THE STRAToSPHERE IS a layer in the atmosphere that extends between about 9 to 31 mi. (15 and 50 km.) in altitude. It is characterized by a vertical temperature structure that is nearly isothermal (no temperature change with altitude) in the lowermost stratosphere and a pronounced inversion (increase of temperature with altitude) above. The stratosphere owes its name to the strong stratification, which is a consequence of this thermal structure.

The stratosphere plays an important role in the climate system. It contains the ozone layer, which shields the Earth's surface from harmful ultraviolet radiation and is responsible for the temperature of the stratosphere. Radiative processes in the infrared part of the electromagnetic spectrum also play an important role. Because, in the stratosphere, chemistry, dynamics, and radiative processes operate under very different conditions than in the troposphere, the stratosphere is susceptible to climatic forcings in a different way than the troposphere. As a consequence, stratospheric processes play an important role for climate variability and change.

The stratosphere was discovered independently by Teisserence de Bort and by Richard Assmann around 1900. It has been explored by balloon-borne observations since around the 1930s and by satellite observations since the 1970s.

The lower boundary of the stratosphere is the tro-popause, the altitude of which varies with latitude (higher over the tropics than over the poles), season, and on a day-to-day scale related to weather systems. The upper boundary of the stratosphere is the stratopause. Below and above the stratosphere are the troposphere and mesosphere, respectively.

Because of its thermal structure (strong static stability), the circulation of the stratosphere is quasihorizontal. The most important features of the zonal circulation are the vortices in the polar regions and the Quasi-Biennial Oscillation (QBO) in the tropics.

The large ozone opening over the poles (dark area). Stratospheric ozone blocks harmful ultraviolet radiation produced by the sun.

The polar vortices form over both poles during the corresponding winter season and vertically extend through the entire stratosphere. The Arctic vortex is subject to strong variability on short timescales (during so-called sudden stratospheric warmings, the vortex can break down completely within days) and on interannular timescales. The Antarctic vortex varies much less. The QBO is an oscillation of the zonal wind in the equatorial stratosphere, with changes from westerlies to easterlies and back to westerlies within approximately 28 months.

Compared with the zonal flow, the meridional circulation and associated vertical motion in the stratosphere are very weak but are, nevertheless, important. The meridional circulation is caused by planetary waves originating from the troposphere, which break and dissipate in the stratosphere and thereby deposit momentum, decelerating the zonal flow and inducing a meridional flow component. The meridional flow is compensated for by vertical motion in the tropics and in the polar areas, forming a single meridional circulation cell, which in the context of trace gas transport is often referred to as Brewer-Dobson circulation. Air enters the stratosphere in tropical areas. On passing the tropopause, the air loses almost all of its moisture; hence, the stratosphere is very dry and mostly cloud free. In the stratosphere, the air slowly moves upward and poleward toward the winter hemisphere (the summer hemisphere is dynamically quiet). In the subpolar and polar region, the air descends and can eventually enter the troposphere in conjunction with midlatitude weather systems. The stratospheric meridional circulation has a turnover time of one to three years.

Chemically, the stratosphere is characterized by a layer of ozone (O3) formed from atomic (O) and molecular (O2) oxygen in the presence of ultraviolet radiation. Ozone can be destroyed by catalytic processes that involve radicals of chlorine, bromine, nitrogen oxides, or hydrogen oxides. The most important source of chlorine radicals are manmade chloro-fluorocarbons (CFCs), which have caused a reduction of the ozone layer since the 1970s, as well as, since the 1980s, the Antarctic ozone hole (a substantial reduction of the total stratospheric ozone amount over Antarctica). The Montreal Protocol of 1987 and its amendments have led to a strong reduction in CFC emissions worldwide. However, because of the long lifetime of CFCs, a full recovery of the ozone layer is only expected for the mid-21st century.

The anthropogenic greenhouse effect, as well as ozone depletion, causes a cooling of the stratosphere, whereas volcanic eruptions lead to warming. The stratosphere plays an important role for climate at the Earth's surface. Perturbations of the stratospheric circulation can propagate downward and affect weather at the ground. This provides a pathway through which some of the forcings can affect climate. For instance, it is now believed that part of the climate effect of strong volcanic eruptions operates via the change in stratospheric circulation induced by the heating effect of volcanic aerosols. Similarly, changes in solar irradiance could affect climate via stratospheric ozone chemistry and their subsequent effects on circulation.

SEE ALSo: Atmospheric Vertical Structure; Climatic Data: Atmospheric Observations; Mesosphere; Stratopause; Tro-popause; Waves, Rossby.

BIBLIoGRAPHY. Mark P. Baldwin and Timothy J. Dunkerton, "Stratospheric Harbingers of Anomalous Weather Regimes," Science (v. 244, 2001); Guy P. Brasseur and Susan Solomon, Aeronomy of the Middle Atmosphere (Springer, 2005); Karin G. Labitzke and Harry van Loon, The Stratosphere. Phenomena, History, and Relevance (Springer, 1999); World Meteorological Organization, Scientific Assessment of Ozone Depletion: 2006 (World Meteorological Organization, 2007).

Stefan Bronnimann ETH Zurich, Swtizerland

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