"El Nino and La Nina occur due to atmosphere and ocean interactions, "said Dr Agus Santoso, senior associate researcher at the UNSW Center for Climate Change Research.
From a La Nina event, strong trade winds blow westward across the Pacific Ocean, pushing warm surface waters towards Asia and seas north of Australia. increased precipitation in northern and eastern Australia.
The second year in a row La Nina announced on Tuesday is the first time consecutive events occur in a decade - since 2010-11 and 2011-12.
But it is not uncommon for multi-year La Nina events to occur, according to BoM For example, La Nina a touche three consecutive years from fall 1998 to fall 2001. What does this mean for this summer?
According to BoM, the six wettest winter-spring periods on record in eastern Australia all occurred during the La Nina years.
The weather event is also linked to cooler daytime temperatures. Australia had a weak La Nina episode last summer, resulting in the coolest summer in nine years and the wettest in four years, with 29% more rain than average.
Despite this, 2020 was the fourth warmest year on record in Australia.
The current La Nina is probably not " as strong "as the 2011-12 event, Santoso said.
He expects" conditions wetter than normal this time around, but hopefully they won't have as much of an impact [an event] "as they did 10 years ago.
The La Nina current could also be weaker than that of last summer. The strength of La Nina is determined by the sea surface temperature deviation from the norm. As a general rule, the stronger an event, the the more precipitation .
However, another climatic factor, called the southern ring mode (Sam), is now in a positive phase, which is associated with increased precipitation over the south -est of Australia.
"If you only have a La Nina, there will be less impact than if you have a La Nina plus a positive phase of Sam, ”Santoso said.
La Nina is also associated with greater cyclonic activity. BoM estimated in Octoberthat there were two chances out of three that the Australian region experiences more than the seasonal average of 11 cyclones.
The weather office confirmed Monday on the first tropical cyclone of the season had formed in the Australian region, which should weaken without touching land.
BoM system upgrade El Nino-Southern Oscillation from the La Nina watch to the La Nina alert in early October, which means there was a 70% chance there was a La Nina training.
The US National Oceanic and Atmospheric Administration hadalready announced the return of La Nina on October 14.
The climate model results in more dry days over the southern third of the United States, increasing the likelihood of drought in the southwest.
"When you have a La Nina ... humidity in the eastern part of the Pacific decreases," Santoso said.
La Nina may also generate higher monsoon rainfall over the Indian subcontinent, Santoso said, increasing the risk of flooding. What about the effect of climate change?
Climate change has an impact on La Nina events, Santoso said. "Particularly if you get warmer sea surface temperatures around Australia, this can improve convective activities which canproduce storms. "
A recent review Santoso co-wrote concluded that the frequency of El Nino and La Nina events would increase under the emission scenarios of greenhouse gases from the status quo.
Unless emissions are reduced, El Nino and La Nina events are expected to decrease from 5.6 events per century in the present to 8.9 and 8.3 events per century in the future, respectively.