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Diane J. Ivy
,
Susan Solomon
, and
David W. J. Thompson

Abstract

Dynamical coupling between the stratospheric and tropospheric circumpolar circulations in the Arctic has been widely documented on month-to-month and interannual time scales, but not on longer time scales. In the Antarctic, both short- and long-term coupling extending from the stratosphere to the surface has been identified. In this study, changes in Arctic temperature, geopotential height, and ozone observed since the satellite era began in 1979 are examined, comparing dynamically quiescent years in which major sudden stratospheric warmings did not occur to all years. It is shown that this approach clarifies the behavior for years without major warmings and that dynamically quiescent years are marked by a strengthening of the Arctic polar vortex over the past 30 years. The associated declines in stratospheric temperatures, geopotential height, and ozone are qualitatively similar to those obtained in the Antarctic (albeit weaker), and propagate downward into the Arctic lowermost stratosphere during late winter and early spring. In sharp contrast to the Antarctic, the strengthening of the Arctic stratospheric vortex appears to originate at a higher altitude, and the propagation to the Arctic troposphere is both very limited and confined to the uppermost troposphere, even when only dynamically quiescent years are considered in the analysis.

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Amy H. Butler
,
David W. J. Thompson
, and
Thomas Birner

Abstract

Climate change experiments run on Intergovernmental Panel on Climate Change (IPCC)–class numerical models consistently suggest that increasing concentrations of greenhouse gases will lead to a poleward shift of the midlatitude jets and their associated eddy fluxes of heat and potential vorticity (PV). Experiments run on idealized models suggest that the poleward contraction of the jets can be traced to the effects of increased latent heating and thus locally enhanced warming in the tropical troposphere. Here the authors provide new insights into the dynamics of the circulation response to tropical tropospheric heating using transient experiments in an idealized general circulation model.

It is argued that the response of the midlatitude jets to tropical heating is driven fundamentally by 1) the projection of the heating onto the meridional slope of the lower tropospheric isentropic surfaces, and 2) a diffusive model of the eddy fluxes of heat and PV. In the lower and middle troposphere, regions where the meridional slope of the isentropes (i.e., the baroclinicity) is increased are marked by anomalously poleward eddy fluxes of heat, and vice versa. Near the tropopause, regions where the meridional gradients in PV are increased are characterized by anomalously equatorward eddy fluxes of PV, and vice versa. The barotropic component of the response is shown to be closely approximated by the changes in the lower-level heat fluxes. As such, the changes in the eddy fluxes of momentum near the tropopause appear to be driven primarily by the changes in wave generation in the lower troposphere.

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David W. J. Thompson
,
Mark P. Baldwin
, and
Susan Solomon

Abstract

This study examines the temporal evolution of the tropospheric circulation following large-amplitude variations in the strength of the Southern Hemisphere (SH) stratospheric polar vortex in data from 1979 to 2001 and following the SH sudden stratospheric warming of 2002. In both cases, anomalies in the strength of the SH stratospheric polar vortex precede similarly signed anomalies in the tropospheric circulation that persist for more than 2 months. The SH tropospheric circulation anomalies reflect a bias in the polarity of the SH annular mode (SAM), a large-scale pattern of climate variability characterized by fluctuations in the strength of the SH circumpolar flow. Consistent with the climate impacts of the SAM, variations in the stratospheric polar vortex are also followed by coherent changes in surface temperatures throughout much of Antarctica. The results add to a growing body of evidence that suggests that stratospheric variability plays an important role in driving climate variability at Earth’s surface on a range of time scales.

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David W. J. Thompson
,
Jason C. Furtado
, and
Theodore G. Shepherd

Abstract

Observational and numerical evidence suggest that variability in the extratropical stratospheric circulation has a demonstrable impact on tropospheric variability on intraseasonal time scales. In this study, it is demonstrated that the amplitude of the observed tropospheric response to vacillations in the stratospheric flow is quantitatively similar to the zonal-mean balanced response to the anomalous wave forcing at stratospheric levels. It is further demonstrated that the persistence of the tropospheric response is consistent with the impact of anomalous diabatic heating in the polar stratosphere as stratospheric temperatures relax to climatology.

