Abstract
The reversed impacts of the Arctic oscillation (AO) on precipitation over the South China Sea and its surrounding areas (SCSA) in October and November during 1979–2014 are investigated. The correlation coefficients between AO and the precipitation in October and November are 0.44 and − 0.31, which are statistically significant at the 99% and 90% confidence levels, respectively. In October (November), the specific humidity exhibits obvious positive (negative) anomalies in the SCSA, and an upward (downward) airflow moving from ground to the upper troposphere (1000–150 hPa) between 10°N and 30°N (10°N and 20°N) is observed with more (less) cloud cover. Moisture budget diagnosis suggests that the precipitation’s increasing (decreasing) in October (November) mainly contributed by zonal moisture flux convergence (divergence). Furthermore, the Rossby wave guided by westerlies tends to motivate positive geopotential height in the upper troposphere over approximately 20°–30°N, 40°–80°E in October, which is accompanied by a stronger anticyclone in the Arabian Sea region. However, in November, the wave train propagating from the Arabian Sea to the Bay of Bengal is observed in the form of cyclones and anticyclones. Further analysis reveal that the AO in October may increase precipitation through the southern wave train (along the westerly jet stream from North Africa to the Middle East and South China). Moreover, air-sea interactions over the North Pacific might also generate horseshoe-shaped sea surface temperature (SST) anomalies characterized by positive SST in the central subtropical North Pacific surrounded by negative SST, which may affect the precipitation in the SCSA. Ensemble-mean results from CMIP6 historical simulations further confirm these relationships, and the models that can better simulate the observed positive geopotential height in the Arabian Sea present more consistent precipitation’s increasing over the SCSA in October.
Highlights
Thompson and Wallace (1998) used the empirical orthogonal function (EOF) analysis method to study the sea level pressure (SLP) poleward of 20°N and found that the first EOF mode shows reversed phase between the mid-latitude and Arctic regions.This pattern is named the Arctic oscillation (AO)
We found that the October AO and November AO are in reverse-phase relationship with precipitation over the South China Sea and its surrounding areas (SCSA, 0°–26°N, 86°–126°E)
These results indicate that the influences of the Oct/Nov AO on the precipitation over the SCSA are closely related to the southern wave train index (SWT)
Summary
Thompson and Wallace (1998) used the empirical orthogonal function (EOF) analysis method to study the SLP poleward of 20°N and found that the first EOF mode shows reversed phase between the mid-latitude and Arctic regions. Concerning the areas at mid- and low latitudes, the AO may affect the temperature and precipitation in East Asia, especially in South China, as well as the westerly jet, wave trains, the Middle East jet (Qu et al 2015; Zhang et al 2008; Zuo et al 2015) and the Pacific SST (Gong et al 2011; Kim and Ahn 2012). The Indian Ocean Dipole (IOD) associated with atmospheric circulation first affects the East Asian summer monsoon and affects the climate at low latitudes, such as the South China Sea (Li and Mu 2011). We found that the October AO and November AO are in reverse-phase relationship with precipitation over the South China Sea and its surrounding areas (SCSA, 0°–26°N, 86°–126°E).
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