Increasing frequency and persistence of the summertime Greenland High regime not captured by a seasonal prediction model very large ensemble

Weather regimes are widely used in weather prediction, but less often to study climate variability and change. Here, we use a year-round North American regime classification to identify summertime circulation trends from 1981 to 2024. We find large increases in the frequency, persistence and interannual variability of the Greenland High (GH) regime, similar to Greenland blocking. A simple Markov model shows that the observed increased GH frequency and variability can arise from increased persistence. We then show that a 10,000-member ensemble using SEAS5 seasonal model data fails to capture the observed trend in GH frequency because persistence trends are too weak. This occurs despite SEAS5 producing summers with more GH days and individual regimes more persistent than observed, so the issue is not simply an overall lack of persistence. Hence, the missing trends must arise from fundamental model deficiencies which develop on subseasonal timescales and are not rectified by initialization.