Major climate drivers INDIAN OCEAN sea surface temperatures PACIFIC OCEAN sea surface temperatures SOUTHERN ANNULAR MODE – north-south shifting of southerly cold fronts
General circulation patterns (BoM 2003) The whole atmosphere is driven by warming at the tropics Dry, descending air produces surface “high pressure” cells at mid latitudes “Low pressure” cells are associated with rising, unstable air – due to warming or converging air streams “Highs” and “lows” track more southerly or northerly as the southern hemisphere tilts towards the sun in summer and away in winter. Warm sea surface temperatures north of Australia promote evaporation and moisture availability for potential rainfall
Sea surface temperatures Sea surface temperatures to the north of Australia are critical to moisture availability and winter/ spring rainfall in SA. Warmer ocean temperatures north of Australia (associated with ‘La Nina’ and ‘Negative Indian Ocean Dipole’ conditions) drive increased evaporation and moisture availability in the atmosphere. Additional conditions, such as the presence of low pressure systems which can drag moisture to the south over SA are then required to actually deliver rainfall. Cooler ocean temperatures north of Australia (ie. ‘El Nino’ and ‘Positive Indian Ocean Dipole’ conditions) suppress winter/ spring rainfall.
Southern Annular Mode North-south shifting of the path of circum-polar cold fronts (over and above normal seasonal shifts). Cold fronts (associated with low pressure systems) are important sources of winter rainfall in South Australia. Climate scientists are concerned about the stronger pressures observed in the Sub-tropical Ridge (the band of high pressure / descending air which falls across Australia in the mid latitudes). Stronger high pressure systems across southern Australia tend to push low pressure frontal systems further south. And systems that push northward become weakened. This appears to be connected to global warming.
For detailed info – see BoM web page BoM > Water and the Land > Australian climate influences http://www.bom.gov.au/watl/about-weather-and-climate/australian-climate-influences.html
“Climate dogs” Animations to communicate climate drivers for southern Australia Acknowledgement: Department of Primary Industries, Victoria