The stratosphere is the second main layer of the Earth's atmosphere, just above the troposphere and below the mesosphere. About 20% of the mass of the atmosphere is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and colder layers closer to Earth. The increase in temperature with altitude is the result of the absorption of the sun's ultraviolet radiation by ozone. This is in contrast to the troposphere, near the Earth's surface, where temperature decreases with altitude.
The stratosphere is a region of intense interactions between radiative, dynamic and chemical processes, in which the horizontal mixture of gaseous components proceeds much more rapidly than the vertical mixture. The general circulation of the stratosphere is called the Brewer-Dobson circulation, which is a unicellular circulation, spanning from the tropics to the poles, consisting of the tropical upwelling of air from the tropical troposphere and the extra-urban influx of air. Stratospheric circulation is a predominantly wave-driven circulation, as the tropical outcrop is induced by wave force by westward-propagating Rossby waves, in a phenomenon called Rossby-Wave pumping.
An interesting feature of the stratospheric circulation is the quasi-biennial oscillation (QBO) in tropical latitudes, which is driven by gravity waves that are generated convectively in the troposphere. L to QBO induces a secondary flow which is important for the overall transport stratospheric tracers such as ozone or water vapor.
Another large- scale feature that significantly influences stratospheric circulation is the breakdown of planetary waves, resulting in intense near-horizontal mixing at mid-latitudes. This break is much more pronounced in the winter hemisphere, where this region is called the surf zone. This break is due to a highly non-linear interactionbetween vertically propagating planetary waves and the isolated high-potential vorticity region known as the Polar Vortex. The resulting breakage causes a large-scale mixing of air and other trace gases along the mid-latitude swell zone. The timescales of this rapid mixing are much smaller than the much slower upwelling timescales in the tropics and the submerged ones in the extratropical.
