The atmospheric circulation changes predicted by climate models are
often described using sea level pressure, which generally shows a
strengthening of the mid-latitude westerlies. Recent observed
variability is dominated by the Northern Annular Mode (NAM) which is
equivalent barotropic, so that wind variations of the same sign are
seen at all levels. However, in model predictions of the response to
anthropogenic forcing, there is a well-known enhanced warming at low
levels over the northern polar cap in winter. This means that there is
a strong baroclinic component to the response. The projection of the
response onto a NAM-like zonal index varies with height. While at the
surface most models project positively onto the zonal index,
throughout most of the depth of the troposphere many of the models
give negative projections. The response to anthropogenic forcing
therefore has a distinctive baroclinic signature which is very
different to the NAM.

Last Updated: 2008-09-08