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David W. J. Thompson
and
David J. Lorenz

Abstract

The extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere. The signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry. It is associated with upper-tropospheric zonal wind anomalies centered about the equator, midtropospheric temperature anomalies located ∼20°N and 20°S, and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres. The linkages between the annular modes and the tropical circulation are only evident during the cold season months, and are most robust in association with the Northern Hemisphere annular mode (NAM).

The coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes. Both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields. The most pronounced tropical anomalies lag indices of the annular modes by ∼2 weeks and are found over the eastern tropical Pacific, where climatological westerlies permit extratropical waves to propagate into the deep Tropics. The linkages between the NAM and the tropical tropospheric circulation are most pronounced during the cold phase of the El Niño–Southern Oscillation cycle.

The recent trend in the NAM is linearly congruent with a ∼0.1-K cooling of the tropical troposphere over the past two decades during the Northern Hemisphere winter season.

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David W. J. Thompson
and
Susan Solomon

Abstract

The long-term, global-mean cooling of the lower stratosphere stems from two downward steps in temperature, both of which are coincident with the cessation of transient warming after the volcanic eruptions of El Chichón and Mount Pinatubo. Previous attribution studies reveal that the long-term cooling is linked to ozone trends, and modeling studies driven by a range of known forcings suggest that the steps reflect the superposition of the long-term cooling with transient variability in upwelling longwave radiation from the troposphere. However, the long-term cooling of the lower stratosphere is evident at all latitudes despite the fact that chemical ozone losses are thought to be greatest at middle and polar latitudes. Further, the ozone concentrations used in such studies are based on either 1) smooth mathematical functions fit to sparsely sampled observations that are unavailable during postvolcanic periods or 2) calculations by a coupled chemistry–climate model.

Here the authors provide observational analyses that yield new insight into three key aspects of recent stratospheric climate change. First, evidence is provided that shows the unusual steplike behavior of global-mean stratospheric temperatures is dependent not only upon the trend but also on the temporal variability in global-mean ozone immediately following volcanic eruptions. Second, the authors argue that the warming/cooling pattern in global-mean temperatures following major volcanic eruptions is consistent with the competing radiative and chemical effects of volcanic eruptions on stratospheric temperature and ozone. Third, it is revealed that the contrasting latitudinal structures of recent stratospheric temperature and ozone trends are consistent with large-scale increases in the stratospheric overturning Brewer–Dobson circulation.

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David W. J. Thompson
and
Ying Li

Abstract

Large-scale variability in the Northern Hemisphere (NH) circulation can be viewed in the context of three primary types of structures: 1) teleconnection patterns, 2) a barotropic annular mode, and 3) a baroclinic annular mode. The barotropic annular mode corresponds to the northern annular mode (NAM) and has been examined extensively in previous research. Here the authors examine the spatial structure and time-dependent behavior of the NH baroclinic annular mode (NBAM).

The NAM and NBAM have very different signatures in large-scale NH climate variability. The NAM emerges as the leading principal component (PC) time series of the zonal-mean kinetic energy. It dominates the variance in the wave fluxes of momentum, projects weakly onto the eddy kinetic energy and wave fluxes of heat, and can be modeled as Gaussian red noise with a time scale of ~10 days. In contrast, the NBAM emerges as the leading PC time series of the eddy kinetic energy. It is most clearly identified when the planetary-scale waves are filtered from the data, dominates the variance in the synoptic-scale eddy kinetic energy and wave fluxes of heat, and has a relatively weak signature in the zonal-mean kinetic energy and the wave fluxes of momentum. The NBAM is marked by weak but significant enhanced spectral power on time scales of ~20–25 days.

The NBAM is remarkably similar to its Southern Hemisphere counterpart despite the pronounced interhemispheric differences in orography and land–sea contrasts.

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David W. J. Thompson
and
Thomas Birner

Abstract

Previous studies have demonstrated the key role of baroclinicity and thus the isentropic slope in determining the climatological-mean distribution of the tropospheric eddy fluxes of heat. Here the authors examine the role of variability in the isentropic slope in driving variations in the tropospheric eddy fluxes of heat about their long-term mean during Northern Hemisphere winter.

