Atmospheric processes governing the northern hemisphere annular mode/north atlantic oscillation

• Main Authors: Thompson, David W.J., Lee, Sukyoung, Baldwin, Mark P.
• Other Authors: Ottersen, Geir, Hurrell, James W., Visbeck, Martin, Kushnir, Yochanan
• Format: Book Part
• Language: unknown
• Published: Blackwell Publishing Ltd 2003
• Subjects: North Atlantic; North Atlantic oscillation
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/87452/; https://doi.org/10.1029/134GM05

The North Atlantic Oscillation, referred to herein as the Northern Hemisphere annular mode (NAM), owes its existence entirely to atmospheric processes. In this chapter, we review the structure of the NAM in the atmospheric general circulation, discuss opposing perspectives regarding its physical identity, examine tropospheric processes thought to give-rise to NAM-like variability, and review the role of the stratosphere in driving variability in the NAM. The NAM is characterized by a deep, nearly barotropic structure, with zonal wind perturbations of opposing sign along -55° and -35° latitude. It has a pronounced zonally symmetric component, but exhibits largest variance in the North Atlantic sector. During the Northern Hemisphere (NH) winter, the NAM is strongly coupled to the circulation of the NH stratosphere. The NAM also affects tropical regions, where it perturbs the temperature and wind fields of both the tropical troposphere and stratosphere. The structure of the NAM is remarkably similar to the structure of the leading mode of variability in the Southern Hemisphere circulation. The processes that give rise to annular variability are discussed. In the troposphere, the NAM fluctuates on timescales of -10 days and is associated with anomalous fluxes of zonal momentum of baroclinic waves across ~45°N. It is argued that the tropospheric component of the NAM exhibits largest variance in the Atlantic sector where the relatively weak thermally driven subtropical flow and the relatively warm lower boundary conditions at subpolar latitudes permit marked meridional excursions by baroclinic waves. In the stratosphere, fluctuations in the NAM evolve on timescales of several weeks. Evidence is presented that long-lived anomalies in the stratospheric NAM frequently precede similarly persistent anomalies in the tropospheric NAM. It is argued that variability in the lower stratospheric polar vortex yields a useful level of predictive skill for NH wintertime weather on both intraseasonal and seasonal timescales. The ...