Missing Increase in Summer Greenland Blocking in Climate Models

Atmospheric blocking events in summer over Greenland promote melting of the Greenland ice sheet, a major contributor to sea level rise. Recent observations indicate that, during the early part of the twenty-first century, summertime atmospheric blocking over Greenland has become markedly more frequent. This increasing trend in blocking activity appears to be missing in climate model simulations. The temporal evolution of Greenland blocking (GB) is assessed here in a larger ensemble of around 500 members from the CMIP6 archive. The observed increase in GB is also not present in the larger ensemble of members considered: the maximum 10-year trend in GB in the reanalysis lies almost outside the distributions of trends in the climate models and a period of such increased GB activity is rarely found in the full historical period of the model simulations.

The climate model simulations do however suggest that variability in GB is partly driven by sea surface temperatures (SSTs) and/or sea ice concentrations (SICs), as well as/or by anthropogenic aerosols, but the response of the models to these forcings may be too weak. To understand if it is forcing from the surface or from aerosols that dominates, a set of climate model experiments is performed with the Met Office climate model. Historical simulations are performed with prescribed SSTs/SICs and both with and without aerosol forcing. Results from the experiments indicate that variability in SSTs/SICs is key for capturing variability in GB. Further work is required to understand why climate models cannot represent a period of increased GB, how SST/SIC variability relates to that in GB, and what implications these have for future projections of Greenland climate change.