Spatially sliding classifications: climatology and trends

The widely used concept of classifications of circulation patterns has recently been extended towards spatially sliding classifications. They consist of large sets (arrays) of classifications conducted independently for individual grid points.

Until today, the Jenkinson-Collison (JC) classification scheme, which is based on threshold values of geostrophic wind speed, direction, and vorticity, has almost exclusively been used in sliding classifications. Nevertheless, other classifications methods are at hand that can be used in the spatially sliding context. In this contribution, we compare sliding JC classifications with sliding Grosswettertypen classifications, the latter being based on correlations with purely zonal, purely meridional, and purely circular flow patterns. The analysis is conducted for sea level pressure; it covers the Euro-Atlantic domain (30°W to 45°E; 30°N to 75°N).

We map frequencies and persistence of individual circulation types as well as estimate their long-term changes (trends). The spatial distribution of frequencies is in line with other characteristics of atmospheric circulation: purely cyclonic types are most frequent along the major stormtrack, westerly and southwesterly types are most frequent south of it, whereas easterly and northeasterly types occur most often north of it. Therefore, sliding circulation classifications can be considered an effective and intuitive tool for describing atmospheric circulation, which aggregates information on various types of circulation features (stormtracks, cyclones, blocking anticyclones, etc.) into a series of maps of occurrence frequencies.

The two classification schemes result in similar spatial distribution of frequencies of circulation types except the type with a weak flow (undetermined type), which occurs excessively often in the JC classification especially in the Mediterranean in summer at the expense of the purely anticyclonic type. This points to the need for allowing the threshold values in the JC method to vary with latitude.