Westward extension of summer atmospheric circulation over the North Pacific after the 1990s

Abstract

This paper documents the westward extension of the summer atmospheric circulation over the North Pacific (NP circulation) around the 1990s and investigates the possible reason, based on the North Pacific index. The NP circulation was centered in the Northeast Pacific during 1961–1983 and in the central North Pacific during 1994–2016. Further results show that the westward movement of the NP circulation was closely linked with an intensification of the relationship between the NP circulation and the previous spring sea surface temperature of the midlatitude North Pacific (SST_NP) after the 1990s. The spring SST_NP anomalies exerted considerable impacts on surface heat flux (i.e., sensible heat flux and latent heat flux) and vertical motion anomalies over the central North Pacific during the following summer, facilitating a westward shift in the summer NP circulation. Additionally, the interannual variability of the sea level pressure over the central North Pacific increased after the 1990s, which was likely a contributing factor to the westward extension of the NP circulation.摘要本文研究了20世纪90年代前后夏季北太平洋大气环流 (NP circulation) 向西扩展, 并探讨了其可能的原因. 结果表明, 在1961–1983年期间NP中心主要位于东北太平洋, 而在1994–2016年期间NP中心发生西移, 至北太平洋中部. 进一步分析指出, 90年代以后NP活动中心的西移和春季中纬度北太平洋海温(SST_NP) 与NP关系的加强有关. 春季SST_NP异常通过引起后期夏季北太平洋中部地表热通量(即感热通量和潜热通量)和垂直运动异常, 有利于NP活动中心向西移动. 此外, 20世纪90年代以后, 北太平洋中部海平面气压的年际变率增加, 这可能是NP向西扩展的另一原因.