Ocean Circulation and Climate Dynamics

Decadal Variability in Tropical Tropopause Temperatures

The tropical tropopause layer (TTL) between 12 and 19km has warmed by 1K during the last decade, as shown in a unique, high vertical resolution GPS radio-occultation (RO) data set (Wang et al., 2013). The TTL is the transition layer between the upper troposphere and the lower stratosphere in the tropics and characterized by distinct tropospheric and stratospheric air properties. The TTL is therefore a key region for stratosphere-troposphere exchange. It controls not only the amount of trace gases entering the stratosphere but also the radiation budget for surface climate through the relative minimum of the tropopause, i.e. the cold point tropopause (CPT). Due to global warming, the troposphere is warming and the stratosphere is cooling but little is known so far about the behavior of the layer in between.

What is the reason for the recent increase in tropical tropopause temperatures? Is this a sign of anthropogenic warming? Is it due to internal or natural climate variability? What are the contributing factors to this warming? Wang et al. (2013) showed for the first time that the increase in TTL temperatures is well captured in chemistry-climate model experiments using observed SSTs and a higher vertical resolution. Increasing TTL temperatures is equivalent to a decrease in static stability in the lower stratosphere and a reduced transport of trace gases such as water vapor. Recent studies with a chemistry-climate model coupled to an interactive ocean suggest that this TTL warming is due to internal climate variability, in particular to tropical SSTs and zonal winds in the tropical lower stratosphere (Wang et al., 2014).

 

References:

  • Wang, W., K. Matthes, T. Schmidt, and L. Neef (2013), Recent variability of the tropical tropopause inversion layer, Geophys. Res. Lett., 40, doi:10.1002/2013GL058350.
  • Wang, W., K. Matthes, and T. Schmidt (2014), Quantifying contributions to the recent temperature variability in the tropical tropopause layer, Atmos. Chem. Phys. Discuss., 14, 22117-22153, doi: 10.5194/acpd-14-22117-2014.

 

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