Northern annular mode and northeast Asian temperature anomalies: exploring causality and the causes of its non-stationarity

To better understand the dynamics and impacts of major atmospheric modes on regional climates, the temporal variability of the causal strength between the Northern Hemisphere Annular Mode (NAM) and winter Surface Air Temperature (SAT) over Northeast Asia from 1950 to 2024 is explored, with particular emphasis on the modulating effect of Pacific Decadal Oscillation (PDO) phase shifts. Decadal variations in causal strength between SAT and NAM at different pressure levels are clearly observed. The influence of the stratospheric NAM on SAT is more pronounced than that of the tropospheric NAM. At lower levels, the causal strength remained relatively weak and exhibited two notable periods of decline: from the late 1950s to the mid-1970s, and after 2008. In the mid-troposphere, a weakening trend occurred from 1975 to 1995, followed by a substantial recovery that peaked around 2008. In the stratosphere, the causal strength remained consistently strong, with 10hPa showing a significant upward trend from the early 1970s to the late 1980s. The relationship between NAM and SAT during winter is strongly modulated by PDO phase shifts, which affect the intensity of the Aleutian Low, thereby impacting the Pacific center of NAM and altering its influence on Northeast Asia. Additionally, variations in the strength of the stratospheric polar vortex (SPV) also modulate the phase of NAM, affecting winter SAT by altering the intensity of the westerlies and the East Asian Trough (EAT).