索马里和澳大利亚越赤道气流的协同变化与我国夏季降水型

Connection of the interannual seesaw of the Somali-Australian cross-equatorial flows with China summer rainfall

利用美国国家环境预报中心-国家大气研究中心（NCEP/NCAR）逐月再分析资料,结合我国国家气候中心提供的160站月平均降水资料（1970～2012年）,在以前工作基础上,进一步讨论了索马里与澳大利亚北部低空越赤道气流的协同变化与我国夏季降水异常型的联系.结果表明：该二支越赤道气流的反相关协同变化（seesaw）与我国西南-东北走向的降水异常型密切相关.当索马里越赤道气流偏强（偏弱）而澳大利亚北部越赤道气流较弱（较强）时,我国夏季降水型呈西南-东北走向,即长江上游和中游、黄河流域以及华北大部分地区降水偏多（偏少）.这种联系的产生同西太平洋副热带高压位置变化有关,在各月份的表现不尽相同.对应索马里越赤道气流增强和澳大利亚越赤道气流减弱,7月副高西伸,8月副高位置偏北.由于这种越赤道气流间的反相关关系从5月开始出现,在整个夏季有很好的持续性,所以它对我国夏季西南-东北走向的降水异常型具有预测意义.

The low-level Cross-Equatorial Flows（CEFs） over the Indian Ocean and western Pacific regions have been recognized as essential components of the Asian-Australian monsoon systems. The variations of the CEFs play a key role in the year-to-year monsoon rainfall anomalies. Recently, several studies have illustrated that the changes of the two CEFs are not independent with each other. A negative（seesaw） correlation at interannual timescale between the Somali CEF and the Australian CEF during boreal summer（Jun-July-August） has been noticed. In contrast to the Indian summer monsoon（ISM）, for which the Somali CEF plays a dominant role in the moisture transport, both the Somali and Australian CEFs show significant contributions to the East Asian summer monsoon（EASM）. Thus, it may be more useful to consider the effect of the combined CEFs on the EASM. To describe the opposite interannual variability between the Somali and Australian CEFs, our previous study has proposed a CEF-seesaw index which is defined by the difference between the standardized Somali and Australian CEFs. A higher（lower） CEF-seesaw index represents an intensified（weakened） Somali CEF but a weakened（intensified） Australian CEF. The CEF-seesaw index shows a closer correlation with EASM than the individual CEF does. In addition, the seesaw-EASM connection is even closer than the ENSO-EASM connection. Thus, the CEF-seesaw and its close connection to Asian summer monsoon may represent an intrinsic variability within the Asian-Australian monsoon system. This contributes to better understanding of the interannual variability of summer rainfall in the Asian-Australian monsoonal region. In the present study, we further analyze the relation of the CEF-seesaw to the China summer rainfall anomaly patterns during 1970–2012 using the NCEP/NCAR reanalysis data and the China's Mainland 160-station precipitation. Since we focus on the interannual time scale, a linear detrend is first removed. Then the fast Fourier transform（FFT） filter is used to remove the components beyond the interannual time scale（greater than 10 years）. The basic methods used here include Empirical Orthogonal Function（EOF）, correlation, and composite analyses. A student＇s t test is applied for checking the significance of the results. Our results indicate that the CEF-seesaw is related to a southwest-northeast oriented rainfall anomaly pattern, which is identified by the second leading Empirical Orthogonal EOF mode（EOF2） of the China summer rainfall. A high（low） CEF-seesaw index, which is characterized by a strengthened（weakened） Somali CEF and a weakened（strengthened） Australian CEF, is related to more（less） rainfall in the upper and middle streams of the Yangtze River, the Yellow River valley and northern China. This rainfall anomaly pattern is related to the anomalous migration of the Western Pacific Subtropical High（WPSH）. The WPSH shows distinct changes in different months. During the high（low） CEF-seesaw index years, the WPSH displays an apparent westward（eastward） propagation in July and shift to north（south） in August. This explains the formation of the above-mentioned southwest-northeast oriented rainfall anomaly. Since the CEF-seesaw starts to appear in early summer（May） and has a high persistence throughout the whole summer, this may provide a useful predictor to the China summer rainfall. It should be pointed out that the frequency of southwest-northeast oriented rainfall anomaly pattern is increasing in recent decades. More comprehensive studies are warranted to explore the physical mechanism and its connection to other rainfall anomaly patterns in the AsianAustralian monsoon areas.