1. Mechanisms behind the Southeast Queensland Summer Rainfall Reduction
Peter van Rensch,
Wenju Cai, Tim Cowan and Arnold Sullivan

2. Outline Trend in southeast Queensland summer rainfall since 1950
IPCC AR4 model representations of the rainfall trends
Influence of various anthropogenic forcing factors
Impact of climate variability on rainfall trends
ENSO, ENSO Modoki, IOD, SAM
The asymmetric nature of ENSO and ENSO Modoki
The changing ENSO/ENSO Modoki teleconnection
CSIRO. 2

3. South East Queensland (SEQ) Rainfall
Summer rainfall trends
CSIRO.

4. IPCC model output: 20th century, trend over 1950-1999
Summer
Most models forced with climate change do not capture the observed SEQ rainfall trend
No distinct difference between ozone depletion and no ozone depletion models
Targeted models show increased aerosols would increase rainfall over SEQ
Suggests the rainfall reduction is not due to climate change or anthropogenic forcing
CSIRO.

5. Possible causes of SEQ rainfall variability
El Nino/ Southern Oscillation (ENSO) shows a strong signal
East Indian Ocean (although this is probably through ENSO)
A Southern Annular Mode signal is not evident
Correlation between summer SEQ rainfall and HadISST
Correlation between summer SEQ rainfall and NCEP/NCAR Reanalysis mean sea level pressure
CSIRO.

6. Coherence between SEQ rainfall and ENSO
An asymmetric relationship is observed
Seasons of high rainfall occur during La Nina conditions
El Nino events show little rainfall response
This is also seen in the Southern Oscillation Index
Same asymmetry occurs during ENSO Modoki
CSIRO.

7. ENSO’s asymmetric relationship with SEQ rainfall
Much of north-eastern Australia is affected by La Nina conditions
Little El Nino relationship with rainfall over much of eastern Australia
Convective systems move south-west during La Nina
No convective system relationship is observed over Australia for El Nino
CSIRO.

8. ENSO Modoki’s asymmetric relationship with SEQ rainfall
Similar results are seen for ENSO Modoki
CSIRO.

9. Interdecadal change in teleconnection (13-year sliding window) as a mechanism for recent rainfall reduction
The relationship between ENSO, ENSO Modoki and SEQ rainfall is changing with time
Modulated by the Interdecadal Pacific Oscillation (IPO)
CSIRO.

10. Changing La Nina-SEQ summer rainfall teleconnection
SEQ rainfall anomaly (mm)
La Nina has a much stronger relationship pre 1980
Eastward shift in convective systems
Consistent with a change in the Walker Circulation
Linear regression using negative NINO3.4 values (La Nina)
CSIRO.

11. Changing La Nina Modoki-SEQ rainfall teleconnection with time
CSIRO.

12. Conclusions
According to current models the decline of SEQ summer rainfall does not appear to be due to climate change or other anthropogenic forcing.
ENSO and ENSO Modoki is a rainfall generating mechanism for SEQ because of the asymmetry in their impacts
The asymmetry seems to have broken-down over the last 30 years, as during : La Nina does not have an impact on rainfall summer totals.
(Paper submitted to Journal of Climate: Asymmetry in ENSO teleconnection, its collapse and impact on southeast Queensland summer rainfall by Wenju Cai, Peter van Rensch, Tim Cowan and Arnold Sullivan)
CSIRO.

13. Thank you CSIRO Marine and Atmospheric Research Peter van Rensch
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