CO 2 -induced heat source changes over the Tibetan Plateau in boreal summer-Part I: the total effects of increased CO 2

The concentration of CO2 in the atmosphere has been increasing, but its effects on the heat source (HS) over the Tibetan Plateau (TP) are unclear. Aimed at understanding these effects, at first, present study evaluated the CMIP5 (phase 5 of the Coupled Model Intercomparison Project) models and found that their multi-model ensemble (MME) reproduces the spatial pattern of the HS over the TP during June–September (hereafter JJAS) in observation reasonably well. Then, we used the MME to investigate the response of the JJAS HS over the TP to increased CO2. In response to increased CO2, the JJAS HS increases significantly. In terms of the response pattern and TP-averaged results, the increase in HS is mainly contributed by the latent heating (LH), which is due to moisture increases (with the lower level stronger than the upper level) and evaporation intensification led by CO2 change. The leading two intermodel spreads feature a nearly uniform structure and a central-southeastern TP dipole structure, respectively, and account for half of the total intermodel variance. The latent heating is mainly responsible for the spreads. The intensified radiative cooling of the atmosphere slightly dampen the TP-averaged HS increases. Over the TP, when CO2 increases, the atmospheric column above warms. Accordingly, the net longwave radiation flux out of the atmosphere column enhances, resulting in the intensified radiative cooling over the TP.