Modulation of the interdecadal variation of atmospheric background flow on the recent recovery of the EAWM during the 2000s and its link with North Atlantic–Arctic warming

The leading mode of the atmospheric background flow over the North Atlantic–Arctic Ocean (i.e., 300-hPa meridional wind; V300-EOF1) exhibited notable decadal variations during 1979–2019, with intensified V300-EOF1 since the early-2000s. The modulation of the interdecadal change in the East Asian winter monsoon (EAWM) by the V300-EOF1 mode and its link with the concurrently warmer North Atlantic and low Arctic sea ice was explored through observational analysis and numerical simulations. The results indicated that the intensified V300-EOF1 mode enhanced the modulation of Rossby wave trains propagating from the North Atlantic–Arctic region to East Asia, which interfered with the climatological quasi-stationary waves to induce a stronger EAWM. The widespread North Atlantic–Arctic warming tended to modulate and interact with this V300-EOF1 background flow, which further enhanced the quasi-stationary Rossby waves along the midlatitude and sub-polar waveguides, resulting in a strengthened EAWM and cold East Asia. This finding was further verified by numerical experiments conducted with an atmospheric model. The model responses generally captured the observed physical processes, albeit weaker in magnitude, suggestive of a modulating role played by external forcings in the enhancement of the V300-EOF1 mode and the EAWM. Noting also the large internal variability in the atmospheric responses, with approximately half of the model years capturing a stronger-than-average East Asian cooling along with deep and strong Arctic warming. This further indicates the importance of atmospheric internal variability in the development of the V300-EOF1 mode. These results have implications for future changes in the EAWM and concomitant East Asian cooling on interdecadal timescales.