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Ying Li
,
David W. J. Thompson
, and
Dirk Olonscheck

Abstract

Cloud radiative effects (CREs) are known to play a central role in governing the long-term mean distribution of sea surface temperatures (SSTs). Very recent work suggests that CREs may also play a role in governing the variability of SSTs in the context of El Niño–Southern Oscillation. Here, the authors exploit numerical simulations in the Max Planck Institute Earth System Model with two different representations of CREs to demonstrate that coupling between CREs and the atmospheric circulation has a much more general and widespread effect on tropical climate than that indicated in previous work. The results reveal that coupling between CREs and the atmospheric circulation leads to robust increases in SST variability on time scales longer than a month throughout the tropical oceans. Remarkably, cloud–circulation coupling leads to more than a doubling of the amplitude of decadal-scale variability in tropical-mean SSTs. It is argued that the increases in tropical SST variance derive primarily from the coupling between SSTs and shortwave CREs: Coupling increases the memory in shortwave CREs on hourly and daily time scales and thus reddens the spectrum of shortwave CREs and increases their variance on time scales spanning weeks to decades. Coupling between SSTs and CREs does not noticeably affect the variance of SSTs in the extratropics, where the effects from variability in CREs on the surface energy budget are much smaller than the effects from the turbulent heat fluxes. The results indicate a basic but critical role of CREs in climate variability throughout the tropics.

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Lina Boljka
,
David W. J. Thompson
, and
Ying Li

Abstract

Baroclinic waves drive both regional variations in weather and large-scale variability in the extratropical general circulation. They generally do not exist in isolation, but rather often form into coherent wave packets that propagate to the east via a mechanism called downstream development. Downstream development has been widely documented and explored. Here we document a novel but also key aspect of baroclinic waves: the downstream suppression of baroclinic activity that occurs in the wake of eastward propagating disturbances. Downstream suppression is apparent not only in the Southern Hemisphere storm track as shown in previous work, but also in the North Pacific and North Atlantic storm tracks. It plays an essential role in driving subseasonal periodicity in extratropical eddy activity in both hemispheres, and gives rise to the observed quiescence of the North Atlantic storm track 1–2 weeks following pronounced eddy activity in the North Pacific sector. It is argued that downstream suppression results from the anomalously low baroclinicity that arises as eastward propagating wave packets convert potential to kinetic energy. In contrast to baroclinic wave packets, which propagate to the east at roughly the group velocity in the upper troposphere, the suppression of baroclinic activity propagates eastward at a slower rate that is comparable to that of the lower to midtropospheric flow. The results have implications for understanding subseasonal variability in the extratropical troposphere of both hemispheres.

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Amy H. Butler
,
David W. J. Thompson
, and
Ross Heikes

Abstract

The steady-state extratropical atmospheric response to thermal forcing is investigated in a simple atmospheric general circulation model. The thermal forcings qualitatively mimic three key aspects of anthropogenic climate change: warming in the tropical troposphere, cooling in the polar stratosphere, and warming at the polar surface. The principal novel findings are the following:

1) Warming in the tropical troposphere drives two robust responses in the model extratropical circulation: poleward shifts in the extratropical tropospheric storm tracks and a weakened stratospheric Brewer–Dobson circulation. The former result suggests heating in the tropical troposphere plays a fundamental role in the poleward contraction of the storm tracks found in Intergovernmental Panel on Climate Change (IPCC)-class climate change simulations; the latter result is in the opposite sense of the trends in the Brewer–Dobson circulation found in most previous climate change experiments.

2) Cooling in the polar stratosphere also drives a poleward shift in the extratropical storm tracks. The tropospheric response is largely consistent with that found in previous studies, but it is shown to be very sensitive to the level and depth of the forcing. In the stratosphere, the Brewer–Dobson circulation weakens at midlatitudes, but it strengthens at high latitudes because of anomalously poleward heat fluxes on the flank of the polar vortex.

3) Warming at the polar surface drives an equatorward shift of the storm tracks. The storm-track response to polar warming is in the opposite sense of the response to tropical tropospheric heating; hence large warming over the Arctic may act to attenuate the response of the Northern Hemisphere storm track to tropical heating.

