Summer Greenland Blocking in observations and in SEAS5.1 seasonal forecasts: robust trend or natural variability?

Summer Greenland blocking - a persistent anticyclone in the mid-troposphere in proximity of Greenland – has shown to severely impact melting of the Greenland ice sheet. Thus, changes in its frequency and characteristics would likely affect the stability of the Greenland Ice sheet, and potentially sea level rise. Despite in recent decades the occurrence of such atmospheric pattern has seen a marked increase, such observed trend is not captured by any simulation from state-of-the-art global climate models.

Here we aim to assess whether the lack of trend is caused by (i) a misrepresentation of key physical mechanisms in climate models or (ii) whether such trend is mainly attributable to decadal variability, or both. We analyze Greenland blocking characteristics in reanalysis (ERA5) and ECMWF seasonal forecasts (SEAS5.1) over the period 1981-2023, showing that about 10% of the 1000 permutations of SEAS5.1 runs can simulate a 43-year trend equal or larger to the ERA5 one. Such capacity of partially replicating the observed trend shows that the initialization of the seasonal forecast – as well as the higher model resolution - contribute to a more realistic representation of the blocking dynamics than in freely evolving climate runs.

We then apply the Peter and Clark momentary conditional independence (PCMCI) algorithm to assess monthly causal pathways. Results show that while the relationship among Arctic temperature, snow cover, Atlantic multidecadal variability and Greenland blocking is consistent both in ERA5 and SEAS5.1, the effect of early snow melt over North America on Greenland blocking is mostly absent in SEAS5.1. Therefore, while it is possible that the observed trend is due to internal decadal variability, the misrepresentation of the snow cover processes may explain the rare occurrence of a positive trend in SEAS5.1. This deficit in representing the snow impact on the atmospheric circulation might also be the culprit of the missing trend in climate models, raising the question whether long-term projections underestimate a future increase in Greenland blocking and ice melt.

Beckmann, J., Di Capua, G., and Davini, P.: Summer Greenland Blocking in observations and in SEAS5.1 seasonal forecasts: robust trend or natural variability?, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3998, 2025.