CMIP6 Models Underestimate Rainfall Trend on South Asian Monsoon Edge Tied to Middle East Warming

In recent decades, an increase in rainfall has been observed on the northwestern edge of the Indian summer monsoon (ISM; NWEISM). However, no studies have focused on model performances over NWEISM, which calls for an urgent evaluation of models. Here, we utilize historical simulations from 24 CMIP6 models to demonstrate that current models tend to underestimate the observed increasing rainfall over NWEISM, with only similar to 30% of the observed intensity. The models broadly capture the spring Middle East land warming, which is the main driver of increased rainfall over NWEISM. Unfortunately, most models fail to reproduce the associated significant decrease in sea level pressure over the surrounding landmasses. This deficiency results in an ineffective trigger of cross-equatorial southwesterly winds, impeding the accurate simulation of the poleward shift of the summer low-level jet (LLJ). Consequently, it leads to a weaker link from the Middle East warming to rainfall enhancement over NWEISM. The Indian summer monsoon (ISM) is one of the most spectacular monsoon systems in the world. In recent decades, the summer rainfall has been found to increase over the northwestern edge of the ISM (NWEISM), such as over Pakistan and northwestern India. Reliable model predictions and early warning systems for floods in such climate-sensitive regions are critical for minimizing the loss of lives, properties and GDPs. Unfortunately, the current climate models are unable to accurately depict the trend of increased rainfall over NWEISM. Although most models can simulate the spring Middle East land warming that triggers rainfall enhancement over NWEISM, they cannot accurately reproduce the associated summer low-level jet (LLJ). In summary, the strong link of LLJ to the spring warming over the Middle East and enhanced rainfall over NWEISM, are not accurately captured by the models. This raises serious concerns about the reliability of future climate projections for these vulnerable regions. Current models tend to underestimate the observed increasing rainfall over the northwestern edge of the Indian summer monsoon (NWEISM) Only 30% of the observed increasing rainfall intensity over NWEISM is reproduced by the CMIP6 models Inaccurate simulation of the poleward shift of the summer low-level jet induces the poor simulation of rainfall enhancement over NWEISM