C20C Workshop Trieste, Italy 19 – 23 April 2004 The impact of stratospheric ozone depletion and CO 2 on AAO trends and regional climate change at surface

C20C Workshop Trieste, Italy 19 – 23 April 2004

The impact of stratospheric ozone depletion and CO2 on AAO trends

and regional climate change at surface of Southern Hemisphere

Syktus J., I. Smith2, G. McKeon1, M. Dix2 & W. Cai2

1Department of Natural Resources, Mines and Energy, Brisbane, Australia

2CSIRO Atmospheric Research, Melbourne, Australia


– the source of observed 20thC trends in the SAM is a matter of scientific debate with stratospheric ozone losses, greenhouse gas increases and natural variability all possible players–my motivation is from practical point of view; does it influence regional climate trends and change– since it is difficult to separate the contribution of these various forcings in the observed record, a global climate model has been used

Motivation


Southern Hemisphere Annular Mode - Discussion

• It is a dominant mode of low frequency variation of the SH circulation. Characterized by zonally symmetric fluctuations in the extra-tropical atmospheric circulation that extends from surface to the stratosphere

• The observation show that AOO index has trend towards a positive values since the late 1970s, coincident with occurrence of ozone depletion (Marshall 2003)

• The issue here is whether AGCM forced with observed SST and radiative forcing factors such as CO2 & O3 can simulate the trends in surface MSLP and other surface characteristics at regional scales

• Climate models forced by changing CO2 or by decreasing stratospheric ozone usually show a trend in the SH annular mode.

• Still an open question how good a representation of stratosphere is needed to accurately model tropospheric climate and climate change, however link to stratosphere is not essential for MSLP to have SH annular mode like response


Martin Visbeck

Todd Mitchell


Problem and approach

• During the past decade large parts of eastern Australia experienced unprecedented and persisting drying trends

• Drying trends in eastern Australia in recent decades Water supply restrictions in major cities of Eastern Australia

• Identification of possible forcing factors; GHG,O3, land cover change …

• Limitations of the approach• Review of the current state of knowledge• Simulation results and discussion


Rainfall Relative to Historical Records Australia– July 1992 to June 2003

Percentiles

Red = Last 10 years are in the bottom 10% of all previous ten July to June year periods from 1890


Trends in annual rainfall expressed in terms of the trend correlation coefficient (r, not b).

1952 to 2002

1952 to 2002MJJASO

1952 to 2002 NDJFMA

Annual


(a)area-averaged total, mean=461

(b)area-averaged mean decile value

1974 1902

2000 1905

1973 2002

1950 1961

1956 1994

1974 1905

1973 1928

2000 1961

1975 1994

1917 2002

r(SOI)=0.50

r(SOI)=0.44

All-Australian annual rainfall indices


•Despite 2002 being one of the driest years on record, Australia appears to have become wetter

•This positive trend in NW Aus appears because of relatively wet years in the second half of the record (e.g.1974, 2000).

•The chances of these trends arising due to random fluctuations of the data are about 5%

We do not know:

•where the trends have occurred

•if there are any compensating negative trends

•which seasons are involved


Second dominant EOT of Australian Observed Rainfall - S2 Central west (15%)

S2 represents variability over that

potion of the continent where rainfall stations are most sparse. It is possible that T2 may be dominated by the relatively small number of stations whose effect has been interpolated over large distances. (Smith 2004)

NDJFMAT2 has also positive & statistically significant trend during the 1900-2002 period, strongest over the past decade


Summary

•Increases in all-Australian annual rainfall over the past 100 years are the result of increases in summer rainfall over much of the western half of the continent over the past 50 years.

•The magnitude and pattern of the increases appears unusual and unlikely to be the result of “natural variability”.

•It is not possible to describe winter rainfall trends as unusual.

•Increases in summer rainfall have been suggested as a response to increased greenhouse gases.

•Temperatures have risen substantially over both Australia and the Indian Ocean over recent decades.

•These may be linked to the rainfall changes.

•These may, in turn, be attributable to EGG - but earlier than expected.



http://iri.columbia.edu/climate/forecast/net_asmt/

• Forecast skill assessment

(Goddard et al., 2004, BAMS)

March 2002 Rainfall Forecast for July to September

IRI Net Assessment ForecastsNR&M contributes NCEP model output at monthly intervals

to IRI consensus forecast since late 1998


Murray-Darling Basin; April-September rainfall & maximum temperature

0

100

200

300

400

500

600

700

800

1952 1957 1962 1967 1972 1977 1982 1987 1992 1997 2002

Year

Ra

infa

ll (

mm

)

15

16

17

18

19

20

21

22

23

Ma

xim

um

te

mp

era

ture

(C

)

Murray-Darling Basin – April-September rainfall and maximum temperature 1952 - 2002


Water Storages LevelsGladstone and Gold Coast water restriction in place for number of years now.

