Projected near term changes in the East Asian summer monsoon and its uncertainty

Abstract

Changes in the East Asian summer monsoon (EASM) during the mid-21st century relative to present day are simulated in two related models GOML1 and GOML2. Both models are the atmospheric components of two state-of-the-art climate models coupled to a multi-level mixed-layer ocean model, following the RCP 4.5 scenario. Both show that the EASM is enhanced due to the amplified land-sea thermal contrast. Summer precipitation over northern China is projected to increase by 5%–10% in both models mainly driven by enhancement of the monsoon circulation. Over south-eastern China the two models project different signs of precipitation change: a decrease in GOML1 with the maximum of about −1.0 mm d−1 and an increase in GOML2 with a maximum of around 1.0 mm d−1. Though the thermal effect of climate warming leads to a projected increase in precipitation over south-eastern China in both models, circulation changes are opposite and dominate the precipitation response. This indicates that uncertainty in changes in projected precipitation largely arises from uncertainly in projected circulation changes. The different circulation changes in the two models are likely related to differences in projected Sea Surface Temperature (SST) in the Western tropical Pacific and North Pacific. In GOML1, the SST warming in the tropical Pacific is associated with an anomalous local Hadley circulation, characterized by anomalous ascent in the tropics and southern subtropics, and anomalous descent with less precipitation over south-eastern China. In GOML2, the large decrease in the meridional SST gradient between the South China Sea and Western North Pacific is associated with an anomalous local Hadley circulation with anomalous ascent at 20°N–30°N and anomalous descent at 5°N–15°N, leading to an anti-cyclonic circulation anomaly over the South China Sea and increased precipitation over south-eastern China.