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Physical processes in the tropical tropopause layer and their roles in a changing climate

Abstract

Tropical climate and the composition of the global upper atmosphere are affected by the tropical tropopause layer — the atmospheric transition zone between the well-mixed, convective troposphere (up to altitudes of 12–14 km) and the highly stratified stratosphere (above about 18 km). Featuring chemical and dynamical properties that are midway between those of the troposphere and stratosphere, the tropopause layer is maintained by a complex interplay between large- and small-scale circulation patterns, deep convection, clouds and radiation. Tropospheric air enters the stratosphere primarily in the tropics. Ozone- and aerosol-related constituents of the global stratosphere, as well as water vapour content, are therefore largely determined by the composition of the air near the tropical tropopause. Over the past years, it has emerged that both slow ascent and rapid deep convection contribute to the composition and thermal structure of the tropical tropopause layer. Ice formation processes at low temperatures affect the efficacy of freeze drying as air passes through the cold tropopause region. Transport and mixing in the tropopause region has been found to be closely linked with the Asian monsoon and other tropical circulation systems. Given these connections, climate change is expected to influence the tropopause layer, for example through enhanced large-scale upwelling of air and potential changes in tropical convection, air temperature, chemical composition and cirrus.

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Figure 1: Schematic of the large-scale structure and circulation of the TTL in the latitude–height plane.
Figure 2: Satellite observations of water vapour in the lower stratosphere demonstrate the transport of air from the TTL to high latitudes.
Figure 3: Large differences exist in current estimates of radiative heating rates in the TTL, and these result in substantial differences in upward transport calculations.
Figure 4: The horizontal structure of ozone in the lower stratosphere during boreal summer shows the influence of monsoonal circulations on the TTL.
Figure 5: Interannual changes in global stratospheric water vapour are closely linked to tropical tropopause temperatures.
Figure B1: Seasonal cycle of cold-point tropical tropopause temperature.

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Randel, W., Jensen, E. Physical processes in the tropical tropopause layer and their roles in a changing climate. Nature Geosci 6, 169–176 (2013). https://doi.org/10.1038/ngeo1733

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