Interannual variability of boreal summer intraseasonal oscillation over the northwestern Pacific influenced by the Pacific Meridional Mode

Abstract

During the boreal summer, intraseasonal oscillations exhibit significant interannual variations in intensity over two key regions: the central-western equatorial Pacific (5°S–5°N, 150°E–150°W) and the subtropical Northwestern Pacific (10°–20°N, 130°E–175°W). The former is well-documented and considered to be influenced by the ENSO, while the latter has received comparatively less attention and is likely influenced by the Pacific Meridional Mode (PMM), as suggested by partial correlation analysis results. To elucidate the physical processes responsible for the enhanced (weakened) intraseasonal convection over the subtropical northwestern Pacific during warm (cold) PMM years, the authors employed a moisture budget analysis. The findings reveal that during warm PMM years, there is an increase in summer-mean moisture over the subtropical northwestern Pacific. This increase interacts with intensified vertical motion perturbations in the region, leading to greater vertical moisture advection in the lower troposphere and consequently resulting in convective instability. Such a process is pivotal in amplifying intraseasonal convection anomalies. The observational findings were further verified by model experiments forced by PMM-like sea surface temperature patterns.

摘要

在北半球夏季, 西北太平洋地区的季节内振荡在两个主要区域呈现出显著的年际变率: 一个区域是赤道太平洋中西部 (5°S–5°N, 150°E–150°W), 另一区域为副热带西北太平洋 (10°–20°N, 130°E–175°W). 通过偏相关分析, 揭示了前者受到厄尔尼诺–南方涛动 (ENSO) 的影响, 而后者与太平洋经向模态 (PMM) 有关. 利用水汽方程诊断, 探讨了在PMM暖 (冷) 年期间, 副热带西北太平洋季节内对流活动增强 (减弱) 的物理过程. 结果表明, 在PMM暖年, 副热带西北太平洋地区的季节平均水汽增加与季节内垂直扰动的增强相互作用, 导致了对流层低层水汽垂直输送的增加, 进而引发对流不稳定性的增强, 促使季节内对流活动增强. 这一发现在以PMM海温为驱动的全球环流模式试验中也得到了验证.