Abstract
Abstract. The recently observed variability in the tropical tropopause layer (TTL), which features a warming of 0.9 K over the past decade (2001–2011), is investigated with a number of sensitivity experiments from simulations with NCAR's CESM-WACCM chemistry–climate model. The experiments have been designed to specifically quantify the contributions from natural as well as anthropogenic factors, such as solar variability (Solar), sea surface temperatures (SSTs), the quasi-biennial oscillation (QBO), stratospheric aerosols (Aerosol), greenhouse gases (GHGs) and the dependence on the vertical resolution in the model. The results show that, in the TTL from 2001 through 2011, a cooling in tropical SSTs leads to a weakening of tropical upwelling around the tropical tropopause and hence relative downwelling and adiabatic warming of 0.3 K decade-1; stronger QBO westerlies result in a 0.2 K decade-1 warming; increasing aerosols in the lower stratosphere lead to a 0.2 K decade-1 warming; a prolonged solar minimum contributes about 0.2 K decade-1 to a cooling; and increased GHGs have no significant influence. Considering all the factors mentioned above, we compute a net 0.5 K decade-1 warming, which is less than the observed 0.9 K decade-1 warming over the past decade in the TTL. Two simulations with different vertical resolution show that, with higher vertical resolution, an extra 0.8 K decade-1 warming can be simulated through the last decade compared with results from the "standard" low vertical resolution simulation. Model results indicate that the recent warming in the TTL is partly caused by stratospheric aerosols and mainly due to internal variability, i.e. the QBO and tropical SSTs. The vertical resolution can also strongly influence the TTL temperature response in addition to variability in the QBO and SSTs.
Highlights
The tropical tropopause layer (TTL) is the transition layer from the upper troposphere to the lower stratosphere in the tropics, within which the air has distinct properties of both the troposphere and the stratosphere
By regressing the temperature differences between model experiments to the respective factors for the whole simulation periods between 1955 and 2099 and projecting the regressed coefficients onto the observed trends of the respective factor during 2001–2011, the contribution of each factor has been quantified in order to explain the causes of the observed recent decadal variability seen in GPS-RO data
The total solar irradiance (TSI) and stratospheric aerosols result in weak positive correlations (0.1–0.2) with TTL temperatures
Summary
The tropical tropopause layer (TTL) is the transition layer from the upper troposphere to the lower stratosphere in the tropics, within which the air has distinct properties of both the troposphere and the stratosphere. A remarkable warming has been captured by Global Positioning System Radio Occultation (GPSRO) data in the TTL region (Schmidt et al, 2010; Wang et al, 2013) This might indicate a climate change signal, with possible important impacts on stratospheric climate: e.g. tropical tropopause temperatures dominate the amount of water vapour entering the stratosphere (Dessler et al, 2013, 2014; Solomon et al, 2010; Gettelman et al, 2009; Randel and Jensen, 2013). Warming of the troposphere and cooling of the stratosphere affect the temperature in the TTL directly, as well as indirectly, by changing chemical trace gas distributions and wave activities (SPARC-CCMVal, 2010). All natural forcings, including transit solar variability, fully coupled ocean, prescribed volcanic aerosols and nudged QBO Like the Natural run but with fixed solar radiation Like the Natural run but with fixed SSTs Like the Natural run but without QBO nudging Like the Natural run
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