A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia

Juan Feng and Jianping LiLASG, IAP, Chinese Academy of Sciences

Yun LiCSIRO Mathematics, Informatics and Statistics

CSIRO Climate Adaptation Flagship

IMSC 2010, Edinburgh12-17 July 2010

Acknowledgement

•Australian-China Climate Change Partnership Program-Department of Climate Change

•Indian Ocean Climate Initiative

•CSIRO Climate Adaptation Flagship

Outline

• Motivations

• Existence of a monsoon-like southwest Western Australia circulation (SWAC)

• Relationship between the winter SWAC and SWWA winter rainfall

• The impacts of large-scale circulations on the SWAC-SWWA winter rainfall relationship

• Possible drivers of the SWAC

• Conclusion

Motivation: A riddle that has puzzled researchers for decades

• ‘Why has south-west Western Australia experienced dramatic declines in rainfall since the 1970s?’

• Possible factors include

1. Natural variability (Cai et al. 2005) 2. Land-use changes (Pitman et al. 2004) 3. Anthropogenic contribution through ozone depletion and carbon

dioxide increase (Cai and Cowan 2006, 2007; Karoly 2003)4. Trend in the Southern Annular Modes (Cai and Cowan 2006, 2007;

Hendon et al. 2007; Li et al. 2005)5. Indian Ocean SST (England et al. 2006; Ummenhofer et al. 2008)

• SWWA rainfall and Antarctic snowfall are reversely linked (van Ommen and Morgan 2010)

Aim: investigate regional circulations and rainfall decline over SWWA

Data

• Observed rainfall and 0.25 x0.25 gird rainfall (National Centre of the Australian Bureau of Meteorology, Lo et al. 2007)

• NCEP-NCAR reanalysis data (Kalnay et al 1997)

• The 40-year ECMWF Reanalysis (ERA-40) data (Uppala et al. 2005): as a comparison to verify results from NCEP-NCAR reanalysis data

• CMAP pentad mean rainfall data (Xie and Arkin 1997)

• IERSST (Smith and Reynolds 2004)

• Nino 3, SAM, IOD and ENSO-Modoki indices

Existence of a Monsoon-Like Southwest Australia Circulation (SWAC)

Evidence 1: The alternating of wet and dry seasons over SWWA

Figure 1: The climatology rainfall distribution (mm/month) in Australia (a) for January and (b) for July. (c) monthly SWWA rainfall climatology, (d) SWWA rainfall departures from the annual mean. The area SWWA of the line joining 30° S, 115° E and 35° S, 120° E is scaled in the red line.

a b c

d

Figure 1: The climatology rainfall distribution (mm/month) in Australia (a) for January and (b) for July. (c) monthly SWWA rainfall climatology, (d) SWWA rainfall departures from the annual mean. The area SWWA of the line joining 30° S, 115° E and 35° S, 120° E is scaled in the red line.

a ba b c

d

c

d

Summer Winter

Existence of a Monsoon-Like Southwest Australia Circulation (SWAC)

Evidence 2: there is a circulation system with a strong seasonal variation controlling the climate in SWWA

Summer Winter

Existence of a Monsoon-Like Southwest Australia Circulation (SWAC)

U925

U925V925

Vertical Velocity at 925 hPa

Zonal vapour trasport at 925 hPa

Existence of a Monsoon-Like Southwest Australia Circulation (SWAC)

Evidence 3: Strong difference and the reversal thermal gradient between land and sea

SS

T/S

AT

(

C)

SA

T-S

ST

(

C)

Difference SAT-SST (C) in the region (70-130E, 35-25S)

A Monsoon-Like Southwest Australia Circulation (SWAC)

•There exists a monsoon-like Southwest Circulation (SWAC) system over the wider Southwest Australia region.

•The conception and the measurement of monsoon strength might be reasonable to delineate the magnitude and variability of the SWAC.

Seasonal March of Winter SWAC and Rainfall

2 3 4 5 6a

b

1 2 3 4 5 6a

b

2 3 4 5 6a

b

1

Winter SWAC phases:

1. “preonset” (1 May-20 May)2. “active” (21 May-29 June)3. “break” (30 June-14 July) 4. “revival” (15 July-24 July)5. “decay” (25 July-13 Aug) 6. “die out” (14 Aug-17 Sep)

SWWA winter rainfall is strongly linked to the winter SWAC evolution phases.

