Calibrating the simulated summer precipitation trend over the southern slope of the Tibetan Plateau in CMIP6 models using a sub-selection method

Abstract

Precipitation on the Tibetan Plateau (TP) is crucial for Asian water balance and global climate patterns. The southern slope of the TP (SSTP), a precipitation center in summer, has experienced a long-term drying trend in recent decades. Accurate simulations and projections of the change in summer precipitation over the SSTP are critical for future sustainable development. However, the multi-model ensemble (MME) from the current state-of-art Coupled Model Intercomparison Project Phase 6 indicates a wetting trend (∼2.58 mm per month per decade) over the SSTP during the period 1965–2014, contradicting the observations (∼−2.95 mm per month per decade). This discrepancy can be attributed to an overestimation of the stimulating impact of greenhouse gas (GHG) on precipitation trend in CMIP6 models. The traditional MME shows limited capability in capturing the response of atmospheric vertical motions and water vapor to the GHG forcing across the SSTP, resulting in wetting biases by the vertical moisture advection and thermodynamically controlled horizontal moisture advection. A sub-selection method is introduced to calibrate the historical simulated wetting trend at each grid point, through which the selected MME (SMME) more reasonably characterizes the precipitation response to GHG forcing and reproduces the observed drying trend, showing a value about −1.95 mm per month per decade. As GHGs are expected to be the major external forcing in the future, this method is extended to future projections. For the anticipated future wetting trend over the SSTP, the SMME results are constrained to 61.8% and 76.4% in the moderate- and high-emission scenarios, respectively, for the period 2050–2099.