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

We investigate concurrent heatwaves across the Northern Hemisphere through the linkage between extremes in Meridional Heat Transport (MHT) and in hemispheric land surface temperature (LST). MHT provides fundamental insight on how large-scale thermodynamics links to atmospheric large-scale dynamics, especially through the action of the eddy planetary-scale circulation in the mid-latitudes. The phase and amplitude of these waves can in fact favor the simultaneous occurrence of heatwaves in remote regions, but how this relates to the amount of heat carried by them has not yet been discussed. In this work, we find that the conditional occurrence of extremely weak MHT and extremely warm hemispheric LSTs is significantly more frequent than other conditional occurrences, both in Summer (JJA) and in Winter (DJF). We argue that the combination of extremely weak, in some cases equatorward, MHTs and warm LSTs in JJA are associated with the reversal of the MHT contribution by zonal wavenumber 3, which is in turn associated with the frequency of atmospheric blocking in western Eurasia and the intensity of blockings and jet stream over the Northwestern Pacific. In DJF, the weak, albeit never equatorward, MHT – warm LST events are characterized by a suppression of the climatologically dominant wavenumber 2, which weakens the overall MHT. The flow is anomalously zonal across much of North America and Eurasia, with reduced frequency of atmospheric blockings and downstream displacement of the jet stream, advecting moist and mild air eastward into the continents. Overall, such dynamical pattern correspond to abnormally warm and widespread temperatures in North America, Eastern Europe, and China. The conditional occurrence of extremely weak MHTs and warm hemispheric LSTs is found to be related to between 30 % and over 40 % of extremely warm hemispheric LST days in both seasons.