A Conceptual Model for Extratropical Atmosphere-Ocean Interaction

Equipped with the current understanding for mid-latitude atmosphere-ocean interaction and the Linear Inverse Modelling (LIM), we build a simple physically-based system of equations to portray the coupling between atmosphere and ocean over the North Atlantic, involving relationships among major atmospheric and oceanic modes, such as the NAO, AMO, AMOC, ENSO and stratospheric NAM. The simple model reproduces the strongly timescale-dependent character of the relationships, which changes in strength and direction on scales ranging from days to centuries. Another emphasis is placed on explicitly resolving the possible feedback from ocean on atmosphere. In constructing and testing the simple model we make use of a multi-millennium-long control integration with a fully coupled climate model. Cross-correlation, spectral analysis are employed to characterize important aspects of the interactions in the full and simple models. It is found that, a) Bjerknes' conjecture on ocean-atmosphere coupling, that is the atmosphere drives climate on high frequencies (days to months) while the ocean acts as the main source of climate variability on interannual and longer timescales, is confirmed; b) the AMOC can be readily understood as a harmonic oscillator driven by the NAO; c) the two-way interaction between NAO and AMO, and also the influence of ENSO on both NAO and AMO are essential for reproducing important correlation features. Our approach not only helps to clarify our understanding for the nature of the atmosphere-ocean interaction problem but also raises new and intriguing questions for future research.