Controls on the Northward Movement of the ITCZ over the South China Sea in Autumn: A Heavy Rain Case Study

Abstract

The autumn Intertropical Convergence Zone (ITCZ) over the South China Sea (SCS) is typically held south of 10°N by prevailing northeasterly and weakening southwesterly winds. However, the ITCZ can move north, resulting in heavy rainfall in the northern SCS (NSCS). We investigate the mechanisms that drove the northward movement of the ITCZ and led to heavy non-tropical-cyclone rainfall over the NSCS in autumn of 2010. The results show that the rapid northward movement of the ITCZ on 1 and 2 October was caused by the joint influence of the equatorial easterlies (EE), southwesterly winds, and the easterly jet (EJ) in the NSCS. A high pressure center on the east side of Australia, strengthened by the quasi-biweekly oscillation and strong Walker circulation, was responsible for the EE to intensify and reach the SCS. The EE finally turned southeast and together with enhanced southwesterly winds associated with an anticyclone, pushed the ITCZ north. Meanwhile, the continental high moved east, which reduced the area of the EJ in the NSCS and made room for the ITCZ. Further regression analysis showed that the reduced area of the EJ and increased strength of the EE contributed significantly to the northward movement of the ITCZ. The enhancement of the EE preceded the northward movement of the ITCZ by six hours and pushed the ITCZ continually north. As the ITCZ approached 12°N, it not only transported warm moist air but also strengthened the dynamic field by transporting the positive vorticity horizontally and vertically which further contributed to the heavy rainfall.

摘要

秋季由于西南风撤退, 东北风建立, 南海赤道辐合带 (ITCZ) 主要位于 10° N 以南, 在此期间天气尺度北移可以造成南海北部强降水. 我们基于 2010 年秋季一次 ITCZ 北移引发的南海北部非台强降水事件, 探究 ITCZ 北移原因及其对降水的影响机制. 结果表明, 2010 年 10 月 1 日–2 日, ITCZ 出现最快一次北移, 该次北移是受到赤道偏东风、 西南风和南海北部东风急流共同影响. 澳大利亚东侧位势高度和太平洋沃克环流增强, 造成赤道偏东风加强并到达南海. 此时强的赤道偏东风与由赤道反气旋加强的西南风合并转向, 推动 ITCZ 北移. 同时, 中国大陆上高压系统东移并入副高, 减小了南海北部的东风急流面积, 促进 ITCZ 北移. 多元线性回归结果表明, 南海北部东风急流面积减小和赤道偏东风加强是造成 ITCZ 北移最主要的两个原因, 其中赤道偏东风加强超前 ITCZ 快速北移约 6 小时且持续推动 ITCZ 北移. 此外, 当 ITCZ 北移到 12° N 附近时, 输送暖湿气流和正涡度平流到南海北部, 加强南海北部水汽通量和大气不稳定性, 利于强降水发生.