摘要:本文主要基于对Webster and Yang（1992）一文的回顾，讨论了年循环在季风和ENSO相互作用中的作用、春季预报障碍（SPB）、Webster-Yang指数（WYI）、以及亚洲夏季风的前期讯号等内容。亚洲季风和ENSO作为全球天气和气候变率的主要来源，它们之间的相互作用存在明显的年变化和季节“锁相”特征：在北半球秋冬季，亚洲季风对流活动最弱，此时ENSO的信号最强；但是到了北半球春季，亚洲季风对流快速爆发，而此时的ENSO信号却迅速衰减。亚洲季风和ENSO位相的错位变化使得热带海—气系统的不稳定性在北半球春季达到最大，此时任意一个微小的扰动都容易快速增长，最终导致基于ENSO的预报技巧减小。亚洲夏季风环流本质上可以看成是大气对副热带地区潜热加热的低频罗斯贝波响应，它具有很强的垂直风切变，这是WYI定义的物理基础。WYI数值越大，代表垂直东风切变越大，即亚洲季风环流增强，反之亦然。利用WYI与前期大气环流场、欧亚雪盖、土壤湿度等物理量进行回归分析，结果表明：当亚洲夏季风增强时，前期冬季和春季，在北印度洋和亚洲副热带地区上空出现东风异常，同时在更高纬度地区伴随出现西风的异常；此外，副热带地区如印度次大陆、中南半岛和东亚的土壤湿度增大；中纬度地区尤其是青藏高原中西部的积雪密度明显减小。这些前期讯号的发现有助于我们构建动力统计模型，进而提高对亚洲夏季风的季节预报水平。
关键词:季风/
ENSO/
年循环/
春季预报障碍/
亚洲季风前期讯号
Abstract:This paper reviews research on the role of annual cycles of monsoon and ENSO in monsoon-ENSO interaction, spring predictability barrier (SPB), Webster-Yang monsoon index (WYI), and precursory signals preceding the Asian summer monsoon mainly based on the study of Webster and Yang (1992). As the major sources of predictability of global weather and climate, the Asian monsoon and ENSO show prominent annual variations and seasonally "phase-locked" features. During the boreal autumn and winter, the weakest Asian monsoon convection occurs, accompanied by the peak intensity of ENSO. During the boreal spring, however, the Asian monsoon strengthens explosively, accompanied by a quick decay of ENSO signals. The out-of-phase variation in the monsoon and ENSO makes the tropical ocean-atmosphere system most unstable during boreal spring. Thus, small random errors could grow unlimitedly and eventually result in a rapid drop of predictive skill of ENSO. Essentially, the Asian summer monsoon circulation is a low-frequency Rossby wave response of the atmosphere to latent heating over the subtropics with strong vertical wind shear, the physical basis for defining WYI. A larger WYI indicates stronger easterly wind shear over South Asia i.e., stronger Asian monsoon circulation, and vice versa. Regressing antecedent physical fields onto the summer WYI reveals several precursory signals preceding a stronger Asian summer monsoon. Significant easterly anomaly occurs over the Indian Ocean and subtropical Asia, while prominent westerly anomaly appears over the higher latitudes. Moreover, significant increase in soil moisture content is found in the subtropics such as the Indian subcontinent, the Indo-China Peninsula, and extratropical East Asia. In addition, decreased snow density occurs in mid-latitude regions especially the middle and western portions of the Tibetan Plateau. These precursors preceding the Asian summer monsoon can be applied to construct dynamical statistical models for improving seasonal prediction of the Asian summer monsoon.
Key words:Monsoon/
ENSO/
Annual cycle/
Spring predictability barrier/
Precursory signals of Asian monsoon
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