Aura Microwave Limb Sounder observations of the Northern Annular Mode: From the mesosphere to the upper troposphere

Abstract

Vertical structure and evolution of the wintertime Northern Hemisphere Annular Mode (NAM), the first Empirical Orthogonal Function (EOF) of geopotential height anomalies, are constructed from the 2005–2009 Aura Microwave Limb Sounder (MLS) measurements in the entire middle atmosphere between 316 hPa (∼9 km) and 0.001 hPa (∼90 km). This is the first report of NAM structure extending into the mesosphere. The mode appears to be robust and it accounts for up to 70% of middle-atmospheric variance before decreasing in the upper mesosphere. The vertical connection of the NAM modes suggests strong dynamic coupling between the mesosphere and stratosphere. Time evolution of the NAM suggests that the significant NAM anomalies typically appear first in the mesosphere and progress downward. NAM patterns derived from MLS observations are consistent with those derived from long-term reanalysis below the middle stratosphere. The NAM indexes show the mesospheric cooling signals during the major stratospheric sudden warmings (SSWs) in 2006 and 2009.