter stratosphere is cooled sufficiently, the tropospheric circulation shifts poleward in a way that projects strongly and positively onto the model's (tropospherically confined) annular mode. The Baldwin- Dunkerton signal remains weak even as the polar vortex is strengthened. Further analysis shows that the response to stratospheric cooling is a 100-300 day timescale adjustment involving "downward-control" influences from the stratosphere, upward influences from the troposphere, and significant changes of the upward propagating synoptic-scale wave activity into the lower straosphere. The model suggests that strong short- timescale ST coupling is not a prerequisite for ST coupling under the influence of climate-timescale perturbations such as global warming and ozone depletion. It also suggests that the stratosphere and troposphere, on climate timescales, are coupled in a way that may be difficult to untangle dynamically.
Jian Lu, with Richard Greatbatch and Andrew Peterson
'Tropical/extratropical forcing of the AO/NAO revisited'
'Modelling the stratospheric polar vortex and its changes for greenhouse gases increase and ozone depletion'
The seasonal evolution of the stratospheric polar vortex in a series of simulations performed with the MEACHAM general circulation model is presented. Four equilibrium simulations are considered, each one differing for the amount of greenhouse gases and the sea surface temperatures specified. Respectively, a simulation representative for past atmospheric amounts of GHGs (1890), a simulation for present condition (1990), and two for possible future amounts of GHGs (2030 and 2100). The latter roughly corresponding to CO2 doubling with respect to the present. Each simulation is 20 year long, to consider internal variability. The ozone distribution is unchanged in the simulations, a ozone climatology for 1980s early 1990s is used. Analysis of the stratospheric polar vortex is focused on the Arctic. The stratosphere cools globally as the GHGs increase, as expected from radiative considerations. The average temperature change between pairs of simulations in the Arctic stratosphere displays a recurrent pattern of less cooling in late autumn and late spring, and more cooling in mid winter - early spring. The reasons for these changes and their possible influence on the troposphere are examined in the simulations. These results are contrasted to temperature changes found in another set of comparable simulations, including an interactive ozone chemistry model.
Paul A. Newman
'Stratospheric-Tropospheric interaction and the 2002 ozone hole'
The 2002 ozone hole was remarkable for its small size and early break-up. This small size resulted from a series of wave events over the course of the 2002 winter. The major event of the 2002 winter was the major warming in late September 2002. This warming resulted from an extremely strong wave event that propagated out of the troposphere, reversed the zonal mean flow, and warmed the polar vortex. This late-September event was the culmination of a series of large wave events which occurred over the course of the 2002 winter. These waves collectively warmed the vortex and decelerated the stratospheric flow. In this talk, we will trace the origin of these wave events, and we will also analyze the feedback of the large disruption of the stratospheric flow on the troposphere.
'Modeling the influence of the stratosphere on tropospheric climate'
Luke Oman, Georgiy Stenchikov, Alan Robock, Brian Soden and Richard Wetherald
'Analysis of Stratospheric and Tropospheric Impacts from the Mt. Pinatubo Eruption in GFDL R30 and GISS GCMs'
Analysis of simulations from the Geophysical Fluid Dynamics Laboratory (GFDL) R30 and Goddard Institute for Space Studies (GISS) GCMs show changes in both the stratospheric and tropospheric circulation resulting from the Mount Pinatubo eruption. The June 15, 1991 Mount Pinatubo eruption produced the largest injection of volcanic aerosols in the 20th Century. By conducting an ensemble of simulations with and without volcanic aerosols, its impact can be assessed. Previous simulations with the GFDL SKYHI and Max Planck ECHAM 4 GCMs have shown that volcanic aerosol heating of the lower stratosphere from thermal IR and solar near-IR radiation causes changes in the circulation of the stratosphere by increasing the meridional tem- perature gradient, resulting in a stronger polar vortex. They also showed that reduced solar radiation in the troposphere caused cooler tempera- tures in the subtropics, which decreased the meridional temperature gradient. This caused a reduction in the amplitude of planetary waves and allowed further strengthening of the polar vortex. The resulting forced positive phase of the Arctic Oscillation (AO) caused changes in tropo- spheric circulation. By examining the response with the low vertical resolution (14 levels) GFDL R30 GCM and the new sophisticated GISS GCM, we explore the dependence of these results on model resolution and physics. Analysis shows a positive AO response in the GISS GCM, but not in the GFDL R30 simulations.
