Our most recent El Niño winter was 2018-2019, which ended our 7-year drought in the last decade.
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The warmer waters cause the Pacific Jet Stream (storm track) to shift south, settling across Southern California/Baja Peninsula.
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There is stronger connection for Southern California seeing an above average winter compared to Northern California because it is closer to the Pacific Jet Stream (storm track).
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El Niño weakens the trade winds, allowing warmer water to arrive off the California coast.
Not every El Niño is the same, but the odds are we will see a wetter than normal winter across California.
Less upwelling also occurs off our coast, providing less nutrients from the deep ocean, which causes less phytoplankton. This means fish who eat phytoplankton have less to consume, which affects everything that eats fish.
El Niño could also suppress formation of tropical cyclones in the Atlantic Ocean.