Milking the Weather - June 2011 Edition
What has SAM got to bark about?
by Daniel Watts, DPI Ellinbank
The Southern Annular Mode, known as SAM is one of the four climate drivers that influence Victoria’s rainfall. SAM influences the strength and frequency of cold fronts over Victoria. SAM is a measure of the strength in the westerly winds that circulate around Antarctica.
When SAM is in a positive phase the polar westerly winds speed up causing low pressure systems and cold fronts to contract towards the South Pole. Generally a positive mode will result in reduced rainfall for the southern fringe of Australia during winter (Figure 1).
Figure 1. A positive phase SAM results in fewer rain bearing frontal systems across the southern coastal region of Australia.
Although a positive phase SAM (more contracted) reduces the chance of rainfall in winter for southern Victoria. It can increase the chance of rainfall in summer, which is shown in Figure 2.
Figure 2. Average impact of a positive phase SAM on rainfall in Australia. In winter it shows a drier anomaly* (mm/day), compared to a wetter anomaly in summer (based on data from 1979-2005). Hendon et al. 2007.
In a negative phase the polar westerly winds slow down and allow low pressure systems to expand northward increasing frontal activity in southern Victoria.
Figure 3. A negative phase SAM involves low pressure systems circulating closer to southern Australia. This results in more cold fronts sweeping up into southern Victoria increasing the chance of rainfall.
SAM is an important driver of winter rainfall for southern Victoria. However over the past 30 years SAM has stayed in a positive phase more frequently resulting in fewer low pressure systems and cold fronts pushing up into Victoria. This is thought to have been a contributing factor to the reduction in rainfall over southeastern Australia over this period.
SAM can change at a rapid rate, therefore its movement can only be predicted up to two weeks out. To keep an eye on the movements of SAM look at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao_index_mrf.shtml
Researchers from Bureau of Meteorology and CSIRO are continuing to find our more about SAM’s behaviour.
|
To view the climate dog animations describing Victoria’s climate drivers and to see SAM in action visit: www.dpi.vic.gov.au/climaterisk
|
