Multistability in a coupled ocean–atmosphere reduced-order model: Nonlinear temperature equations

Abstract

Multistabilities in the ocean–atmosphere flow were found in a reduced-order ocean–atmosphere coupled model, by solving the nonlinear temperature equations numerically. In this article, we explain how the full nonlinear Stefan–Bolzmann law was implemented numerically and the resulting change to the system dynamics was compared with the original model where these terms were linearised. Multiple stable solutions were found that display distinct ocean–atmosphere flows, as well as different Lyapunov stability properties. In addition, distinct low-frequency variability (LFV) behaviour was observed in multiple attractors. We investigated the impact on these solutions of changing the magnitude of the ocean–atmospheric coupling, as well as the atmospheric emissivity, to simulate an increasing greenhouse effect. Where multistabilities exist for fixed parameters, the possibility for tipping between solutions was investigated, but tipping did not occur in this version of the model where there is a constant solar forcing. This study was undertaken using a reduced-order coupled quasigeostrophic ocean–atmosphere model.