'My friend the stratosphere'
'Observational Evidence of a Stratospheric Influence on the Tropo- sphere by Planetary Wave Reflection'
Observational evidence for an effect of downward planetary wave reflection in the stratosphere on Northern Hemisphere tropospheric waves is given by combining statistical and dynamical diagnostics. A time-lagged singular value decomposition analysis is applied to daily tropospheric and stratospheric height fieldsrecomposed for a single zonal wavenumber. A wave geometry diagnostic for wave propagation characteristics which separates the index of reflection into vertical and meridional components is used to diagnose the occurrence of reflecting surfaces. For zonal wavenumber 1, this study suggests that there is one characteristic configuration of the stratospheric jet which reflects waves back into the troposphere - when the polar night jet peaks in the high latitude mid-stratosphere. This configuration is related to the formation of a reflecting surface for vertical propagation at around 5ޢhPa as a result of the vertical curvature of the zonal mean wind and a clear meridional waveguide in the lower to middle stratosphere, that channels the reflected wave activity to the high latitude troposphere. The possible effects of reflection on the tropospheric circulation will be discussed.
'Dynamical mechanisms and diagnosis of stratospheric influence on the troposphere'
'Surface observed trends: possible stratospheric role?'
The Northern Annular mode structure shows a dependence on the polarity of the ENSO cycle. During warm ENSO winters its signature on wind and temperature extends deeper into the stratosphere, both in extratropical and tropical regions. During the same warm ENSO winters the surface observed trends of SLP and SAT appear to be much stronger than average, with a structure consistent with the warm-ENSO NAM structure. Here we address the question whether there is a relation between this troposphere-stratosphere link and the increased strength of extratropical trends. We investigate whether the phase of the QBO can modulate the mentioned effect of ENSO on trends, since both QBO and ENSO have comparable impacts on the extratropical stratosphere.
'Laboratory images of polar vortices, Rossby-wave momentum forcing and up-down interactions'
Barotropic laboratory simulations with a polar ?-plane provide animated images of Rossby-wave propagation, geostrophic turbulence/potential-vorticity stirring and resulting bands zonal acceleration, with a prominent polar anticyclonic vortex, 'PV-elasticity' and a resistance to lateral mixing of polar tracers. The fluid is initially at rest, with all eddy- and zonal flow produced by the 'Green-function' forcing centered at lower latitudes. Resonance conditions are surprisingly sharp with respect to forcing frequency. Weakly damped barotropic numerical simulations driven by super-rotation past a mountain also illustrate the life-cycle of Rossby wave generation, zonal-flow modification and polar vortex formation (reminiscent of the Polvani-Plumb (J.Atmos. Sci. 1992) study of mountain wake instability and its excitation of hemispheric EOF patterns) . Orographic generation is further explored using laboratory bowl-shaped basins spanned by mountain ridges. In these simulations driven by global oscillatory forcing, the Rossby-wave east-west asymmetry is used to drive (analogous-) anticyclonic hemispheric circulation. It can be idealized as a 3-variable 'oscillatory' Charney-deVore problem. The mean easterly zonal-flow tendency carries far into the turbulent regime as lee-cyclones continue to favor the westerly flow phase.
While these experiments emphasize lateral PV interactions, life-cycles of stratified geostrophic turbulence involve strong barotropization (work of Dritschel, Juarez and Ambaum, Phys. Fluids 1999, notwithstanding, which shows long-term equilibration of lightly damped turbulence in near-Prandtl-ratio baroclinic states). Up-down interactions at synoptic scale in the more heavily damped atmosphere can involve downwind group propagation as recently reviewed by Chang, Lee and Swanson (J. Climate, 2002). These interactions will be described in context with the important oceanic diabatic heat- source in the Atlantic and Pacific storm-tracks. Wintertime oceanic forcing is derived from poleward heat-transport by the ocean circulation, in much of the N. Atlantic and a smaller fraction of the N. Pacific. It acts on several scales: explosive storms, the Icelandic and Aleutian lows (background structures analogous to oceanic gyres), and the low-zonal wavenumber hemispheric poleward flux of moisture and dry static energy. All of these signals involve up-down interaction which can reach the stratosphere.
