Milking the Weather - December 2010 Edition
Seasonal update and outlook
Summary as of 24 November, 2010
La Niña persists in the Pacific
- La Niña conditions have weakened slightly but remain firm across the tropical Pacific.
- The 2010 La Niña event has contributed to Australia’s wettest August to October period on record.
- Night time temperatures during La Niña periods are historically warmer than average.
- Long-range models suggest that this La Niña event will persist through the southern hemisphere summer and into the first quarter of 2011.
- The Indian Ocean Dipole (IOD) has been negative (Figure 2) for spring, but is now its in normal cycle of decay in the late spring/early summer.
Indian Ocean Dipole this season
During this winter and spring period we have experienced a La Niña, and an Indian Ocean Dipole (IOD) negative event. The last time this occurred was in 1975. Both of these events are associated with an increased chance of winter/spring rainfall for Victoria, and have been responsible for the wet conditions over the last few months.
The IOD is related to small changes in sea surface temperature (SST) in the Indian Ocean off the Sumatran and Kenyan coasts (see Figure 2). Changes in SST in this region influence the moisture source for the formation of cloud bands that stretch down to Victoria.
This spring there has been a negative IOD. Warmer than average SSTs in the northeast Indian Ocean has lead to increased the cloud band formation across Australia, resulting from the negative IOD. This has allowed moist tropical air to be drawn from the north west bringing rain. A negative IOD generally develops during May and June, peaking from August through to October and will then rapidly decay. This years IOD has recently decayed in November. The last IOD negative event was in 1992.
Figure 1 and Figure 3 show the rainfall for winter/spring for 2010 and 1975. Both show increased rainfall for Victoria and through central Australia, brought about by La Niña and IOD negative conditions.
