季节转换期副热带西风急流变化对云南降水的影响

Yunnan situates in the buffer zone of the two Asian giant monsoon systems, showing prominent dry and wet seasons. It is the most important task to forecast the onset of rainy season and the precipitation during the transitional season for the short term climate prediction in Yunnan. The subtropical westerly jet (SWJ) is one of the major circulation components influencing the weather and climate of East Asia. Especially, the variation of East Asian SWJ is closely related to the commencement of rainy seasons as well as the abnormal of rain belts in China during the transitional season. Preliminary study shows that the East Asian SWJ is companied with obvious meridional oscillation when it is advancing northward seasonally during the span of the commencement of Yunnan rainy season, and it contributes to the commencement of Yunnan rainy season when it swings southward, which is a new discover compared with the general concept in the past that Yunnan rainy season began when the SWJ jumped northward. However, the mechanisms of the meridional oscillation and its relationship with the upstream circulation changes as well as the precipitation in Yunnan are not clear yet. This project aims to use long term meteorological data to study the relationship between the variation of SWJ and Yunnan abnormal rainfall, to reveal the roles of Rossby wave propagation in the changes in the position and intensity of the SWJ, to recognize the interactions among the East Asian SWJ, the cold air activity and the lower latitude circulation through statistical and dynamical diagnosis methods, and ultimately obtain a conceptual model that might provide theoretical foundation for the diagnosis and prediction of abnormal precipitation in Yunnan.

云南干湿季转换明显，对季节转换期云南雨季开始和降水量的预测是短期气候预测的重中之重。副热带西风急流（SWJ）是影响东亚天气气候重要的环流系统，季节转换期它的变化更与我国大部分地区雨季的开始以及雨带的异常密切相关。前期研究发现，云南雨季开始期东亚SWJ在季节性北推过程中还伴有明显的经向摆动，并且急流南摆对雨季开始有重要贡献，这有别于过去笼统地将SWJ北抬作为云南雨季开始标志的认识。然而，东亚SWJ摆动的机理及其与上游环流变化的关联，以及与季节转换期云南降水变化的关系还不清楚。本项目拟用长时间序列气象资料，通过多种统计和动力诊断方法，研究季节转换期SWJ变化和云南降水异常的关系，揭示Rossby波传播对SWJ位置和强度变化的影响，弄清东亚SWJ振荡与冷空气活动以及与中低纬环流系统的相互作用，最终得到季节转换期SWJ活动影响云南降水的概念模型，为云南降水异常的诊断预测工作提供理论依据。

云南干湿季分明，从春至夏季节转换期雨季开始的早晚和降水量的多寡对当地工农业生产具有重要影响，是科学研究和气候预测业务十分关注的问题。本项目重点研究了季节转换期东亚副热带西风急流（SWJ）变化、冷空气活动和云南降水异常的关系，分析了云南雨季开始期的气候特征及前期信号，研究结果为雨季开始和季节转换期降水异常的诊断预测提供了新的理论依据。此外项目对云南降水资料均一性的检验订正、MJO和水汽输送对云南5月降水影响等问题也作了一些有益的探讨。.本项目研究显示，季节转换期SWJ的经向位置和强度具有显著的10-30天低频变化特征，在热带季风爆发和逐渐北推的大背景下，SWJ增强南移是云南雨季开始的一个重要触发机制。季节转换期SWJ增强南移、东亚大陆冷空气活跃、云南所在区域上升气流增强等一系列事件不是孤立的，欧亚中高纬度EU型低频波列在东亚地区向东南方向的传播是这些事件关联的一个重要纽带。EU型波列在东亚地区的传播不仅在动力和热力两个方面影响着云南雨季开始，对4-7月整个季节转换期的降水也有显著影响。上述研究突破了过去笼统将月平均SWJ北跳作为云南雨季开始指标之一的认识，加深了对季节转换期降水和雨季开始物理过程的理解。.气候分析表明，云南雨季主要从东、南、西三个方向呈半包围态势向云南西北腹地推进，具有明显的季风交汇区特征，而前期3-4月热带中东太平洋海温对云南雨季的推进具有显著的影响——暖事件年雨季推进偏晚，冷事件年推进偏早，同时雨季开始期对暖事件的响应大于对冷事件——这种响应主要是通过影响亚洲低纬地区环流完成的。.此外分析发现，云南雨季开始的早晚还与前冬北极涛动指数（AO）存在负相关关系，即当前冬AO为正（负）时，云南雨季开始偏早（晚），5月降水量偏多（少）。前冬AO正异常时，5月欧亚中高纬为“-+-”环流型，这种环流型与云南雨季开始时的环流型十分相似，有利于SWJ增强和雨季正常偏早开始。