Tropospheric wave response to decelerated stratosphere seen as downward propagation in northern annular mode

Abstract

Baldwin and Dunkerton [1999] found that negative northern annular mode (NAM) anomalies sometimes descend all the way from the stratosphere into the lower troposphere. However, no viable mechanism has been proposed so far to account for the magnitude of the anomalies in the denser troposphere. Further, analysis shows that the character of the anomaly changes across the tropopause. Above the tropopause the NAM pattern is approximately zonal, and its descent represents the descent of decelerated zonal mean winds. This stratospheric change is explainable using theories similar to those for the descent of the zero-wind line associated with a major stratospheric sudden warming. However, such a reversal in the zonal mean wind rarely reaches the denser troposphere. The descent of the NAM anomalies into the troposphere may be implying a different relationship between the stratosphere and the troposphere. We note that in the troposphere the structure of the NAM has a large wave component. In some cases, this wave component appears to react to the decelerated wind configuration aloft. Here we show observations of the wave component and the zonal mean component in comparison to corresponding NAM events to show that the wave response is a sizable component of the NAM anomaly in the troposphere. We will also present a simple model calculation to show that tropospheric waves forced by topography can react to changing stratospheric winds. These tropospheric waves can project directly onto the tropospheric NAM patterns and produce anomalies in the index which appear to be connected to the negative NAM anomalies in the stratosphere. Copyright 2005 by the American Geophysical Union.