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

Mechanisms for the Acceleration of the Brewer Dobson Circulation in a Climate Change Scenario Natalia Calvo

(1,2)

, and Rolando R. Garcia

(1)

(1) Atmospheric Chemistry Division, NCAR (2) Universidad Complutense de Madrid

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

Motivation:

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.

Trends in the upward mass flux in the tropical lower stratosphere. Butchart et al., (2006).

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?

Butchart et al.,

Clim. Dyn.

(2006)

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:

Vertical: Horizontal: 1.9x2.5 or

4x5

(lat x lon) 60 levels 0-140km < 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)

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

Simulations:

REF1: Retrospective simulation of the 20 th century (1950-2003)

Observed SST, GHG and CFCs

REF 2: Prognostic run (1975-2049)

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 (1995-2049)

Identical to REF2 but with GHGs concentrations held fixed since 1995 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

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

Age of Air: Tropical Average at 10hPa

REF1 NCC REF2

Decreasing in AOA related to the increase in GHGs

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

Trend in AOA and Residual Circulation

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 *( -q 0 , + q 0 ) of Downward Control

w

*: black: all contributions red: resolved waves only

Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)

Trend in EP flux and EP flux divergence

 *  q Which are the resolved waves involved in these trends?

Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)

Wave analysis

Eliassen-Palm flux components:

F y

 0 

u z

q q

z u

'

F z

 0  

f

1 

u y



v

q

z

'    Stationary and transient decomposition + Fourier analysis

u



u

 +

u

+ , , , , , ) 

k

, ) 

k v k

 2 , )

ikx e

, )

ikx e

 2  2

k

 

Mechanisms for the Acceleration of the BD circulation in Climate Change (Calvo et al.)

Wave analysis

Similar results for REF1 simulations

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

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 .

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.

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

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

Seasonal study

MAM REF2 JJA REF2 SON REF2

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

Seasonal study

JJA REF2 SON REF2

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

Seasonal study JJA REF2

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

Seasonal study DJF REF2