El Niño only happens every few years, and there's a fair amount of variability between events, so just getting a baseline is tough. How El Niño will change with global warming is a big and open question. But this is still an emerging field of research. Part of that, presumably, is related to the whole tropics being very, very warm. It's taking much longer to develop, and it's not as strong.

This year, in comparison with other large El Niño events – such as 1982-83, 1997-16 – we're not seeing the same change in where the precipitation is happening. During El Niño, the engine driving tropical circulation shifts to the east. These ripples affect the jet stream, which steers weather patterns in the U.S. The atmosphere reacts to this change like ripples in a pond when you throw a stone in.

When the warm pool's water shifts eastward, that also shifts where the rising and sinking motions happen. The rising and sinking creates giant loops in the atmosphere called the Walker Circulation. Heat from the warm ocean water causes the air above it to warm and rise, which fuels precipitation. The atmosphere is what transmits El Niño's impact. But with the atmosphere not responding to the warmer water right away, the impact on Atlantic hurricanes was lessened and it turned out to be a busy season. Forecasters often use El Niño as a predictor of wind shear, which can tear apart Atlantic hurricanes. The 2023 Atlantic hurricane season is a good example. Is that why El Niño didn't affect the 2023 hurricane season the way forecasts expected? What has been surprising about this year's El Niño – and still is – is that the atmosphere hasn't responded as much as we would have expected based on the rising sea surface temperatures.

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