Aerosol- and greenhouse gas-induced changes in summer rainfall and circulation in the Australasian region: a study using single-forcing climate simulations 1Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Vic, Australia
23 Jul 2012
2Queensland Climate Change Centre of Excellence, Dutton Park, Qld, Australia
Received: 27 Jan 2012 – Published in Atmos. Chem. Phys. Discuss.: 15 Feb 2012Abstract. We use a coupled atmosphere-ocean global climate model (CSIRO-Mk3.6) to
investigate the drivers of trends in summer rainfall and circulation in the
vicinity of northern Australia. As part of the Coupled Model Intercomparison
Project Phase 5 (CMIP5), we perform a 10-member 21st century ensemble driven
by Representative Concentration Pathway 4.5 (RCP4.5). To investigate the
roles of different forcing agents, we also perform multiple 10-member
ensembles of historical climate change, which are analysed for the period
1951–2010. The historical runs include ensembles driven by "all forcings"
(HIST), all forcings except anthropogenic aerosols (NO_AA) and forcing only
from long-lived greenhouse gases (GHGAS). Anthropogenic aerosol-induced
effects in a warming climate are calculated from the difference of HIST minus
Revised: 17 May 2012 – Accepted: 01 Jul 2012 – Published: 23 Jul 2012
CSIRO-Mk3.6 simulates a strong summer rainfall decrease over north-western
Australia (NWA) in RCP4.5, whereas simulated trends in HIST are weakly
positive (but insignificant) during 1951–2010. The weak rainfall trends in
HIST are due to compensating effects of different forcing agents: there is a
significant decrease in GHGAS, offset by an aerosol-induced increase.
Observations show a significant increase of summer rainfall over NWA during
the last few decades. The large magnitude of the observed NWA rainfall trend
is not captured by 440 unforced 60-yr trends calculated from a 500-yr
pre-industrial control run, even though the model's decadal variability
appears to be realistic. This suggests that the observed trend includes a
forced component, despite the fact that the model does not simulate the
magnitude of the observed rainfall increase in response to "all forcings"
We investigate the mechanism of simulated and observed NWA rainfall changes
by exploring changes in circulation over the Indo-Pacific region. The key
circulation feature associated with the rainfall increase in reanalyses is a
lower-tropospheric cyclonic circulation trend off the coast of NWA, which
enhances the monsoonal flow. The model shows an aerosol-induced cyclonic
circulation trend off the coast of NWA in HIST minus NO_AA, whereas GHGAS
shows an anticyclonic circulation trend. This explains why the
aerosol-induced effect is an increase of rainfall over NWA, and the
greenhouse gas-induced effect is of opposite sign.
Possible explanations for the cyclonic (anticyclonic) circulation trend in
HIST minus NO_AA (GHGAS) involve changes in the Walker circulation or the
local Hadley circulation. In either case, a plausible atmospheric mechanism
is that the circulation anomaly is a Rossby wave response to convective
heating anomalies south of the Equator. We also discuss the possible role of
air-sea interactions, e.g. an increase (decrease) of sea-surface
temperatures off the coast of NWA in HIST minus NO_AA (GHGAS). Further
research is needed to better understand the mechanisms and the extent to
which these are model-dependent.
In summary, our results suggest that anthropogenic aerosols may have
"masked" greenhouse gas-induced changes in rainfall over NWA and in
circulation over the wider Indo-Pacific region. Due to the opposing effects
of greenhouse gases and anthropogenic aerosols, future trends may be very
different from trends observed over the last few decades.
Citation: Rotstayn, L. D., Jeffrey, S. J., Collier, M. A., Dravitzki, S. M., Hirst, A. C., Syktus, J. I., and Wong, K. K.: Aerosol- and greenhouse gas-induced changes in summer rainfall and circulation in the Australasian region: a study using single-forcing climate simulations, Atmos. Chem. Phys., 12, 6377-6404, doi:10.5194/acp-12-6377-2012, 2012.