Seasonal March of Winter SWAC and Rainfall

a b

c d

e

a b

c d

e

Phase difference in winds at 850 hPa Phase difference in Rainfall

(a) Active – Preonset(b) Break – Active(c) Revival – Break(d) Decay – Revival(e) Die-out – Decay

Active - Preonset Break - Active

Revival - Break Decay - Rivival

Die-out - Decay

A New Climate Index Explains Winter Rainfall Decline in SWWA

The SWAC Index (SWACI): derived from a unified Dynamic Normalized Seasonality (DNS) monsoon index (GRL, Li and Zeng 2002)

The SWAC domain: (100°-145E, 35 °- 25S )

1( , ) ( , )( , ) 2 (1)

( , )

nm

nm

V i j V i ji j

V i j

1where ( , ) (m/s) is the January climatology wind vector (1968-1996),

( , ) (m/s) is the wind vectors at grid point in the th month of the th year,

( , ) (m/s) is the mean of January a

nm

V i j

V i j m n

V i j

nd July climatology wind vectors at grid point ( , ). i j

The SWACI is avereged ( , ) over the SWAC domain

ave ( , ) (2)nm

nm nm

i j

MI i j

A New Climate Index Explains Winter Rainfall Decline in SWWA

Cor SWACI & Rain (MJJ) Cor SWACI & Rain (ASO)

MJJ

ASO

Normalized Winter SWACI & SWWA Rain Cor SWACI & Rain (MJJ) Cor SWACI & Rain (ASO)

MJJ

ASO

MJJ

ASO

Normalized Winter SWACI & SWWA Rain

MJJASOR=0.73 (0.73)

R=0.75(0.74)

R=0.68(0.68)

No ENSO

No SAM No SAM

No ENSO

Estimated coefficients of regression models and associated AIC values for the relationship between SWWA rainfall and various normalized

indices

Model Constant SWAC SAM IOD EM Niño3 R2 AIC

MJJ-I 101(1.9) 16.4(1.9) 0.569 317.8

MJJ-II 101(1.9) 16.5(2.2) 0.08(2.2) 0.569 319.8

MJJ-III 101(1.9) 16.6(2.2) 0.2(2.2) 0.8(2.0) 0.570 321.6

MJJ-IV 101(1.9) 16.7(2.2) -0.8(2.3) 1.2(1.9) -3.5(2.0) 0.594 320.3

MJJ-V 101(1.9) 16.2(2.3) -1.0(2.3) 1.7(2.0) -3.8(2.0) -1.8(2.1) 0.600 321.5

ASO-I 65.4(1.5) 10.5(1.5) 0.456 291.4

ASO-II 65.4(1.5) 11.2(1.7) 1.9(1.7) 0.468 292.1

ASO-III 65.4(1.5) 11.5(1.8) 2.0(1.7) 0.7(1.6) 0.470 293.9

ASO-IV 65.4(1.5) 11.3(1.8) 1.9(1.7) 1.0(1.7) -1.0(1.7) 0.474 295.4

ASO-V 65.3(1.5) 11.2(1.8) 1.8(1.7) 1.4(1.9) -1.0(1.7) -1.0(1.8) 0.477 297.1

A New Climate Index Explains Winter Rainfall Decline in SWWA

Late winter (ASO) in 1979-2006

No IODCor SWACI & Rain

No EM No ENSO

Composite SLP and 850 hPa wind differences between high and low SWACI polarities

MJJ ASO

No SAM years

No IOD years

No EM years

No ENSO years

The SWAC is largely Independent of well-known large-scalecirculations.

Possible drivers of the SWAC

Driver 1: Polarward shift of the subtropical high -the strength of the SWAC is related to the location of subtropical high ridge

The stronger SWAC in early and late winter imply the ridge of subtropical high locates to the north of the SWWA region and brings more synoptic rainfall events impinging on this region.

Early winter (MJJ) Late winter (ASO)

Possible drivers of the SWAC

Driver 2: Warming SSTs in equatorial northern Australia and westward of SWWA over the Indian-Ocean

The warming/cooling SSTs westward of SWWA over the Indian-Ocean are associated with the weak/strong SWAC which as a result leads to negative/positive rainfall anomalies over SWWA

Conclusion

• There is a monsoon-like Southwest Western Australia Circulation (SWAC) based on evidences characterizing a monsoonal system.

• SWWA winter rainfall is strongly linked to the winter SWAC evolution phases.

• The well-coupled SWAC-SWWA rainfall relationship seems to be largely independent of the well-known effects of large-scale atmospheric circulations such as the SAM, ENSO, IOD and ENSO and ENSO-Modoki.

• The weakening of the SWAC may contribute to SWWA rainfall decline in early winter.

• A new climate index (SWACI) to explain rainfall decline of SWWA

On going work

• Further investigation using global climate models to confirm whether warming oceans or natural fluctuations are responsible for the weakening SWAC circulation;

• Investigating the linkage between reduction of extreme rainfall and the monsoon-like SWAC;

• Investigating the relationship between the summer SWAC and hot spells and heat waves in SWWA;

• The SWACI will be used to see if it can improve skill in forecasting seasonal rainfall and crop growth potential.

Contact: Yun Li Phone: (08) 93336388Email: Yun.Li@csiro.au; Web:http://www.cmis.csiro.au/Yun.Li/

Thank you and welcome your comments!

Feng, J., J. Li, and Y. Li, 2010: A monsoon-like Southwest Australian circulation and its relation with rainfall in Southwest Western Australia.

Journal of Climate 23, 1334-1353

Web: http://www.cmis.csiro.au/yun.li/paper.htm