Tropical Tropopause Layer

Studies of the dynamics of the tropical tropopause layer are of great interest for quantitative estimates of climate change and an understanding of the mechanisms... 

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). 

The tropical tropopause, essentially defined by the 380 K potential temperature surface (see Chapter 21), is located at about 16-17 km. Tropical convection occurs up to an altitude of about 11-12 km. Between these two levels lies a transition region between the tropopause and the stratosphere, call the tropica tropopause layer (TTL). Together with the lowermost stratosphere, the region is referred to as the upper troposphere/lower... 

It was pointed out above that the 18r W tracer observations, the excess 210Pb in the tropical stratosphere, and the radiochemical evidence from fallout samples collected by balloon can be reconciled with the slow mean motion of air upward across the equatorial tropopause and within the tropical stratosphere and outward toward higher latitudes at stratospheric levels up to about 35 km. Above 23 km. altitude, such mean motions poleward can explain the lack of significant equatorwards transport of fallout debris from Novaya Zemlya. In the stratosphere, below 23 km., meridional eddy mixing obscures the pattern of slow mean motions. However, even in this layer, little Soviet test debris mixes south of about 30 °N, suggesting that poleward mean motions restrict equator-wards transport by eddy mixing within the lower stratosphere between the equator and 30 °N. 

Rather than being an infinitely thin surface, the tropopause should be regarded as a highly dynamic layer over which the mixing ratios of chemical compounds (such as water vapor or ozone) change rapidly from their tropospheric to their stratospheric values. This layer behaves as a quasi-material entity in the tropics, it nearly coincides with the isentropic surface of 380 K. In the extratropics, it corresponds to the iso-surface of 2-4 potential vorticity units, and includes a mixture of air with tropospheric and stratospheric characteristics (e.g., tracer mixing ratios). 

It needs to be emphasized that the vertical temperature profile varies depending on the geographic latitude and solar activity. For example, the tropopause is essentially higher and it has lower temperatures above tropical areas as compared to those above the polar regions. In the lowest layers of the stratosphere, the temperature profile depends on the geographic situ-... 

---