Climate drivers
Less than the headlines suggest, and not the thing most people assume. El Niño’s clearest Australian rainfall signal is a drier winter and spring across the eastern and northern parts of the continent, and the Bureau of Meteorology’s own statement of El Niño impacts does not extend that signal to the south-west. In Western Australia the seasonal outcome is usually settled by the Indian Ocean Dipole, the Southern Annular Mode and where the mid-latitude storm track sits, with ENSO as one input among several. And whatever the season does, it moves no design number: a 1% annual exceedance probability flood is a long-record statistic, not a forecast, and it does not become rarer because the Pacific is warm.
What the Bureau actually says El Niño does
El Niño is the warm phase of the El Niño–Southern Oscillation. The equatorial trade winds that normally blow east to west across the Pacific weaken or reverse, warm surface water builds in the central and eastern tropical Pacific, and the deep convection that usually sits north of Australia shifts east with it. The Bureau’s stated consequences for Australia are less cloudiness and rainfall north of the continent, usually below average winter–spring rainfall for eastern parts of the country, and a later start to the northern wet season. Events occur on average every three to five years, typically develop in autumn and winter, strengthen through winter and spring, and decay over the following summer and autumn.
How it is measured changed recently, and the change is worth knowing. From early September 2025 the Bureau moved from a traditional Niño index, which compared tropical Pacific sea surface temperatures against a fixed baseline period, to a relative Niño index that compares them against the averaged sea surface temperature of the broader global tropics. The point is to separate the ENSO signal from the background warming trend — against a fixed baseline, a steadily warming ocean drifts towards El Niño thresholds on its own. An event is indicated when relative Niño3.4 exceeds +0.80 °C and the thirty-day Southern Oscillation Index sits below −7.
As at its 1 September 2026 issue, the Bureau describes El Niño as firmly established, with both oceanic and atmospheric indices consistent with a strong event: relative Niño3.4 at +2.45 °C for the week ending 30 August 2026, and a thirty-day SOI of −14.6. Models forecast further warming through spring, a peak in late spring or summer, and persistence into autumn 2027. The Indian Ocean Dipole was neutral at +0.22 °C on the same date, having eased back below the positive threshold, with models still suggesting a positive IOD could develop during spring. That is the state of the drivers on the day this was written. It is not the state of anything a design is anchored to.
The rainfall signal is eastern, and the south-west is not in it
The strength of the eastern signal is not in doubt. Nine of the ten driest winter–spring periods on record for eastern Australia occurred during El Niño years. Averaged over every El Niño event since 1900, winter–spring rainfall in the Murray–Darling Basin ran 28 per cent below the long-term average. But where the Bureau names regions in its El Niño impact statements, it names the eastern and northern parts of the continent. South-west Western Australia is not among them, and the absence is the useful part.
Even in the east the relationship is loose. The Bureau’s own position is that widespread drought does not occur with every event, and that the strength of an El Niño is not directly proportional to its rainfall impacts: the very strong 1997–98 event produced effects largely confined to coastal south-eastern Australia and Tasmania, while the relatively weak 2002–03 event delivered widespread and significant drought. The current monitoring page states it flatly — a strong El Niño signal in the Niño3.4 region does not necessarily mean strong impacts on Australian rainfall and temperature, because ENSO is only one of many factors influencing Australia’s seasonal climate.
Western Australia has already demonstrated the reverse. The consecutive La Niña events of 2010 and 2011 produced heavy rainfall across most of the country and gave Australia its wettest two-year period on record — with the far south-west excepted. The wettest ENSO phase in the record did not reach the corner of the continent most of this state’s population lives in.
What does drive south-west rainfall is a combination the state’s own water utility describes plainly: unlike eastern Australia, where El Niño often has a strong influence on rainfall, Western Australia’s weather is shaped by several climate systems working together, and warmer, drier conditions become more likely across southern WA when El Niño coincides with a positive Indian Ocean Dipole or a positive Southern Annular Mode. July 2026 was the worked example. Perth received 56.8 millimetres against a July average of 144.3 millimetres, attributed to a strong positive SAM pushing rain-bearing systems south of the continent, with El Niño contributing rather than causing.
What it does change — cyclones in the north, water balance in the south
For a Pilbara or Kimberley site the ENSO signal arrives as cyclones rather than as winter rain. Historically the relationship has been significant: during El Niño years the number of tropical cyclones in the Australian region has typically been below average, and during La Niña years above average. The Bureau attaches a caveat worth carrying — those patterns now appear to be changing, and more data is needed to confirm the shift. A positive IOD points the same way, with below-average cyclone numbers typical in positive IOD years and the influence most pronounced in November and December.
