Controls on projected climate extremes in two regional ensembles for the UK

As part of the UK Climate Projections 2018, a perturbed physics ensemble of regional climate model simulations was created. Each of the 11 member was created using a variant of the Hadley Centre's global and regional models, using an RCP8.5 scenario and a Europe-wide 12km grid. Here we compare the projected climate changes of this ensemble to those projected by the EuroCORDEX multi-model ensemble over the UK. The two ensembles exhibit biases of comparable magnitudes during the historical period, but project increasingly divergent trends in future climate change. We show that for some indices, the ensembles sample the same future space, but in some cases do not even overlap - despite the wide spread in each ensemble. The effects of these diverging trends are illustrated in a case study of compound indices of extreme weather, such as the Fire Weather Index (derived from temperature, wind, humidity and precipitation variables), for which bias correction is known to be particularly problematic. Future GCM projections can be constrained by the fidelity of each ensemble member’s representation of the observed climate. We suggest how this approach might be extended to RCM ensembles, given the ability to match the spatial pattern seen in observations can be determined by the RCM whilst the magnitude of the climate change response is more related to the driving global model.