David Rind, J. Perlwitz, P. Lonergan
'Stratospheric Effects on Tropospheric Climate from the GISS Model(s) Perspective'
We address the question "What effect do Climate-Related Changes in the Stratosphere have on Tropospheric Climate?" from the perspective of GISS Global Climate/Middle Atmosphere Model simulations. The climate changes considered include solar variations, CO2 increases, stratospheric ozone depletion, volcanic aerosol injections and stratospheric water vapor increases. The potential impacts are divided into radiative, the direct forcing of tropospheric climate changes through changes in stratospheric constituents; and dynamic, the impact changes in the stratosphere have on tropospheric circulations and therefore the related climate. Experiments are run in two ways. First, we allow the sea surface temperatures to respond, which helps determine the model's climate sensitivity to the stratospheric perturbations, which will be compared to the corresponding sensitivity to well-mixed gases. We then repeat the experiments without allowing the sea surface temperatures to respond; the radiative impact on the troposphere is thus minimized, and the resulting tropospheric dynamical changes arise primarily from the stratospheric response itself. In particular we investigate the impact on the Hadley Circulation, tropospheric planetary wave generation, and planetary wave propagation, including AO/NAO effects. Results are compared with previous GISS model simulations to climate changes, using coarse and finer resolution.
How Does the Stratosphere Influence the Troposphere in Mechanistic GCMs?
Nineteen years ago, Boville showed that changing the stratospheric climate in a general circulation model has a significant effect on the tropospheric circulation. Recent mechanistic GCM studies by Polvani & Kushner, Taguchi, and Song & Robinson obtained similar results. Throughout this recent work, stratospheric influences are associated with altered planetary wave driving (or an imposed source of zonal momentum) in the stratosphere, and the tropospheric response appears primarily as a projection on the leading modes of internal variability - the annular modes of these models.
The dynamical mechanisms through which stratospheric changes influence tropospheric annular modes are however, not yet completely understood, even within these mechanistic models. Changes in stratospheric planetary wave driving affect the lower atmosphere through their induced secondary circulations ("downward control)", but this is a weak forcing in the troposphere. Tropospheric eddy feedbacks may shape and amplify the tropospheric response, but strong eddy feedback also increases internal variability within the troposphere. The difficulty is explaining how the responses to weak stratospheric forcing emerges from the noise of tropospheric internal variability.
Here the results of these recent experiments are reviewed and compared, in an effort to determine the mechanism or mechanisms responsible for the robust tropospheric responses to stratospheric forcing displayed by these models.
Alan Robock and Georgiy L. Stenchikov
'Mechanisms of Forced Arctic Oscillation Response to Volcanic Eruptions'
Large volcanic eruptions inject sulfur gases into the stratosphere, which convert to sulfate aerosols with an e-folding residence time of about 1 year. Large ash particles fall out much quicker. The radiative and chemical effects of this aerosol cloud produce responses in the climate system. One of the most interesting is the "winter warming" of Northern Hemisphere continents following major tropical eruptions. During the winter in the Northern Hemisphere following every large tropical eruption of the past century, surface air temperatures over North America, Europe, and East Asia were warmer than normal, while they were colder over Greenland and the Middle East. This pattern and the coincident atmospheric circulation correspond to the positive phase of the Arctic Oscillation. Using the Max Planck Institute ECHAM4 and the Geophysical Fluid Dynamics Laboratory SKYHI GCMs, we have successfully simulated this response following the 1991 Mount Pinatubo eruption. In spite of the decrease in surface solar heating, surface air temperature increases in high and midlatitudes of the Northern Hemisphere in the winter because of changes in tropospheric circulation caused by stratosphere-troposphere dynamical coupling. By scattering some solar radiation back to space, the aerosols cool the surface, but by absorbing both solar and terrestrial radiation, the aerosol layer heats the stratosphere. For a tropical eruption, this heating is larger in the tropics than in the high latitudes, producing an enhanced pole-to-equator temperature gradient, especially in winter. Ozone depletion caused by heterogeneous chemistry on volcanic aerosols further enhances this mechanism. The phase of the Quasi- Biennial Oscillation also affects the dynamical response. The QBO in its westerly phase strengthens the AO response. Therefore the polar vortex in the winter of 1992-1993 was stronger than in the winter of 1991-1992 although the aerosol radiative forcing declined. Because of nonlinear interactions, aerosols and the QBO together produce a stronger response than a linear superposition of responses to each of these forcings. A positive phase of the AO was also produced in an experiment with only the tropospheric effect of aerosols, showing that aerosol heating in the lower tropical stratosphere is not necessary to force positive AO response, as was previously assumed. Aerosol-induced tropospheric cooling in the subtropics decreases the meridional temperature gradient in the winter troposphere between 30oN and 60oN. The corresponding reduction of mean zonal energy and amplitudes of planetary waves in the troposphere decreases wave activity flux into the lower stratosphere. The resulting strengthening of the polar vortex forces a positive phase of the AO.
