Stratospheric Harbingers of Boreal Summer Climate: The role of lower stratospheric processes in the trans-seasonal connection between spring and summer northern annular modes

Research over recent decades has fundamentally shifted our understanding of the polar stratosphere's role in the boreal surface climate from a passive layer to an active driver of weather and climate extremes. The polar stratosphere's influences often project onto major climate patterns such as the northern annular mode (NAM), leading to predictable surface impacts such as cold air outbreaks. However, such top-down influences are considered contingent on the existence of the stratospheric polar vortex in the winter half-year. In this study, we report a significant negative correlation between the March and high summer (July and August) NAM during the past four decades. The lower stratosphere acts as a signal transmitter and converter: the negative NAM-like anomalies in the troposphere in March induce a stratospheric response in April that evolves into a positive NAM-like pattern in late spring propagating downward to the upper troposphere in June and maintaining through the summer. The nudged simulations with separate tropospheric and stratospheric states reveal the dominant role of lower stratosphere in this trans-seasonal linkage. These results provide new insights into the seasonal prediction of summer NAM and boreal surface climate.