Summer heat extremes in northern continents linked to developing ENSO events

Understanding the variations of extreme weather/climate events is important to improve the seasonal forecast skill of such harmful events. Previous studies have linked boreal summer hot extremes to decaying El Nĩo-Southern Oscillation (ENSO) events at the interannual scale, but how these hot extreme episodes respond to developing ENSO events remains unclear. Using observational analyses, we demonstrate strong linkages between developing ENSO and extreme heat events in northern continents. In particular, heat extremes in North America, Eastern Europe-Central Asia and Northeast Asia tend to be more frequent during La Nĩa developing summers and less frequent during El Nĩo developing phases. Associated atmospheric changes reveal that developing ENSO events feature a circumglobal teleconnection (CGT) pattern over the mid-latitudes. In the La Nĩa developing summer, this CGT pattern exhibits enhanced geopotential height and anomalous anticyclones over North Pacific, North America, Eastern Europe-Central Asia and Northeastern Asia, and the jet stream generally shifts northward. The atmospheric circulation changes lead to more persistent weather conditions that favor extreme heat events in mid-latitudes. Conversely, opposite changes associated with developing El Nĩo can inhibit heat extremes in the above locations. The responses of heat extremes to different types (i.e., conventional Eastern Pacific and Modoki Central Pacific) and durations (1 and 2 year) of ENSO events are also discussed.