Extratropical pump

As air is transported rapidly upward, for example in a convective system, cooling occurs (see Chapter 2), leading to the condensation of water as ice crystals. Because of this removal of water as moist tropospheric air rises, air in the stratosphere is very dry, of the order of a few ppm. Some water is also produced directly in the stratosphere from the oxidation of CH4 and H2. The so-called extratropical pump then moves the air poleward and downward at higher latitudes (Path I), warming the air as it descends. 

FIGURE 12.3 Schematic of wave-driven extratropical pump that drives global-scale transport from the tropical troposphere to the stratosphere and then poleward. Three possible paths for stratosphere-troposphere exchange (STE) are shown. (Adapted from Holton et al., 1995.)... 

The formalism of the transformed Eulerian mean circulation shows that meridional transport in the middle atmosphere is generated primarily by non-local momentum forcing associated with wave dissipation. This forcing, represented by the Eliassen-Palm flux divergence in equation (3.67), acts as an extratropical pump producing strong upward air motions in the tropics and downward return ... 

The classic depiction of stratospheric transport is that material enters the stratosphere in the tropics, is transported poleward and downward, and finally exits the stratosphere at middle and high latitudes. The mean meridional stratospheric circulation, known as the Brewer-Dobson circulation, is generated by stratospheric wave forcing, with the circulation at any level being controlled by the wave forcing above that level (recall Figure 5.25). This process is also called the extratropical pump. The composition of the lowermost stratosphere varies with season, suggesting a seasonal dependence in the balance between the downward transport of stratospheric air and the horizontal transport of air of upper tropospheric character. The stratosphere and troposphere are actually coupled by more dynamically complex mechanisms than the traditional model of... 

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