Influence of the North American Dipole on ENSO onset as simulated by a coupled ocean-Atmosphere model

The North American Dipole (NAD) is a north–south seesaw pattern of sea level pressure anomalies over the western tropical North Atlantic and northeastern North America. Previous observational studies have demonstrated that the NAD can affect the outbreak of El Niño–Southern Oscillation (ENSO) events. The present study analyzed the NAD–ENSO relationship as simulated by a coupled ocean–atmosphere model—namely, the Flexible Global Ocean–Atmosphere–Land System model, gridpoint version 2 (FGOALS-g2). Results indicated that the model can replicate a distinct dipole comprised of a low over northeastern North America and a high over the western tropical North Atlantic, which is the signature feature of the NAD. Further analysis verified that the winter NAD can initiate the central equatorial Pacific warming in the subsequent winter by effectively forcing an anticyclonic flow and sea surface temperature (SST) warming over the northeastern subtropical Pacific (NESP) during late winter or early spring. In addition, the probability of an El Niño event was increased by a factor of 1.8 in the assimilation experiment with the NAD. By comparison, the winter Northern Atlantic Oscillation had no significant impact on the occurrence of ENSO a year later owing to its failure to induce the SST and surface wind anomalies over the NESP.摘要北美偶极子(NAD)是热带北大西洋西部和北美东北部的南北向海平面气压异常偶极型模态.以往的观测研究表明, NAD可以有效地影响ENSO事件的爆发. 本文利用全球耦合模式FGOALS-g2, 评估了NAD与ENSO的关系. 结果表明, 该模式能较好地重现NAD模态. 进一步的分析验证了冬季NAD可以通过强迫冬末春初副热带东北太平洋上空的反气旋和暖海温的出现, 在随后的冬季触发El Niño事件. 此外, 在同化NAD实验中, 发生El Niño事件的概率增加了将近一倍. 相比之下, NAO未能在副热带东北太平洋上空引起表面风和海温的异常, 因而不能有效地激发次年冬季ENSO事件.