Role of the South China Sea in Southern China rainfall: meridional moisture flux transport

Chunzai Wang,Jian Lan,Xiangting Li

doi:10.1007/s00382-020-05603-w

Abstract

The South China Sea (SCS) serves as the main source of moisture for rainfall in Southern China (SC) and the meridional moisture transport to SC is dominated by wind changes during the first rainy season (April–June). El Niño-Southern Oscillation (ENSO) and Tropical Northwestern Pacific (TNWP) variability modulate the SC rainfall through anomalous anticyclonic circulation over the western North Pacific by strengthening the SCS meridional moisture transport to SC. However, our study indicates that the SCS is not only the intermediary in which ENSO or the TNWP affects the SC rainfall but also plays an independent role in the modulation of the SC rainfall. Notably, the SCS meridional moisture transport has a lower impact on the SC rainfall during the second rainy season (July–September), especially in July. At that time, the main cause of the SC rainfall is the southward moisture flux anomaly across its northern boundary with the anomalous cyclone over SC. This cyclone suppresses the moisture flux out of SC and leads to moisture convergence in SC. Moreover, we present a new concept by analyzing internal differences of moisture circulation during the second rainy season. Either strengthening the meridional moisture flux into SC across its southern boundary or suppressing the moisture flux out of SC across its northern boundary is important depending on whether or not the moisture from the SCS can converge in SC, which is mainly determined by the amplitude of moisture transport fluxes in SC.

Highlights

Located in the East Asia monsoon region and south of the Yangtze River, Southern China (SC) is a tropical-subtropical region with lengthy rainy seasons
The precipitation data used in this study are from the basic meteorology historical dataset of stations released by the National Meteorological Information Centre (NMIC; http://data.cma.cn) of the China Meteorological Administration (CMA) and NOAA’s Precipitation Reconstruction over Land (PREC/L; https://psl.noaa.gov/data/gridded/ data.precl.html) (Chen et al, 2002)
A high positive correlation between the South China Sea (SCS) meridional moisture flux and the SC rainfall is observed in the first rainy season (AMJ)

Summary

Introduction

Located in the East Asia monsoon region and south of the Yangtze River, Southern China (SC) is a tropical-subtropical region with lengthy rainy seasons. Most of the moisture transport channels from tropical zones that affect rainfall in the East Asian monsoon region are moving across the South China Sea (SCS). As the wind variation plays an important role in moisture circulation (Herdies et al 2002; Zhang and Sumi 2002; Wang et al 2013), the behavior of the SCS moisture flux changes greatly together with the SCS monsoon during different seasons. For the first rainy season, we investigate how the SCS meridional moisture flux influences the SC rainfall together with other factors, such as ENSO and the Tropical Northwest Pacific (TNWP). Role of the South China Sea in Southern China rainfall: meridional moisture flux transport

Data and indices

Moisture flux and decomposed moisture flux anomaly

Partial correlation

Characteristics of SC rainfall and SCS meridional moisture transport

Composites of SC rainfall and SCS moisture flux anomalies

Moisture flux across the boundaries of SC in AMJ

Why is the correlation weak in the second rainy season?

Findings

Conclusions and summary