It's been raining again, and it's got me thinking about what we can expect from the weather and the climate over the coming spring and summer.

After a couple of years with very heavy rain, I think a lot of people are probably quite worried about what's ahead. The impacts on housing are still with people across the region, with some places still uninhabitable. Costs are still being borne by community and by council. For our waterways, many of which have suffered badly, the future of many recent replantings and much natural regeneration is at stake, as more heavy rain could wash everything away. 

The Bureau of Meteorology has recently put out its latest long-range forecast (BoM 2025(a), indicating that rainfall is likely to very likely (60% to over 80%) to be above average for most of eastern Australia, including the Illawarra. In small areas, unusually high rainfall is possible, but it looks like Illawarra is not in that category. 

But I wanted to say something about those averages, and the variability that they conceal. The averages are composed from records taken over many decades, in which multiple cycles and patterns play out, such that very few years are actually 'average.' I saw this in graphic detail recently when analysing rain gauge records in my local area. A few factors are at play, leaving aside the additional complexity of human-made global warming, which is expected to make conditions warmer and drier, and rainfall more intense (per the below image).

The first cycle is the fairly familiar El Niño Southern Oscillation (or ENSO), with its roughly seven-year pattern of alternating El Niño years (characterised by generally warmer and drier conditions) and La Niña years (characterised by generally cooler and wetter conditions).

Drought periods are often (but not always) associated with El Niño years. And over the last few years, the BoM recorded La Niña years in 2020-21, 2021-2022 and 2022-23, while El Niño was only recorded in 2023-24 (against predictions).

There are also other patterns in play. One is the Indian Ocean Dipole (IOD), defined as 'sustained changes in sea surface temperatures of the tropical western and eastern Indian Ocean'. This can impact temperatures and rainfall across the country, particularly in winter and spring.

Warmer than average Indian Ocean surface temperatures are associated with higher winter and spring rainfall (BoM 2025(c).

And even less known, and less studied, is the Southern Annular Mode (SAM), which is the '(non-seasonal) north-south movement of the strong westerly winds that blow almost continuously in the mid- to high-latitudes of the southern hemisphere'. Like the ENSO and the IOD, the SAM has negative, neutral and positive modes that affect rainfall levels, but it runs over weeks rather than whole seasons. 

Then last, but not least, is a decadal cycle that almost nobody has heard of, but that may have massive implications for our urban waterways, and our city as a whole, in coming years (see, for example, R.F. Warner's 1995 article 'Predicting and managing channel change in Southeast Australia'). This is the alternation between Drought-dominated Regimes (DDRs) and Flood-dominated Regimes (FDRs) in eastern Australia, which is associated with 20-40 periods of much lower and much higher rain.

Documented since at least the 1970s, and with research ongoing, there is now some official concern that eastern Australia may be heading back into a Flood-dominated Regime, as per this guidance from the NSW State Emergency Service for residents in the Hawkesbury-Nepean catchment, released in 2023. 

If this is correct, then we may expect that the big rains and floods of the last few years may continue, further exacerbated by the impacts of climate change, a scenario that Robin Warner described as 'a situation yet to be experienced' (Warner 1995, p.412). This should give everyone pause for thought, particularly those responsible for considering development anywhere near any of the Illawarra creeks.  

References

• Bureau of Meteorology 2025(a), Climate outlooks - weeks, months and seasons, online, viewed 11 August 2025. 
• Bureau of Meteorology 2025(b), Australian rainfall during El Niño and La Niña events, online, viewed 11 August 2025.
• Bureau of Meteorology 2025(c), Indian Ocean climate influences, online, viewed 12 August 2025.
• Bureau of Meteorology 2025(d), Southern Annular Mode, online, viewed 11 August 2025.
• NSW Department of Climate Change, Environment, Energy and Water, 2025, Interactive climate change projections map, online, viewed 12 August 2025.
• NSW State Emergency Service (2023) How often does it flood here, online, viewed 11 August 2025.
• Warner, R.F. 1995, Predicting and managing channel change in Southeast Australia, Catena 25 (1995), 403-418.