Imperial College London

Transcription

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 Page 1

2 Temperature changes over the 11year cycle: Non Uniform. Increase of ~1K in equatorial stratosphere at Solar Max, decreasing towards the poles. Bands of heating in mid-latitude troposphere. T(Solar max) T(Solar min) from NCEP-NCAR reanalysis data Page 2 Figure: Haigh (2003)

3 Tropospheric Circulation Changes: Figure: Haigh et al (2005) Page 3 Weakening and poleward shift of the mid-latitude jets. Weakening and expansion of the Hadley cells. Poleward shift of the Ferrell cells.

4 Previous Modelling Results (Haigh et al (2005)) Simplified General Circulation Model. Applied heating perturbation to the stratosphere: Equatorial heating (5K) 5K 0K 10K A similar response in the troposphere as seen at Solar maximum is obtained by heating the stratosphere preferentially at the equator: Weakening and poleward shift of mid-latitude jets. Weakening and expansion of the Hadley cells. Poleward shift of the Ferrel cells. Banded temperature increase in mid-latitude troposphere. Page 4

5 Spin-up ensemble runs: Aim: Investigate how the stratospheric heating perturbation could result in tropospheric circulation changes. Simplified GCM (T42, L15) day runs 5K 4.5K 0.5K 0K Start from control run conditions Newtonian temperature relaxation Alter stratospheric relaxation temperature distribution Page 5 Investigate how the model responds over the following 50days.

6 Control Change in temperature over the spin-up Equilibrium (Equatorial heating (5K) Control) Page 6

7 Control Change in zonal wind over the spinup Equilibrium (Equatorial heating (5K) Control) Page 7

8 Control Change in mean meridional circulation over the spin-up Equilibrium (Equatorial heating (5K) Control) Page 8

9 Stratospheric and Tropopause level accelerations: Consistent with altered temperature gradients. Thermal wind balance: [ u] [ T ] z φ Zonal mean Temperature Zonal-mean zonal wind [T ] φ Increase Decreased [ u] z [T ] φ Decreased Increased [ u] z Page 9

10 Momentum Balance: Acceleration of zonal mean zonal wind Horizontal Eddy Momentum Flux [u v ] Mean meridional circulation [ u] [ u' v' = f [ v] ] t y Coriolis force on meridional wind decreases [v] decreases Convergence of horizontal eddy momentum flux Changes in Eddy momentum flux must be important. [ u' v' ] y [ u' v' ] y [v] increases increases Page 10

11 Anomalous meridional circulations are accompanied by zonal wind accelerations in the troposphere: Mean meridional circulation Zonal mean zonal wind [u] [ u] [ u' v' = f [ v] ] t y [v] [u] increases increases [v] [u] decreases decreases Page 11

12 Comparison with zonally symmetric model. Eddy forcing remains fixed at the values for the control run. Temperature perturbation applied and the model run as before. Full 3D model No change in Eddy fluxes [mmc] [u] Not much response in the troposphere,particularly at mid/high latitudes it is the altered eddy momentum fluxes that are important in driving tropospheric circulation changes. Page 12

13 What s causing the change in eddy momentum flux? E-P Flux F F φ [ u' v'] p [ v' θ '] Using c=12ms -1 n 2 = Refractive Index [ q] y [ u c] k a cosφ 2 2 f NH 2 a 2 Zonal wind changes around the tropopause alter q y refract eddies Page 13

14 Outline of mechanism: Altered temperature gradients Zonal wind accelerations Change in eddy momentum fluxes Changes mean meridional circulations? Zonal wind accelerations in the troposphere Page 14

15 Conclusions The tropospheric response to increased solar activity could be produced by a dynamical response to increased heating of the equatorial stratosphere. Changes in eddy momentum fluxes are important in transmitting the response to the troposphere below. Possible feedback with zonal wind changes in the troposphere altering the eddy momentum flux there. Page 15

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