Many of you know that I'm a fan of MILD weather. Well, I don't want to risk jinxing it, but the ENSO report at the Australian Bureau of Meteorology is starting to show another slight movement into a La Niña direction.
For those of you who don't know, El Niño is a time where a great amount of rainfall and storms hits central America, and also the West coast of the USA. You may not realise it but El Niño causes droughts and low rainfall here in Australia.
An El Niño event is particularly bad for Australian farmers. A sustained event will lead to long-term drought, which obviously leads to lower crop yields. El Niño also reduces rainfall on the coast, which means that dam levels tend to drop.
La Niña has the opposite effect - less storms in America and more rain in Australia, particularly the Eastern half.
Many Sydney residents will remember the Giardia and Cryptosporidium warnings issued by Sydney Water many years ago. This was due to high rainfall in the Warragamba Dam catchment area which raised dam levels from about 50% to 80% in a matter of weeks. The huge amount of water pouring into Lake Burragorang led to a disturbance in silt at the bottom of the reservoir, and made these parasites more likely to enter the water supply. This occurred during 1997 (I think) during a particularly wet winter. If you check out the stats, you'll find that there was a sustained La Niña event occurring at the time.
Where I live on the East coast of Australia, our rainfall is mainly tropical - that is, most of our rainfall comes during the summer. But the further you go inland (West) the less that rainfall is during the summer months.
La Niña, because of the warmer waters to Australia's north, allows the formation of low pressure systems in Central Australia during the Southern winter (June-August). This means more winter rain, especially in inland areas. Sometimes this weather pattern has been severe, and major flooding has occurred. If you check the times that Eastern Australia has had severe inland flooding, you'll find that it corresponds with a La Niña event.
La Niña also makes tropical cyclones (hurricanes) in Australia's north more likely during the Summer (December-February). Although cyclones appear every year, during La Niña they are more likely, due to lower atmospheric pressure.
The ENSO wrap-up page at the BOM has a graph called the "30 day moving SOI", based on the Southern Oscillation Index. Anything below zero appears to favour El Niño, while any sustained reading below -10 indicates that an El Niño event is occurring. Above zero, conditions favour La Niña, and any sustained reading above 5 indicates a La Niña event. At the present moment in time, conditions are favourable for La Niña, but only sustained readings above zero will make me happy.
You can read all about these things at Wikipedia.