Understanding the recent changes in summer atmospheric circulation on the Northern Hemisphere: the roles of external forcing and sea surface temperature variability.

The summer climate in the Northern Hemisphere during recent decades has shown distinct trend patterns, with warming hotspots that spatially match with the ridges of a circumpolar atmospheric wave pattern. The drivers behind this wave-like trend and warming pattern are not yet well understood. On the one hand, the CMIP6 multi-model ensemble mean presents a high degree of spatial pattern correlation over some regions but at a very small magnitude relative to observations. When considering individual single-model ensembles, however, we find: (i) a substantial spread in the forced response across models and (ii) a large spread in pattern similarity across the different ensemble members of the same models. This suggests that a combination of both forcing and internal climate variability may have contributed to the observed changes in atmospheric circulation. In ongoing work we are aiming to better understand the specific roles of forcing and climate variability, e.g., by investigating specific composites of those simulations most closely resembling the observed trends or by constraining ocean temperature variability patterns.