Recent Winter Precipitation Increase in the Middle–Lower Yangtze River Valley since the Late 1970s: A Response to Warming in the Tropical Indian Ocean

The middle–lower valley of the Yangtze River (MLY), located in the middle of eastern China, has been one of the largest economic centers of China since ancient times. Winter precipitation variability over the MLY is important for China because of its significant influence on the local economy. However, few studies have focused on the long-term variability of winter precipitation over the MLY. This study reports a significant wetting trend over the MLY in winter during the three decades since the late 1970s, forming a “mid-east-China winter wetting” pattern, which has become an important feature of precipitation change under the weakening East Asian winter monsoon. This wetting trend in the MLY also implies the poleward extension of the precipitation belts of southern China. Further investigation reveals that the increasing sea surface temperature (SST) in the tropical Indian Ocean (TIO) is the dominant factor responsible for recent increases in precipitation over the MLY. The thermal forcing driven by warming of the TIO SST gives rise to an anomalous cyclonic circulation along the coast of eastern China. This transports more water vapor onto the Chinese mainland, shifts and causes anomalous convergence over the MLY, and generates the increase in precipitation there. As such, the increasing SST in the TIO induces over 80% of the observed wetting trend over the MLY. This mechanism was verified by results obtained from two sets of sensitivity experiments using a numerical spectral atmospheric general circulation model. Thus, increasing SST in the TIO has made a dominant contribution to the recent winter precipitation increase over the MLY.