A Study of the Impact of South Indian Ocean Dipole on the Summer Rainfall in China

The relationship between the Indian Ocean sea surface temperature(SST) in spring and Chinese 160-station rainfalls during summer is investigated by using singular value decomposition(SVD) method.The results show: the first mode of SVD,that is,the most significant mode between Indian Ocean SST(the left field for SVD) and the summer rainfalls(the right field for SVD) is interdecadal effect.That is,the warming trend exhibited in most of the Indian Ocean SST in recent fifty years has important impacts on rainfalls over China and the degree of similarity between right field heterogeneous correlation pattern of the first SVD mode and the climatological trend of the rainfalls over China is very high(reaches 0.765). Besides,the distribution of the significant eigenvalues of the second SVD mode of the Indian Ocean SST in spring in extratropical regions of the Indian Ocean also has notable impacts on the rainfalls over China.The Southwest Indian Ocean(50°S-30°S,50°E-80°E) is defined as West Pole and the region northeast of West Pole(20°S-10°S,60°E-100°E) is defined as East Pole.When the area-averaged SST anomalies are negative(positive) in West Pole and positive(negative) in East Pole,a positive(negative) South Indian Ocean Dipole(SIOD) happens.The composition analysis shows that both the PSIOD(positive SIOD) and NSIOD(negative SIOD) events have important impacts on Chinese rainfalls in late spring(May) and summer(averaged from June to August). In PSIOD years,the rainfalls in Southwest China,the middle and lower reaches of the Yangtze River and its southern flank are intensive in late spring,and the rainfalls over Northeast China,North China,the middle reaches of the Yangtze River and South China are intensive while the rainfalls over the region between North China and Southeast China are subdued in summer.In NSIOD years,the rainfalls over most of China decrease in late spring,and the rainfalls in Southwest China,the south of the lower reaches of the Yangtze River,the Yellow River and the Huaihe River basin increase in summer.The mechanism influencing the rainfalls over China in spring and summer is rather different during SIOD years.In late spring,the strengthened(weakened) Mascrene high can intensify(weaken) cross-equatorial flow off Somali coast in the lower troposphere,that is,Somali jets,and then leads to the total water vapor transport from the Indian Ocean to South and Southeast Asia increasing(decreasing) during PSIOD(NSIOD) years. The thermal states of the maritime continent are different in the opposite phases of SIOD in summer.During PSIOD(NSIOD) years,the SST anomalies near the maritime continent are generally positive(negative). The strengthening(weakening) convective activity induced by positive(negative) SST anomalies makes the water vapor transport converge(diverge) over the maritime continent,and then increases(decreases) the northward total water vapor transport to China.Moreover,the intensity of the western Pacific subtropical high would strengthen(weaken) during PSIOD(NSIOD) years because of the strengthening(weakening) convection over the maritime continent,which makes its position shift northward(southward) and then induces the rainbands over China to shift northward(southward).