Journal Article

Meridional Wind in the Upper Stratosphere: A Source of Winter NAO Predictability

King, John, Collingwood, Elizabeth, Marsh, Robert, Sinha, Bablu, Lu, Hua, Marshall, Gareth, Scaife, Adam A.

2024

Overview

Improvement of subseasonal to seasonal North Atlantic winter forecasting requires better prediction of the North Atlantic Oscillation (NAO), the dominant mode of variability in the Northern Hemisphere. Despite recent research demonstrating the importance of stratosphere‐troposphere coupling for NAO predictability, the driving mechanisms and implications are not fully understood. This study reveals that the October upper stratosphere is highly relevant to polar vortex development and predictability of winter NAO. We derive a simple index based on the strength of meridional wind in the upper stratospheric surf zone and find that anomalously poleward motion is associated with a significantly stronger polar vortex, which predicts the subsequent winter surface NAO with a correlation coefficient of r = 0.40. Plain Language Summary The North Atlantic Oscillation (NAO) is a large‐scale atmospheric system that significantly affects the weather and climate in the North Atlantic basin, especially in winter. Accurately forecasting the NAO 1–3 months ahead is challenging. However, on these timescales, more predictable factors like the stratosphere play a crucial role in modulating the NAO. The upper stratosphere plays a significant role in stratospheric dynamics, however it remains poorly understood and its potential to improve winter NAO predictions is largely untapped. Here, we create a simple index to measure the north‐south winds in the upper stratosphere during October and find that a positive index predicts a stronger winter polar vortex, leading to a more positive NAO. This results in warmer, wetter, and stormier conditions in northern Europe and the eastern US, and colder, drier conditions in southern Europe and Canada. Conversely, a negative index indicates a weaker winter polar vortex and an increased likelihood of sudden stratospheric warming events, which can often lead to extreme and prolonged cold conditions at the surface. Our findings highlight the importance of monitoring the upper stratosphere in October to improve winter NAO predictions and better understand stratosphere‐troposphere coupling. Key Points The meridional wind in the midlatitude upper stratosphere in October contains significant seasonal predictability for the winter NAO The strength of the meridional wind in this region also predicts changes in the occurrence of midwinter SSWs The winter surface impact of the October upper stratospheric wind occurs partly, but not entirely, via changes to the polar vortex

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Atmospheric forcing / Correlation coefficient / Correlation coefficients / Coupling / Meridional wind / NAO / North Atlantic Oscillation / Northern Hemisphere / Ocean-atmosphere system / Polar vortex / Predictions / Seasonal forecasting / Stratosphere / stratosphere‐troposphere coupling / Stratospheric dynamics / Stratospheric warming / subseasonal‐to‐seasonal / Surf zone / Troposphere / Upper stratosphere / Vortex development / Vortices / Weather forecasting / Winds / Winter