The Tibetan Plateau, a vast and high-altitude region primarily in southwestern China, stands at an elevation of about 14,800 feet. Scientists have discovered that abnormally strong warming on this plateau early in winter can trigger powerful storms and heavy precipitation in California, thousands of kilometers away.
California experienced heavy rains during the winter periods of 2016–2017 and 2022–2023, which led to the destruction of roads and bridges and caused flooding. Typically, the western coast of America experiences storms in winter known as 'atmospheric rivers'—long, narrow streams of water vapor in the atmosphere carrying enormous amounts of moisture.
When these atmospheric rivers reach California, they cause intense rainfall and snowfall. However, in the two mentioned years, these storms were unusually severe despite the presence of La Niña, which usually brings drought to California and is associated with cold ocean water.
Traditionally, weather forecasts are based on sea surface temperature, but in those years, no major computer model could accurately predict such high levels of precipitation in California using only water temperature data. This demonstrated that relying solely on oceanic data is insufficient.
Scientists first analyzed historical data and established a correlation: higher temperatures on the Tibetan Plateau early in winter correspond to increased precipitation in late winter in California. Nevertheless, data analysis alone does not prove causation. Therefore, researchers tested the hypothesis using computer models.
Models run exclusively on sea surface temperature failed to reproduce either the warming in Tibet or the heavy rains in California. However, when real land temperatures from Tibet were introduced into the model, both phenomena matched observations. Since the Tibetan Plateau is a very large and elevated feature, an increase in its surface temperature leads to the heating of the air above it. This alters the temperature gradient between the plateau itself and the surrounding areas.
This alteration disrupts the trajectory of the jet stream, which is a fast-moving air current circulating in the Northern Hemisphere and influencing weather. This disruption takes the form of a large wave moving eastward, called the Tibetan Plateau-Rocky Mountain wave. When this wave reaches the Eastern Pacific Ocean, it breaks like a sea wave hitting the shore. This, in turn, changes the atmospheric conditions over the Pacific Ocean, which intensifies atmospheric rivers.
Atmospheric rivers are the primary cause of heavy rains in California. This study indicates that warming in Tibet early in winter can intensify these storms, potentially improving the accuracy of meteorological forecasts. Previously, forecasts mainly depended on sea surface temperature.
The new findings suggest that the temperature of distant landmasses can serve as an important indicator. The influence of the Tibetan Plateau extends to other regions, such as the East Asian monsoons. It was previously observed that the temperature of the Rocky Mountains affects the weather in southern parts of America.
By considering Tibet's temperature in early winter along with sea and atmospheric conditions, a more accurate forecast of heavy precipitation on the west coast can be achieved. Including these indicators in forecasting systems will give residents, water users, and emergency services more time to prepare. This finding underscores the deep interconnectedness of the Earth's climate system: warmth in one place can trigger a severe storm in a completely different one.