'Interannual Changes in the Stratosphere: Relationship to Troposhperic Changes and Ozone'
Richard Scott and L.M. Polvani
'Does the stratosphere control the upward flux of wave activity from the troposphere?'
Recent studies (Baldwin and Dunkerton, 2001; Thompson et al., 2002) have suggested that large stratospheric anomalies can propagate downward into the troposphere, and that such downward propagation may be useful in improving the predicability of the tropospheric circulation on timescales of the order of a several weeks. Observations (Polvani & Waugh, 2003) also indicate that the eddy heat flux near the tropopause lag correlates with the zonal mean winds in the middle stratosphere, indicating that the stratospheric anomalies are controlled by anomalous wave acticity fluxes from the troposphere. It is still not clear, however, to what degree the stratosphere simply responds to upward wave activity fluxes from the troposphere, or actually modulates such wave activity fluxes.
Here we consider to what extent the stratosphere can control its own dynamics, by modulating the eddy heat flux at the tropopause. Although highly truncated, low-order models have previously indicated internal stratospheric variability, persistence of such variability in higher resolution models does not follow automatically. Using a primitive equation model of the stratosphere-troposphere system, in which the troposphere and the tropospheric wave forcing are held constant, we show that variability of the stratospheric circulation and of the eddy heat flux at the tropopause, arise naturally from the internal dynamics of the stratosphere. Our simplified model allows for a careful examination of the mechanisms that control the wave flux into the stratosphere, as well as the downward propagation of zonal wind anomalies following major stratospheric sudden warmings. Allowing the troposphere to evolve according to its own baroclinic dynamics indicates the relative importance of the internal stratospheric variability compared with that arising from the natural variability of the tropospheric forcing from below.
Theodore G. Shepherd
'Mechanisms for stratospheric influences on tropospheric climate'
David B. Stephenson
'Potential Predictability of the NAO: From Days to Decades'
The NAO/AO is an inherently noisy phenomenon that varies on all time scales. This talk will review the variability and persistence of the NAO/AO on different time scales and will identify how much of the variation is due to inherent weather noise. It will be shown that the NAO/AO contains more than weather noise on both very short (<60 day) and very long (> 3 year) time scales. This potential predictability offers windows of opportunity for forecasting the NAO/AO. A stratospheric explanation of some of the short-range predictability will also be presented.
'Tropospheric response to stratospheric sudden warmings in a simple global circulation model'
A composite analysis is made of 132 stratospheric sudden warming (SSW) events obtained in a 10000-day integration with a simple global circulation model under a perpetual-winter condition. The analysis confirms general features of the SSWs, such as enhanced upward propagation of planetary wave activity from the troposphere to the stratosphere before the SSWs and downward propagation of warming signals to the lower stratosphere after the events.
