SAM and regional rainfall in IPCC AR4 models: Can anthropogenic forcing account for southwest Western Australian winter rainfall reduction?

Abstract

Winter rainfall over southwest Western Australia (SWWA) has decreased by 20% since the late 1960s. Why has the reduction occurred in the Southern Hemisphere (SH) winter months but not in summer? To what extent is this reduction attributable to anthropogenic forcing and congruent with the Southern Annular Mode (SAM)? Using reanalysis data and the Intergovernmental Panel on Climate Change 4th Assessment Report (IPCC AR4) 20th century model experiments, we show that a SAM‐SWWA relationship exists in winter and not in other seasons. An ensemble result from 71 experiments reveals that anthropogenic forcing contributes to about 50% of the observed rainfall decline. Approximately 70% of the observed trend is congruent with the SAM trend, whereas for the models it is 46%. Our result suggests that other forcing factors must be invoked to fully account for the observed rainfall reduction.

Impact of the Indian Ocean high pressure system on winter precipitation over western and southwestern Australia

“…Several SAM indices are available; we use the index based on Marshall (2003) available from David Thompson's website. This SAM index is close to the index previously used in studies of SWWA precipitation by Cai and Cowan (2006).…”

“…The results clearly show that the unique contributions of IOH pressure and longitude to the variability of SWWA winter precipitation exceed those of the SOI, IOD and SAM. Cai and Cowan (2006) have studied the relationship between SWWA JJA rainfall and the Southern Annular Mode during 1950-1999 by using a modification of the SAM data of Marshall (2003). They did not use the principal component of the EOF which is commonly considered as the SAM index, but instead considered the inter-annual change in the average sea-level pressure in the EOF pattern and found its correlation with SWWA rainfall to be −0.30 which is statistically significant at the 95 per cent confidence level.…”

“…The Southern Annular Mode (SAM) is the leading empirical orthogonal function of winter sea-level pressure over the southern hemisphere and its principal component is correlated with rainfall over SWWA (Cai et al 2005;Cai and Cowan 2006;Meneghini et al 2006;Hendon et al 2007). …”

A possible cause of decreasing summer rainfall in northeast Australia

“…The rainfall decrease in SWWA has been studied intensely in recent years (e.g. Smith et al, 2000;Cai and Watterson, 2002;IOCI, 2002;Pitman et al, 2004;Cai et al, 2005;Cai and Cowan, 2006;Hope et al, 2006;Meneghini et al, 2007;Feng et al, 2010aFeng et al, , 2010b, as has the increasing trend in NWA (Wardle and Smith, 2004;Rotstayn et al, 2006;Shi et al, 2008aShi et al, , 2008bTaschetto and England, 2008). In contrast, relatively little is known of the cause and mechanism of the decreased NEA rainfall.…”

Multi-century cool- and warm-season rainfall reconstructions for Australia's major climatic regions

“…Our analysis showed that the most recent decline in rainfall in the Southern Slopes is within the range of variability given by the reconstruction. The cool-season decline in the south is often associated with the intensification of the subtropical ridge (Timbal and Drosdowsky, 2012;Timbal et al, 2006), changes in large-scale atmospheric circulation such as the Indian Ocean (Ummenhofer et al, 2011), and the observed upward trend in SAM (Cai and Cowan, 2006). All of these processes are significantly linked to interannual rainfall variability in the south (Fig.…”