Limited nonlinearity of joint ENSO–QBO effects from the stratosphere to the troposphere

Previous studies have explored the possible impact of joint El Niño–Southern Oscillation (ENSO)–Quasi-Biennial Oscillation (QBO) forcings on the northern winter stratospheric circulation. This study continues to investigate the relative contributions of the nonlinearity and linearity of joint ENSO–QBO forcings in the total circulation anomaly. The possible interference of ENSO (QBO) signals within QBO (ENSO) is removed before the pure QBO (ENSO) signals in the circulation are calculated. A linear superposition of the pure QBO+ENSO signals is compared with the joint ENSO–QBO composite patterns. It is revealed that the combined ENSO–QBO effect is nearly a linear summation of the pure ENSO+QBO signals in the stratosphere, displaying strong linearity. This linearity decreases slightly from the stratosphere to the troposphere, and the nonlinearity of the joint ENSO–QBO impact increases in regional tropospheric circulation. The linearity and nonlinearity of the joint ENSO–QBO impact in the troposphere are nonuniformly distributed: Lower-to-mid latitude circulation changes associated with configured ENSO–QBO forcings are nearly a linear combination of pure ENSO and QBO signals, while nonlinearity soon dominates the total circulation anomaly pattern at high latitudes, with the ENSO signals overwhelming the QBO signals, especially for the El Niño–EQBO (QBO easterly phase) configurations. This study provides evidence that linearity of the joint ENSO–QBO impact prevails in the stratosphere and tropical troposphere, while nonlinearity exists in the Arctic troposphere. 已有研究表明:厄尔尼诺-南方涛动 (ENSO) 与热带平流层准两年振荡 (QBO) 对北半球冬季平流层环流的协同影响主要表现为ENSO信号和QBO信号的线性叠加.本文进一步探讨ENSO–QBO联合强迫对大气环流总异常的非线性分量和线性分量相对贡献.结果表明, ENSO–QBO对平流层的协同影响几乎是ENSO信号和QBO信号的线性叠加, 即表现出强线性叠加关系.从平流层到对流层, 线性解释分量有所减弱.ENSO–QBO对对流层协同影响的线性与非线性分量表现出一定的空间非均匀性:线性分量在低纬至中纬地区依然较强.在高纬地区非线性分量很快主导了总环流异常;此外, 对流层ENSO信号远强于QBO信号, 尤其是在厄尔尼诺—QBO东风位相情况下.