Any day now, the Bureau of Meteorology is likely to declare a La Niña event in the Pacific Ocean – the third in a row since spring 2020. Emergency services, farmers and flood-affected communities along Australia’s east coast are nervously awaiting the forecast that will determine whether another dramatic summer of flooding is on the way.
Australia’s natural climate variability is driven by the Pacific, Indian and Southern oceans. In particular, sustained periods of cooling and warming in the tropical Pacific and Indian oceans determine the background climate that influences the weather conditions we experience from year to year. For decades, scientific research has focused on describing changes in individual oceans separately, with the majority of work focused on the Pacific Ocean. But in recent years, there has been a shift towards trying to understand interactions between ocean basins.
It is complex work, as the seasonal behaviour of these vast oceanic regions differs, and determining past cycles of ocean variability is hampered by a lack of long-term monitoring data, particularly before 1950. These imperfect estimates of past climate mean we are likely to be underestimating the true extent of future climate variability in many parts of the world.
Further complicating matters is the fact that human-caused global warming is now altering the natural circulation of ocean patterns and atmospheric winds across the planet. One of the key priorities in climate change research is to understand exactly how natural climate variability is shifting in a warmer world. Thermodynamics tells us that as our planet warms, the water-holding capacity of the lower atmosphere increases by about 7 per cent for every 1 degree of warming. This can cause heavier rainfall, which, in turn, increases flood risk. The oceans are also warming, especially at the surface, driving up both evaporation rates and the transport of moisture into weather systems. This makes wet seasons and wet events wetter than usual. Because global temperatures are rising, evaporation is also increasing during dry periods, leading to an intensification of drought conditions. This means we are likely to experience a future of intensifying rainfall extremes – baking droughts broken by torrential rains.
The extremes experienced in Australia during recent years have scientists concerned about how natural weather and climate variability will change as our planet continues to warm. We have seen unusual conditions that are starting to differ from patterns we have historically observed in our region. For example, the flooding experienced in northern New South Wales in late February to early March this year was not associated with the cyclonic conditions that characterised the major floods of the 1950s and ’70s in the region. Is this a preview of “normal” summer conditions in a fossil-fuelled world, or simply a case of natural climate cycles interacting to produce extremes that are just rarely observed in our weather records?
To answer these questions, Australian scientists are focused on investigating how changing conditions in our surrounding oceans are influencing the processes that generate extreme rainfall. To do this, we first need to investigate the behaviour of natural cycles embedded in our oceans. Aside from the seasons, the El Niño–Southern Oscillation (ENSO) is the planet’s largest natural climate cycle and it influences year-to-year climate variability across about 60 per cent of Earth. The term “El Niño” refers to unusual warming or cooling of ocean water in the Pacific, while “Southern Oscillation” is a measure of the seesawing of high and low atmospheric pressure across the planet’s largest ocean basin. These changes influence ocean temperatures and wind patterns, causing a significant redistribution of major rainfall-producing systems that periodically swing from one extreme to the other. Here, ENSO is what makes Australia the land of “droughts and flooding rains”.
During normal conditions, trade winds blow from east to west across the Pacific Ocean along the equator. These easterly winds push warm surface ocean water from South America towards Asia and Australia, collecting around Indonesia to form the warmest seawaters in the world, known as the “western Pacific warm pool”. Over in the eastern Pacific, cold water wells up along the South American coastline, creating a temperature gradient across the Pacific. That is, warmer water to the north of Australia and cooler water near the Americas cause a redistribution of heat across the planet. The accumulation of warm water in the western Pacific adds heat to the air, causing it to rise and generate the warm and rainy weather that characterises the tropical rain belt of the Indo-Pacific region.
During an El Niño year, the easterly trade winds weaken or even break down and reverse direction. The warm water that is normally pooled in the western Pacific flows back across the Pacific, piling up on the eastern Pacific coastline from California to Chile, increasing rain and stormy weather in those regions. During an El Niño event, ocean temperatures along eastern Australia are cooler than average, suppressing the formation of rain-bearing clouds. These conditions generate weaker than normal summer monsoons, resulting in reduced rainfall and river flows and fewer tropical cyclones. During these periods, the country experiences hot and dry conditions that are associated with the biggest droughts and bushfire seasons in Australian history. A classic example is the El Niño-driven drought of 1982–1983, which culminated in the dust storm that blanketed Melbourne on February 8, 1983.