A further dynamical diagnosis shows that the tropospheric circulation is quite different between pre-SSW and post-SSW periods in terms of the zonal mean zonal wind, planetary wave and synoptic-scale waves. In the pre-SSW period, planetary wave is more active than normal in relation to the tropospheric westerly jet that shifts poleward. In the post-SSW period, on the other hand, planetary wave is less active, while the mean zonal wind is close to the climatology. Synoptic-scale waves also exhibit anomalous features in both periods corresponding to the anomalous planetary-scale flow.
The less active planetary wave in the post-SSW period is a return signal of the SSWs, or tropospheric response to the SSWs, since the signal disappears as SSWs are absent by increased thermal damping in the stratosphere.
'The role of the stratosphere in tropospheric climate: observational evidence'
K. -K. Tung
'Decadal tropospheric changes caused by radiative-dynamical feedbacks from stratospheric perturbations'
The increase in greenhouse gases and the decrease in stratospheric ozone are two external pertubations of decadal duration to the radiative budget of the stratosphere. Through radiative-dynamical feedbacks, they affect the dynamics of not just the stratosphere, but also the troposphere. The character of the pertubation of each is different, and it is possible to detect the decadal changes caused by the different perturbations. Thompson and Solomon (2002) discussed some of the trends in the Antarctic troposphere attributable to Antartic stratospheric polar ozone loss. We shall discuss the possible radiative-dynamical feedbacks from greenhouse gases and polar ozone in the Northern Hemisphere.
'A Mechanism and Simple Dynamical Model of the NAO and Annular Modes'
We will present a mechanism and simple model for the basic spatial and temporal structure of the large-scale modes of intraseasonal variability and associated variations in the zonal index. We suggest that these patterns are a direct consequence of the stirring effects of baroclinic eddies, and we explicitly show how such stirring, as represented by a simple random forcing in a barotropic model, leads to a variability in the zonal flow via a variability in the eddy momentum flux convergence, and to patterns similar to those observed. If the stochastic forcing is statistically zonally uniform, then the resulting patterns of variability (i.e., the empirical orthogonal functions) are zonally uniform and the pressure pattern is dipolar in the meridional direction, resembling an annular mode. If the forcing is enhanced in a zonally localized region, thus mimicking the effects of a stormtrack over the ocean, then the resulting variability pattern is zonally localized, with a pattern resembling the North Atlantic Oscillation. Our results suggest that the North Atlantic Oscillation and annular modes are produced by the same mechanism, and are manifestations of the same phenomenon. The basic mechanism is tropospheric, and depends on the presence of neither the ocean nor the stratosphere. However, we will speculate on how they might, or might not, affect the observed patterns.
'Upward Wave Flux as a Precursor to Extreme Stratospheric Events and its Connection to Anomalous Weather Regimes'
Extremely values of the Arctic Oscillation (AO) Index in the stratosphere have recently been linked to anomalous weather regimes at the surface for periods of 60 days following the anomalous events. This has lead to the suggestion that stratospheric variability might be useful for extending weather prediction beyond current time scales. However, the cause of extreme stratospheric events has not been identified.
In order to elucidate the cause of the extreme AO index events, we examine, using NCEP reanalyses from 1958 to 2001, the upward wave flux near the tropopause levels, integrated of periods of 30 days prior to extreme stratospheric events. We find that the AO timeseries the stratosphere (10hPa) and 30-day-integrated wave flux time series at 100 hPa correlate with an extremely high coefficient (typically 0.8) over the entire record, and that there is a clear difference in the upward wave flux from the troposphere between the periods preceding extremes in the phases of AO index at 10hPa. In particular, the eddy heat flux at 100 hPa integrated over a month is anomalously large (small) preceding the onset of a weak (strong) polar vortex. Furthermore, the evolution of the AO index following periods when the integrated 100 hPa wave fluxes are anomalous is very similar to that of the AO events discussed in Baldwin and Dunkerton (2001): The anomalous AO index occurs first in the upper stratosphere, migrates down into lower stratosphere and troposphere, and persists for 30-60 days at the surface.
Our results thus indicate that, although the AO signal appears first in the upper stratosphere, the "forcing" for these events can be detected in the upper troposphere first, and that the 30-day integrated eddy heat flux at 100 hPa is the key quantity for understanding the occurrence of extreme stratospheric vortex episodes, and their related impacts on surface weather.