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.
