Abstract
Modes of low-frequency variability of atmospheric circulation undergo decadal variations, which affect their spatiotemporal impact on climatic variables on long time scales. Previous studies focused either to one circulation mode or to limited geographical areas. Here, the topic is substantially extended as we provide an overview of long-term variations of all detected circulation modes in the northern extratropics in winter and their impact on relationships with temperature during the twentieth century. Circulation modes are identified by rotated principal component analysis of 500-hPa geopotential heights in the ERA-20C reanalysis; gridded surface temperature data are gained from the Climatic Research Unit Time Series (CRUTS) dataset. Temporal variations of relationships are evaluated by 15-yr running correlations between the circulation modes and surface temperature at all land grid points. Time series of running correlations with all nine circulation modes at all grid points are clustered using the partitioning around medoids (PAM) method into 18 clusters. Both composite maps and temperature advection at the 850-hPa level during specific periods of strengthened, weakened, and normal relationships with surface temperature are used for the determination of mechanisms responsible for the variation of relationships. The main mechanisms are changes in the location, shape, and intensity of centers, and formation or split of centers. These mechanisms affect mainly the intensity and direction of advection, which translate into the magnitude of relationships. Possible causes of changes in the spatial structure of 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.
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