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date: 20 September 2017

Atmospheric Blocking in Observation and Models

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Climate Science. Please check back later for the full article.

Blocking is a typical feature of the atmospheric circulation of the mid-latitudes of both hemispheres. It occurs when the usual zonal flow is hindered by the establishment of a large-amplitude, quasi-stationary, high-pressure meridional circulation structure that “blocks” the westerlies. The lifetime of such structures can vary from a few days to several weeks (in the most extreme cases), and their presence and persistence can strongly affect the propagation of synoptic systems and the weather of large portions of the mid-latitudes. Blocking tends to occur preferentially in certain longitudinal sectors. In the Northern Hemisphere, such sectors are the locus of Euro-Atlantic blocking and of Pacific blocking. In the Southern Hemisphere, blocking frequencies are generally lower and the longitudinal localization is less pronounced, but Australian and Andean Blocking are particularly relevant.

Since the pioneering observational works of Berggren, Bolin, & Rossby, and of Rex, between 1949 and 1950, atmospheric blocking has been the object of innumerable observational and theoretical studies that tried to sharpen the picture of the synoptic phenomenon (starting from a commonly accepted structural definition), study its climatology in terms of the geographical distribution of its frequency of occurrence and the associated seasonal and inter-annual variability, and find a satisfactory theoretical model of its dynamic development that could account for its observational characteristics. Such theoretical studies focused on a wide range of possible dynamic mechanisms, from large-amplitude planetary-scale waves dynamics, multiple equilibria circulation regimes, large-scale anticylones forcing by synoptic-scale eddies, finite-amplitude non-linear instability theory, influence of sea surface temperature (SST) anomalies, and others.

Since the advent of widely available, numerically produced, global medium-range weather predictions, with the establishment, in the late seventies, of the European Centre for Medium-Range Weather Forecasts, it became relevant to assess the ability (or the lack of it) of models to forecast blocking with the correct space-time characteristics, such as location, time of onset, lifespan, and decay. Early studies showed how models had substantial difficulties in correctly representing blocking, possibly in connection with their large systematic (mean circulation) errors.

Although such model errors have been progressively and continuously decreasing in the course of time, blocking remains a challenge as an initial-value problem for global weather prediction models. The difficulties of correctly predicting blocking shown by weather forecasting models are partially reflected in the problems that global climate models have in representing blocking climatology. Such problems have negative consequences not only on the ability of the models to represent the observed climate, but also on the possibility of producing high quality seasonal-to-decadal climate predictions, for which representing the correct space-time statistics of blocking occurrence is, especially for certain geographical areas, of paramount importance.