© Imperial College LondonPage 1 Solar Influence on Stratosphere-Troposphere Dynamical Coupling Isla Simpson, Joanna D. Haigh, Space and Atmospheric Physics, Imperial College London Mike Blackburn, Department of Meteorology, University of Reading

© Imperial College LondonPage 2 Introduction to the Solar influence on climate over the 11-year cycle. Model experiments to investigate how the tropospheric response over the 11-year cycle could be produced by a dynamical response to stratospheric heating. Comparison of two different stratospheric heating perturbation cases.

© Imperial College LondonPage 3 Temperature changes over the 11-year cycle Non-uniform. Increase of ~1K in equatorial stratosphere, decreasing towards the poles. Banded increase in temperature in mid-latitudes. Figure: Haigh (2003) Multiple regression analysis of NCEP/NCAR reanalysis,

© Imperial College LondonPage 4 Circulation changes over the 11-year cycle Weakening and poleward shift of the mid-latitude jets. Weakening and expansion of the Hadley cells. Poleward shift of the Ferrell cells. Figure: Haigh and Blackburn (2006) Multiple regression analysis of NCEP/NCAR reanalysis, DJF,

© Imperial College LondonPage 5 Model Experiments: Simplified General Circulation Model (Reading IGCM2.2) –T42L15 –Newtonian Relaxation –No Orography Haigh et al (2005) - Equatorial Heating gave a similar response to that seen over the Solar cycle. Spin-up ensemble: –200, 50-day runs 5K 0K 4.5K 0.5K

© Imperial College LondonPage 6 Change in temperature over the spin-up Control Equilibrium (Equatorial heating (5K) – Control) 0909 20   49

© Imperial College LondonPage 7 Change in zonal wind over the spin- up Equilibrium (Equatorial heating (5K) – Control) Control 0909 20   49

© Imperial College LondonPage 8 increases  increases decreases  decreases Changes in eddy momentum fluxes are in the right sense to drive meridional circulation changes. Mean meridional circulation Horizontal Eddy Momentum Flux [u’v’]

© Imperial College LondonPage 9 Anomalous meridional circulations are accompanied by zonal wind accelerations in the troposphere: increases  increases decreases  decreases Mean meridional circulation Zonal mean zonal wind [u]

© Imperial College LondonPage 10 Comparison with zonally symmetric model. Eddy forcing remains fixed at its value of the control run. Heating perturbation applied and the model run as before. Not much response in the troposphere, particularly at mid/high latitudes.  it is altered eddy momentum fluxes that are important in driving the tropospheric circulation changes. Full 3D modelNo change in Eddy fluxes [mmc] [u]

© Imperial College LondonPage 11 What’s causing the change in eddy momentum fluxes? E-P Flux Refractive Index C=8ms -1

© Imperial College LondonPage 12 Days 0 to 9 of the spin-up: Change in E-P Flux and Change in Change in : a) Only changing b) Only changing

© Imperial College LondonPage 13 Days 40 to 49 of the spin-up: Change in E-P Flux and Change in Change in : a) Only changing b) Only changing

© Imperial College LondonPage 14 Contributions to the change in PV gradient (days 0  9): Meridional Curvature Third term (only changing ) Total change in PV gradient

© Imperial College LondonPage 15 Outline of mechanism: Altered vertical temperature gradients Zonal wind accelerations stratosphere/tropopause Change in horizontal eddy momentum flux Changes in mean meridional circulation Zonal wind accelerations in the troposphere. Altered horizontal temperature gradients

© Imperial College LondonPage 16 Comparing Uniform and Equatorial Heating: 5K 0K 5K Equatorial heating (5K) (E5) Uniform heating (5K) (U5) Weakening and polewards jet shift. Weakening and equatorwards jet shift.

© Imperial College LondonPage 17 E-P Flux E5 (days 0  9 )U5 (days 0  9 )

© Imperial College LondonPage 18 E5 (days 40  49 )U5 (days 40  49 ) E-P flux and n 2

© Imperial College LondonPage 19 Conclusions The tropospheric response to increased Solar activity could be produced by a dynamical response to increased heating in the stratosphere. Changes in eddy momentum flux are important in driving circulation changes in the troposphere. Feedback with changing zonal wind in the troposphere influencing eddy propagation. Change in vertical temperature gradient around the tropopause and its localisation in latitude is important in determining the direction of the jet shift.

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© Imperial College LondonPage 25 + = control * = E5

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© Imperial College LondonPage 27 Equilibrium (Equatorial heating (5K) – Control) Control Change in mean meridional circulation over the spin-up 0909 20   49

© Imperial College LondonPage 28 Stratospheric and Tropopause level accelerations: Consistent with altered temperature gradients. Thermal wind balance: Zonal mean Temperature Zonal-mean zonal wind Increase  Decreased Decreased  Increased

© Imperial College LondonPage 29 Momentum Balance: Acceleration of zonal mean zonal wind Coriolis force on meridional wind Convergence of horizontal eddy momentum flux [u] days control[v] days control [u] and [v] terms don’t balance  changes in eddy momentum fluxes must be important.

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© Imperial College LondonPage 32 U5