Quasi-3-yr Cycle of Rainy Season Precipitation in Tibet Related to Different Types of ENSO during 1981–2015

Abstract

The rainy season precipitation in Tibet (RSPT) is a direct cause for local floods/droughts. It also indirectly affects the thermal conditions of the Tibetan Plateau, which can result in anomalous patterns of atmospheric circulation over East Asia. The interannual variability of the RSPT is often linked with the El Nino–Southern Oscillation (ENSO), but the relevant mechanisms are far from being understood, particularly for different types of ENSO events. We investigated the interannual variation of the RSPT in association with different types of ENSO. A quasi-3-yr period of the RSPT (less–more–more precipitation) was significant at the 95% confidence level. A joint multi-taper method with singular value decomposition analysis of the coupled field between the RSPT and the sea surface temperature (SST) revealed that the developing eastern Pacific type El Nino was accompanied by a decrease in the RSPT. The shift from the central Pacific type El Nino to the eastern Pacific La Nina was accompanied by an increase in the RSPT. Weakening of the central Pacific La Nina was accompanied by an increase in the RSPT. Analysis of the mechanism of this coupling, using the same analysis method but other climatic factors, indicated that the gradually strengthening eastern Pacific El Nino can inhibit the Walker circulation, weakening the South Asian summer monsoon, and resulting in transport of less water vapor from the Bay of Bengal to Tibet. The change from the central Pacific El Nino to the eastern Pacific La Nina led to continued strengthening of the Walker circulation with westward movement of the ascending area. This enhanced the South Asian summer monsoon over the Arabian Sea and transported more water vapor to Tibet. The decreasing central Pacific La Nina accompanied by persistent cooling of SSTs in the equatorial Pacific led to a strong eastern North Pacific summer monsoon, causing an anomaly in the easterly transport of water vapor from the Sea of Japan to Tibet and increased RSPT.