The stratospheric circulation 421 Mean thermal structure

The Arctic stratosphere extends upward from about 10 km to 40-50 km altitude. It is essentially a very dry, statically stable region where ozone dominates the absorption of solar ultraviolet radiation and emission of infrared radiation. In the troposphere (the region between the stratosphere and the surface, which contains about 80% of the atmospheric mass), the mean thermal structure is maintained by an approximate balance between infrared radiative cooling to space, surface radiative heating and the subsequent vertical transport of sensible and latent heat by turbulent processes, and large-scale horizontal heat transport by synoptic-scale disturbances. The dominance of surface radiative heating has the result that surface and tropospheric temperatures are highest in the equatorial regions and decrease toward the poles. As the troposphere is primarily heated from below, there is also a rapid decrease in temperature with altitude (however, the potential temperature - the temperature that an air parcel would have if raised or lowered to the 1000 hPa surface, generally increases with height). There are of course prominent seasonal and regional departures, such as strong low-level temperature inversions in the Arctic during winter (Chapter 5). By contrast, for mean conditions in the stratosphere, the infrared cooling to space is balanced primarily by radiative heating owing to absorption of ultraviolet radiation by ozone. As a result of ozone absorption, the mean temperature increases upward in the stratosphere (hence potential temperature increases strongly with height, meaning very stable conditions). Above roughly 50 km, we enter the mesosphere, where temperature again decreases with height due to reduced ozone absorption. In the stratosphere above about 30 hPa (roughly 25 km), the temperature decreases from the summer pole to the winter pole, in qualitative accord with radiative equilibrium conditions (in June, total solar radiation at the top of the atmosphere is greatest at the North Pole, and least at the South Pole). Winter temperatures at around 30 km in the Arctic are below -70 °C while in midsummer with 24-hour sunlight they exceed -40 °C (Andrews et al., 1987). However, in the lower stratosphere, the temperature is strongly influenced by upper-tropospheric processes. This region is characterized by a mean temperature minimum at the equator, and maxima at the summer pole and in the mid-latitudes of the winter hemisphere (Holton, 1992).

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