On month-to-month time scales, the lower-tropospheric isentropic slope and eddy fluxes of heat are not significantly correlated when all eddies are included in the analysis. But the isentropic slope and heat fluxes are closely linked when the data are filtered to isolate the fluxes due to synoptic (<10 days) and low-frequency (>10 days) time scale waves. Anomalously steep isentropic slopes are characterized by anomalously poleward heat fluxes by synoptic eddies but anomalously equatorward heat fluxes by low-frequency eddies. Lag regressions based on daily data reveal that 1) variations in the isentropic slope precede by several days variations in the heat fluxes by synoptic eddies and 2) variations in the heat fluxes due to both synoptic and low-frequency eddies precede by several days similarly signed variations in the momentum flux at the tropopause level.

The results suggest that seemingly modest changes in the tropospheric isentropic slope drive significant changes in the synoptic eddy heat fluxes and thus in the generation of baroclinic wave activity in the lower troposphere. The linkages have implications for understanding the extratropical tropospheric eddy response to a range of processes, including anthropogenic climate change, stratospheric variability, and extratropical sea surface temperature anomalies.

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David W. J. Thompson
and
Susan Solomon

Abstract

The global structure of recent stratospheric climate trends is examined in radiosonde data. In contrast to conclusions published in previous assessments of stratospheric temperature trends, it is demonstrated that in the annual mean the tropical stratosphere has cooled substantially over the past few decades. The cooling of the tropical stratosphere is apparent in both nighttime and adjusted radiosonde data, and seems to be robust to changes in radiosonde instrumentation. The meridional structure of the annual-mean stratospheric trends is not consistent with our current understanding of radiative transfer and constituent trends but is consistent with increased upwelling in the tropical stratosphere.

The annual-mean cooling of the tropical stratosphere is juxtaposed against seasonally varying trends in the extratropical stratosphere dominated by the well-known springtime cooling at polar latitudes. The polar stratospheric trends are accompanied by similarly signed trends at tropospheric levels in the Southern Hemisphere but not in the Northern Hemisphere.

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Ying Li
and
David W. J. Thompson

Abstract

The signatures of large-scale annular variability on the vertical structure of clouds and cloud radiative effects are examined in vertically resolved CloudSat and other satellite and reanalysis data products. The northern and southern “barotropic” annular modes (the NAM and SAM) have a complex vertical structure. Both are associated with a meridional dipole in clouds between subpolar and middle latitudes, but the sign of the anomalies changes between upper, middle, and lower tropospheric levels. In contrast, the northern and southern baroclinic annular modes have a much simpler vertical structure. Both are linked to same-signed anomalies in clouds extending throughout the troposphere at middle to high latitudes. The changes in cloud incidence associated with both the barotropic and baroclinic annular modes are consistent with dynamical forcing by the attendant changes in static stability and/or vertical motion. The results also provide the first observational estimates of the vertically resolved atmospheric cloud radiative effects associated with hemispheric-scale extratropical variability. In general, the anomalies in atmospheric cloud radiative effects associated with the annular modes peak in the middle to upper troposphere, and are consistent with the anomalous trapping of longwave radiation by variations in upper tropospheric clouds. The southern baroclinic annular mode gives rise to periodic behavior in longwave cloud radiative effects at the top of the atmosphere averaged over Southern Hemisphere midlatitudes.

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Laura M. Ciasto
and
David W. J. Thompson

Abstract

The authors provide a detailed examination of observed ocean–atmosphere interaction in the Southern Hemisphere (SH). Focus is placed on the observed relationships between variability in SH extratropical sea surface temperature (SST) anomalies, the Southern Annular Mode (SAM), and the El Niño–Southern Oscillation (ENSO). Results are examined separately for the warm (November–April) and cold (May–October) seasons and for monthly and weekly time scales. It is shown that the signatures of the SAM and ENSO in the SH SST field vary as a function of season, both in terms of their amplitudes and structures. The role of surface turbulent and Ekman heat fluxes in driving seasonal variations in the SAM- and ENSO-related SST anomalies is investigated. Analyses of weekly data reveal that variability in the SAM tends to precede anomalies in the SST field by ∼1 week, and that the e-folding time scale of the SAM-related SST field anomalies is at least 4 months. The persistence of the SAM-related SST anomalies is consistent with the passive thermal response of the Southern Ocean to variations in the SAM, and seasonal variations in the persistence of the SAM-related SST anomalies are consistent with the seasonal cycle in the depth of the ocean mixed layer.

