Evaluation of summer Greenland blocking index of the CMIP6 models

The Greenland blocking index (GBI), an indicator of the synoptic-scale circulation over Greenland, has been anomalously positive during summer since the end of the 1990s. The GBI is negatively correlated with the North Atlantic Oscillation (NAO), which has been consistently negative during the same period. These changes in atmospheric circulation have led to an increase in Greenland summer temperatures, a decrease in cloud cover and greater ice sheet surface melt. The GBI is therefore a key indicator of melting in Greenland. However, the fifth phase of the Coupled Model Intercomparison Project (CMIP5) models do not represent any increase in GBI as indicated by observations. Until 2100, no oscillation of the GBI, and therefore no circulation changes, are projected. In this study we evaluate the new generation CMIP6 models in order to analyze the evolution of the future GBI. Our initial analysis reveals a general trend towards a decrease of the GBI until 2100. Additionally, still no model reproduces the strong increase in GBI observed over the last few decades, similar to the CMIP5 models. It is therefore important to note that significant melting events related to a highly positive GBI, as observed this summer 2019 for example, are still not considered by CMIP6 models and therefore the projected mass balance decrease could be underestimated if such general circulation change will occur in future.