The mean meridional circulation (MMC) — representing the planetary-scale movement of energy and air between the equator and poles — is expected to change in a warmer world. Many observational and model studies clearly identify an expansion of the tropical Hadley Cell along with a poleward contraction of the mid-latitude westerly wind belts in response to anthropogenic climate change.
Over recent decades, the state of Victoria in south-east Australia has seen a decrease in cool season rainfall with a coincident increase in rainfall over the warm part of the year in some regions. In this study we set out to examine how these global and regional changes are linked.
We use a daily representation of the zonally-averaged MMC from 1979–2017 to examine this global pattern across multiple time scales. First, we employ clustering techniques to group similar MMC patterns. These clusters clearly identify the seasonal progression of the MMC over the course of the year.
Using the timing and length of each cluster, we then explore changes in the timing of the annual cycle as opposed to more traditional seasonal or monthly averages, offering a unique perspective on the changing circulation. Preliminary results suggest that the summer and winter-type clusters are persisting for longer now than they did in the early part of the record, at the expense of the transition seasons.
Next, we use the same approach to split cool and warm season clusters into 'sub-clusters', allowing for the identification of subtle variations in the circulation patterns. Using frequency analysis of the sub-cluster types, we can identify changes to the MMC that occur within a given season. These results can be combined with datasets of global synoptic systems and surface conditions to identify any changing relationships between the MMC and synoptic variability and the subsequent impact on Victorian rainfall.