Concurrent heat waves and their linkage to large-scale meridional heat transports through planetary-scale waves

There is increasing interest within the community in the mechanisms behind the development of concurrent heatwaves, i.e., heatwaves that occur simultaneously in geographically remote regions. This interest is motivated by their socio-economic implications and by the fact that they are occurring more frequently with global warming.

While the large-scale atmospheric dynamical drivers of concurrent heatwaves have often been emphasized, with a focus on quasi-stationary wave patterns favoring the formation of blockings, particularly in Summer, the thermodynamic drivers have so far received less attention, despite the recognized role of moisture and latent heat transport for the development of blockings, especially in Winter.

Here, we relate extremes in hemispheric meridional heat transport (MHT) to occurrences of hemispheric land-surface temperature (LST) warm and cold extremes. We find that the combination of extremely weak MHT and extremely warm hemispheric LST days occurs significantly more often than other combinations, and that these events are associated with a substantial amount of concurrent heatwaves in the Northern Hemisphere mid-latitudes, both in boreal Winter and Summer. We highlight that, in Summer, the phase and amplitude of high-latitude blockings associated with these occurrences lead to vanishing, and sometimes even equatorward, overall MHT, together with an intensification of the Pacific branch of the jet stream. In Winter, MHT is largely suppressed by an excessively zonal flow, bringing mild and moist air towards continental regions, both in Eurasia and North America. The reversal or suppression of zonal wavenumber-2 and -3 contributions to MHT is found to be related to these MHT extremes, pointing towards the predominant role of ultra-long planetary-scale waves.