Quantitative attribution of vertical motions responsible for the early spring drought conditions over southeastern China

Abstract The interannual variability and long-term trend of the drought conditions over southeastern China during early spring (from February to April) are investigated by analyzing the standardized precipitation evapotranspiration index in 1979–2020. Results from an attribution analysis show that precipitation deficiency and atmospheric water demand contributes about 96.5 and 6.7% to the drought conditions on interannual time scale, and about 72.8 and 22.3% to the long-term trend, respectively. The precipitation deficiency is primarily contributed by moisture divergence via the descending anomalies throughout the troposphere over southeastern China. A further diagnosis with the omega equation reveals that the descending anomalies are dominated by the strong negative zonal vorticity advection in the upper troposphere and the enhanced meridional cold advection throughout the troposphere. They are controlled by a barotropic anomalous anticyclone over the eastern Tibetan Plateau and an anomalous cyclone over the western North Pacific. Non-negligibly, the contribution of potential evapotranspiration to the drying trend in early spring over southeastern China is about four times larger than that to the interannual variability. Given that potential evapotranspiration may increase in a warming climate, it may be critical for the change in drought conditions in future. This study serves as a basis for fully understanding the severity of recent droughts and for model simulation of the drought conditions over southeastern China.