In the past few years in Australia, seasonal rainfall and temperatures have left a lot of people confused. Sometimes, the hot, dry conditions usually associated with an El Niño have not eventuated. Similarly, there have been years where a La Niña did not lead to the cool, wet conditions expected.
It’s important for scientists to better understand all weather processes at play, so we can manage expectations around what Australia might experience when climate drivers such as El Niño and La Niña are forecast in future. That’s where our new research comes in.
We examined the state of play in November 2020 and November 2021. La Niña conditions occurred in both years. November 2021 followed the La Niña script and was wet and cool, but November 2020 departed from expectations and was dry and warm. We set out to determine why.
We found the differences could be explained by fluctuations in the path of storms over the Australian continent. These fluctuations can be hard to predict well in advance, which makes it difficult to say for certain how a particular La Niña or El Niño event will affect Australia.
A tale of two Novembers
For large parts of Australia, the presence of a La Niña or El Niño shifts the odds of experiencing wet or dry conditions. Our analysis of Novembers 2020 and 2021 shows how actual outcomes can differ from, or align with, expectations.
The first step in our analysis was to examine other climate drivers, including the Indian Ocean Dipole and Southern Annular Mode. We wanted to know if these drivers were in the same phase – negative, neutral or positive – during those two Novembers a year apart.
So what did we find? In addition to La Nina, both Novembers occurred during positive Southern Annular Mode phases and very weak negative Indian Ocean Dipole phases. These phases are typically associated with more rainfall in Australia. So this didn’t explain why November 2020 was hot and dry.
Next, we looked to the Madden Julian Oscillation. When this driver is located in the Australian region, it has been linked to more rainfall in Australia. Although the oscillation was in different phases during November 2020 and 2021, we found in general, this driver does not strongly influence rainfall across all of Australia in November.
It was time to look for answers elsewhere.
Jet streams: a key piece in the puzzle
Next, we examined weather systems – in other words, the movement of high and low pressure systems across the globe.
These systems are partly controlled by jet streams, which are bands of wind in the upper atmosphere. The effect of jet streams on weather systems, including storms, means they influence rainfall in the regions they pass over.
We found there was a strong jet stream over Australia in November 2021. This would have assisted the development of any rain-bearing low-pressure systems moving in from the west, allowing these systems to travel across the Australian continent. These systems brought rain and contributed to the very wet conditions.
In November 2020, the jet stream was largely absent over Australia. Instead, it was pushed south of the continent, which means rainfall systems received little help and were also largely steered south. That contributed to the dry month.
But why did the jet streams develop in the first place? They form in part due to temperature differences, and are found in the zones where the temperature contrast between warm and cool air is strongest.
In November 2021, Australia experienced cooler temperatures over land, but above-average sea surface temperatures in the waters off northern Australia. This pattern set up the zone of strongest temperature contrasts over the continent, which led to a persistent jet stream there.
In November 2020, Australia was relatively warm both over land and on the sea surface to the north. This meant that the strongest temperature contrasts (and the jet stream) now sat at the junction between the warm continent and cooler Southern Ocean.
But wait, there’s more
So why were temperatures over Australia so different?
To help answer this question we shifted our analysis from the monthly timescale to the daily timescale. That’s because atmospheric features such as jet streams vary strongly from day to day.
We found for about the first three weeks of November 2021, a large low-pressure system – also known as a trough – was sitting south of Australia. It pumped cold air onto the continent, cooling it down.
This maximised the north-south temperature difference between the warm sea surface temperatures to the north of Australia and the cool of the continent. And as we know, this aided the development of the jet stream over Australia.
In November 2020, the continent started off relatively warm. And for a large portion of the month, there was a large high-pressure system over Australia, pulling warmer air from the tropics over the continent.
This system would have also promoted clear skies over Australia and enhanced heating coming from the sun, contributing to the warm Australian continent in November 2020.
More puzzle pieces to fit
November 2020’s hot, dry conditions were not the only time a climate driver has failed to bring the conditions some had anticipated. Just last year El Niño did not deliver expected dry conditions, leaving many people scratching their heads.
Climate drivers play an important role in shaping rainfall. But they’re not the whole story. As our research shows, sometimes they are confounded by changes in weather patterns, which might mean that our expectations of a wet month or season don’t always play out.
When it comes to Australia’s climate puzzle, these findings show there’s more to understand about the role of weather.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
