Intraseasonal variability of the extratropical tropopause

It is still a subject of discussion which processes determine the tropopause height, especially at extratropical latitudes. The basic underlying problem is the stabilization of the troposphere at these latitudes, as an idealized tropopause model implies both a radiative and a dynamical constraint, the latter being related to the mechanism of stabilization. The role and importance of both constraints have been widely discussed in the literature although there is still no consensus on what determines the height of the tropopause.

In this presentation, we describe the intraseasonal variability of the zonal-mean extratropical tropopause in the Northern Hemisphere extended winter by applying a principal component analysis to 30 years of deseasonalized daily tropopause-height data. Additionally, an index is computed to measure the variability in the extratropical-mean tropopause. The results show that changes in the extratropical mean tropopause are much smaller than the variability represented by the leading EOFs, which have dipolar or tripolar structures and very small means. This suggests that it is much easier to deform the tropopause than to change its mean height. The leading tropopause EOF is strongly correlated with the Artic Oscillation.

We then investigate the relation between these modes of tropopause variability and the variability of stratification. The leading mode of stratification variability peaks at and changes sign across the tropopause, and may thus be described as a sharpening of the tropopause. The second mode is deep and has fairly uniform tropospheric structure, with compensating changes at tropopause levels. A strong correlation is found between this mode of variability and changes in the extratropical mean tropopause, consistent with expectations based on the radiative constraint.