How North Atlantic SST Anomalies Modify the Occurrence of Atmospheric Blocking

Atmospheric blocking describes a quasi-stationary weather pattern in midlatitudes which is characterised by a disruption of the westerly flow. Within the blocking anticyclone, large-scale subsidence of air effects dissipation of clouds whereas the precipitation rate increases around the block. The stationarity of the weather system increases the probability of extreme events as heatwaves, droughts or floods. Reanalyses show (multi)decadal variability of blocking frequency. This finding encourages the investigation of potential drivers of blocking, i.e. parameters in the climate system favouring or hampering the occurrence of blocking. One important driver is the variability of sea surface temperatures (SST) in the North Atlantic. In this study, we focus on a region in the south of Greenland and investigate the relationship between SST anomalies in that region and blocking in reanalyses and a subset of selected CMIP6 climate simulations. The region in the south of Greenland is known as the so-called “North Atlantic Warming Hole” because the SST trend is negative unlike in overwhelming parts of the ocean. The negative SST trend in the south of Greenland increases the temperature gradient to surrounding, warming areas resulting in a modification of the atmospheric circulation over the Atlantic and Europe and consequently in the frequency of blocking. Our results show a decreased blocking frequency over the North Atlantic in the case of a strong warming hole, i.e. in the case of a strong negative SST anomaly in the south of Greenland. The effect is strongest in winter. Over Europe, blocking frequency tends to be higher than on average. A comparison of reanalyses and CMIP6 simulations reveals that the climate models are neither able to simulate the SST trend in the North Atlantic properly nor the link between SST anomalies and blocking. Furthermore, different realisations of one model show different SST trends and different blocking patterns indicating an incomplete modelling of physical processes and feedbacks.