Mechanisms of elevation-dependent warming over the Tibetan plateau in quadrupled CO2 experiments

Observations have shown that the Tibetan Plateau (TP) has experienced elevation-dependent warming (EDW) during recent decades, that is, greater warming at higher elevations than at lower elevations. However, the factors and their mechanisms driving these changes remain unclear, due to scarce radiation-related observations. In the present study, four CCSM3 experiments using the 1990 control and quadrupled (4×) CO2 levels, with fine and coarse resolutions, were examined to shed light on the mechanisms driving EDW. The differences in annual and seasonal surface temperatures (TS) between the 4× CO2 and 1990 control runs, using T85 resolution, feature clear changes with elevation. In addition, EDW 500 m above the ground surface is much weaker and almost disappears at the surface elevations higher than 2000 m. This implies that the greater warming mainly occurs at the near surface with higher elevations and should be attributed to changes in the surface energy budget. In the 4× CO2, there are greater increases (compared to the 1990 control run) in the net solar, net longwave and sensible heat fluxes at the surface at higher elevations, but lower levels of the parameters are simulated at lower elevations. These differences lead to increases in the heat storage at the surface and finally result in greater warming at higher elevations. Compared with the net longwave flux, the relative net shortwave flux at the surface increases more evidently at higher elevations, implying that the increase in net shortwave flux at the surface plays a dominant role in producing greater warming at higher elevations. The elevation range between 2000 and 3000 m appears to be a turning point; below 2000 m, the total cloud increases and subsequently constrains the surface net solar radiation. Above 3000 m, the total cloud decreases but shows little elevation dependency, favoring the increases in the surface net solar radiation, and decreases in the snow depth, with more differences with increasing elevation, lead to the reduced surface albedo. This further facilitates the absorption of solar radiation at higher elevations. Therefore, the combined effects of changes in the snow depth and cloud cover in response to 4× CO2 levels result in greater heat storage at the surface at higher elevations than at lower elevations, leading to EDW over and around the TP.