Stratospheric variability and tropospheric annular‐mode timescales

Article English OPEN
Simpson, I. R. ; Hitchcock, P. ; Shepherd, T. G. ; Scinocca, J. F. (2011)

Climate models tend to exhibit much too persistent Southern Annular Mode (SAM) circulation anomalies in summer, compared to observations. Theoretical arguments suggest this bias may lead to an overly strong model response to anthropogenic forcing during this season, which is of interest since the largest observed changes in Southern Hemisphere high‐latitude climate over the last few decades have occurred in summer, and are congruent with the SAM. The origin of this model bias is examined here in the case of the Canadian Middle Atmosphere Model, using a novel technique to quantify the influence of stratospheric variability on tropospheric annular‐mode timescales. Part of the model bias is shown to be attributable to the too‐late breakdown of the stratospheric polar vortex, which allows the tropospheric influence of stratospheric variability to extend into early summer. However, the analysis also reveals an enhanced summertime persistence of the model’s SAM that is unrelated to either stratospheric variability or the bias in model stratospheric climatology, and is thus of tropospheric origin. No such feature is evident in the Northern Hemisphere. The effect of stratospheric variability in lengthening tropospheric annular‐mode timescales is evident in both hemispheres. While in the Southern Hemisphere the effect is restricted to late‐spring/early summer, in the Northern Hemisphere it can occur throughout the winter‐spring season, with the seasonality of peak timescales exhibiting considerable variability between different 50 year sections of the same simulation.
  • References (21)
    21 references, page 1 of 3

    Baldwin, M. P., and D. W. J. Thompson (2009), A critical comparison of stratosphere‐troposphere coupling indices, Q. J. R. Meteorol. Soc., 135, 1661-1672.

    Baldwin, M. P., D. B. Stephenson, D. W. J. Thompson, T. J. Dunkerton, A. J. Charlton, and A. O'Neill (2003), Stratospheric memory and skill of extended‐range weather forecasts, Science, 301, 636-640, doi:10.1126/ science.1087143.

    Butchart, N., et al. (2011), Multimodel climate and variability of the stratosphere, J. Geophys. Res., 116, D05102, doi:10.1029/2010JD014995.

    Dee, D. P., et al. (2011), The ERA‐interim reanalysis: Configuration and performance of the data assimilation system, Q. J. R. Meteorol. Soc., 137, 553-597, doi:10.1002/qj.828.

    Fogt, R. L., J. Perlwitz, A. J. Monaghan, D. H. Bromwich, J. M. Jones, and G. J. Marshall (2009), Historical SAM variability. Part II: Twentiethcentury variability and trends from reconstructions, observations, and the IPCC AR4 models, J.Clim., 22, 5346-5365, doi:10.1175/ 2009JCLI2786.1.

    Fyfe, J. C., and O. A. Saenko (2006), Simulated changes in the extratropical Southern Hemisphere winds and currents, Geophys. Res. Lett., 33, L06701, doi:10.1029/2005GL025332.

    Gerber, E. P., and L. M. Polvani (2009), Stratosphere‐troposphere coupling in a relatively simple AGCM: The importance of stratospheric variability, J. Clim., 22, 1920-1933.

    Gerber, E. P., L. M. Polvani, and D. Ancukiewicz (2008), Annular mode time scales in the Intergovernmental Panel on Climate Change Fourth Assessment Report models, Geophys. Res. Lett., 35, L22707, doi:10.1029/2008GL035712.

    Gerber, E. P., et al. (2010), Stratosphere‐troposphere coupling and annular mode variability in chemistry‐climate models, J. Geophys. Res., 115, D00M06, doi:10.1029/2009JD013770.

    Keeley, S. P. E., R. T. Sutton, and L. C. Shaffrey (2009), Does the North Atlantic Oscillation show unusual persistence on intraseasonal timescales?, Geophys. Res. Lett., 36, L22706, doi:10.1029/2009GL040367.

  • Metrics
    views in OpenAIRE
    views in local repository
    downloads in local repository

    The information is available from the following content providers:

    From Number Of Views Number Of Downloads
    Central Archive at the University of Reading - IRUS-UK 0 22
Share - Bookmark