Dynamics of East Asian Spring Rainband and spring-autumn contrast: Environmental forcings of large-scale circulation
AbstractDynamics of the East Asian spring rainband are investigated with reanalysis dataset and station observations. Here, it is revealed that the rainband is anchored by external forcings. Midtropospheric jet core stays quasi-stationary around Japan. It has two branches in its entry region, which originate from the south and north flanks of Tibetan Plateau and then run northeastward and southeastward, respectively. The southern branch advects warm air from the Hengduan-Tibetan plateaus northeastwards, forming rainband over southern China through both causing adiabatic ascent motion and triggering diabatic feedback. The rainband is much stronger in spring than in autumn due to the stronger diabatic heating over Hengduan-Tibetan Plateau, more southward-displaced midtropospheric jet and resultant stronger warm advection over southern China. The northern jet branch forms a zonally-elongated cold advection belt, which reaches the maximum around northern China, and then weakens and extends eastwards towards east of Japan. The westerly jet also steers strong disturbance activities roughly collocated with the cold advection belt via baroclinic instability. The high disturbance activities belt causes large cumulative warm advections (CWA) through drastically increasing extremely warm-advection days in its eastern and south flank, where weak cold advection prevails. CWA is more essential for monthly/seasonally rainfall than conventionally-used time-average temperature advection because it is revealed that strengthened warm advection can increase rainfall through positive diabatic feedback, while cold advection cannot cause negative rainfall. Thus, the rainband is collocated with the large CWA belt instead of the 48 warm advection south of it. This rainband is jointed to the rainband over southern China, forming the long southwest-northeast-oriented East Asian spring rainband. Southeastward-increasing moisture slightly displaces the rainband southeastwards.