December: 21 NSWtowns required water carting

Canberra: Firstwater restrictionssince 1966

Melbourne: First water restrictionssince 1982

Follows a 6-year dry spell in southernAustralia

Gladstone

Gold Coast


Natural Annual Flows into Hume Weir (MDBC)

0

2,000,000

4,000,000

6,000,000

8,000,000

10,000,000

12,000,000

14,000,000

1891 1911 1931 1951 1971 1991

Year

Ann

ual F

low

Lowest 4-year period

on recordNatural Annual Flows into Hume Weir (MDBC)


CSIRO Mk3 AGCM Model• Atmosphere

Grid: T63 (1.88o x 1.88o) 18 levels - hybrid ,p with top at 4.5 hPa (~ 36 km)

Semi-Lagrangian moisture transport

UKMO convection (Gregory & Rowntree)

Liquid water clouds (Rotstayn)

• Land surface Soil model - 6 levels

Temperature, water, ice9 soil types

13 land surface and/or vegetation typesSnow-cover model - 3 layers


CSIRO T63/L18 C20C Experiments

• 5 runs: 1949-2003 SST only• 5 runs: 1961-2003 SST and O3

• 5 runs: 1871-2003 SST only• 5 runs: 1871-2003 SST and solar• 5 runs: 1871-2003 SST, solar and CO2

• 5 runs: 1961-2003 SST, solar, CO2 and O3

• 10 runs: 1949-2003 SST, solar, CO2 and O3 in progress

•


Ozone scenario used in CSIRO AGCM simulations

Observations


Ozone recovery projections


Trends in the Southern Hemisphere annular mode in CSIRO Mk2 transient greenhouse runs reverse when the forcing is stabilised. However, this may take several centuries to take place, depending on the emission scenario used.


CSIRO Mk2 Model, Warming Run


CSIRO Mk2 Model, greenhouse run, showing when the AAO goes uprainfall over SWWA decreases, and vice-versa!

Correlation between the AAO and JJA Rainfall


ANN

MAM

DJF JJA

SON

MSLP change in Mk 3


SWWA RainfallSAM

Winter

Spring


Queensland summer rainfall in GHG scenario with Mk3 coupled AGCM


Model Simulated and NCEP R1 JJA MSLP for JJA 1971-2000


Annual Trend NCEP PSL 1971-2001


Model Simulated MSLP difference for JJA 1971-2000










Trends in the SAM: impact on Antarctic temperaturesTrends in the SAM: impact on Antarctic temperatures

Time series of the SAM based on NCEP-NCARZ850 data and Z500 Antarctic radiosonde data(Thompson & Solomon, 2002). Data are normal-ized to 1968-98.Source: Todd Mitchell (http://tao.atmos.washington.edu/aao/)

Regression of Antarctic surfacetemperature anomalies vs unitchange in the SAM, 1982-98.(Kwok & Comiso, 2002)


Response of surface temperature

Warming over Antarctic Peninsula and cooling over Eastern Antarctica seen in both CO2 and O3 forcing, but stronger in O3


Response of surface temperature

DJF 1993-2003 cooling over Eastern Australia due to CO2 effect and an opposite effect when O3 is added


Response of precipitation

NDJFM 1993-2003 wetter over Eastern Australia due to CO2 effect and an opposite effect when O3 is added, follows from surface temperature changes in previous slide. SPCZ changes


Response of JJA precipitation

JJA 1993-2003 weaker Asian monsoon due to CO2 effect and an lesser effect when O3 case is considered. The area of precipitation decrease/increase in Southern Ocean consistant with AAO trend is visible


Summer Rainfall Difference between1961-1994 and 1995-2002

Observed Difference SSTs only

SSTs + Solar + CO2 SSTs + Solar + CO2 + Ozone


Observed Difference SSTs only

SSTs + Solar + CO2 SSTs + Solar + CO2 + Ozone

Summer Rainfall Difference between1961-2003 and 1998/9-2000/1


Conclusions

Both CO2 and O3 forcing have contributed to pressure changes in Southern Hemisphere

These changes are similar to SH Annular Mode

The model has rather weak response to a radiative forcing when 30-40 years long ensemble mean climatology climatology is compared

Model show some systematic response in surface air temperature and precipitation, significance of which needs to be assessed

At regional scale contribution of CO2 and O3 to simulated rainfall anomalies is positive

Documents

C20C Workshop Trieste, Italy 19 – 23 April 2004 The impact of stratospheric ozone depletion and CO 2 on AAO trends and regional climate change at surface