Sometimes after an El Niño year, the pendulum swings in the other direction, generating La Niña conditions. The early trade winds intensify, upwelling more cold water along the South American coastline. This pushes warm water from the eastern Pacific towards Australia, resulting in conditions that are essentially the reverse of an El Niño.
During typical La Niña events, warmer-than-average ocean temperatures pool along Australia’s east coast. As the ocean warms, water evaporates, rises and condenses to form clouds and rain. The warmer the temperature, the more moisture is available to fuel rain-bearing systems. Under La Niña conditions, the summer monsoon is more active than normal, leading to greater flooding due to increased tropical cyclone and storm activity. For example, the number of tropical cyclones that hit the Queensland coast roughly doubles during La Niña summers, causing widespread flooding, especially along the eastern seaboard. The majority of the wettest years in Australian history have taken place against a background of La Niña conditions.
Since the 1990s, scientists also began describing similar patterns in the Indian Ocean that influence Australian rainfall from late autumn to early spring. Like ENSO, they are sustained changes in the difference between sea surface temperatures and atmospheric pressure between the western Indian Ocean near Africa and the eastern Indian Ocean near Indonesia. These are referred to as Indian Ocean Dipole (IOD) events. When there are warmer-than-average ocean conditions to the north-west of Australia – essentially like a La Niña condition in the Pacific – a negative IOD event brings wet winters and springs to the south-eastern corner of the country. Conversely, when cooler-than-average conditions prevail in the north, a positive IOD event brings El Niño-like dryness to the region.
Following the catastrophic Black Summer bushfires of 2019–2020, rain finally began to fall across our drought-ravaged country. A weak La Niña was declared by spring 2020, resulting in mostly wet conditions that persisted until early autumn 2021. During this period we saw major flooding in eastern NSW, including in the Hawkesbury–Nepean region in Western Sydney. When that La Niña finally decayed, a weak negative IOD event developed during the winter of 2021, continuing the drenching of much of south-eastern Australia, resulting in parts of NSW flooding yet again.
By spring 2021, a second La Niña formed, reinforcing the wet conditions already prevailing in the Indian Ocean. When a negative IOD coincides with La Niña, the impact of warm ocean temperatures is amplified, generating very wet conditions. As a result, Australia experienced its wettest November on record, and we saw catastrophic flooding in places such as Lismore in northern NSW, and the repeated inundation of the Hawkesbury–Nepean region in western Sydney in late February and early March this year. Wet conditions continued into early winter, when the La Niña finally petered out again, much to the relief of flood-displaced communities.
But just as neutral conditions returned to the Pacific in winter, the Bureau of Meteorology announced another negative Indian Ocean Dipole event had formed, increasing the chances of a wet spring across much of south-eastern Australia – for the second year in a row. So now, as spring begins, we are facing the prospect of another round of combined La Niña–negative IOD conditions that could result in the continuation of record-breaking rainfall and flooding, with catchments across eastern Australia are already saturated by relentless rain.
While consecutive La Niña events are rare, there are three instances since 1950 when this has occurred; 1954–1957, 1973–1976 and 1998–2001. Each period resulted in widespread flooding and record-breaking rainfall. What is interesting about the current protracted La Niña event is that it is noticeably weaker than those of the past but involves complex interactions with the Indian Ocean. This double whammy of La Niña–negative IOD conditions has happened before, resulting in the exceptionally wet conditions experienced across the country during the 1970s. Since this event is still unfolding, it is too early to determine just how the current wet period may differ from historical conditions. But as the past few years have shown us, we should expect the unexpected as natural variability continues to shift in a rapidly warming world.
This article was first published in the print edition of The Saturday Paper on September 3, 2022 as "Weathering heights".
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