Changes in the Interannual Impact of the May North Atlantic Oscillation on Eurasian Surface Air Temperature and Its Extremes Around the Late 1990s

The changes in the interannual impact of the May North Atlantic Oscillation (NAO) on May Eurasian surface air temperature (SAT), extreme warm days (EWD), and extreme cold days (ECD) during 1979–2021, along with the underlying mechanisms involved, were investigated in this study based on observation, reanalysis, and numerical experiments. Results showed that since the late 1990s, the response of May SATs, EWDs, and ECDs over the mid‐ and high‐latitudes of Eurasia to the May NAO changed from a southwest–northeast‐oriented dipole to a zonal tripole. During 1979–1998 (2000–2021), the May NAO led to northeastward‐ and southeastward‐propagating (eastward‐propagating) Rossby waves in May over the region and a resultant dipole (tripole) pattern of atmospheric circulation anomalies with a barotropic structure, which favored a dipole (tripole) pattern of SAT/lower‐tropospheric air temperature anomalies, mainly via anomalous horizontal temperature advection (anomalous vertical adiabatic heating/cooling and horizontal temperature advection). From 1979 to 1998 to 2000–2021, the changes in Rossby wave activity induced by the May NAO were caused by the enhancement of atmospheric circulation anomalies in May around Iceland in response to the May NAO. The enhanced response of May atmospheric circulation around Iceland was partly attributed to the enhancement and southward movement of the mid‐ and upper‐tropospheric northern action center of the May NAO, which was located west of Greenland, and the changes in the atmospheric mean state around Iceland.