Study of troposphere-stratosphere coupling through the Northern Annular Mode

Coupling between troposphere and stratosphere through the Northern Annular Mode (NAM) is studied. Cluster analysis of NAM indices at various levels is used in order to reduce the number of the NAM series to two: one tropospheric and one stratospheric. Once the periods when the tropospheric and stratospheric clusters have similar variability (called "coupled") are detected, EOF analysis provides the principal modes of variability when the troposphere and the stratosphere are considered coupled and uncoupled. The spatial patterns of variability are very different depending on coupling. The most important difference is found in the spatial pattern in the troposphere (which shows a wave 3 pattern for coupled periods and a NAO-like pattern for uncoupled periods). Then, we distinguish between coupled periods with dominant stratosphere (when the influence of the stratosphere over the troposphere is greater than the influence of the troposphere over the stratosphere) or troposphere (the inverse situation) and analyze the behavior of Eliassen-Palm and Plumb fluxes in each case. (c) 2006 Elsevier Ltd. All rights reserved.