past and future changes in southern hemisphere tropospheric circulation and the impact of...

Past and Future Changes in Southern Hemisphere Tropospheric Circulation and the

Impact of Stratospheric Chemistry-Climate Coupling

Collaborators: Steven Pawson,

Richard Stolarski,

Paul Newman,

Eric Nielsen,

Judith PerlwitzCIRES-Climate Diagnostics CenterUniversity of Colorado

Martin HoerlingBill Neff

Outline

• Introduction– Southern Hemisphere Annular Mode (SAM)

• Answers of)Fourth Assessment Report (AR4) on past and future changes in Southern Hemisphere Circulation

• Will the answers be different when analyzing experiments with Chemistry Climate models?

• Summary and Implications

Southern Hemisphere Annular mode

Thompson and Wallace, 2000

Reconstruction of SH summer SAM index(Jones and Widman, 2004)

Change in Southern Hemisphere Polar Cap Temperature and Geopotential Heights

1969-1998Based on Radiosonde Data

(Thompson and Solomon, 2002)

Change in 500 hPa geopotential height (1979-2000), surface temperature (1969-2000) and 925 hPa wind (1979-2000)

Thompson and Solomon, 2002

Response to stratospheric ozone loss in the Southern Hemisphere (Gillett and Thompson, 2003)

• Strengthening of westerly flow from the stratosphere to the troposphere

• Cooling in the troposphere occurs one to two month after the maximum cooling in the stratosphere

• Model response agrees well with observations.

Z 500hPa [m]DJF

Z [m]

T [K]

Fourth Assessment Report (AR4) of Intergovernmental Panel on Climate Change

• Released 2007

• Describes our improved understanding in observed changes, attribution, and estimates of future change

• Based on AR4 model simulations

Past Changes of SAM index

IPCC report: • On average, a larger positive trend is projected during the late 20th

century by models that include stratospheric ozone changes (red) than those that do not (blue),

Miller et al, 2006

Attribution of recent changes?

IPCC Report:• The cause of the positive

SAM trend in the second half of the 20th century is mainly attributed to stratospheric ozone depletion, evidenced by the fact that the signal is largest in the lower stratosphere in austral spring through summer (Thompson and Solomon, 2002; Arblaster and Meehl, 2006).

Attribution of recent SAM trend

• However, increases in greenhouse gases are also important factors (Shindell and Schmidt, 2004; Arblaster and Meehl, 2006) for the year-round positive SAM trend induced by meridional temperature gradient changes (Brandefelt and Källén, 2004).

Future Change of SAM index

IPCC report: • during the 21st century, when ozone changes are smaller, the SAM trends of

models with and without ozone are similar.

Miller et al. 2006

Future Change of SAM?IPCC Report: During the 21st century, although the ozone amount is expected to stabilise or recover, the polar vortex intensification is likely to continue due to the increases in greenhouse gases (Arblaster and Meehl, 2006).

How realistic are simulated SST changes?

How realistic are future projections of stratospheric

polar ozone changes used in AR4 models? • Ozone Assessment Report 2006:

Black Observations:

Over the Antarctic, most models predict that ozone amounts will increase to 1980 values close to the time when Antarctic equivalent effective stratospheric chlorine (EESD) decreases to 1980 values. A new empirical model, based on observations, indicates a return of Antarctic EESD values between 1960-1975

Stratosphere is not well resolved in these models

• Stratospheric dynamical processes are not well represented• AR4 Models do not simulate feedback mechanism of chemistry climate coupling

GEOS4-CCM Model

• GEOS4-GCM– Model Top: 0.01 hPa (~80km)– 55 levels– Horizontal resolution: 2ox2.5o

• Stratospheric Chemistry Module

•Role of SST changes and changes in atmospheric composition in recent changes of tropospheric SAM

•Role of greenhouse gas increase and ozone recovery in future SAM changes

Correlation coefficients between SAM index at 10 hPa and at all pressure levels between 1000 and 1 hPa for

time lags between -60 and 60 days.

Experiments:•Two Historic Simulations with

Observed SST (1950-2004)• Two simulations using SST

from CCM2, HadGem1 (1971-2049)

•Two Future Simulations using CCM3, HadGem1 SST (2000-2099)

•Historic and Future simulations where Clorine is kept at 1960 values

Past Climate Change1960-2004

•Monthly Changes in Polar Cap Temperature (90S-65S)•Monthly changes in Zonal Wind 70S-50S•Maps of 500 hPa and surface pressure changes

Monthly Linear Trend in Polar Cap Temperatureand Zonal Wind 70S-50S (1960-2004)

Polar Cap Temperature

Zonal Wind70S-50S

Change in tropospheric circulation

500 hPaGeopotential Height

Surface Pressure

1950-1999 500 hPa

geopotential heights [gpm]DJF

GFDL-AM2.0 GFDL-AM2.1 NCAR-CAM3

OF AM2.0Response to observed

SST changes In tropospheric models

AMIP

Tropical Indian Ocean warming

Simulations of Recent Past

• Consistent with many other studies: Observed increase of SAM index is caused by stratospheric polar ozone depletion

• In GEOS4-CCM observed SST changes from 1960-2004 cause negative SAM trend, due to increased upward flux of wave activity into the stratosphere during winter

• Impact of SST changes is model dependent

Future Climate Change2000-2099

Monthly Linear Trend in Polar Cap Temperatureand Zonal Wind 70S-50S (2000-2099)

Polar CapTemperature

Zonal Wind70S-50S

Change in tropospheric circulation

500 hPa Geopotential Height

Surface Pressure

Seasonal development of SAM trend

2000-2049

2000-2099

500 hPa 50 hPa

Changes in SAM index during 21st century

• CCM simulations without changes in stratospheric polar ozone:– Consistent with AR4 models and previous

studies, CCM shows increase of tropospheric SAM index

• CCM simulation with ozone recovery: – Ozone recovery dominates future development of

tropospheric SAM during summer– leading to a negative trend – Changes are most pronounced during first half of

21 century.

• Two runs with same SST are compared.

2000-2049• In lower

stratosphere, positive trend at South Pole is larger in C21D than in CCSM3.

2050-2099• In C21D, positive

trend continues. • Small negative trend

in CCSM3.

Implications• One-model-result

– Multi-model approach necessary similar to IPCC– will be realized in SPARC CCMval project

(Validation of Chemistry Climate Models)

• SST changes were prescribed using simulations that give a different answer (Implications of ozone recovery on ocean circulation and possible atmosphere/ocean interactions are not simulated correctly)– Simulations with coupled CCM/Ocean model are

necessary

• Ozone recovery scenario for 5th IPCC report should be defined