But it's important to remember that the observed increasing trend in positive SAM phases mainly only applies to summer and not all the year round.
I've read that there's been a decreasing trend in winter rainfall over southern Australia as a result of the belt of westerly winds and associated rain bearing synoptic systems (cold fronts and mid latitude depressions) tending to be moving further south over the last few decades, and therefore dropping more of their rain on the southern ocean and less on the mainland, as the result of a combination of (increasing atmospheric levels of CO₂ and other greenhouse gases) and (the depletion of stratospheric ozone due to CFCs). My understanding has been that the SAM is an index which indicates the latitude of this belt of westerly winds and hence of rain bearing systems, with more positive values of SAM indicating higher (more southerly) latitudes of this belt. If my understanding is correct, then wouldn't that mean that the SAM has been in an increasing trend over the last few decades in winter as well as in summer? What am I missing here?