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

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 mainland China 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 Asian- Australian monsoon areas.