Briefing Note: Climate Variability

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What is Climate Variability?

How does it influence weather systems, and how will weather systems change in a warmer world?

Climate modes themselves do not cause the weather. They set the scene for weather systems such as fronts and low-pressure systems, or in the tropics, clusters of tropical thunderstorms or stormy episodes within the monsoon.

What do we mean by Climate Variability?

Our climate varies naturally. This natural climate variability explains why one year might be drier or wetter than the next, and why farmers can experience bumper crops one year and more meagre productivity the next.

Climate variability refers to the differences from the expected conditions that can occur over various timescales, from days to weeks, months and even years and longer.

Scientists have observed and measured these fluctuations from the typical climate state and have defined those that vary on particular timescales as ‘modes of climate variability’ or simply, ‘climate modes’.

The main modes of climate variability influencing Australia’s weather include the El Niño-Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Madden-Julian Oscillation (MJO) and the Southern Annular Mode (SAM).

There is no single physical process that explains the variability of the weather and climate system. The sources of variability derive from both the ocean and the atmosphere, and in some cases, from the interaction between the two.

Multiple sources of variability occur at the same time, influencing one another

At any given time, multiple types of variability are occurring simultaneously. For example, during a wet winter associated with La Niña, there might be short, dry spells associated with fluctuations in rainfall that are not persistent or intense enough to affect the seasonal outlook.

Weather systems are inherently highly variable. However, variability in weather systems can also result from modes of variability, external influences such as seasonal changes, as well as long-term changes in temperature, clouds and rainfall.

Many factors are involved, which is why not all El Niño events lead to drought, nor do all La Niña events guarantee wet conditions. This means that it is never enough to consider one mode of variability by itself, which explains why in 2023 we experienced a wet El Niño.

How does this variability affect our daily weather?

The weather we experience–wind, rain and clouds–is an expression of the movement of air and moisture organised in a wide range of weather systems.

Weather systems shape the weather, but when climate variability comes into play, the characteristics of these weather systems, including spatial distribution, frequency and intensity, can change.

While we know that climate variability impacts regions differently, and high-impact weather events often occur at the extremes of this variability, the cascading influence of competing modes of climate variability on weather systems remains unclear.

What will happen in a Warmer World?

Climate change is more than just rising temperatures.

As the world warms, both weather systems and climate variability change and are expected to continue changing, shaping our daily weather in cities and regional areas.

Statistics based on past climate data, such as ‘one-in-a-100 year’ events, no longer apply as the changing climate influences the likelihood of these events occurring, and the environmental and societal impacts of different types of weather will change as well. Understanding the physics and dynamics of weather systems is central to understanding how high-impact weather events, and weather more generally, may change in a warmer world. Just because an extreme event hasn’t happened for some time doesn’t mean it can’t.

The weather and climate are complex, and some questions remain unanswered

Scientists use observations, such as those from satellites and weather stations, together with computational models made up of mathematical equations to describe the movement of air and water in the atmosphere and ocean. Models are tools that help us estimate future weather and climate, including weekly weather forecasts, seasonal outlooks, or projections decades in advance.

Due to the chaotic nature of the atmosphere, individual weather systems have limited predictability beyond seven days; hence, weather forecasts are typically provided a week in advance. However, statistical changes in weather systems can be shaped by large-scale climate modes, such as El Niño, as well as by long-term changes in the climate system associated with increasing atmospheric carbon dioxide levels..

Climate models must be able to simulate both individual phenomena (such as El Niño and La Niña events) and the interactions between them. For example, we know that sea surface temperature variability in the tropics differs between El Niño and La Niña years; this means that any errors in simulating El Niño and La Niña conditions could map onto errors on shorter timescales.

Natural climate variability is ‘noise’ that obscures the ‘signal’ of climate change. In fact, climate change will almost certainly result in changes to climate variability itself; that’s why it is hard to quantify and disentangle variability from climate change. As time goes by, we will increasingly be able to observe changes in the patterns of variability, even if a single extreme event cannot be definitively linked to climate change.

Preparation to address the challenges posed by high-impact weather can range from risk assessment to infrastructure adaptation.

How might your day-to-day life change in a warmer world, and what can you do?

Across the country, we will increasingly need to make decisions that depend on the future of our weather. But a lack of understanding of climate variability could lead to incorrect choices with knock-on effects on communities and businesses.

To effectively address the challenges posed by high-impact weather, it is essential to prepare in advance and strengthen our resilience.

What is 21st Century Weather doing?

We are working on identifying the mechanisms that link modes of climate variability to weather systems. For example, linking historical variability in rainfall, temperature and wind to known modes of variability will help us define the importance of variability to future weather, as well as their predictability.

With an enhanced understanding of small-scale processes, we will examine why models may differ from our observed weather and climate. These advancements will enable us to enhance model capabilities and better understand how weather systems are changing in a warmer world.

Climate variability is important to our industry partners because it governs volatility and risk in many sectors, including energy, agriculture and insurance.

Download a PDF copy of this briefing note here.

We want to hear your thoughts on key weather systems that influence your decision-making. Reach out via hello@21weathercentury.org.au