The results contradict previous studies that suggest that variations in stratospheric wave drag are too weak to account for the attendant changes in the tropospheric flow. However, the results also reveal that stratospheric processes alone cannot account for the observed meridional redistribution of momentum within the troposphere.

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David W. J. Thompson
,
Brian R. Crow
, and
Elizabeth A. Barnes

Abstract

Wave activity in the Southern Hemisphere extratropical atmosphere exhibits robust periodicity on time scales of ~20–25 days. Previous studies have demonstrated the robustness of the periodicity in hemispheric averages of various eddy quantities. Here the authors explore the signature of the periodicity on regional spatial scales.

Intraseasonal periodicity in the Southern Hemisphere circulation derives from out-of-phase anomalies in wave activity that form in association with extratropical wave packets as they propagate to the east. In the upper troposphere, the out-of-phase anomalies in wave activity form not along the path of extratropical wave packets, but in their wake. The out-of-phase anomalies in wave activity give rise to periodicity not only on hemispheric scales, but also on synoptic scales when the circulation is sampled along an eastward path between ~5 and 15 m s−1. It is argued that 1) periodicity in extratropical wave activity derives from two-way interactions between the heat fluxes and baroclinicity in the lower troposphere and 2) the unique longitude–time structure of the periodicity in upper-tropospheric wave activity derives from the contrasting eastward speeds of the source of the periodicity in the lower troposphere (~10 m s−1) and wave packets in the upper troposphere (~25 m s−1).

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Kevin M. Grise
,
David W. J. Thompson
, and
Piers M. Forster

Abstract

Climate change in the Southern Hemisphere (SH) polar stratosphere is associated with substantial changes in the atmospheric circulation that extend to the earth’s surface. The mechanisms that drive the changes in the SH troposphere are not fully understood, but most previous hypotheses have focused on the role of atmospheric dynamics rather than that of radiation.

This study quantifies the radiative response of temperatures in the SH polar troposphere to the forcing from long-term temperature and ozone trends in the SH polar stratosphere. A novel methodology is employed that explicitly neglects changes in tropospheric dynamics and hence isolates the component of the tropospheric temperature response that is radiatively driven by the overlying stratospheric trends. The results reveal that both the amplitude and seasonality of the observed cooling of the middle and upper SH polar troposphere over the past few decades are consistent with a reduction in downwelling longwave radiation induced by cooling in the SH polar stratosphere. The results are compared with analogous calculations for trends in the Northern Hemisphere (NH) polar stratosphere. Both the observations and radiative calculations imply that the comparatively weak trends in the NH polar stratosphere have not played a central role in driving NH tropospheric climate change.

Overall, the results suggest that radiative processes play a key role in coupling the large trends in SH polar stratospheric temperatures to tropospheric levels. The tropospheric radiative temperature response documented here could be important for triggering the changes in internal tropospheric dynamics associated with stratosphere–troposphere coupling.

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Varavut Limpasuvan
,
David W. J. Thompson
, and
Dennis L. Hartmann

Abstract

Motivated by recent evidence of strong stratospheric–tropospheric coupling during the Northern Hemisphere winter, this study examines the evolution of the atmospheric flow and wave fluxes at levels throughout the stratosphere and troposphere during the composite life cycle of a sudden stratospheric warming. The composite comprises 39 major and minor warming events using 44 years of NCEP–NCAR reanalysis data. The incipient stage of the life cycle is characterized by preconditioning of the stratospheric zonal flow and anomalous, quasi-stationary wavenumber-1 forcing in both the stratosphere and troposphere. As the life cycle intensifies, planetary wave driving gives rise to weakening of the stratospheric polar vortex and downward propagation of the attendant easterly wind and positive temperature anomalies. When these anomalies reach the tropopause, the life cycle is marked by momentum flux and mean meridional circulation anomalies at tropospheric levels that are consistent with the negative phase of the Northern Hemisphere annular mode. The anomalous momentum fluxes are largest over the Atlantic half of the hemisphere and are associated primarily with waves of wavenumber 3 and higher.

