Seasonal Climate Outlook Message for July to October 2008

The bottom line

22 July 2008

MJO can influence winter rains

The MJO has been better organised, with a stronger signal over the past week or so. The area of active MJO convection is now located over the eastern Indian Ocean, or Phase 3. The MJO should continue to propagate slowly east over the next couple of weeks, although the current moderate strength may tail off to a weaker signal.

This passage of the MJO has contributed to the progress of the monsoon in India. The monsoon penetrated to the western extent of India early July, about a week earlier than normal.

At this time of year the MJO usually tracks north, towards the South China Sea, as it approaches the Maritime Continent (Phase 4). However there is still a potential for the passage of the MJO to influence Australian rainfall.

During Phase 4 southern and south western Queensland can expect increased chances of rain. While in Phases 4 and 5 south eastern Australia has an enhanced probability of rain. To explore seasonal MJO rainfall impacts go to

http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/RMM/composites/ind ex.htm

The Madden Julian Oscillation (MJO) is named after the two scientists who located fluctuations in station records, at more or less forty days. The MJO is also colloquially known as the forty day wave, but can have an interval of between 20 and 90 days.

The MJO is a tropical atmospheric disturbance. It's first obvious in the western Indian Ocean, near Africa. While the passage of an MJO event is restricted to the tropics it affects areas well out of the tropics via teleconnections. This means that systems like the MJO interact with other weather and climate systems. The MJO influences and is influenced by these other factors.

Based on statistical tests of rainfall distribution we can tell which MJO Phase will enhance or suppress rain where, and how much. The MJO tends to have the strongest impact on Australian rainfall over summer.

The Bureau of Meteorology have an index for following the MJO. This MJO Phase Space Diagram (also known as the 'spiderweb') shows a square divided into eight parts: each octant (Phase) represents the location and strength of the MJO. When the MJO signal strength is low this is indicated by the plot line tracking into the central circle of the diagram. The MJO is in each octant an average of 6 to 12 days.

The 30 day average of the SOI as of 22nd July was plus 3.0. The SOI has been fluctuating, however the biggest improvement in seasonal conditions across eastern Australia especially leading into spring and summer, will be if the SOI remains in consistently positive values for a couple of months at least.