On the coupled interdecadal variations of the extra-tropical surface air temperature and the isentropic meridional airmass transport in Northern winter

Based on the ERA5 reanalysis since the 1950s, this study investigates the coherent inter-decadal variations of the winter continental surface air temperature (SAT) between the Eurasian (EUA) and the North American continents in the past 70 years. The SAT variation over the two continents can be characterized with five typical periods successively from a 'both-warmer' (1950s-1960s), to a 'both-colder' (1960s-1980s), and then a 'EUA-warmer' (1980s-1990s) followed by another 'both-warmer' (1990s-2000s) and a 'EUA-colder' (2000s-2010s) period. Such periodic variation of the SAT is closely coupled with changes of the low isentropic-level meridional mass transport (IMMT), as manifested by the coupling relationship between the midlatitude SAT and the net IMMT out of the polar circle, in terms of their zonal patterns and geographical patterns. The SAT variation can be well reconstructed with the first two leading maximum covariance analysis (MCA) modes between the SAT and IMMT. The first leading MCA mode (MCA1) mainly dominates a 'EUA-warmer/colder' pattern, while the second (MCA2) dominates the 'both-warmer/colder' pattern. There exists a significant lead-lag correlation between the MCA1 and MCA2, which determines the changing SAT pattern in the five successive periods. Furthermore, the MCA1 and MCA2, especially their lead-lag coupling after the 1975 is significantly associated with the Pacific decadal oscillation (PDO) which dominates the much-intensified MCA1 and the related 'EUA-warmer/colder' SAT pattern since then. The Atlantic multi-decadal oscillation mainly dominates the third leading MCA mode with it's effects on the SAT over Europe coincidently to amplify the PDO-related SAT changes.