In 2017, California experienced its wettest winter on record. Record-breaking rain in January of that year caused major flooding, inundating hundreds of homes, blocking roads and causing power blackouts for more than half a million people. But the torrential rain puzzled researchers because it occurred during a weak La Niña phase, where sea-surface temperatures in the Pacific Ocean are generally below normal.
This would usually increase the risk of a dry winter for California, not a very wet one. Six years later, the same thing happened again. The Pacific Ocean was in a La Niña but, contrary to expectations, torrential rain poured over California between December 2022 and March 2023.
Californian highway flooded by the king tide For years, this strange inversion of winter weather baffled scientists. Now, an international team of researchers thinks the answer may lie in unusually warm weather on the Tibetan plateau, a vast mountain plateau in Asia with an average elevation greater than 4500 metres above sea level. Weather in high-elevation regions like this can have an effect on weather “comparable to sea-surface temperatures”, says Yongkang Xue at the University of California, Los Angeles, who led the study.
The team used climate models to simulate the effect of record warm weather over the Tibetan plateau in December 2016 and the wet weather that arrived in California in January 2017. They repeated the same modelling for February 2023 over the Tibetan plateau and March 2023 in California. Xue says in both cases, the warm temperatures over the plateau led to meteorological effects well known to drive extreme rainfall.
They caused a perturbation in the westerly jet, a fast-flowing atmospheric river, which, in turn, affected other atmospheric rivers and triggered the breaking of Rossby waves – large meanders in high-altitude winds – near the Californian coast. According to the paper, the modelling suggests heat over the Tibetan plateau can explain 56 per cent of the extreme rainfall anomaly over California in January 2017, and 30 per cent of the rainfall anomaly in March 2023. The connection between the high mountains and California emerges in the data only after 1980, adds Xue, a sign that rapidly rising temperatures in mountainous regions are causing unexpected effects.
The amount of rainfall in the southern Amazon basin has declined by 8 to 11 per cent since 1980, largely due to the impact of deforestation The conclusions form part of growing scientific efforts to identify the role high-elevation land temperatures might play in driving extreme or unusual weather events around the world. However, he says the exact causal mechanisms linking the Tibetan plateau to California require further study. Arun Kumar, formerly of the US National Oceanic and Atmospheric Administration, agrees.
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