4) The signs of the tropospheric and stratospheric responses to all thermal forcings considered here are robust to seasonal changes in the basic state, but the amplitude and details of the responses exhibit noticeable differences between equinoctial and wintertime conditions. Additionally, the responses exhibit marked nonlinearity in the sense that the response to multiple thermal forcings applied simultaneously is quantitatively different from the sum of the responses to the same forcings applied independently. Thus the response of the model to a given thermal forcing is demonstrably dependent on the other thermal forcings applied to the model.

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Kathryn L. Verlinden
,
David W. J. Thompson
, and
Graeme L. Stephens

Abstract

The authors exploit three years of data from the CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellites to document for the first time the seasonally varying vertical structure of cloudiness throughout Antarctica and the high-latitude Southern Ocean. The results provide a baseline reference of Southern Hemisphere high-latitude cloudiness for future observational and modeling studies, and they highlight several previously undocumented aspects and key features of Antarctic cloudiness.

The key features of high-latitude Southern Hemisphere cloudiness documented here include 1) a pronounced seasonal cycle in cloudiness over the high-latitude Southern Hemisphere, with higher cloud incidences generally found during the winter season over both the Southern Ocean and Antarctica; 2) two distinct maxima in vertical profiles of cloud incidence over the Southern Ocean, one centered near the surface and another centered in the upper troposphere; 3) a nearly discontinuous drop-off in cloudiness near 8 km over much of the continent that peaks during autumn, winter, and spring; 4) large east–west gradients in upper-level cloudiness in the vicinity of the Antarctic Peninsula that peak during the austral spring season; and 5) evidence that cloudiness in the polar stratosphere is marked not by a secondary maximum at stratospheric levels but by a nearly monotonic decrease with height from the tropopause.

Key results are interpreted in the context of the seasonally varying profiles of vertical motion and static stability and compared with results of previous studies.

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Ying Li
,
David W. J. Thompson
, and
Sandrine Bony

Abstract

The influence of clouds on the large-scale atmospheric circulation is examined in numerical simulations from an atmospheric general circulation model run with and without atmospheric cloud radiative effects (ACRE). In the extratropics of both hemispheres, the primary impacts of ACRE on the circulation include 1) increases in the meridional temperature gradient and decreases in static stability in the midlatitude upper troposphere, 2) strengthening of the midlatitude jet, 3) increases in extratropical eddy kinetic energy by up to 30%, and 4) increases in precipitation at middle latitudes but decreases at subtropical latitudes. In the tropics, the primary impacts of ACRE include 1) eastward wind anomalies in the tropical upper troposphere–lower stratosphere (UTLS) and 2) reductions in tropical precipitation. The impacts of ACRE on the atmospheric circulation are interpreted in the context of a series of dynamical and physical processes. The changes in the extratropical circulation and precipitation are consistent with the influence of ACRE on the baroclinicity and eddy fluxes of momentum in the extratropical upper troposphere, the changes in the zonal wind in the UTLS with the influence of ACRE on the amplitude of the equatorial planetary waves, and the changes in the tropical precipitation with the energetic constraints on the tropical troposphere. The results make clear that ACRE have a pronounced influence on the atmospheric circulation not only at tropical latitudes, but at extratropical latitudes as well. They highlight the critical importance of correctly simulating ACRE in global climate models.

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Ying Li
,
David W. J. Thompson
, and
Yi Huang

Abstract

Previous studies have explored the influence of atmospheric cloud radiative effects (ACRE) on the tropospheric circulation. Here the authors explore the influence of ACRE on the stratospheric circulation. The response of the stratospheric circulation to ACRE is assessed by comparing simulations run with and without ACRE. The stratospheric circulation response to ACRE is reproducible in a range of different GCMs and can be interpreted in the context of both a dynamically driven and a radiatively driven component.

