Whiplash weather in ENSO Transition Years Identified by A Novel Cascading Extremes Index

Cascading extreme weather events, characterized by sequential occurrences of distinct extremes such as heatwaves, floods or droughts, pose increasing risks in a warming climate. However, existing approaches for identifying such events focus either on temporal persistence or spatial coherence alone, and are thus unable to identify the most severe events with both characteristics. Here, we propose a new approach based in dynamical systems theory that treats variables as coupled systems, with a view to enable their mechanistic understanding. We illustrate the application of the method to temperature and relative humidity data during the period 1979-2020, identifying cascading heat-drought extremes over the Mississippi, southeastern China and France. While these events are controlled by different large-scale climate modes and blocking patterns, nine of the events occurred during rapid transitions (<12 months) from El Niño to La Niña. In China, these transitional events were consistently preceded by heavy rainfall approximately two weeks earlier. Key drivers include the prolonged presence of the western north Pacific subtropical high and land-atmosphere feedbacks. Our findings uncover the speed and severity of cascading wet-dry transitions within as little as two weeks during El Niño transition years, and the need for a greater understanding of their driving mechanisms.