IMPACT OF ENSO ON THE PRECIPITATION OVER CHINA IN WINTER HALF YEAR
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摘要: 利用1979—2010年观测和再分析资料，诊断分析ENSO对中国华南冬半年降水的影响及其机制。结果表明，在El Niño冬半年期间，东亚沿岸上空对流层低层南风的增强导致了水汽输送明显偏多，水汽在华南辐合，使得大气可降水量和比湿增加，降水显著偏多。而在La Niña冬半年期间，这些大气要素并没有呈现显著的相反变化，负异常的量值很弱并在统计上不显著。通过进一步分析El Niño和La Niña冬半年期间季节内振荡的特点，给出一种华南冬半年降水对ENSO信号不对称响应的物理解释。El Niño期间，热带西太平洋到南海地区的季节内振荡不活跃，与El Niño相联系的西北太平洋反气旋性环流异常造成的水汽输送以及水汽辐合在华南能够稳定维持，致使华南降水明显偏多。但在La Niña冬半年期间，季节内振荡很活跃，与La Niña相联系的西北太平洋气旋性环流异常受到季节内时间尺度的扰动影响，ENSO的年际变化信号被季节内振荡破坏，使得西北太平洋和华南的年际异常信号不能得到稳定维持，导致与ENSO信号相联系的年际变化在统计上不显著。因此，热带西太平洋到南海地区的季节内振荡强度在El Niño和La Niña冬半年期间的差异，是华南冬半年降水对ENSO信号不对称响应的一个主要原因。Abstract: The impact of ENSO on the precipitation over China in winter half year is investigated by using observed and reanalysis datasets in 1979—2010. The results show that positive precipitation anomalies with statistical significance appear over southern China in El Niño episodes, which are caused by the enhanced warm and humid southwesterliesalong the East Asian coast in the lower troposphere. The enhanced southwesterlies transport more water vapor to southern China, and the convergence of water vapor over southern China increases the precipitable water and specific humidity in the atmosphere. In La Niña episodes, no reverse precipitation anomalies with statistical significance appear over southern China. Although atmospheric elements are changed reversely, they are not statistically significant as those in El Niño periods. The possible physical mechanism of the asymmetric impact of ENSO cycles on the precipitation over southern China is investigated by analyzing the intraseasonal oscillations (ISOs) in El Niño and La Niña, respectively, in winter half year. By comparing the characteristics of ISOs in El Niño and La Niña, a physical mechanism is proposed to explain the asymmetric responses of the precipitation over China toENSOin winter half year. In El Niño episodes, over western North Pacific (WNP) and South China Sea (SCS) the ISOs are inactive and exert little effect on the water vapor transport and convergence, inducing the positive precipitation anomalies with statistical significance over southern China in El Niño episodes. In La Niña episodes, however, the ISOs are active, which weaken the interannual variation signals of ENSO over WNP and southern China and lead to the insignificance of the interannual signals related to ENSO. Therefore, the asymmetric responses of precipitation over China to ENSO in winter half yearare possibly caused by the difference of intraseasonal oscillations over WNP and SCS between El Niño and La Niña.
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图 3 El Niño (a) 和La Niña (b) 的冬半年水汽输送矢量距平 (矢量，单位：g/(cm·hPa·s)) 及其散度 (等值线，单位：10-6 g/(cm2·hPa·s))
阴影区表示统计检验达到5%的信度区域。
图 5 El Niño (a、c) 和La Niña (b、d) 的冬半年OLR距平 (单位：W/m2) (a、b) 和方差 (单位：W2/m4)(c、d) 方框范围为110~140 °E，5.0~17.5 °N。
图 6 El Niño (实线) 和La Niña (虚线) 冬半年期间5.0~17.5 °N之间平均的850 hPa季节内振荡动能距平 (单位：m2/s2)
a. 20~70 d；b. 10~20 d。
图 7 El Niño (a、c) 和La Niña (b、d) 冬半年20~70 d (a、b) 和10~20 d (c、d) 的OLR季节内振荡方差距平
单位：W2/m4。方框说明同图 5。
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