The cooperative impacts of the El Nino-Southern Oscillation and the Indian Ocean Dipole on the interannual variability of autumn rainfall in China

The El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are high degree synergic air-sea interactions greatly affecting the interannual variability of rainfall in China. This study reveals the possible cooperative impacts of the ENSO and IOD on boreal autumn rainfall anomalies in China, on the basis of data analysis collected during 1951-2010. Our results suggest that the positive phase of the combined ENSO and IOD events may result in an anomalous lower-level anticyclone over the western North Pacific (WNP) region, with a southward shift of the strengthened East Asian jet stream (EAJS). An enhanced moisture supply is transported to increase autumn rainfall over southern China by the southwesterly winds. In the negative phase, however, southerly anomalies are observed between the anomalous cyclone and anticyclone over the Indo-China Peninsula and the WNP, which carry more moisture to the Loess Plateau (LP) area with a northward shift of the weakened EAJS, leading to an increase of autumn rainfall in this area. Note that the negative IOD plays an important role in the cooperative influences on the autumn rainfall anomalies over the LP in that the associated anomalous lower-level cyclone shifts westward compared to the impacts of the pure La Nina. The regional asymmetric responses of autumn rainfall to the positive and negative phases of the ENSO and IOD could be attributed to the different responses of lower-level circulation over WNP and the meridional shift of the upper-level EAJS. Results demonstrate that the combined ENSO and IOD induce an anomalous zonal gradient of sea surface temperature (SST) over the tropical Indo-Pacific Ocean to alter the convection anomalies over the Indo-Pacific warm pool that further affect the upper and lower atmospheric circulations by means of a Rossby wave response.