La Ni帽a has been declared. Why should we care?
With La Ni帽a prolonging the wet season, there is an increased risk of flooding along the north, east and southeast regions.
Published on the 24 Nov 2021 by Andrea Taschetto, Agus Santoso
It is quite common that La Ni帽a events occur two years in a row, a case that we are seeing now, which means more wet weather in large parts of Australia over summer. Photo: Avalon rock pool in Sydney by Shutterstock.
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The Australian Bureau of Meteorology has just declared a La Ni帽a in the tropical Pacific.
But what is a La Ni帽a?
La Ni帽a is part of a natural phenomenon called the (ENSO) that develops in the tropical Pacific.聽
During a La Ni帽a event, the sea surface in the central Pacific is cooler than average, and the water closer to Australia warmer than usual.
The warm temperatures in the west Pacific and cool in the east intensify the trade winds which blow from South America to Indonesia.
These equatorial winds pile up warm water towards Australia, allowing cool water to upwell from the subsurface in the eastern Pacific.
The opposite generally occurs for El Ni帽o, the warm phase of ENSO.聽 The tropical Pacific swings from La Ni帽a to El Ni帽o in a cycle that happens every 3 to 7 years.
Sea surface temperature anomalies based on NOAA Optimum Interpolation (OI) Version 2 SST data. Graphic: NOAA PSL.
You may recall that we had a La Ni帽a at the end of last year.
Typically, La Ni帽a and El Ni帽o develop during the Southern Hemisphere聽spring, mature at the end of the year in summer, and decay away in autumn.
NSW and QLD experienced heavy rain in March this year at the end of the 2020 La Ni帽a event.
It is quite common that La Ni帽a occurs two years in a row, a case that we are seeing now.
From 22 La Ni帽a appearances from 1958 to 2020, 10 of those developed in the year following a . 聽
A back-to-back El Ni帽o is less common.聽
This is because the equatorial Pacific cools rapidly following an El Ni帽o due to the intense air-sea interactions that act to dissipate the warm El Ni帽o condition.聽
For La Ni帽a, the heat accumulation and the associated air-sea interactions tend to be more sluggish, allowing for another cold event to develop. 聽
What is in store for summer?
La Ni帽a events are generally associated with wet weather in large parts of Australia, particularly in the north, east and southeast regions.
The warm sea surface north and northeast of Australia tends to increase moisture, produce clouds and rainfall to Australia.
The warm oceans around Australia also increase the chances for tropical cyclone development. We saw the first tropical cyclone (Paddy) of the season this week.聽
The wet weather in east Australia has been lingering since winter.
Schematic of La Ni帽a shows a cooler Pacific, stronger trade winds, and warmer than normal water north of Australia increases convection and the chances for rain in Australia. Graphic: Australian Bureau of Meteorology.
After the 2020 La Ni帽a, a negative Indian Ocean Dipole (IOD) developed during winter and contributed to the .
A negative IOD means that ocean temperatures in the east Indian Ocean, close to Australia, are warmer than normal, while ocean temperatures are cooler in the west, closer to Africa.
This pattern tends to enhance rainfall for the southern and south-eastern parts of Australia.
However, the negative IOD has weakened in the past two weeks and is now back to neutral conditions.
While the IOD will not directly affect the next season, it might have helped keep the soil wet and streamflows high since winter and spring.聽
With La Ni帽a prolonging the wet season, the risk of flooding risk is increased.
A third phenomenon
Another contributor is the Southern Annular Mode (SAM) which has been in its positive phase since mid-October.
The SAM is a shift in the location of storm track in the mid-latitudes.
While a positive SAM means that storms are shifted poleward, it also enhances onshore winds to east Australia and thus is generally associated with increased rainfall in the east and southeast during spring and summer.
So, this year we experienced a combination of climate phenomena that are conducive to extreme rain over large parts of Australia.
What about the future?
Climate models are a useful tool to understand the processes behind climate variability and climate change.
Most climate models that represent ENSO characteristics well project an increase in the frequency of extreme El Ni帽o and .
This suggests that under greenhouse warming, we will see more often appearances of strong and potentially increased rainfall in the future.
is聽an Associate Professor聽at the Climate Change Research Centre,聽and a chief investigator at the ARC Centre of Excellence for Climate Extremes, 国产精品; Dr Agus Santoso is a senior research associate at the Climate Change Research Centre, 国产精品.