Impact of stochastic physics on the representation of atmospheric blocking in EC-Earth3

Atmospheric blocking is a synoptic-scale phenomenon that consists in an obstruction of the normal easterly progression of weather patterns in the midlatitudes, leading to persistent atmospheric conditions sometimes associated with extreme weather. State-of-the-art climate models systematically underestimate winter atmospheric blocking frequency, especially over Europe. This is often attributed to a poor representation of small-scale processes that are fundamental for the onset and maintenance of blocking events. Here, we explore how the implementation of two stochastic parameterizations, namely the Stochastically Perturbed Parameterization Tendencies (SPPT) scheme and the Stochastic Kinetic Energy Backscatter (SKEB) scheme, influences the representation of Northern Hemisphere winter blocking in EC-Earth3. Surprisingly, the activation of the two stochastic schemes has detrimental effects on blocking representation. Such deterioration is attributed to changes in the mean winter atmospheric circulation, primarily manifested in a strengthening of the mid-latitude jet stream and an intensification of the Hadley Cell. Ultimately, these circulation differences arise from a modified condensation process in tropical clouds that impacts the tropical stationary eddy activity, which in turn modifies the zonal momentum balance. Our findings reconnect with earlier literature on similar experiments and suggest that the activation of stochastic parameterizations may require a retuning of the model to correct for significant biases in the mean atmospheric circulation.