Interannual Variation of the Onset of Yunnan’s Rainy Season and Its Relationships with the Arctic Oscillation of the Preceding Winter

Yan Chen,Juzhang Ren,Shichang Guo,Yu Liu,Jianhua Ju

doi:10.4236/acs.2017.72015

Abstract

Based on an analysis of the circulation in May associated with the interannual variation of the onset of Yunnan’s rainy season, this study examined the rela-tionship between Arctic Oscillation (AO) and the onset timing of the rainy sea-son by using the NCEP/NCAR reanalysis and observational precipitation data for 1961-2010. The results indicated that, on an interannual time scale, intense Asian summer monsoon and an active EU-pattern wave train circulation in its positive phase, associated with a cold cyclonic cell covering the western part of the East Asian subtropical westerly jet (EASWJ), jointly contributed to the onset of the rainy season in May. Otherwise, the onset might be suppressed. The cold cyclonic cell over East Asia likely led to the southward shift and enhancement of EASWJ as well as its secondary circulation around the jet entrance, which could provide a favorable dynamic and thermal condition for rainfalls in Yunnan as was revealed in previous studies on 10 - 30-day time scale. Further examination showed that the preceding wintertime AO played a significant role in the timing of the onset of the rainy season before the mid-1980s’ by mostly modulating the wave-train-like circulation over East Asia in May. During that time period, when the AO index of the previous winter was positive (negative), Yunnan’s rainy season tended to begin earlier (later) than normal. Correspond-ingly, the precipitation in May was also closely linked to wintertime AO.

Highlights

One of the most prominent climatological features of Yunnan Province is the alternation of dry and wet seasons, with low-level westerly wind prevailing all year round [1]
Based on an analysis of the circulation in May associated with the interannual variation of the onset of Yunnan’s rainy season, this study examined the relationship between Arctic Oscillation (AO) and the onset timing of the rainy season by using the NCEP/NCAR reanalysis and observational precipitation data for 1961-2010
This study analyzed the anomalous circulations related to interannual variation of the onset of Yunnan’s rainy season using long-term data, and examined the relationship with the preceding AO index

Summary

Introduction

One of the most prominent climatological features of Yunnan Province is the alternation of dry and wet seasons, with low-level westerly wind prevailing all year round [1]. Chen et al 212 transition of the atmospheric circulation from spring to summer, the southward shift of EASWJ can trigger the onset of Yunnan’s rainy season through intensifying its secondary vertical circulation around the jet entrance and enhancing the interaction between the summer monsoon and cold air It is not clear whether the onset of Yunnan’s rainy season on an interannual time scale is related to the abnormal displacement of EASWJ. This study is designed to explore the anomalous circulation leading to the interannual onset of Yunnan’s rainy season using long-term data, investigating the abnormal signals of EASWJ variation, and examining the possible relationship between AO and the timing of the onset of Yunnan’s rainy season in the expectation of obtaining some antecedent clues for prediction

Data and Methods

Circulations Associated with the Onset of the Rainy Season in May

Findings

Summary