Institute of Atmospheric Physics, Chinese Academy of Sciences
之前根据观测资料的研究指出春季北极涛动（Arctic Oscillation, AO）对随后冬季厄尔尼诺-南方涛动（El Ni?o-Southern Oscillation, ENSO）的影响具有明显不对称性。春季AO处于正位相时，它对随后冬季厄尔尼诺（El Ni?o）事件的影响显著，然而春季AO负位相对随后冬季拉尼娜（La Ni?a）的影响不明显。本研究分析了30个来自CMIP5的耦合模式对春季AO与随后冬季ENSO不对称性关系的模拟能力。30个CMIP5耦合模式中，只有CNRM-CM5和GISS-E2-H-CC能较好地抓住春季AO与冬季ENSO的联系。进一步分析这两个模式中春季AO与冬季ENSO的不对称性关系。研究发现CNRM-CM5能较好地再现春季AO与冬季ENSO的非对称关系，即春季AO正（负）位相会导致赤道中东太平洋出现El Ni?o（La Ni?a）型海表温度增暖（冷却）。然而， GISS-E2-H-CC的模拟结果显示，春季AO对随后冬季ENSO的影响是对称的。本文随后解释了CNRM-CM5（GISS-E2-H-CC）能（不能）模拟出春季AO与冬季ENSO不对称关系的原因。对于CNRM-CM5，在春季AO正位相年，副热带西北太平洋上空存在明显的异常气旋和正降水异常，正降水异常通过Gill型大气响应对赤道西太平洋异常西风的形成和维持起着重要作用，异常西风通过激发向东传播的暖赤道Kelvin波对随后冬季El Ni?o事件的发生产生显著的影响；然而，在春季AO负位相年，副热带北太平洋的异常反气旋和负降水异常较弱，导致赤道西太平洋的异常东风不明显，因此，春季AO负异常对随后冬季La Ni?a的影响不显著。所以，CNRM-CM5模式能够较好地抓住春季AO对随后冬季ENSO事件的非对称性影响。相比之下，对于GISS-E2-H-CC模式，春季AO正（负）位相年副热带西北太平洋上存在显著的正（负）降水异常，通过Gill型大气响应在赤道西太平洋激发出明显的异常西（东）风从而影响随后冬季的El Ni?o（La Ni?a）事件。因此，在GISS-E2-H-CC模式中，春季AO对随后冬季ENSO具有对称性影响。另外，模式捕捉春季AO对随后冬季ENSO非对称性影响的能力与模式对春季AO空间结构的模拟能力有一定的联系。
Previous observational study has demonstrated that spring Arctic Oscillation (AO) has a significant asymmetric impact on the El Ni?o-Southern Oscillation (ENSO) during the following winter. In particular, positive spring AO year can exert a notable impact on the following winter El Ni?o event. However, impact of the negative spring AO on the following winter La Ni?a is weak. In this study, we examine ability of the 30 coupled models participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in reproducing the observed asymmetric impact of the spring AO on the ENSO during the following winter. Results showed that only two (i.e. CNRM-CM5 and GISS-E2-H-CC) out of the 30 models can well capture the observed significant spring AO-ENSO relation. These two models were further employed to examine the asymmetric impact of the spring AO on the following winter ENSO. It is showed that the CNRM-CM5 can reasonably well reproduce the observed asymmetric relationship between the spring AO and the winter ENSO. In particular, positive (negative) spring AO could (not) lead to an El Ni?o-like (La Ni?a-like) sea surface temperature (SST) warming in the tropical central-eastern Pacific in the CNRM-CM5. By contrast, the significant impact of the spring AO on the subsequent winter ENSO is symmetry in the GISS-E2-H-CC. Namely, positive (negative) spring AO could result in significant positive (negative) SST anomalies in the tropical central-eastern Pacific during following winter in the GISS-E2-H-CC. The possible factors responsible for the asymmetric/symmetric relation of the spring AO with following winter ENSO in the CNRM-CM5/GISS-E2-H-CC were further examined. For the CNRM-CM5, a significant anomalous cyclone and positive precipitation anomalies can be seen over the subtropical western-central North Pacific during positive spring AO year. The positive precipitation anomalies play an important role in maintaining the westerly wind anomalies over the tropical western Pacific via Gill-type atmospheric response. The westerly wind anomalies over the tropical western Pacific further impact following winter El Ni?o event via triggering eastward propagating and downwelling Kelvin wave. However, during the negative spring AO year, the associated anomalous anticyclone and negative precipitation anomalies over the subtropical North Pacific are weak. Hence, significant easterly wind anomalies cannot be induced over the tropical western Pacific, leading to weak connection of the negative spring AO with the following La Ni?a event. Hence, the CNRM-CM5 can well reproduce the observed asymmetric impact of the spring AO on the following winter ENSO. In comparison, for the GISS-E2-H-CC, significant positive (negative) precipitation anomalies can be seen in the subtropical western North Pacific during the positive (negative) spring AO year, which could induce clear westerly (easterly) wind anomalies in the tropical western Pacific via Gill-type atmospheric response. The resultant westerly (easterly) wind anomalies over the tropical western Pacific further contribute to formation of the El Ni?o (La Ni?a)-like SST anomalies in the tropical central-eastern Pacific during following winter. Hence, the spring AO has a symmetric impact on the following winter ENSO in the GISS-E2-H-CC. Further analysis suggests that the model"s ability in capturing the observed asymmetric impact of the spring AO on the following winter ENSO may be also partly related to the model"s ability in capturing the observed spatial structure of the spring AO.