Thermal characteristics of the cold-point tropopause region in CMIP5 models

Abstract

The climatology, seasonality, and intraseasonal to interannual variability of the temperature field near the cold-point tropopause (CPT) are examined using the state-of-the-art climate models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5). Both historical simulations and future projections based on the Representative Concentration Pathway (RCP) 8.5 scenario are used to evaluate model performance and to identify potential changes at the CPT focusing on the 100hPa and zero-lapse-rate (ZLR) temperatures. It is found that historical simulations successfully reproduce the large-scale spatial structure and seasonality of observed temperature and reasonably capture variability associated with El Nino-Southern Oscillation and equatorial waves near the CPT. However, the models show nonnegligible biases in several aspects: (1) most models have a warm bias around the CPT, (2) large intermodel differences occur in the amplitude of the seasonal cycle in 100hPa temperature, (3) several models overestimate lower stratospheric warming in response to volcanic aerosols, (4) temperature variability associated with the quasi-biennial oscillation and Madden-Julian oscillation is absent in most models, and (5) equatorial waves near the CPT exhibit a wide range of variations among the models. In the RCP 8.5 scenario, the models predict robust warming both at the 100hPa and ZLR levels, but cooling at the 70hPa level. A weakened seasonal cycle in the temperature is also predicted in most models at both the 100 and 70hPa levels. These findings may have important implications for cross-tropopause water vapor transport and related global climate change and variability.