The poleward shift of the Subtropics in the Northern Hemisphere winter: time of emergence of the climate change signals

Here we consider the areas located below the descending branch of the Hadley cell and characterized by arid or semiarid climate in the northern hemisphere. There are controversial suggestions that they are shifting northwards and this trend will continue during the 21st century.  We investigate the time-of-emergence (ToE) of corresponding climate change signals using  the Max Planck Institute Grand Ensemble (MPI-GE), a model experiment which allows by design to disentangle the role of external forcing from the internal climate variability on climate signals. Here we show that the ToE and the regions where it will occur are strongly dependent on the variable that is adopted for the analysis. For most  variables, ToE of regional subtropical expansion would occur only after the end of the 21st century and only in few irregularly distributed areas. Therefore, in spite of the consensus among projections on the future boreal subtropical broadening, the strong role played by the internal climate variability prevents to consider the observed trends in reanalyses over last decades a robust signal of anthropogenic forcing.