Warm Tibetan Plateau winters account for 56% of California's January 2017 rain anomaly in models

Those two percentages come out of experiments in which the observed plateau warmth is the input, not the unknown. According to New Scientist, the team simulated the effect of record December 2016 warmth over the plateau on the wet weather that arrived in California in January 2017, then repeated the exercise for February 2023 and March 2023 ^[13]. That design isolates a mechanism rather than testing prediction, since an operational system would first have to forecast the plateau anomaly, then propagate it across the Pacific without smearing it. Live Science reports the models replicated the extreme weather with surprising accuracy ^[20]. Reproducing an event whose driver you have already specified is the easier half of the job.

Subtract and the accounting is more modest than the link itself: 44 per cent of the January 2017 anomaly and 70 per cent of the March 2023 anomaly are left to everything else ^[22]. The two shares differ by 26 percentage points, close to a factor of two, across the only two cases in the paper ^[23]. With two events there is no basis for saying which is the representative one.

The physics is the sturdiest part. As the plateau, which averages more than 4,500 metres above sea level, warmed, a strong gradient formed against the regions north and south of it, generating a disturbance that travelled east along the jet stream as a Rossby wave train toward the Rockies ^[18][11]. Over the eastern Pacific the wave broke, which Xue says reduced static stability and sharply intensified atmospheric rivers, the narrow corridors of water vapour transport that drive much of the flooding in the western United States ^[12][24]. Matthew Huber of Purdue, who was not involved, called the analysis a very specific and impactful case study and said it builds on the established idea that extra heat on a mountaintop affects Rossby waves almost as if the mountain had suddenly grown ^[17].

The false alarm rate is missing from this record. Neither published account describes the converse test: early winters when the plateau ran hot and California's rainfall stayed unremarkable. Xue notes the connection appears in the observational record only after 1980, which he attributes to rapid warming in mountain regions ^[14]; that also caps how many analogue winters exist to score a predictor against. Andries-Jan de Vries of the University of Lausanne told New Scientist the study nicely demonstrates the effect but that the exact causal mechanisms need further study ^[15]. Arun Kumar, formerly of NOAA, called the findings intriguing and said more work with fresh data is needed before they can prove useful for forecasting ^[16].

The test that would settle it is unglamorous: add plateau temperature as a predictor to a subseasonal hindcast over the post-1980 record and score it against the ocean-only baseline, counting misses and false alarms. Until that exists, this is a well-argued mechanism with two cases behind it, and roughly $6.6 billion in damage and at least nine deaths across those two winters ^[6] is a fair reason to spend the compute on the harder experiment.