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

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

South East Queensland (SEQ) Rainfall Summer rainfall trends 1950-2008 CSIRO.

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.

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 1950-2008 Correlation between summer SEQ rainfall and NCEP/NCAR Reanalysis mean sea level pressure 1950-2008 CSIRO.

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.

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.

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

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.

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.

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

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 1921-1950: 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.

Thank you CSIRO Marine and Atmospheric Research Peter van Rensch Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au CSIRO Marine and Atmospheric Research Peter van Rensch Email: peter.vanrensch@csiro.au Web: www.cmar.csiro.au Thank you