Spatially sliding classifications of atmospheric circulation patterns: a tool to explain skewness of day-to-day temperature differences

Classifications of circulation patterns have been widely used in synoptic climatology for decades. Recently, this concept has been extended towards spatially sliding classifications, i.e., large sets (arrays) of classifications conducted independently for individual grid points.

We make use of spatially sliding classifications in an attempt to explain asymmetry of statistical distributions of day-to-day temperature difference (DTD). Over most of Europe, negative skewness of DTD prevails in summer, while positive skewness dominates in winter. The asymmetry is reflected in small temperature increases occurring more often than small temperature decreases in summer, the opposite holding for winter. Unlike for temperature itself, mechanisms governing the asymmetry of DTDs have only been hypothesized but investigated neither in sufficient detail nor on scales larger than local.

We employ the Jenkinson-Collison (JC) method to classify atmospheric circulation for grid points covering Europe and North Atlantic. The version of the JC method with 11 types (8 directional, 2 (anti)cyclonic, and one with weak pressure field) is adopted. For each grid point, we identify circulation types with the largest asymmetry of small DTDs (i.e., with the largest difference between the number of small day-to-day warmings and coolings). ‘Small’ is defined here as the inner 50% of values of a DTD distribution. Although the circulation types most conducive to the DTD asymmetry vary regionally, there is a pronounced tendency for the anticyclonic type, the type with a weak pressure field, and types with warm advection from the southern quadrant to contribute to the DTD asymmetry in summer. In winter, the anticyclonic type and types with cold northerly to easterly advection largely contribute to the DTD asymmetry.

Next, we analyze surface energy budget, and particularly radiation and energy fluxes, for the circulation types conducive to the DTD asymmetry at selected representative grid points. We demonstrate that it is indeed radiative processes that strongly contribute to the prevalence of small day-to-day warmings in summer and of small day-to-day coolings in winter.

We use ERA5 reanalysis as input dataset, with grid step of 1.25° x 1.25°. Sea level pressure is used as a circulation variable. Maximum temperature in summer and minimum temperature in winter are used to describe DTD. Analyzed period is 1940-2022.