[A-CG36] Extratropical oceans and atmosphere

[ACG36-10] An air-sea interactive annular mode formed by the Pacific Decadal Variability and the Northern Annular Mode

*Tsubasa Kohyama¹, Yoko Yamagami², Shoichiro Kido², Fumiaki Ogawa³, Hiroaki Miura⁴ (1.Department of Information Sciences, Ochanomizu University, 2.Japan Agency for Marine-Earth Science and Technology, 3.Mie University, 4.The University of Tokyo)

Keywords:jet stream, western boundary current, Pacific Decadal Variability, Northern Annular Mode

The atmospheric jet stream governs the distribution and intensity of midlatitude weather systems and climate variability. In the Northern Hemisphere, meridional migrations of the jet stream are directly linked to the frequency and magnitude of extreme weather events. While previous studies have established that jet stream fluctuations are strongly modulated by spatio-temporal variations in diabatic heating, the relationship between low-frequency modes of atmospheric and oceanic variability remains unclear. Here we propose a hypothesis that the Northern Annular Mode (NAM) and the Pacific Decadal Variability (PDV), the most dominant modes of climate variability in the Northern Hemisphere, constitute an annular-shaped air-sea coupled mode. The singular value decomposition analysis of sea surface temperatures and lower tropospheric zonal winds shows that the most dominant covariability over the Northern Hemisphere closely corresponds to both the conventional PDV and NAM patterns. The extracted PDV-like and NAM-like modes explain 46% of each other's variance, suggesting substantial coupling, and we refer to this hypothesized air-sea coupled phenomenon as the interactive Annular Mode (iAM). Through a heat budget analysis and a pacemaker experiment using a high-resolution global climate model, we discuss a positive feedback mechanism that amplifies iAM to be a dominant mode. The unprecedented heat waves in 2024 observed all over the Northern Hemisphere are explained as a record-high positive event of iAM.