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 classification s of circulation type are often used. In this study, the classification sys tem developed by Lamb (1950. Quarterly Journal of the Royal Meteorological Society 76: 393-438) is applied to obtain circulation information for Swede n on a monthly basis. For that purpose, monthly mean sea-level pressure (MS LP) data from 1873 to 1995 is used to derive six circulation indices and to provide a circulation catalogue with 27 circulation types. The frequency o f circulation types over different periods is computed and described. Four major types (cyclonic, C; west, W; southwest, SW; anticyclonic, A) have bee n identified. The catalogue and the associated indices provide a tool for i nterpreting 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 cl imatology. It deals with the linkage between the mean regional temperature in southwestern Sweden and the large-scale circulation. It is shown that di rectional flows with a westerly component clearly favour a positive tempera ture anomaly, while the anticyclonic condition often induces a negative ano maly. Moreover, it is the westerly or southwesterly flows that accompany ex treme positive temperature anomalies, whereas anticyclonic conditions play an important role in creating extreme negative temperature anomalies. Furth er, the derived indices have a relationship with temperature anomalies, est ablished via a statistical model. Stepwise multiple regression was used, le ading to a successful model with only three indices. Using the model, 70% o f the total variance in the temperature anomalies between 1887 and 1994 has been reconstructed from the pressure, which leads to the conclusion that t he circulation is a critical determinant of the January temperature and tha t the derived climatology is useful in explaining the temperature anomalies . Copyright (C) 2000 Royal Meteorological Society.