Possible impacts of mega‐El Niño/Southern Oscillation and Atlantic Multidecadal Oscillation on Eurasian heatwave frequency variability

Identifying predictability sources of heatwave variations is a scientific challenge and of practical importance. This study investigates the summertime heatwave frequency (HWF) over Eurasia for 1950–2014. The Eurasian HWF is dominated by two distinct modes: the interdecadal (ID) mode, featured by an increasing pattern overall, centred around eastern Europe–central Asia and Mongolia–southwestern China; and the interannual (IA) mode, resembling a tripole anomaly pattern with three centres over western–northern Europe, northeastern Asia and East Asia. The ID mode is found to be influenced by mega‐El Niño/Southern Oscillation (mega‐ENSO) and the Atlantic Multidecadal Oscillation (AMO), and the latter has far more effect, whereas the IA mode is connected with mega‐ENSO. Further analysis suggests that mega‐ENSO variations can incite a Gill‐type response spreading to Eurasia, while the AMO changes cause eastward‐propagating Rossby wave trains toward Eurasia. These two teleconnection patterns together contribute to the large‐scale circulation anomalies of the ID mode, and those related to the IA mode arise from the teleconnection pattern excited by mega‐ENSO. A strong mega‐ENSO triggers subsidence with high pressure anomalies, warms the surface and increases the HWF significantly over northeastern Asia particularly. Likewise, the warm AMO‐induced circulation anomalies engender surface radiative heating and HWF growth in most of the Eurasian continent except some localized Siberian and Asian regions. The situation is opposite for a weak mega‐ENSO and AMO. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which realistically capture the features of the ID mode can reproduce the AMO‐like sea‐surface temperature anomalies (SSTAs), while signals resembling mega‐ENSO are found in those with favourable capability of simulating the IA mode. On the contrary, these relevant SSTAs linked to the respective modes vanish in the models with little skills. Thus, mega‐ENSO and the AMO might provide two critical predictability sources for heat waves over Eurasia.