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Laura M. Ciasto
and
David W. J. Thompson

Abstract

Observations of subsurface temperatures are used to examine the winter-to-winter “reemergence” of sea surface temperature (SST) anomalies in the extratropical South Pacific Ocean. Reemergence is the mechanism through which SST anomalies formed in the late winter are sequestered beneath the relatively shallow summer mixed layer and then reentrained into the deepening mixed layer during the following autumn/winter. Although several studies have extensively examined reemergence in the Northern Hemisphere (NH), this is the first study to use observations of subsurface temperatures to document reemergence in the extratropical Southern Hemisphere (SH). The SH subsurface data reveal a pronounced reemergence signal in the western extratropical South Pacific. In this region, surface thermal anomalies formed during late SH winter are observed to persist below the summertime mixed layer and reemerge at the surface during the following early winter months. As such, SST anomalies formed during late winter are strongly correlated with SST anomalies during the following early winter but are not significantly correlated with SST anomalies during the intervening summer months. The results based on subsurface data are robust to small changes in the period of analysis and are qualitatively similar to existing evidence of reemergence in the NH. Analyses of independent SST data reveal that reemergence is widespread in the western extratropical South Pacific basin but is less discernible in SST anomalies over the eastern part of the basin.

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David W. J. Thompson
and
Jonathan D. Woodworth

Abstract

The leading patterns of large-scale climate variability in the Southern Hemisphere are examined in the context of extratropical kinetic energy. It is argued that variability in the Southern Hemisphere extratropical flow can be viewed in the context of two distinct and largely independent structures, both of which exhibit a high degree of annularity: 1) a barotropic structure that dominates the variance in the zonal-mean kinetic energy and 2) a baroclinic structure that dominates the variance in the eddy kinetic energy. The former structure corresponds to the southern annular mode (SAM) and has been extensively examined in the literature. The latter structure emerges as the leading principal component time series of eddy kinetic energy and has received seemingly little attention in previous work.

The two structures play very different roles in cycling energy through the extratropical troposphere. The SAM is associated primarily with variability in the meridional propagation of wave activity, has a surprisingly weak signature in the eddy fluxes of heat, and can be modeled as Gaussian red noise with an e-folding time scale of approximately 10 days. The baroclinic annular structure is associated primarily with variations in the amplitude of vertically propagating waves, has a very weak signature in the wave fluxes of momentum, and exhibits marked quasi periodicity on time scales of approximately 25–30 days. Implications for large-scale climate variability are discussed.

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Elizabeth A. Barnes
and
David W. J. Thompson

Abstract

Do barotropic or baroclinic eddy feedbacks dominate the atmospheric circulation response to mechanical forcing?

To address this question, barotropic torques are imposed over a range of latitudes in both an idealized general circulation model (GCM) and a barotropic model. The GCM includes both baroclinic and barotropic feedbacks. The barotropic model is run in two configurations: 1) only barotropic feedbacks are present and 2) a baroclinic-like feedback is added by allowing the stirring region to move with the jet. The relationship between the latitude of the forcing and the response is examined by systematically shifting the torques between the tropics and the pole. The importance of the mean state is investigated by varying the position of the control jet.

Five main findings are presented: 1) Barotropic feedbacks alone are capable of producing the structure of the GCM response to mechanical forcing but are not capable of accounting for its full magnitude. 2) Baroclinic processes generally increase the magnitude of the response but do not strongly influence its structure. 3) For a given forcing, the largest response in all model configurations occurs 5°–10° poleward of the forcing latitude. 4) The maximum response occurs when the forcing is located approximately 10° poleward of the control jet. 5) The circulation response weakens as the mean jet is found at higher latitudes in all model configurations.

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