Does El Niño cause Cape Town's droughts?

What El Niño is

El Niño and La Niña are swings in the surface temperature of the tropical Pacific Ocean. NOAA tracks them with the Oceanic Niño Index: a reading of +0.5°C or more is El Niño, -0.5°C or less is La Niña, and anything in between is neutral.

The swings change weather patterns far from the Pacific. In South Africa, El Niño mostly makes itself felt in spring and summer, often as hotter, drier conditions in the summer-rainfall parts of the country. That's where the El Niño drought headlines usually come from.

Why Cape Town is different

Cape Town gets most of its rain in winter, from cold fronts and mid-latitude storms off the Southern Ocean between April and September. That winter rain is what fills the six big dams. So the question for Cape Town is what steers those cold fronts, and El Niño has little to do with it.

Research at the University of Cape Town points instead to the Southern Annular Mode, a belt of westerly winds around Antarctica. When it is strongly positive, cold fronts tend to pass further south, or weaker and fewer, and the Cape's winters are drier. The link is real but loose: plenty of positive years are not dry.

What the data since 2000 shows

We set every winter since 2000 against its El Niño or La Niña phase, averaged over May to September. The three El Niño winters include both the driest and the wettest winter in the record: 2015, at about 62% of average rain, and 2023, at about 165%.

The Day Zero winters of 2016 and 2017 were neither El Niño nor La Niña. And in September 2023, during an El Niño, the Department of Water and Sanitation reported the system's dams at over 100% after winter storms.

So what does decide a dry year?

Winter rain, above all. A study of the 2015 to 2017 drought found that low rainfall, rather than heat, was the main cause, and that climate change made a drought like it about three times more likely. For a sense of how this winter is going, watch the rain and the dam levels, not the El Niño forecast.

And the El Niño forecast for summer 2026 to 2027?

An El Niño was building through 2026, and the South African Weather Service expected its usual hotter, drier summer for South Africa. Cape Town's summer is dry anyway: the dams fall every summer, and what matters is how full they were at the end of winter.

A hot summer can push water use up, and use is the part Capetonians control. Every litre saved now is still in the dam in April.