Reply on RC2

Abstract

The Tibetan Plateau (TP) exerts strong influence on both regional and global climate through thermal and mechanical forcings. The TP also contains the headwaters of large Asian rivers that sustain billions of people and numerous ecosystems. Understanding the characteristics and changes to the hydrological regimes on the TP during the mid-Holocene (MH) will help understand the expected future changes. Here, an analysis of the hydroclimates over the headwater regions of three major rivers originating in the TP, namely the Yellow, Yangtze and Brahmaputra rivers is presented, using an ensemble of climate simulations, which have been dynamically downscaled to 10-km resolution with the Weather Research and Forecasting Model (WRF) coupled to the hydrological model WRF-Hydro. Basin-integrated changes in the seasonal cycle of hydroclimatic variables are considered. In the global model, we have also incorporated Green Sahara (GS) boundary conditions in order to compare with standard MH simulations (which do not include GS) and to capture interactions between the GS and the river hydrographs over the TP. Model-data comparisons show that the dynamically downscaled simulations significantly improve the regional climate simulations over the TP in both the modern day and the MH, highlighting the crucial role of downscaling in both present-day and past climates, although both global and regional models have a cold bias in modern-day simulations and underestimate the wet anomalies inferred from proxy data in the east and southeast part of the TP. TP precipitation is also greatly influenced by the inclusion of a GS, with a particularly large increase predicted over the southern TP, as well as a delay in the monsoon withdrawal. The model performance was first evaluated over the upper basins of the three rivers before the hydrological responses to the MH forcing in streamflow as well as temperature, rainfall and snowmelt for the three basins were quantified via the WRF simulations.