Australia: The Land Where Time Began

A biography of the Australian continent 

Climate Change - The Roles of Physical processes in the Tropical Tropopause Layer

The tropical tropopause layer, the transition zone in the atmosphere that separates the troposphere, that extends up to 12-24 km altitude, the well-mixed, convective zone from the stratosphere that extends above about 18 km altitude, that is highly stratified. The tropopause is a layer that has dynamical and chemical properties the lie midway between those of the troposphere and the stratosphere, that is maintained by a complex interplay between large- and small-scale circulation patterns including deep convection, clouds and radiation. The entrance of tropospheric air into the stratosphere occurs mostly in the tropics, therefore the atmosphere near the tropopause in tropical regions largely determines the constituents of the global stratosphere such as ozone, aerosols and water vapour. The composition and thermal structure of the tropical tropopause layer has been found over the past 3 years to result from both slow ascent and deep convection. The efficacy of freeze drying as air passes through the cold tropopause region is affected by the formation of ice at low temperatures. It has been found that in the tropopause region transport and mixing are closely linked to the Asian monsoon, as well as to other tropical circulation systems. The tropopause layer is predicted to be influenced by climate change, as a result of these connections, such as the enhancement of large-scale upwelling of air and the potential tropical convection changes, air temperature, chemical composition and cirrus.

Coupling of the extratropics with transport within the tropical tropopause layer (TTL) is better quantified as a result of chemical observations and chemical transport models. Air in the TTL is often supersaturated, as suggested by the weight of evidence, and the assumption that is often made in global models, of complete removal of water vapour in excess of saturation should be modified. Novel perspectives on the TTL circulation have been provided by satellite observations of constituents and clouds which have allowed calculations such as comprehensive rates of radiative heating. The Asian monsoon anticyclone has been identified as the dominant feature of circulation in the boreal summer, which has effects on the TTL and global stratosphere that are far-reaching.

Sources & Further reading

1. Randel, William J., and Eric J. Jensen. "Physical Processes in the Tropical Tropopause Layer and Their Roles in a Changing Climate." Nature Geosci 6, no. 3 (03//print 2013): 169-76.