The dynamic component is linked to ACRE-induced changes in the vertical and meridional fluxes of wave activity. The ACRE-induced changes in the vertical flux of wave activity into the stratosphere are consistent with the ACRE-induced changes in tropospheric baroclinicity and thus the amplitude of midlatitude baroclinic eddies. They account for a strengthening of the Brewer–Dobson circulation, a cooling of the tropical lower stratosphere, a weakening and warming of the polar vortex, a reduction of static stability near the tropical tropopause transition layer, and a shortening of the time scale of extratropical stratospheric variability. The ACRE-induced changes in the equatorward flux of wave activity in the low-latitude stratosphere account for a strengthening of the zonal wind in the subtropical lower to midstratosphere.

The radiative component is linked to ACRE-induced changes in the flux of longwave radiation into the lower stratosphere. The changes in radiative fluxes lead to a cooling of the extratropical lower stratosphere, changes in the static stability and cloud fraction near the extratropical tropopause, and a shortening of the time scales of extratropical stratospheric variability.

The results highlight a previously overlooked pathway through which tropospheric climate influences the stratosphere.

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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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Samantha M. Wills
,
David W. J. Thompson
, and
Laura M. Ciasto

Abstract

The advent of increasingly high-resolution satellite observations and numerical models has led to a series of advances in understanding the role of midlatitude sea surface temperature (SST) in climate variability, especially near western boundary currents (WBC). Observational analyses suggest that ocean dynamics play a central role in driving interannual SST variability over the Kuroshio–Oyashio and Gulf Stream extensions. Numerical experiments suggest that variations in the SST field within these WBC regions may have a much more pronounced influence on the atmospheric circulation than previously thought.

In this study, the authors examine the observational support for (or against) a robust atmospheric response to midlatitude SST variability in the Gulf Stream extension. To do so, they apply lead–lag analysis based on daily mean data to assess the evidence for two-way coupling between SST anomalies and the atmospheric circulation on transient time scales, building off of previous studies that have utilized weekly data. A novel decomposition approach is employed to demonstrate that atmospheric circulation anomalies over the Gulf Stream extension can be separated into two distinct patterns of midlatitude atmosphere–ocean interaction: 1) a pattern that peaks 2–3 weeks before the largest SST anomalies in the Gulf Stream extension, which can be viewed as the “atmospheric forcing,” and 2) a pattern that peaks several weeks after the largest SST anomalies, which the authors argue can be viewed as the “atmospheric response.” The latter pattern is linearly independent of the former and is interpreted as the potential response of the atmospheric circulation to SST variability in the Gulf Stream extension.

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Kevin M. Grise
,
David W. J. Thompson
, and
Thomas Birner

Abstract

Static stability is a fundamental dynamical quantity that measures the vertical temperature stratification of the atmosphere. However, the magnitude and structure of finescale features in this field are difficult to discern in temperature data with low vertical resolution. In this study, the authors apply more than six years of high vertical resolution global positioning system radio occultation temperature profiles to document the long-term mean structure and variability of the global static stability field in the stratosphere and upper troposphere.

The most pronounced feature in the long-term mean static stability field is the well-known transition from low values in the troposphere to high values in the stratosphere. Superposed on this general structure are a series of finer-scale features: a minimum in static stability in the tropical upper troposphere, a broad band of high static stability in the tropical stratosphere, increases in static stability within the core of the stratospheric polar vortices, and a shallow but pronounced maximum in static stability just above the tropopause at all latitudes [i.e., the “tropopause inversion layer” (TIL)].

The results shown here provide the first global survey of static stability using high vertical resolution data and also uncover two novel aspects of the static stability field. In the tropical lower stratosphere, the results reveal a unique vertically and horizontally varying static stability structure, with maxima located at ∼17 and ∼19 km. The upper feature peaks during the NH cold season and has its largest magnitude between 10° and 15° latitude in both hemispheres; the lower feature exhibits a weaker seasonal cycle and is centered at the equator. The results also demonstrate that the strength of the TIL is closely tied to stratospheric dynamic variability. The magnitude of the TIL is enhanced following sudden stratospheric warmings in the polar regions and the easterly phase of the quasi-biennial oscillation in the tropics.

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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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