Interactive annular mode links jet stream-ocean coupling to decadal Northern Hemispheric warmth

Tsubasa Kohyama , Yoko Yamagami , Shoichiro Kido, Fumiaki Ogawa, Hiroaki Miura

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 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). We detect a positive feedback mechanism that amplifies iAM to be a dominant mode by conducting a heat budget analysis and performing a pacemaker experiment with a high-resolution global climate model. The record-breaking heat waves observed across the Northern Hemisphere in 2024 are attributed to recent prolonged positive phase of iAM.