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  • 1
    Online Resource
    Online Resource
    Testing the Annular Mode Autocorrelation Time Scale in Simple Atmospheric General Circulation Models
    American Meteorological Society ; 2008
    In:  Monthly Weather Review Vol. 136, No. 4 ( 2008-04-01), p. 1523-1536
    Details
    In: Monthly Weather Review, American Meteorological Society, Vol. 136, No. 4 ( 2008-04-01), p. 1523-1536
    Abstract: A new diagnostic for measuring the ability of atmospheric models to reproduce realistic low-frequency variability is introduced in the context of Held and Suarez’s 1994 proposal for comparing the dynamics of different general circulation models. A simple procedure to compute τ, the e-folding time scale of the annular mode autocorrelation function, is presented. This quantity concisely quantifies the strength of low-frequency variability in a model and is easy to compute in practice. The sensitivity of τ to model numerics is then studied for two dry primitive equation models driven with the Held–Suarez forcings: one pseudospectral and the other finite volume. For both models, τ is found to be unrealistically large when the horizontal resolutions are low, such as those that are often used in studies in which long integrations are needed to analyze model variability on low frequencies. More surprising is that it is found that, for the pseudospectral model, τ is particularly sensitive to vertical resolution, especially with a triangular truncation at wavenumber 42 (a very common resolution choice). At sufficiently high resolution, the annular mode autocorrelation time scale τ in both models appears to converge around values of 20–25 days, suggesting the existence of an intrinsic time scale at which the extratropical jet vacillates in the Held and Suarez system. The importance of τ for computing the correct response of a model to climate change is explicitly demonstrated by perturbing the pseudospectral model with simple torques. The amplitude of the model’s response to external forcing increases as τ increases, as suggested by the fluctuation–dissipation theorem.
    Type of Medium: Online Resource
    ISSN: 1520-0493 , 0027-0644
    URL: Article
    DOI: 10.1175/2007MWR2211.1
    RVK:
    RA 1000
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2008
    detail.hit.zdb_id: 2033056-X
    detail.hit.zdb_id: 202616-8
    SSG: 14
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  • 2
    Online Resource
    Online Resource
    Abrupt Circulation Responses to Tropical Upper-Tropospheric Warming in a Relatively Simple Stratosphere-Resolving AGCM
    American Meteorological Society ; 2012
    In:  Journal of Climate Vol. 25, No. 12 ( 2012-06-15), p. 4097-4115
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 25, No. 12 ( 2012-06-15), p. 4097-4115
    Abstract: The circulation response of the atmosphere to climate change–like thermal forcing is explored with a relatively simple, stratosphere-resolving general circulation model. The model is forced with highly idealized physics, but integrates the primitive equations at resolution comparable to comprehensive climate models. An imposed forcing mimics the warming induced by greenhouse gasses in the low-latitude upper troposphere. The forcing amplitude is progressively increased over a range comparable in magnitude to the warming projected by Intergovernmental Panel on Climate Change coupled climate model scenarios. For weak to moderate warming, the circulation response is remarkably similar to that found in comprehensive models: the Hadley cell widens and weakens, the tropospheric midlatitude jets shift poleward, and the Brewer–Dobson circulation (BDC) increases. However, when the warming of the tropical upper troposphere exceeds a critical threshold, ~5 K, an abrupt change of the atmospheric circulation is observed. In the troposphere the extratropical eddy-driven jet jumps poleward nearly 10°. In the stratosphere the polar vortex intensifies and the BDC weakens as the intraseasonal coupling between the troposphere and the stratosphere shuts down. The key result of this study is that an abrupt climate transition can be effected by changes in atmospheric dynamics alone, without need for the strong nonlinearities typically associated with physical parameterizations. It is verified that the abrupt climate shift reported here is not an artifact of the model’s resolution or numerics.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-11-00166.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2012
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 3
    Online Resource
    Online Resource
    Understanding Hadley Cell Expansion versus Contraction: Insights from Simplified Models and Implications for Recent Observations
    American Meteorological Society ; 2013
    In:  Journal of Climate Vol. 26, No. 12 ( 2013-06-15), p. 4304-4321
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 26, No. 12 ( 2013-06-15), p. 4304-4321
    Abstract: This study seeks a deeper understanding of the causes of Hadley Cell (HC) expansion, as projected under global warming, and HC contraction, as observed under El Niño. Using an idealized general circulation model, the authors show that a thermal forcing applied to a narrow region around the equator produces “El Niño–like” HC contraction, while a forcing with wider meridional extent produces “global warming–like” HC expansion. These circulation responses are sensitive primarily to the thermal forcing’s meridional structure and are less sensitive to its vertical structure. If the thermal forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recently observed trends of rapid tropical widening. The shift of the HC edge is explained using a very simple model in which the transformed Eulerian mean (TEM) circulation acts to diffuse heat meridionally. In this context, the HC edge is defined as the downward maximum of residual vertical velocity in the upper troposphere ; this corresponds well with the conventional Eulerian definition of the HC edge. In response to a positive thermal forcing, there is anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This causes the HC edge () to shift toward the descending anomaly, so that a narrow forcing causes HC contraction and a wide forcing causes HC expansion.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-12-00598.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2013
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 4
    Online Resource
    Online Resource
    Assessing and Understanding the Impact of Stratospheric Dynamics and Variability on the Earth System
    American Meteorological Society ; 2012
    In:  Bulletin of the American Meteorological Society Vol. 93, No. 6 ( 2012-06), p. 845-859
    Details
    In: Bulletin of the American Meteorological Society, American Meteorological Society, Vol. 93, No. 6 ( 2012-06), p. 845-859
    Type of Medium: Online Resource
    ISSN: 0003-0007 , 1520-0477
    URL: Article
    DOI: 10.1175/BAMS-D-11-00145.1
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2012
    detail.hit.zdb_id: 2029396-3
    detail.hit.zdb_id: 419957-1
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  • 5
    Online Resource
    Online Resource
    Stratosphere–Troposphere Coupling in a Relatively Simple AGCM: The Importance of Stratospheric Variability
    American Meteorological Society ; 2009
    In:  Journal of Climate Vol. 22, No. 8 ( 2009-04-15), p. 1920-1933
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 22, No. 8 ( 2009-04-15), p. 1920-1933
    Abstract: The impact of stratospheric variability on the dynamical coupling between the stratosphere and the troposphere is explored in a relatively simple atmospheric general circulation model. Variability of the model’s stratospheric polar vortex, or polar night jet, is induced by topographically forced stationary waves. A robust relationship is found between the strength of the stratospheric polar vortex and the latitude of the tropospheric jet, confirming and extending earlier results in the absence of stationary waves. In both the climatological mean and on intraseasonal time scales, a weaker vortex is associated with an equatorward shift in the tropospheric jet and vice versa. It is found that the mean structure and variability of the vortex in the model is very sensitive to the amplitude of the topography and that Northern Hemisphere–like variability, with a realistic frequency of stratospheric sudden warming events, occurs only for a relatively narrow range of topographic heights. When the model captures sudden warming events with fidelity, however, the exchange of information both upward and downward between the troposphere and stratosphere closely resembles that in observations. The influence of stratospheric variability on variability in the troposphere is demonstrated by comparing integrations with and without an active stratosphere. A realistic, time-dependent stratospheric circulation increases the persistence of the tropospheric annular modes, and the dynamical coupling is most apparent prior to and following stratospheric sudden warming events.
    Type of Medium: Online Resource
    ISSN: 1520-0442 , 0894-8755
    URL: Article
    DOI: 10.1175/2008JCLI2548.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2009
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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