Projected future changes in Omega blocking and subtropical ridges and their relationship to European heatwaves in two SMILEs

Over the last few decades, Europe has emerged as a hotspot for heatwaves (HWs), with prominent examples such as 2003, 2010, 2018 and 2022. The development of European HWs is often linked to atmospheric blocking, in summer most notably in the form of a so-called Omega blocking. However, not all HWs necessitate atmospheric blocking, particularly over Southern and Central Europe, where they can also be caused by poleward extensions of the subtropical high pressure belt, so-called subtropical ridges. These can be positioned such that they induce southward flow anomalies of hot and dry air, which have been suggested before as an explanation of the overproportional increase in heat extremes over Europe.

Future projections show a clear increase in the number and intensity of HWs but are inconclusive with respect to changes in atmospheric blocking. Moreover, subtropical ridges are generally not considered, although they may play a greater role in a warmer climate. We present ongoing research into CMIP6-projected changes of both Omega (and other) atmospheric blocking and subtropical ridges for Europe. Besides overall trends, we are particularly interested in the most intense and most persistent HWs and whether their link to large-scale atmospheric flow anomalies such as Omega blocking or ridges might change.

Preliminary results based on two large ensembles (SMILEs; MPI-GE and SMHI-LENS) suggest that subtropical ridges are projected to increase in frequency during summer in Western and Central Europe, while for Omega and other atmospheric blocking a small reduction or no change is identified. Particularly during HWs, the frequency of ridge detection increases substantially for a warmer climate in both ensembles. This is of particular interest as such ridge-type HWs are generally found to more intense (albeit not more persistent) than Omega-type HWs, both in the present and the projected future climate.