A primer on atmospheric rivers
Climate change runs through it
Atmospheric rivers are low-level jets of air that flow out of the tropics to the mid-latitudes. They are associated with extratropical cyclone systems. For more details, read this.
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Sometimes atmospheric rivers originate in the tropics, near Hawaii, and those are also called the Pineapple Express. Their origin in the tropics also explains why they carry enormous amounts of water vapor.
When these atmospheric rivers hit California, they can cause enormous rainfall. The mechanism is straightforward: As an atmospheric river encounters the land, particularly the Sierra Nevada mountain range, the moist air is forced upwards — a process known as orographic lift. As the air rises, it cools and condenses, and rain falls out.
Because the ocean never runs out of water, this weather pattern can bring neverending rain to the State. One storm in the 1860s brought continuous rain for nearly 43 days, leading to catastrophic flooding across much of California, particularly in the Central Valley, which transformed into an inland sea, reportedly up to 30 miles wide and 300 miles long. If such an event occurred today, the damage could top $1 trillion.
What about climate change, you ask? A warmer planet has more water vapor in the atmosphere. And, everything else being the same, an atmospheric river carrying more water vapor will cause more rainfall when it hits land and starts rising.
Because of the simplicity of this physical argument, the IPCC concluded that global warming will increase the precipitation from these events:
Precipitation associated with extratropical storms and atmospheric rivers will increase in the future in most regions (high confidence).1
Thus, we can conclude with confidence that climate change is making the event occurring in California right now worse than it would be without climate change.
But by how much? If we assume a purely thermodynamic response (e.g., Clausius-Clayperon scaling), rain would increase by about 7% for every degree Celsius of warming of the atmosphere. But this neglects the dynamic response — e.g., impact of climate change on atmospheric circulations.
My reading of the literature suggests that we don’t really have a good handle on this. Thus, a firm quantification of the impact of climate change on the rain in this event will await formal attribution analysis.
In summary, atmospheric rivers are a high-consequence weather system, with significant implications for water resources and flood risk. The interaction between atmospheric rivers and climate change is complex, but the increase in moisture in the atmosphere will certainly lead to more intense rainfall. Changes in atmospheric circulation could ameliorate or enhance this, which future research should clarify.