Title: Extratropical low-frequency variability with ENSO forcing: A reduced-order coupled model study Show Chinese title

Title: 中纬度低频变化与ENSO强迫：降阶耦合模型研究

Abstract: The impact of the El Ni\~no-Southern Oscillation (ENSO) on the extratropics is investigated in an idealized, reduced-order model that has a tropical and an extratropical module. Unidirectional ENSO forcing is used to mimick the atmospheric bridge between the tropics and the extratropics. The variability of the coupled ocean-atmosphere extratropical module is then investigated through the analysis of its pullback attractors (PBAs). This analysis focuses on two types of ENSO forcing generated by the tropical module, one periodic and the other aperiodic. For a substantial range of the ENSO forcing, two chaotic PBAs are found to coexist for the same set of parameter values. Different types of extratropical low-frequency variability are associated with either PBA over the parameter ranges explored. For periodic ENSO forcing, the coexisting PBAs exhibit only weak nonlinear instability. For chaotic forcing, though, they are quite unstable and certain extratropical perturbations induce transitions between the two PBAs. These distinct stability properties may have profound consequences for extratropical climate predictions: in particular, ensemble averaging may no longer help isolate the low-frequency variability signal. Show Chinese abstract

Abstract: 厄尔尼诺-南方涛动（ENSO）对中纬度的影响在一种理想化、降阶模型中被研究，该模型包含一个热带模块和一个中纬度模块。使用单向ENSO强迫来模拟热带和中纬度之间的大气桥梁。然后通过分析其拉回吸引子（PBAs）来研究耦合的海洋-大气中纬度模块的变异性。此分析专注于由热带模块生成的两种类型的ENSO强迫，一种是周期性的，另一种是非周期性的。对于ENSO强迫的一个相当大的范围，发现对于相同的参数值，两个混沌的PBAs共存。不同类型的中纬度低频变异性与所探索参数范围内的PBA相关联。对于周期性ENSO强迫，共存的PBAs仅表现出较弱的非线性不稳定性。然而，对于混沌强迫，它们非常不稳定，某些中纬度扰动会引发两个PBAs之间的转换。这些不同的稳定性特性可能对中纬度气候预测产生深远影响：特别是，集合平均可能不再有助于隔离低频变异性信号。