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What controls dynamical timescales near the Tropopause?

What controls dynamical timescales near the Tropopause?. Andrew J. Charlton-Perez and Alan O’Neill Dept. of Meteorology Univ. of Reading. Seasonal predictability from the Stratosphere ?. Long timescales of the annular mode in mid-winter in the lower stratosphere. Baldwin et al. (2003).

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What controls dynamical timescales near the Tropopause?

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  1. What controls dynamical timescales near the Tropopause? Andrew J. Charlton-Perez and Alan O’Neill Dept. of Meteorology Univ. of Reading

  2. Seasonal predictability from the Stratosphere ? Long timescales of the annular mode in mid-winter in the lower stratosphere. Baldwin et al. (2003)

  3. Stratospheric influence on Tropospheric timescales Changing dynamical timescales in the middle stratosphere changes dynamical timescales in the troposphere 10hPa 1000hPa Norton (2003)

  4. Project aims • Are the long timescales in the lower stratosphere due to radiative processes only or related to dynamics? • What impact does changing the dynamical timescale above the Tropopause have on the troposphere? Conventions: NAM is annular mode in geopotential height AO is annular mode in surface pressure

  5. Model Setup • Reading IGCM T42L20. • Physics is Newtonian Cooling and Rayleigh Friction. • Relaxation temperature is as Held and Suarez (1994) and Polvani and Kushner (2001). • Idealised wave two topography.

  6. Imposed timescales in middle stratosphere have strong influence Dynamical timescales Longer radiative timescale in the stratosphere lead to longer timescales in the troposphere E-folding timescale of NAM at each pressure level

  7. Robustness Check Run model with different tropospheric configurations. Stars - No vortex Squares - No mountains Diamonds - More northerly mountains Correlation between NAM timescales in the Lower Stratosphere and the AO

  8. AO size/importance AO pressure signature is larger in model climates with longer dynamical timescales Explained variance of AO and dynamical timescale related

  9. PV Reservoir Model The AO is associated with flux of PV between a mid-latitude and polar reservoir (Czaja and Goodman, 2007) Regression of AO index with zonal mean PV

  10. What influences timescales? Larger feedback between reservoir and PV flux correlated with timescale Lorenz and Hartmann (2001)

  11. Conclusions • Changing the stratospheric radiative timescale in the model changes the dynamical timescale throughout the lower stratosphere and troposphere. • Longer dynamical timescales are associated with larger feedback between the mean-state and eddies. • Future work will investigate how changes to the stratospheric state influence the feedback between the mean-state and eddies. http://www.met.reading.ac.uk/~sws05ajc a.j.charlton@reading.ac.uk

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