Simulating Eastern- and Central-Pacific Type ENSO Using a Simple Coupled Model

Abstract

Severe biases exist in state-of-the-art general circulation models (GCMs) in capturing realistic central-Pacific (CP) El Niño structures. At the same time, many observational analyses have emphasized that thermocline (TH) feedback and zonal advective (ZA) feedback play dominant roles in the development of eastern-Pacific (EP) and CP El Niño–Southern Oscillation (ENSO), respectively. In this work, a simple linear air–sea coupled model, which can accurately depict the strength distribution of the TH and ZA feedbacks in the equatorial Pacific, is used to investigate these two types of El Niño. The results indicate that the model can reproduce the main characteristics of CP ENSO if the TH feedback is switched off and the ZA feedback is retained as the only positive feedback, confirming the dominant role played by ZA feedback in the development of CP ENSO. Further experiments indicate that, through a simple nonlinear control approach, many ENSO characteristics, including the existence of both CP and EP El Niño and the asymmetries between El Niño and La Niña, can be successfully captured using the simple linear air–sea coupled model. These analyses indicate that an accurate depiction of the climatological sea surface temperature distribution and the related ZA feedback, which are the subject of severe biases in GCMs, is very important in simulating a realistic CP El Niño.

摘 要

现今的环流模式(GCMs)在模拟中部型El Niño时存在严重的偏差. 与此同时, 很多基于观测的分析指出温跃层反馈和纬向平流反馈分别对东部型和中部型El Niño–Southern Oscillation (ENSO)起着主导作用. 本文利用一个简单的海气耦合模式对两类El Niño进行了研究. 基于观测信息, 该模式能够准确给出温跃层反馈和纬向平流反馈的强度沿赤道太平洋的分布. 研究结果表明, 当关闭模式中的温跃层反馈而仅保留纬向平流反馈项时, 模式能够模拟出中部型ENSO的主要特征. 这验证了纬向平流反馈对中部型ENSO的支配作用. 接着, 通过在模式中引入一个简单的非线性调控项, 很多ENSO特征都能被这一简单的海气耦合模式抓住, 包括同时产生两类El Niño以及El Niño与La Niña的非对称性. 该模式分析表明, 若要模拟出接近真实的中部型El Niño, 海表温度的气候态分布及其相联系的纬向平流反馈必须足够准确. 而这恰恰是如今环流模式存在的严重偏差之一.