Synergistic effects of El Niño-Southern Oscillation and Arctic Oscillation on the winter northern Hadley circulation edge

The Hadley circulation edge (HCE) is an important component of the HC, modulating the subtropical high-pressure distribution and global precipitation pattern. This study investigates the synergistic influence of winter El Niño and Arctic Oscillation negative phase (AO–) on the locations of the northern HCE (NHCE). Results show that both El Niño and AO– are individually associated with equatorward NHCE shifts, but their concurrent occurrence results in a significantly amplified displacement, reaching 1.34° latitude. Mechanism analysis indicates that El Niño and AO– jointly modulate the variations in eddy momentum flux (EMF), meridional temperature gradient (MTG), subtropical tropopause height (STH), and baroclinic instability criterion (BIC). Under the co-occurrence of El Niño and AO–, anomalous EMF divergence is observed over 15°–30°N, thereby causing an equatorward shift of the convergence center. The El Niño related warming over 5°–30°N and AO– related cooling over 30°–55°N weaken the MTG, combined with a significant decrease in STH over 30°–35°N and enhanced BIC in the latitude belt of 25°–35°N. These anomalies together contribute to the equatorward displacement of the NHCE. The synergistic impacts of El Niño and AO– are further established by using 1950–1980 datasets. The ridge regression analysis shows that variations in MTG are the dominant contributor to the anomalous NHCE shift. The study underscores the importance of the synergistic effects of climate variabilities in shaping the variations of NHCE latitude, which is important for the crucial role of NHCE latitude in impacting global climate.