[A-CG36] 中緯度大気海洋相互作用

[ACG36-P16] Weakening of the North Atlantic Oscillation under global warming in atmospheric large ensemble simulations

*川村 岳¹、小坂 優¹ (1.東京大学 先端科学技術研究センター)

キーワード：北大西洋振動、地球温暖化、エネルギー変換

The North Atlantic Oscillation (NAO) is a meridionally dipolar pressure anomaly pattern across the North Atlantic polar-front jet, which induces anomalous winter weather conditions in surrounding regions. This study investigates modulations of the winter NAO under global warming based on the large-ensemble atmospheric simulation dataset d4PDF. While the spatial structure exhibits a slight eastward extension, its amplitude significantly decreases over the North Atlantic Ocean basin. This weakening is examined in light of the energetics framework. Recent studies highlight the crucial role of baroclinic energy conversion (CP) from the climatological mean field in maintaining the NAO, which is well captured in d4PDF. The heat flux associated with the baroclinic structure of the NAO acts to relax the climatological temperature gradient, transferring the available potential energy to the NAO. This energy conversion mainly occurs in association with the zonal climatological temperature contrast between northeastern North America and the subpolar North Atlantic. Under global warming, the efficiency of CP declines markedly due to a weakening of the climatological zonal temperature gradient. The pronounced warming over northeastern North America, combined with relatively muted warming over North Atlantic, weakens the climatological land-sea thermal contrast, which is robust across CMIP6 models. The extent of the NAO weakening thus depends on the background warming pattern in the North America-North Atlantic sector. Despite the weakened NAO-associated atmospheric circulation anomalies, their eastward extension and increased background water vapor can enhance precipitation anomalies over western Europe under global warming.