La Niña on the Horizon?
Recent weeks have seen the eastern Pacific Ocean resume its stalled cooling trend. With warmer-than-average Sea Surface Temperatures (SSTs) across the western Pacific, the Pacific Ocean is edging closer to slipping into a La Niña state.
Loop of SST anomalies for the previous month showing the cool water expanding and extending across the central eastern Pacific Ocean. Source: NOAA
Forecast modelling has also aligned. In July, global climate models were mixed on the outcome, but by September, nearly every global climate model has the Pacific Ocean reaching La Niña thresholds in the coming months.
Comparison between forecasts from July and September for this December. More models are hinting at a La Nina (or pushing it very close to La Nina thresholds). Source: BoM.
What is La Niña and How Does It Work?
La Niña events often correlate with a higher chance of rainfall and flooding. Many significant flood events in Australia, such as those in 2022, 2011, and 1974, occurred during La Niña years. However, not all La Niña events lead to extreme weather. To understand what we can expect, it’s essential to first grasp how La Niña works.
The Pacific Ocean traditionally holds warmer water across the western Pacific (near Australia) and cooler water in the eastern Pacific (near South America). This is due to trade winds along the South American coastline, which cause upwelling—bringing cooler water from several hundred meters below the sea surface to rise.
This cooling effect is a key reason why tropical cyclones are rare in the eastern Pacific but common in the western Pacific. This pattern establishes a system where air sinks over the eastern Pacific but rises over the western Pacific, reinforcing the easterly trade winds across the Pacific. As these winds travel over thousands of kilometers of ocean toward Australia, they collect humidity, which can increase moisture levels across eastern Australia.
La Nina's tend to increase easterly trade winds over eastern Australia. Source: BoM.
The Role of Trade Winds and Moisture
A La Niña occurs when the water in the eastern Pacific Ocean is colder than normal, while the water in the western Pacific becomes warmer than usual. This temperature difference strengthens the easterly trade winds—in other words, La Niña tends to bring more easterly winds than normal across eastern Australia. Rainfall is therefore, an indirect result of this pattern.
When higher levels of moisture are present over eastern Australia, troughs and upper systems are more likely to produce rainfall. However, it’s also possible to have high levels of moisture and easterly winds without a trigger to generate rain. Conversely, there may be plenty of trigger mechanisms to produce rainfall, but without moisture (as can happen with westerly winds coming off the continent), clouds may not form.
2022 (La Nina) and 2019 (El Nino) rainfall percentage anomalies for Australia. In 2022, parts of Queensland had below average rainfall while in 2019, parts of Queensland recorded above average rainfall. However, overall 2022 was wetter and 2019 was drier - even though some locations defied the trend. Source: BoM.
The 2022 La Niña Legacy
Since the 2022 La Nina, the Western Pacific has remained unusually warm and is one of the factors that drove elevated rainfall last summer (despite being in an El Nino pattern). Warm ocean temperatures around Australia often correlate with higher levels of rainfall - so this factor alone points to increased rainfall potential as we move into spring and summer. Combine this with the cooling trend across the eastern Pacific, the odds for a La Nina (and conversely, a higher potential for above average rainfall during the spring-summer period) across eastern Australia are increasing.
SST anomalies in January 2022 showed a very strong La Nina signal in the Pacific Ocean, much stronger than the current La Nina signal. Source: NOAA
La Nina (and ENSO) Isn’t the Only Factor
When considering long-range forecasts, while El Niño and La Niña (the ENSO cycle) receive much attention, other oscillations also influence Australia’s weather. For instance, the Indian Ocean plays a significant role in rainfall patterns, though this year, neutral conditions are expected in that region which means it’s not expected to play a significant role in the long-term outlook.
IOD Forecast from the ACCESS model. Source: BoM.
Rainfall and Flooding: What to Expect
Long-range climate outlooks provide broad predictions - that is, they provide an "average" outlook across a region and not every location in the region will experience the same trend. Nonetheless, with so much warm water surrounding Australia and the building pool of cool water over the central to eastern Pacific, the most likely pattern will be an increase in easterly trade winds in the coming months.
