The Correlation Between IOD And ENSO And Its Impact On Rainfall Over China

By using the methods of statistical and diagnostic analysis, variation characteristics of IOD and the relationship between it and ENSO are studied. The paper also discusses the impact of IOD on the rainfall both in summer and autumn over China, and the reasons for rainfall anomaly over china during IOD time are found out.Main results are as follows:(1) By means of composite analysis, it is found that Indian Ocean Dipole (IOD) appears in spring, matures in autumn, decay in winter. Such characteristic is called as season-lock. The result of delay dependence indicates that IOD is well correlated to wind field. When IOD is in its positive (negative) phase, the southeast (northwest) wind anomalies appear in the eastern equatorial Indian Ocean, while the north (south) wind anomalies show in the center and south of tropical Indian Ocean.(2) IOD is closely related to ENSO on the periods of quasi-1-2 year, quasi-3 year, quasi-4 year. The correlations, belonging to different periods, are different.(3)The relationship is analyzed from the perspective of vertical circulation. In the three periods, characteristics of vertical circulation have the same points, that is to say, when IOD is in positive (negative) phase, there are anticlockwise (clockwise) Walker circulation anomaly over Pacific and clockwise (anticlockwise) circulation anomaly over Indian Ocean. For all that, the location of circulation center, the ascending branches, the descending branches are different. According to horizontal wind field, sea surface temperature (SST) variations at the east of IOD are induced by meridional wind change in the east of 80°E, leading to the reversal of the zonal SST gradient in east-west Indian Ocean. Then, the direction of zonal wind anomaly reverses, finally resulting in SST change at the west pole of IOD. Moreover, in the three periods, all the SST variations in Indian Ocean lead that in Pacific. Meanwhile, there exists zonal wind anomalies propogating from western Indian Ocean to Pacific. The two points investigate that IOD events may attribute to the appearance of ENSO.(4) There is one IOD mode independent of ENSO. This feature embodies mainly on the period of quasi-8 month. In the time scale, IOD appears, but there is no ENSO event following. Through analyzing zonal wind anomaly, it is found that wind anomaly in Indian Ocean does not propogate eastward. Meridional wind variations are similar to that of quase-1-2 year, but it is limited in the westof 100°E.(5) Rainfall anomalies, influenced by IOD events, are analyzed. In summer and autumn, the positive phase of IOD may increase rainfall over the south of China, while the negative phase of IOD may weaken the rainfall. This phenomenon is most remarkable in autumn. But in winter, the impact of IOD on rainfall is opposite. Considering the impact of ENSO on the relationship between IOD and rainfall, the rainfall variation in summer and autumn, are similar to the former. But in winter, ENSO plays a negative role to the correlation of IOD and precipitation in the south of China. This results in the different distributions of rainfall anomaly in winter. When both of ENSO and IOD are in the positive phase, the rainfall over the southeast of China increases. Vice versa.(6) In the mid-high latitudes, the positive phase of IOD may strengthen meridional circulation, while the negative phase does the same. Current field in autumn is also analyzed. The results indicate that the positive phase of IOD enhances cross-equatorial flow along 105°E and increase the rainfall over southeast of China, while the negative reduce the flow and rainfall. So ENSO does. Under action of IOD, Somali jet strengthens and transport of moisture over Indian increase. The influence of ENSO is just on the contrary.