Extratropical low-frequency variability with ENSO forcing: A reduced-order coupled model study

TL;DR

This study uses a reduced-order coupled model to explore how ENSO forcing influences extratropical climate variability, revealing coexistence of multiple attractors and implications for climate predictability.

Contribution

It introduces an idealized, coupled model analyzing the impact of ENSO forcing on extratropical variability through pullback attractors and their stability.

Findings

Coexistence of two chaotic pullback attractors under ENSO forcing.

Different stability properties of attractors depending on ENSO forcing type.

Transitions between attractors induced by extratropical perturbations.

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.