Analysis of tropopause variability in observations and in an idealized model

Abstract

The focus of this thesis is the study of the extratropical tropopause. The tropopause is a distinctive feature of the vertical structure of the atmosphere, separating two regions, troposphere and stratosphere, with very different dynamical and chemical characteristics. The abrupt change in air properties found when moving from one layer to another serves as the basis for a wide variety of definitions. The first and most widely used definition is the thermal tropopause, based on the discontinuity in the lapse-rate. The dynamical tropopause is based on the discontinuity in the isentropic potential vorticity. Chemical definitions are also possible, based on the discontinuity in the concentration of tracer gases above and below the tropopause. The greatest variability in tropopause height is found in midlatitudes in synoptic scales, associated with the potential vorticity anomalies caused by the eddies at levels close to the tropopause. Seasonal variability, of lesser amplitude, has the form of a single wave in midlatitudes of both hemispheres and in the southern polar latitudes –where the cycle is inverted with respect to midlatitudes, while some regions of the Arctic exhibit a double-wave seasonal cycle. Previous studies attributed this interhemispheric asymmetry to the different strength of the residual circulation in both hemispheres, due to the different land-sea distributions. The tropopause has been traditionally explained as the transition from a stratospheric temperature profile in radiative equilibrium to a dynamically adjusted profile in the troposphere. Radiative-convective models seem to work well for the tropical regions, while in the extratropics it is less clear which dynamical mechanism adjusts the troposphere. Several possibilities, such as dry and moist baroclinic equilibration, have been proposed.