Abstract
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.
Highlights
The weather and climate of Europe is strongly influenced by the large-scale circulation over the North Atlantic (e.g. Wanner et al, 2001), which controls the mean temperature and precipitation and their extremes, in winter (e.g. Trigo et al, 2004; Santos et al, 2007; Kenyon and Hegerl, 2010; Efthymiadis et al, 2011; Cattiaux et al, 2012)
The European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble prediction system, which contributes to the TIGGE project, has been shown to be more skilful than the deterministic control forecasts at all forecast lead times, both short range and medium range, despite a slight under-forecasting bias (Pelly and Hoskins, 2003b); this study evaluated only Euro-Atlantic and Pacific sector blocking
The bias develops very early in the forecast period and spreads throughout the Northern Hemisphere (NH) by the end of the first day of both forecasts. These results indicate that the bottom-up process, whereby vertically propagating Rossby waves from the troposphere disrupt the polar vortex in the stratosphere, may not be well captured by the ECMWF forecast model
Summary
The weather and climate of Europe is strongly influenced by the large-scale circulation over the North Atlantic (e.g. Wanner et al, 2001), which controls the mean temperature and precipitation and their extremes, in winter (e.g. Trigo et al, 2004; Santos et al, 2007; Kenyon and Hegerl, 2010; Efthymiadis et al, 2011; Cattiaux et al, 2012). In contrast to the quasi-stationary anticyclones that block the ambient westerly winds and associated weather systems in the midlatitudes, high-latitude blocking is characterized by an anticyclone on the poleward side of the storm track and jet stream locations, which tends to divert rather than block the westerly flow Such anticyclones are often identified as blocks by objective blocking indices. Objective analyses of weather regimes in the North Atlantic frequently distinguish a pattern with an anticyclone poleward of the storm track near southern Greenland (Vautard, 1990; Cheng and Wallace, 1993; Kimoto and Ghil, 1993) This regime results in the diversion of the storm track and the axis of maximum precipitation, with regional implications for winter rainfall amounts over Europe (Santos et al, 2013).
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