1. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
CCSM Working Group Meeting, February 2008
Mechanisms for the Acceleration of the Brewer Dobson Circulation in a Climate Change Scenario
Natalia Calvo(1,2), and Rolando R. Garcia (1)
Atmospheric Chemistry Division, NCAR
Universidad Complutense de Madrid

2. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Motivation:
Butchart et al., Clim. Dyn. (2006)
The acceleration of the stratospheric mean meridional circulation (Brewer Dobson circulation) is a robust result in GCMs as a response to increase in GHG.
Increases in wave driving arising from enhanced tropospheric excitation of waves in the warmer tropospheric climate.
No general agreement:
What are the waves that might have a strongest influence?
Do the waves act mainly in subtropics or in middle and high latitudes?
Trends in the upward mass flux in the tropical lower stratosphere. Butchart et al., (2006).

3. Garcia et al., (2007), Kinnison et al., (2007), Marsh et al., (2007)
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Model:
WACCM3 : Whole Atmosphere Community Climate Model
developed at NCAR
extension of the NCAR Community Atmospheric Model version 3 (CAM3)
Resolution: Horizontal: 1.9x2.5 or 4x5 (lat x lon)
Vertical: 60 levels km
< 1.0 km in UTLS
1-2 km mid-upper stratosphere
3 km in M/LT
WACCM3 does not produce a QBO spontaneously. No QBO was included in the simulations discussed here
Interactive chemistry (MOZART, 57 species)
Mesoscale Gravity Wave are parameterized
Garcia et al., (2007), Kinnison et al., (2007), Marsh et al., (2007)

4. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Simulations:
REF1: Retrospective simulation of the 20th century ( )
Observed SST, GHG and CFCs
REF 2: Prognostic run ( )
GHG and halogen abundances specified following A1B scenario
SSTs specified from CAM coupled with a full ocean model also following A1B scenario
NCC: No climate change run ( )
Identical to REF2 but with GHGs concentrations held fixed since SSTs from CAM coupled with ocean with GHGs constant since 1995
Simulations were made in support of CCMVal and WMO
Each simulation comprises an ensemble of 3 realizations
Results presented here are ensemble averages

5. Age of Air: Tropical Average at 10hPa
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Age of Air: Tropical Average at 10hPa
REF1
REF2
NCC
Decreasing in AOA related to the increase in GHGs

6. Trend in AOA and Residual Circulation
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Trend in AOA and Residual Circulation

7. Residual circulation is wave driven: what are the waves involved?
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Residual circulation is wave driven: what are the waves involved?
Tropical Average *(-q0, + q0) of Downward Control w*:
black: all contributions
red: resolved waves only

8. Trend in EP flux and EP flux divergence
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
Trend in EP flux and EP flux divergence
Which are the resolved waves involved in these trends?

9. Wave analysis Eliassen-Palm flux components:
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
Wave analysis
Eliassen-Palm flux components:
Stationary and transient decomposition + Fourier analysis

10. Wave analysis Similar results for REF1 simulations
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
Wave analysis
Similar results for REF1 simulations

11. Trend in w*: Downward Control Principle
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
Trend in w*: Downward Control Principle
Influence of the planetary and synoptic waves
K=1-3
K=4-6
Similar results for REF1 simulations

12. What causes the trend in div (EP) in the lower stratosphere?
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
What causes the trend in div (EP) in the lower stratosphere?
Trends in EPflux coincide with regions of increasing westerlies in the lower stratoshere.
Trends in zonal mean U associated to trends in zonal mean T.
Tropospheric warming and stratospheric cooling driven by GHGs sharpens zonal-mean T gradients in the lower stratosphere.

13. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)
Conclusions
An acceleration of the residual circulation is observed in WACCM3 during the last 50 years and is predicted for the future under A1B scenario.
This acceleration occurs as a result of an increased Rossby wave driving in the subtropical lower stratospheric region.
Stationary planetary waves exert the strongest influence in the wave driving trend in the tropical region. In the subtropics boths planetary and synoptic transient waves are important.
The increased wave driving in the subtropics appears due to changes in the tropospheric and lower stratospheric zonal mean winds which follow from changes in the zonal mean T distribution caused by the greenhouse effect.

14. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)

15. Seasonal study DJF REF2 MAM REF2 JJA REF2 SON REF2
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Seasonal study
DJF REF2 MAM REF2
JJA REF2 SON REF2

16. Seasonal study JJA REF2 SON REF2
Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Seasonal study
JJA REF2 SON REF2

17. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Seasonal study JJA REF2

18. Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo & Garcia)
Seasonal study DJF REF2