A non-ENSO driver of the South China Sea winter monsoon: North Pacific sea ice

Abstract

The South China Sea winter monsoon (SCSWM), an integral component of the East Asian winter monsoon, connects extratropical and tropical regions. Utilizing ERA5 reanalysis and PAMIP simulations, the relationship between Arctic sea ice and the SCSWM is investigated. The authors reveal that its strongest relationship with Arctic sea ice occurs in the North Pacific sector, i.e., the Sea of Okhotsk and western Bering Sea. This link persists throughout the cold season, peaks when sea ice precedes the SCSWM by one month, and is independent of ENSO. North Pacific sea-ice loss weakens the meridional temperature gradient (MTG) and vertical wind shear in midlatitudes, reducing baroclinic eddy formation. Given the reduced zonal wind according to the thermal wind relation, the reduced wave activity flux in the upper troposphere must be balanced by equatorward wind based on the quasi-geostrophic momentum equation. This generates an anomalous meridional overturning circulation with descent and low-level divergence around 30°N, which intensifies the divergent component of the SCSWM. The divergent northerly anomalies also lead to cold advection and subtropical cooling. The enhanced MTG due to the subtropical cooling and weakened MTG due to high-latitude warming closely tied to reduced North Pacific sea ice displace the westerly jet southward, creating cyclonic shears over the North Pacific and intensifying the rotational component of the SCSWM. These findings establish North Pacific sea ice as a non-ENSO driver of the SCSWM, holding substantial implications for the predictability of the SCSWM.

摘要

南海冬季风作为东亚冬季风系统的重要组成部分, 在热带与热带外地区的相互作用中发挥着重要作用. 使用大气再分析数据和环流模式试验, 本研究探讨了北极海冰与南海冬季风之间的关系. 研究表明, 影响南海冬季风的北极海冰关键区在鄂霍次克海. 以鄂霍次克海和西白令海为主的北太平洋海冰减少, 可通过调节垂直经向环流和副热带急流, 显著增强南海冬季风的辐散分量和旋转分量. 这一联系不受ENSO影响, 对南海冬季风的预测以及理解北极与热带之间的联系具有重要的科学意义.