Finding anthropogenic global warming (AGW) shapes in the fog of variability is a major challenge, simply because natural climate variability is large. And then the problem becomes attributing those changes to a climate mechanism in an interlinked dynamic climate system.
One of those climate detective stories taking some intriguing twists and turns is the rainfall decline in Western Australia which has been the impetus for the Indian Ocean Climate Initiative (IOCI). Their web site explains:
“In southwest WA, a drying trend has been observed … The rainfall decline has been most apparent in late autumn and early winter, with a major drop in rainfall totals occurring in the 1970s, and possibly another more recently in the 1990s. Averaged across southwest WA, a step decrease in total annual rainfall of almost 10% was seen in the mid-1970s, though individual locations would have experienced a greater decrease.”
Antarctica, in particular both stratospheric ozone decline and tropospheric greenhouse forcing over Antarctica impacting a positively trending southern annular mode (SAM) with the strengthening circumpolar vortex, have been the focus of explanations for the rainfall decline (see for example Thompson and Solomon, 2002, in “Interpretation of recent southern hemisphere climate change”).
For a description of the SAM – http://www.atmos.colostate.edu/ao/introduction.html
Shindell and Schmidt (2004) claim that both Antarctic ozone depletion and increasing greenhouses gases have contributed to these trends. And with an interactive climate model including the stratosphere and both composition changes reproduces the vertical structure and seasonality of observed trends.
Arblaster and Meehl (2006) asserted that a recovery in the ozone hole would be matched by inexorable increase in greenhouse gases:
“Although stratospheric ozone losses are expected to stabilize and eventually recover to preindustrial levels over the course of the twenty-first century, these results show that increasing greenhouse gases will continue to intensify the polar vortex throughout the twenty-first century, but that radiative forcing will cause widespread temperature increases over the entire Southern Hemisphere.”
Fundamentally, the SAM is an expression of the meridional pressure gradient between the sub-Antarctic and middle latitudes. The mode has been increasing towards its positive polarity (in the annual, summer, and autumn means) since the late 1960s, leading to lower (higher) surface pressures over Antarctica (southern mid-latitudes) (Polar Meteorology Group). Although Arblaster et al (2010) in a modelling exercise now suggest that ozone hole recovery will indeed lead to SAM reversing trends in coming “decades”.
In an apparent vindication of the unusual nature of the SWWA rainfall decline, van Ommen reported in Nature Geoscience (2010) a correlation between Law Dome snowfall and SW WA rainfall. (Law Dome – 66°44′S 112°50′E, a large ice dome which rises to 1,395 m directly south of Cape Poinsett, Antarctica) And that the SW WA drought sequence was unusual as being the largest anomaly in the last 750 years and outside the field of variability.
So back in 2006, the trail seemed to be closing on anthropogenic climate change in the Antarctic and associated changes in southern hemisphere circulation (SAM) as being responsible for the decline in SW WA winter rainfall.
Cai and Cowan (2006) in GRL stated:
“It is not clear why the reduction occurs in the winter months, when the observed SAM trend is weak, but not in the summer months, when the observed SAM trend is strongest. It is also not clear to what extent the reduction is attributable to anthropogenic forcing and is congruent with the SAM. Using IPCC AR4 20th century model experiments and available observations, we show that in winter the mid-latitude center-of-action of the SAM is closest to SWWA latitudes, compared to other seasons. As a result, there is a statistically significant relationship between the SAM and SWWA rainfall in winter, but not in other seasons. The observed winter SAM trend, though not statistically significant, accounts for two thirds of the observed winter rainfall reduction.”
At a 2007 conference Cowan reported ” Interannual / multidecadal variability linked to the SAM” and “about ½ of rainfall decline attributable to anthropogenic forcings”.
However, in 2007, Meneghini et al reported:
“The AOIR (Antarctic Oscillation Index regional version) accounts for more of the winter rainfall variability in southwest Western Australia, southern South Australia, western and southern Victoria, and western Tasmania than the Southern Oscillation Index. Overall, our results suggest that changes in the SAM may be partly responsible for the current decline in winter rainfall in southern South Australia, Victoria, and Tasmania, but not the long-term decline in southwest Western Australian winter rainfall”.