An increase in easterly trade winds brings more moisture - that's more fuel available for rainfall on the provision that there are other factors (surface troughs, upper systems etc) that can interact with this moisture and turn it into rain-bearing systems and storms. This brings a higher probability of rainfall - and the slightly elevated potential for flood risks. However, conditions do not currently appear to be as significant as the 2022 La Nina, with the Pacific Ocean only expected to be a weak La Niña (or cool neutral, which is a cross between a La Niña and neutral state). Of course - we need to keep monitoring the Pacific Ocean in case this changes.
ECMWF probability of exceeding the median rainfall for December. Source: ECMWF
ACCESS-S forecast for exceeding the median rainfall from October - December, 2024. Source: BoM.
Storms and Temperature Extremes
La Niña can increase the frequency of storms but bring an earlier start to the storm season (through increased availability of moisture). This doesn’t always translate to more intense storms. In some cases, the increased cloud cover and rain can reduce instability, which is necessary for the development of severe storms. As such, we can sometimes see an earlier finish to the storm season (particularly in traditional hotspots such Southeast Queensland and Northeast NSW). Storms can also become a little more focused over inland areas in the latter part of the storm season as easterly winds can dominate and keep the main trough systems further inland.
One complication is the Sudden Stratospheric Warming (SSW) event on Antarctica - a further (weak) SSW event is underway, which could see renewed bursts of westerly winds and cold fronts mid-Spring resulting in a clash of airmasses. As SSW events are rarer in the Southern Hemisphere (compared to the Northern Hemisphere), there is limited understanding of their impacts though.
Additionally, La Niña’s influence can help moderate extreme heat in eastern Australia by blocking the hot northwesterly winds. Given the late development in the Pacific Ocean, we may see a delayed impact though - this means, Spring will likely see above average maximums before possibly tempering somewhat into summer (though easterly winds can bring plenty of humidity and make temperatures feel just as warm!)
However, late-season heatwaves can still occur in parts of South Australia, Victoria, and inland NSW due to dominant ridging along the east coast helping to anchor the heat trough over these regions.
Impacts on Western Australia
While La Niña brings humid and mild conditions to eastern Australia, the opposite can occur in the west. Easterly winds passing over the hot desert can bring hotter-than-usual conditions to Western Australia, increasing the likelihood of extreme temperatures.
Tropical Cyclones
Tropical Cyclone forecasts have become increasingly complex over the past 10-15 years with a steady decline in tropical cyclone numbers. Conversely, the tropical cyclones that have occurred have generally been more intense. Tropical cyclones are fueled by warm oceans and with the oceans around Australia remaining anonymously warm - conditions remain prime for tropical cyclone development.
Fortunately, there is no strong signal that there will be significantly more tropical cyclones than normal for the 2024-25 season. But we do expect a higher potential (and ratio) of intense or severe tropical cyclones (that is, category 3 or higher).
Tropical Cyclone Ilsa in April 2022 approaching the West Australian Coastline. Source: Weatherwatch Dashboard.
Navigating the Challenges of La Niña
One of the challenges in navigating La Niña is evaluating rainfall threats. A common mistake is relying too heavily on online weather model data, leading to knee-jerk decisions that can cost businesses time and money.
Weather forecasting remains an art, and experienced meteorologists who work with these models daily understand the nuances and limitations. This is why having meteorologists as a part of your daily cadence truly pays dividends - they can provide professional advice to ensure you make the right decisions.
Read our recent discussion on the limitation of modelling in a recent rainfall event in August this year here.
Weatherwatch: Keeping You Informed
At Weatherwatch, we offer a range of services to help businesses and individuals manage weather-related risks:
Online weather briefings
Accurate weather forecasts
Climatic outlooks
Weather Dashboards
Rainfall alerts, including via our cutting-edge Virtual Rain Gauge Network, which can detect previously undetected flood events.
Real-time thunderstorm mapping and severe thunderstorm detection via the Weatherwatch HailTracker® platform.
If you’d like to discuss how we can assist with your weather-related needs, please reach out to our team of meteorologists!
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