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David W. J. Thompson
,
John M. Wallace
, and
Gabriele C. Hegerl

Abstract

The authors exploit the remarkable similarity between recent climate trends and the structure of the “annular modes” in the month-to-month variability (as described in a companion paper) to partition the trends into components linearly congruent with and linearly independent of the annular modes.

The index of the Northern Hemisphere (NH) annular mode, referred to as the Arctic Oscillation (AO), has exhibited a trend toward the high index polarity over the past few decades. The largest and most significant trends are observed during the “active season” for stratospheric planetary wave–mean flow interaction, January–March (JFM), when fluctuations in the AO amplify with height into the lower stratosphere. For the periods of record considered, virtually all of the JFM geopotential height falls over the polar cap region and the strengthening of the subpolar westerlies from the surface to the lower stratosphere, ∼50% of the JFM warming over the Eurasian continent, ∼30% of the JFM warming over the NH as a whole, ∼40% of the JFM stratospheric cooling over the polar cap region, and ∼40% of the March total column ozone losses poleward of 40°N are linearly congruent with month-to-month variations in the AO index. Summertime sea level pressure falls over the Arctic basin are suggestive of a year-round drift toward the positive polarity of the AO, but the evidence is less conclusive. Owing to the photochemical memory inherent in the ozone distribution, roughly half the ozone depletion during the NH summer months is linearly dependent on AO-related ozone losses incurred during the previous active season.

Lower-tropospheric geopotential height falls over the Antarctic polar cap region are indicative of a drift toward the high index polarity of the Southern Hemisphere (SH) annular mode with no apparent seasonality. In contrast, the trend toward a cooling and strengthening of the SH stratospheric polar vortex peaks sharply during the stratosphere’s relatively short active season centered in November. The most pronounced SH ozone losses have occurred in September–October, one or two months prior to this active season. In both hemispheres, positive feedbacks involving ozone destruction, cooling, and a weakening of the wave-driven meridional circulation may be contributing to a delayed breakdown of the polar vortex and enhanced ozone losses during spring.

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Varavut Limpasuvan
,
David W. J. Thompson
, and
Dennis L. Hartmann
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Michelle L. L’Heureux
and
David W. J. Thompson

Abstract

There is increasing evidence indicating that the climate response to variations in the El Niño–Southern Oscillation (ENSO) includes not only thermally forced zonal wind anomalies in the subtropics but also eddy-driven zonal wind anomalies that extend into the mid–high latitudes of both hemispheres. In this study, new insights into the observed seasonally varying signature of ENSO in the extratropical zonal-mean circulation are provided and the associated linkages with the dominant patterns of extratropical variability are examined.

The zonal-mean extratropical atmospheric response to ENSO is characterized by two principal features: an equivalent barotropic dipole in the Southern Hemisphere (SH) zonal-mean zonal flow with centers of action located near ∼40° and ∼60° during austral summer, and a weaker, but analogous, dipole in the Northern Hemisphere (NH) with centers of action located near ∼25° and ∼45° during early and late boreal winter. Both structures are accompanied by eddy momentum flux anomalies that exhibit a remarkable degree of hemispheric symmetry. In the SH, the extratropical signature of ENSO projects strongly onto the primary mode of large-scale variability, the southern annular mode (SAM). During the austral summer, roughly 25% of the temporal variability in the SAM is linearly related to fluctuations in the ENSO cycle. An analogous relationship is not observed in association with the principal mode of climate variability in the NH, the northern annular mode (NAM).

It is argued that the seasonally varying impact of ENSO on the extratropical circulation is consistent with the impact of the thermally forced subtropical wind anomalies on the dissipation of equatorward-propagating wave activity at subtropical latitudes.

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