Temporal variation of relationships between circulation modes and temperature during the 20th century

Circulation modes undergo substantial decadal variability, which also affects spatiotemporal impact of modes on climatic variables on long time scales. Previous studies focused either to one circulation mode or to limited geographical area. Here, the topic is substantially extended as we provide an overview of long-term variations of nine circulation modes in winter and their impact on relationships eith temperaure during the 20^th century over the Northern Extratropics. Circulation modes are identified by rotated Principal Component Analysis of 500 hPa geopotential heights in the ERA-20C reanalysis; gridded surface climatic data are gained from the CRUTS dataset. Temporal variations of relationships are evaluated by 15-year running correlations between circulation modes and climatic variables at all land gridpoints. Time series of running correlations with all nine circulation modes at all gridpoints are clustered using the partitioning around medoids (PAM) method into 18 clusters for temperature. Both composite maps and temperature anomalies in the 850 height during specific periods of strengthened, weakened, and normal relationships with climatic variables are used for determination of mechanisms responsible for the variation of relationships. The main mechanisms are changes in the location, shape, and intensity of centres, and formation or split of centres. These mechanisms affect mainly the intensity and direction of advection, which translate into the magnitude of relationships. Possible causes of changes in circulation modes are linked with El Niño‐Southern Oscillation (ENSO), shifts of the Pacific Decadal Oscillation (PDO), and long-term changes in the sea ice extent. The results of this study may be used for climate modelling.