摘要:基于多种再分析资料和观测资料，分析El Ni?o衰退年北半球夏季西北太平洋异常反气旋（NWPAC）季节内演变特征及其机制。结果表明，较之El Ni?o衰退年6月，NWPAC在7月与8月北移，且其强度在7月与8月显著增强。NWPAC通过影响对流层低层水汽通量散度对东亚夏季季节内降水产生影响，使东亚夏季异常雨带随NWPAC北移而逐渐北移；与NWPAC相伴随的降水异常减弱与入射太阳短波辐射增强，可引起夏季中南半岛、菲律宾及我国南部地区地表气温正异常，且随着NWPAC北移，东亚地表气温异常也随之北移。局地海气相互作用过程可能是NWPAC夏季季节内北移的成因之一。NWPAC北侧短波辐射的减弱和海表潜热与感热释放的增强会造成其下海温负异常，而海温负异常有利于NWPAC的维持。与之相反，NWPAC南侧短波辐射的增强与海表潜热与感热释放的减弱会造成其下海温正异常，而海温正异常可能会激发对流不利于NWPAC的维持。
关键词:西北太平洋异常反气旋/
季节内演变/
局地海气相互作用
Abstract:Characteristics and mechanisms of the sub-seasonal evolution of the Northwest Pacific Anomalous Anticyclone (NWPAC) phenomenon during a decaying El Ni?o in summer were analyzed based on a variety of reanalysis data and observation data. The results showed that the NWPAC had a more northern shift and increased intensity in both July and August compared with June during a decaying El Ni?o in summer. NWPAC affects precipitation in East Asia during boreal summer by influencing the divergence of water vapor fluxes in the low-level troposphere. The abnormal rain belt in East Asia shifts northward along with the NWPAC. The anomalous precipitation decline and increase in incidental solar short-wave radiant flux under the ridge of the NWPAC can result in positive surface air temperature anomalies in the Indochina Peninsula, the Philippines, and southern China during a decaying El Ni?o in summer. With the northward shift of the NWPAC, the positive surface air temperature anomalies in East Asia also shift northwards. Local air-sea interaction processes may be one of the causes of NWPAC’s northern jump during a decaying summertime El Ni?o. Increased short-wave radiant fluxes and reduced upward sensible and latent heat fluxes on the southern side of the NWPAC can cause anomalous positive sea surface temperature anomalies (SSTAs), and positive SSTAs, in turn, could be detrimental to NWPAC’s maintenance by inducing convective instability. In contrast, weakened short-wave radiation fluxes and increased upward sensible and latent heat fluxes on the northern side of NWPAC can cause negative anomalous SSTAs, which favors the maintenance of NWPAC.
Key words:Northwest Pacific Anomalous Anticyclone/
Sub-seasonal evolution/
Local air-sea interaction
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