Draft:Tropical tropopause layer

The tropical tropopause layer is a feature of atmospheric circulation.

The main process for transport of air between troposphere and stratosphere as well as within the stratosphere is described by the Brewer-Dobson circulation. The circulation starts with large-scale upward transport in the tropics. The downward transport happens through a shallow cell to the low-middle latitudes and through a deeper cell to the poles, where most of the air is transported down to the troposphere. The cells are driven by breaking planetary-scale waves. The circulation explains extreme low water vapor concentrations in the stratosphere, extremely high ozone values in the troposphere near the poles or high values of sulfate aerosols in the stratosphere after tropical volcanic eruptions.

The uplifting process of the Brewer-Dobson circulation takes place at the tropical upper troposphere and lower stratosphere. This region between around 12 km, the altitude of the main convective outflow and 17 km, the altitude of the cold point tropopause is one of the definitions of the tropical tropopause layer, a transition layer which shares stratospheric and tropospheric properties, bounded horizontally by the subtropical jets.

The tropical tropopause layer is often referred to as the ”gate to the stratosphere“, being the main entrance for atmospheric tracers from the troposphere to the stratosphere. It is characterized by a decrease of water vapor and the increase of ozone across the layer with increasing height. In addition, its temperature is strongly controlled by the impact of radiative heating through absorption of solar and thermal radiation, depending on the amount of atmospheric gases. The complex interplay of radiation, temperature, convection and atmospheric gases defines the net transport of atmospheric gases to the stratosphere and the troposphere. In particular, the amount of those gases that are lifted up in the stratosphere defines the composition on a global scale, because the air entering the stratosphere in the tropics is transported poleward and thereby spreads around the globe.