Robust Increases in Extreme Pacific North American Events Under Greenhouse Warming.

The Pacific North American (PNA) pattern is an internal mode of the Northern Hemisphere atmosphere, which strongly affects the hydroclimate and ecosystems of the Pacific‐North American sector. Recent studies have suggested a more positive PNA pattern in response to greenhouse warming, but how extreme PNA events will change remains unclear. Based on results from the Coupled Model Intercomparison Project Phase 5 multimodel ensemble, we find a 50% increase in the frequency of extreme positive PNA events and a 32% increase in extreme negative PNA events in the 21st century relative to the twentieth century. The increased frequency arises from a shift toward a more positive PNA pattern and is enhanced by more frequent ENSO events under greenhouse warming. Our study suggests that a continued increase in greenhouse‐gas emissions is likely to cause increased occurrence of extreme PNA events, thus increasing the risk of extreme weather and climate in North America. Key Points: Increased frequency of extreme positive and negative PNA events is detected in response to greenhouse warmingIncreased extreme PNA events are associated with a shift toward a positive PNA pattern and more frequent ENSO eventsIncreased extreme PNA occurrences aggravate the risk of weather and climate extremes in North America [ABSTRACT FROM AUTHOR]