What causes some La Nina's to be so severe?
"A La Nina within a La Nina" - the Pacific Decadal Oscillation

It's been an exceptional few months of rainfall across parts of NSW and Queensland. Brisbane recorded its third highest flood since the late 1800s, western Sydney has now experienced multiple floods, Lismore flood heights reached catastrophic levels and it seems that barely a week has gone by without at least some flood event occurring since mid February in either NSW and Queensland.

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It's easy to identify the cause - a La Nina pattern in the Pacific Ocean. However La Nina's are not rare by any means. So what causes some La Nina's to be relatively benign, while other La Nina's cause significant devastation?



There are two main factors that have made this La Nina event stand out from previous La Nina's. But first and foremost, we have to understand why La Nina's generate an increased potential of rainfall. La Nina's cause the easterly trade winds in the Pacific Ocean to increase. This pushes higher levels of easterly winds (and thus higher levels of moisture) across the country. In the right conditions, this generates rainfall. Increased moisture is only one part of the equation when it comes to the generation of rainfall - if we think of baking a cake, moisture may be the flour (ie the main ingredient), but we still need other ingredients to be present. One critical ingredient is a trigger to generate rainfall. This is often in the form of upper troughs and upper lows that wander across the continent periodically. In other words, La Nina's only increase the probability of rainfall, and is not a guarantee of increased rainfall. Other factors need to combine too - this is why not all areas of NSW and Queensland have experienced above average rainfall (the north Queensland wet season has been well below average, and the tropical cyclone season has been all but non-existent).

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Total rainfall since the start of the year across Australia

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Rainfall as a percentage of the mean since the start of the year. Well above average rainfall has occurred in NSW and SE QLD (and SA), but the tropics have generally been much drier than average.

This year we've seen plenty of easterly winds across Australia. This has been the main driving force behind the hot summer in Western Australia (as the easterly winds travel across the continent and push the heat from the interior in Western Australia). This brings us to the first feature of this La Nina that's different to others - it's lasted longer than usual.

1) A Prolonged La Nina

La Nina and El Nino events typically "reset" by the end of summer. This means their influence often weakens. For a La Nina, this is important because while upper troughs and lows wander across the continent in summer, this is the period they're typically at their least active (they tend to remain further south as any cold influence contracts poleward during the height of summer). However as autumn rolls in, we begin to see this colder influences begin to extend northwards again. Autumn typically begins to see an increase in upper troughs and upper level instability pushing across. This is why autumn can still be a fairly wet period across NSW and Queensland, particularly when combined with the southeasterly trade winds that occur. However if a La Nina is still operating in autumn, these easterly winds can be "turbocharged" and this increases the levels of moisture significantly. Once you mix the moisture with instability, you find the perfect recipe for rain events to occur.

In other words, the prolonged La Nina event has caused this La Nina to interact with autumn systems causing them to generate more rain than you would even expect in December and January (keeping in mind that the heaviest rain occurred just days from the official start of Autumn).

Both the 2011 and 2022 Brisbane (and SE QLD) flood events experienced similar patterns in the upper atmosphere where an upper low sat to the northwest of the region. This not only added instability (that combined with the easterly winds and moisture) to generate rainfall, but also meant that the upper and surface systems combined to push rain and storm activity continuously over the same region for an extended period of time.

However there's one other factor that's likely played a significant role in the increased rainfall patterns - and that's a lesser know circulation called the Pacific Decadal Oscillation (PDO).

2) "A La Nina within a La Nina" - the Pacific Decadal Oscillation (PDO)

La Nina and El Nino cycles typically last months, but the PDO cycles last years (and sometimes decades). These are much slower, and broader moving oceanic patterns that can enhance or detract from either La Nina and El Nino cycles.

Currently we're in a "cool PDO" cycle. This means that the water in the northwestern Pacific is consistently warmer and above average and the water in the northeastern Pacific is consistently cooler and below average. A cool PDO emphasises the current La Nina patterns - in fact, many of the big Brisbane flood events have occurred during both a cool PDO and La Nina combination.

A "warm PDO" is the opposite, and if an El Nino occurred during a warm PDO cycle then it's likely to be further enhanced. Conversely, a warm PDO may weaken the impacts of a La Nina and a cool PDO may weaken the impacts of an El Nino.

A cool PDO is defined by a prolonged period (many years) of warm water across the northwestern Pacific and a prolonged period of cool water across the northeastern Pacific. Cool PDOs often coincide with increased rainfall across eastern Australia.

Since 1887, nearly every major flood in Brisbane has occurred during both a cool PDO period and a La Nina (the exceptions were during the early 1900s but these were more borderline major floods). Also note that the construction of Wivenhoe Dam after the 1974 flood has helped reduce flood heights in the Brisbane region.

The oceans play an enormous role in the global climate and it comes as no surprise that we see these strong correlations occur. However long range forecasting of events like these will long be challenging. All we're doing is identifying a range of factors that may either be favourable (or not favourable) for rainfall. To use the same cake analogy, a La Nina and cool PDO cycle may give us all the ingredients to bake a cake, but it's another thing for all those ingredients to come together and actually create one. The same thing happens in our climate - we need those ingredients to come together, and where, if and when they come together will always be nearly impossible to determine when it comes to long-range weather forecast.