Perhaps the downfall of what seemed to be a good story on SAM came from Feng et al (2010) in Journal of Climate. There landmark paper on a new index, the southwest Australian circulation, begins:
“Previous studies have raised the possibility that the recent decline in winter rainfall over southwest Western Australia (SWWA) is related to the concurrent upward trend in the southern annular mode (SAM). On the basis of an analysis of 60-yr (1948–2007) reanalysis and observed data, the authors suggest that the apparent inverse relationship between the SAM and SWWA winter rainfall (SWR) is caused by a single extreme year—1964. It is shown that both the negative and positive phases of the SAM have little impact on SWR in the case that data for 1964 are excluded from the analysis. In addition, for periods prior to and after 1964 in the case that data for 1964 are excluded, the apparent relationship between the SAM and SWR becomes insignificant, and the circulation anomalies with respect to SWR appear to be an SAM-like pattern for which the anomalies at high latitudes are not significant. The result indicates that the SAM does not significantly influence the winter rainfall over SWWA. Instead, the variation of SWR would be more closely linked to the variability in regional circulations…
Feng et al (2010) continue:
“It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is called the southwest Australian circulation (SWAC) because of its several distinct features characterizing a monsoonal circulation: the seasonal reversal of winds, alternate wet and dry seasons, and an evident land–sea thermal contrast. The seasonal march of the SWAC in extended winter (May–October) is demonstrated by pentad data. An index based on the dynamics’ normalized seasonality was introduced to describe the behavior and variation of the winter SWAC. It is found that the winter rainfall over SWWA has a significant positive correlation with the SWAC index in both early (May–July) and late (August–October) winter. In weaker winter SWAC years, there is an anticyclonic anomaly over the southern Indian Ocean resulting in weaker westerlies and northerlies, which are not favorable for more rainfall over SWWA, and the opposite combination is true in the stronger winter SWAC years. The SWAC explains not only a large portion of the interannual variability of SWWA rainfall in both early and late winter but also the long-term drying trend over SWWA in early winter.
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 southern annular mode (SAM), El Niño–Southern Oscillation (ENSO), Indian Ocean dipole (IOD), and ENSO Modoki (EM). The result offers qualified support for the argument that the monsoon-like circulation may contribute to the rainfall decline in early winter over SWWA. The external forcing of the SWAC is also explored in this study”.
And what more do we know about what might become of the SWAC?
IPCC climate change simulations diagnose that the Hadley circulation will weaken and expand polewards as a result of climate change. Associated with this is an expansion of the subtropical dry zone. Additional work by Ummenhofer (2008) has shown changes in the Indian Ocean SST may provide another forcing to SWAC.
The potential for the SWAC to become a seasonal climate forecasting tool for SW WA remains an exciting possibility.
http://www.lasg.ac.cn/staff/ljp/data-NAM-SAM-NAO/SAM(AAO).htm SAM index
http://polarmet.osu.edu/acd/sam/sam_recon.html Polar Meteorology Group reconstructions
http://www.ioci.org.au/pdf/Fact%20Sheet%204.pdf IOCI on rainfall decline
http://www.agu.org/pubs/crossref/2004…/2004GL020724.shtml Shindell and Smith
http://www.nature.com/ngeo/journal/v3/n4/full/ngeo761.html#a1 van Ommen
http://www.agu.org/pubs/crossref/2006…/2006GL028037.shtml Cai and Cowan 2006
http://www.clw.csiro.au/conferences/GICC/cowan.pdf Cowan 2007
http://www.earthsci.unimelb.edu.au/~ihs/publication_pdfs/Meneghini%2BSimmonds%2BSmith_2007_IJC_27_109-121.pdf Meneghini et al
http://journals.ametsoc.org/doi/abs/10.1175/2010JCLI3667.1 Feng on SAM in SWWA 2010
http://www.cmis.csiro.au/Yun.Li/Papers/reprints/2010_JCLI_SWWA%20Monsoon_Feng_Li_Li.pdf Feng 2010 on the SWAC
http://www.csiro.au/news/New-climate-index-solves-south-west-WA-rainfall-riddle.html CSIRO on SWAC