Investigating the inability of climate models to reproduce observed increases in summer Greenland Blocking

Summertime atmospheric blocking over Greenland promotes melting of the Greenland ice sheet, which is known to be a major contributor to sea level rise. In the early twenty-first century, strong, persistent Greenland blocking was observed, coincident with significant melt events. However, extended blocking events of comparable magnitude to those recently observed appear to be missing in climate model simulations. It is therefore crucial for modelling future sea level rise to understand where the deficiency in modelled Greenland blocking lies. 

To this end, we explore the temporal evolution of Greenland blocking (GB) in the CMIP6 archive, leveraging all available historical runs and four future socio-economic pathways. We investigate the models’ forced response to greenhouse warming, their magnitude of natural variability, and their ability to simulate persistent blocking events. We put a particular emphasis on initial condition large ensembles to separate forced responses from internal variability. The observed evolution of GB is rarely found in the full historical period of the model simulations, and forced responses suggest that GB will be less, not more, frequent in the future. Preliminary results therefore suggest that the models may be underestimating variability, especially multi-year GB events. Previous work has suggested that variability in GB is partly driven by sea surface temperatures (SST) and/or sea ice concentrations (SIC), as well as/or by anthropogenic aerosols, but the response of the models to these forcings may be too weak. To understand the discrepancy between observed and modelled GB, we run an ensemble of prescribed SST/SIC experiments, with and without aerosol forcing, using the Met Office climate mode. Preliminary results indicate that variability in SST/SIC, rather than aerosols, is key for capturing variability in GB.