Intermodel comparison of upwelling in the tropical tropopause layer by using CMIP5 models and MRI-AGCM

Annual mean climatology of the temperature and upwelling in the tropical tropopause layer (TTL) is examined by using the models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Meteorological Research Institute atmospheric general circulation model (MRI-AGCM 3.2). We focus on an issue that what causes intermodel differences in present climate simulations (1979-2003) and in future climate changes (2075-2099) based on the Representative Concentration Pathway (RCP) 8.5 scenario. Compared with the ERA-Interim reanalysis data, many present climate simulations have warm bias and weak residual mean upwelling in the tropics at around 100 hPa. The models that have weaker upwelling in the TTL tend to have smaller temperature lapse rate, associated with weaker advective cooling. Tropical mean sea surface temperature (SST) bias does not correlate with the tropical temperature bias at 100 hPa. It is plausible because the tropical upwelling at 100 hPa is not constrained by the tropical mean SST bias significantly. According to the downward control diagnosis, difference in the tropical upwelling is mainly induced by that in the meridional eddy momentum flux. In the future climate, the tropical temperature at 100 hPa increases by around 3 K in the multimodel mean. While the amount of the tropical temperature increase at 150 hPa is strongly related to that of tropical mean SST increase, the relationship between them at 100 hPa is not robust comparatively. It might be responsible for weak correlation between change in the tropical SST and that in the tropical upwelling. Intermodel difference in the tropical upwelling changes is generated by that in the meridional eddy momentum flux change, in a similar manner to the present climate. Interestingly, the models with stronger tropical upwelling at 100 hPa in the present climate have tendency to predict larger upwelling change in the future through a contribution of the meridional eddy momentum flux, even if tropical SST change is relatively small.