A monthly circulation climatology for Sweden and its application to a winter temperature case study

Atmospheric circulation is important in determining the surface climate and environment. To quantify its effect, circulation indices or classifications of circulation type are often used. In this study, the classification system developed by Lamb (1950. Quarterly Journal of the Royal Meteorological Society 76: 393-438) is applied to obtain circulation information for Sweden on a monthly basis. For that purpose, monthly mean sea-level pressure (MSLP) data from 1873 to 1995 is used to derive six circulation indices and to provide a circulation catalogue with 27 circulation types. The frequency of circulation types over different periods is computed and described. Four major types (cyclonic, C; west, W; southwest, SW; anticyclonic, A) have been identified. The catalogue and the associated indices provide a tool for interpreting the regional climate and for developing statistical downscaling models to derive regional climate change scenarios for Sweden. An example is given to illustrate the application and usefulness of this climatology. It deals with the linkage between the mean regional temperature in southwestern Sweden and the large-scale circulation. It is shown that directional flows with a westerly component clearly favour a positive temperature anomaly, while the anticyclonic condition often induces a negative anomaly. Moreover, it is the westerly or southwesterly flows that accompany extreme positive temperature anomalies, whereas anticyclonic conditions play an important role in creating extreme negative temperature anomalies. Further, the derived indices have a relationship with temperature anomalies, established via a statistical model. Stepwise multiple regression was used, leading to a successful model with only three indices. Using the model, 70% of the total variance in the temperature anomalies between 1887 and 1994 has been reconstructed from the pressure, which leads to the conclusion that the circulation is a critical determinant of the January temperature and that the derived climatology is useful in explaining the temperature anomalies. Copyright (C) 2000 Royal Meteorological Society.