Intraseasonal variations of precipitation over southern China during boreal winter and their two primary influencing factors

This paper utilizes reanalysis data and statistical methods to elucidate the physical mechanisms of intraseasonal variations of precipitation over southern China during the boreal winter, as well as the two primary factors influencing these variations. Anomalous southerly (northerly) wind over southern China will result in the strong moisture convergence (divergence) in enhanced (suppressed) scenarios, and the stronger positive (negative) horizontal advection of absolute vorticity at upper troposphere induces the anomalous ascending (descending) motion in enhanced (suppressed) scenarios. The Madden-Julian oscillation (MJO) in the tropics and a wave train over the North Atlantic and Eurasia with four activity centres in the mid-high latitudes (referred as NAE-wave train) are the two primary factors that modulate the intraseasoanl precipitation over southern China. The intraseasonal circulation induced by MJO and NAE-wave train exhibits distinct features, resulting in different physical mechanisms of their influences. The moisture flux divergence anomalies over southern China associated with MJO mainly stem from the meridional moisture convergence, whereas the meridional moisture advection is the first contributor in the influence of NAE-wave train. The anomalous vorticity advection at upper troposphere associated with MJO is primarily influenced by the meridional advection, but that associated with NAE-wave train is dominated by the zonal advection. Anomalous precipitation induced by MJO is concentrated mainly over South China, while that caused by NAE-wave train is primarily located in the mid-lower Yangtze River basin. Therefore, different phases locking of MJO and NAE-wave train could lead to various precipitation anomalies over southern China. Madden-Julian Oscillation (MJO) and North Atlantic and Eurasia wave train (NAE-wave train) are the two primary factors that modulate the intraseasonal variations of precipitation over southern China during boreal winter. Anomalous precipitation induced by MJO is mainly over South China, while that caused by NAE-wave train is primarily located in the mid-lower Yangtze River basin. The intraseasonal circulation induced by MJO and NAE-wave train exhibits distinct features, resulting in different physical mechanisms of their influences on the precipitation. Figure shows schematic diagram for the influences of MJO and NAE-wave train on the intraseasonal precipitation over southern China. (a) Precipitation-enhanced scenarios and (b) precipitation-weakened scenarios. The red dashed line represents the NAE-wave train, and the blue and orange arrows indicate the circulation.image