Fewer is a statement about frequency, and frequency is not what damages a site. One system does that, and it need not cross the coast nearby: cyclones or their effects are felt as far south as Perth, and the most common effect is heavy rain and associated flooding after the system has weakened. Climate models point towards fewer but more intense tropical cyclones, with greater rainfall than previously observed for comparable systems. A season with a below-average count and one severe crossing is worse than an average season without one.
Monsoon onset is the other northern consequence, and it is a programme fact rather than a design one. Onset in tropical Australia runs generally two to six weeks later in El Niño years than in La Niña years, with rainfall in the northern tropics typically well below average early in the wet season and close to average later. That lengthens the dry-season construction window at the front and compresses it at the back, which matters when a diversion, a levee or a sediment basin has to be complete before the first rains.
In the south the consequence is a water balance rather than a flood. Warmer and drier conditions, where they eventuate, mean drier soils, lower streamflow and reduced runoff into storages. That bears on dam yield, farm water supply, licence reliability, dust and fire, and on the antecedent moisture assumptions inside a catchment model. It does not bear on the flood level the same catchment produces when a storm arrives.
Why none of it moves a design flood
Design rainfalls in Australia are intensity–frequency–duration estimates derived from the full observed record at each location and expressed as an annual exceedance probability. An AEP is a long-run probability, not a seasonal forecast. A 1% AEP event is a 1% AEP event in an El Niño year and in a La Niña year alike; the estimate is not conditioned on the state of the Pacific, and there is no accepted method that conditions it that way. Nothing in a seasonal outlook licenses a smaller culvert, a lower floor level or a thinner freeboard.
Where the standards do respond to climate, they respond to the trend rather than the phase. The Climate Change Considerations chapter that replaced the 2019 edition of Book 1 Chapter 6 of Australian Rainfall and Runoff, published in August 2024, applies adjustment factors to design rainfall IFD curves derived from temperature projections, with the factors differing across storm durations. That is a deliberate, documented uplift on a design estimate. It is a different operation from reading a seasonal outlook.
The two also point in opposite directions, which is the part most easily lost. State of the Climate 2024 records a decrease of around 16 per cent in April to October rainfall in the south-west of Australia since 1970, with the May to July reduction the largest at around 20 per cent, alongside a decrease in streamflow at most gauges across Australia since 1970. The same report finds that heavy short-term rainfall events are becoming more intense, and projects more intense short-duration heavy rainfall even in regions where average rainfall decreases or stays the same. Mean down, extremes up. A drying region is not a region with smaller floods.
The south-west decline is also neither gradual nor proportionate. The Bureau describes the winter rainfall drop since around 1970 as a step change or series of step changes rather than a slope, accompanied by much larger reductions in streamflow than in rainfall. Runoff is the residual of a catchment water balance, so a modest rainfall reduction against a drier landscape produces a disproportionate loss of yield. That is a real problem for supply. It is not evidence about the design storm, which is a different question asked of the same catchment.
The local reminder is recent. Albany’s highest daily rainfall in a record beginning in 1877 was set on 14 March 2025 — 126.4 millimetres in twenty-four hours, in a month whose total reached 382 per cent of the March average. Records are set by single storms, and single storms do not consult the seasonal outlook.
Where a seasonal outlook is genuinely useful
The distinction that makes an outlook usable is between operational decisions and design decisions. Operationally the list is real: when to stage bulk earthworks, when to open and close a creek diversion, when to draw a storage down ahead of the wet season, how long temporary sediment controls will be exposed and therefore how they should be sized, when to run dewatering, when to schedule survey and gauging so the fieldwork is not lost to a wet month, and what to assume about monsoon onset on a northern site. Those are all questions about the coming months, and a seasonal forecast is the right instrument for them.
The design list is untouched. Freeboard, culvert and spillway capacity, floodway and finished floor levels, basin volumes, flood immunity of access roads, the design storm behind a tailings or water storage — every one of them is anchored to an annual exceedance probability derived from the record, and none of them moves because the Pacific is warm this year.
The interesting ground is the gap between the two lists, which is temporary works. A partially constructed embankment, a diversion bund, an open trench or a stripped catchment carries a lower standard than the permanent work, deliberately, because it is only meant to exist for a season. Its exposure is a function of how long it stands and what arrives while it does, and that genuinely is a seasonal question. A drier-than-average outlook is a reason to revisit the exposure window; it is not a reason to reduce the temporary standard.
And a caution running the other way. A seasonal outlook is a statement about a total across months. It says nothing about how that total arrives, and a below-average season can contain the storm that sets the record. Reading a dry outlook as a quiet season is the same error as reading absence from the flood mapping as absence of flood risk.