A framework to characterize the contribution of upstream land-atmosphere interactions to cold spells and heatwaves

Energy fluxes between the surface and the atmosphere are known contributors to the genesis and the amplification of temperature extremes: a classic example is the enhancement of land-to-atmosphere sensible heat fluxes during heatwaves over dry soils, boosting the already high surface temperatures to extreme values. Recent work on the Lagrangian analysis of temperature extremes has pinpointed that, in some specific continental regions, diabatic processes do not just act as amplifiers, but play a dominant role in the genesis of positive and negative extreme temperature anomalies. This observation suggests a distinction between world regions where extremely warm or cold air masses are locally generated by non-adiabatic processes, acting as warm or cold air "reservoirs", and other neighboring regions where such extreme air masses are exported adiabatically by the large-scale circulation.

In this work we propose a methodology to identify, in the ERA5 reanalysis data set, the surface energy balance regimes that correspond to the local generation of hot and cold air during summer and winter, respectively, and to separate them from cold/warm air advection regimes. The generation of cold air during winter is favored during clear, calm nights over continental or ice-covered regions, that leads to sustained radiative cooling. The regions where such conditions are most frequent are Siberia and the Canadian Arctic, which can be depicted as the two "boreal cold air reservoirs" of the northern hemisphere. Hot air generation during summer is more geographically spread than cold air, but occurs more frequently in subtropical areas including regions surrounding the Mediterranean Sea.

The framework is illustrated in detail through two case studies. The first is a cold air outbreak that affected eastern Asia during January 2023, which led to the new absolute negative temperature record for China. This event was preceded by particularly favorable conditions for cold air generation over northern Siberia. The second is the July 2022 heatwave, that led to temperatures exceeding 40°C over central England. In this case, a Lagrangian analysis suggests that the extremely high temperatures were related to strong diabatic heating not over the British Isles, but over the Iberian Peninsula in the days preceding the event.