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Abstract
The midlatitude westerly jet and the eastward progression of synoptic systems are frequently interrupted by prolonged periods of atmospheric blocking, a large-scale quasi-stationary extratropical flow regime.
Here, a new blocking index based on potential vorticity and potential temperature diagnostics is defined to enable the objective identification of blocking from a dynamical standpoint. A 5-year climatology of northern hemisphere blocking is presented using this index. Furthermore, we use the ECWMF Ensemble Prediction System to give indications of the sensitivity of the blocking process to differing synoptic developments and to produce probabilistic forecasts of blocking.
Blocking episodes of 4 days or more are shown to exhibit different persistence characteristics to shorter events, showing that blocking is not just the long time-scale tail end of a distribution. Furthermore, the use of a variable blocking latitude is found to be crucial for the correct representation of blocking by a blocking index. Consequently, Pacific blocking is seen to occur further east than was previously thought. By means of two case studies of block onset, it is shown that large amplitude, upper-level synoptic waves are vital during block onset and we find evidence for the preconditioning of planetary waves towards blocking. An analysis of a year of EPS probability forecasts of blocking indicate that probabilistic blocking forecasts are more skillful than deterministic blocking forecasts at all lead times. In addition, it is shown that the onset of a blocking episode is harder to predict than its decay.
