Ensemble sensitivity analysis of Greenland blocking in medium-range forecasts

The North Atlantic Oscillation (NAO) is the leading mode of variability in the large-scale circulation over the North Atlantic in winter, and strongly influences the weather and climate of Europe. On synoptic time-scales, the negative phase of the NAO often corresponds to the occurrence of a blocking episode over Greenland. Hence, the dynamics and predictability of these blocking events is of interest for the prediction of the NAO and its related impacts over a wide region. Ensemble sensitivity analysis utilises the information contained in probabilistic forecast ensembles to calculate a statistical relationship between a forecast metric and some precursor condition. Here the method is applied to 15-day forecasts of a set of 26 Greenland blocking events using the state-of-the-art European Centre for Medium-Range Weather Forecasts (ECMWF) forecasting system. The ensemble sensitivity analysis shows that Greenland blocking does not develop in isolation in these forecasts, but instead the blocking is sensitive to remote precursors, such as 500 and 50 hPa geopotential height, particularly in the low-frequency flow. In general, there are more significant sensitivities to anomalies in the Tropics than in the polar regions. Stratospheric sensitivities tend to emerge at later lead times than tropospheric sensitivities. The strongest and most robust sensitivities correspond to a Rossby wave precursor reaching from the Pacific basin across North America.