Brewer-Dobson circulation

Transport from the troposphere to the stratosphere occurs primarily in the tropics and is associated with the upward branch of the Brewer-Dobson circulation. Folkins et al. (1999) argue that the top of the tropospheric Hadley circulation in the tropics occurs at 14 km, he., well below the tropopause, and that a barrier to vertical mixing therefore exists in the tropical tropopause layer (TTL) 2 or 3 km below the thermal tropopause. Air injected above this barrier subsequently participates in generally slow vertical ascent into the stratosphere. Small-scale exchanges also take place at mid-latitudes through filamentary structures that are drawn poleward in relation with anticyclonic circulation in the upper troposphere (Chen, 1995). 

A latitude-height schematic cross section of the upper troposphere and lower stratosphere is shown in Figure 5.34. The lowermost stratosphere is sometimes referred to as the middleworld and that above the 380 K potential temperature level as the overworld. Tropospheric air enters the stratosphere principally in the tropics, as a result of deep cumulus convection, and then moves poleward in the stratosphere. By overall mass con-servation, stratospheric air must return to the troposphere. This return occurs in midlatitudes the overall circulation is called the Brewer-Dobson circulation. The midlatitude... 

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... 

Figure 13 shows the zonal-mean meridional circulation during Northern Hemisphere winter and summer calculated from the net heating rate. The latter was in turn obtained from observations of temperature and ozone made by the Upper Atmosphere Research Satellite (UARS). In the lower stratosphere, the mean meridional circulation is upward in the tropics and downward in extratropical latitudes throughout the year (this is the Brewer-Dobson circulation mentioned earlier). Above about 30 mb (25 km), the circulation exhibits a single-cell structure, directed mainly from the summer to the winter hemisphere. 

The global circulation of the stratosphere thus plays a major role in determining its temperature distribution calculations indicate that, in the absence of adiabatic warming, the winter polar stratosphere would cool to temperatures of 150-170 K, much colder than what is actually shown in Fig. 5. The stratospheric circulation also plays a crucial role in determining the distribution of chemical species. In fact, well before the stratospheric thermal budget was understood, the sense of the global circulation of the lower stratosphere was deduced by A. W. Brewer and G. B. Dobson from observations of the distribution of stratospheric water vapor and ozone, respectively. 

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