Abstract
Abstract. The multiple equilibria are an outstanding characteristic of the Atlantic meridional overturning circulation (AMOC) that has important impacts on the Earth climate system appearing as regime transitions. The AMOC can be simulated in different models, but the behavior deviates from the real world due to the existence of model errors. Here, we first combine a general AMOC model with an ensemble Kalman filter to form an ensemble coupled model data assimilation and parameter estimation (CDAPE) system and derive the general methodology to capture the observed AMOC regime transitions through utilization of observational information. Then we apply this methodology designed within a “twin” experiment framework with a simple conceptual model that simulates the transition phenomenon of AMOC multiple equilibria as well as a more physics-based MOC box model to reconstruct the “observed” AMOC multiple equilibria. The results show that the coupled model parameter estimation with observations can significantly mitigate the model deviations, thus capturing regime transitions of the AMOC. This simple model study serves as a guideline when a coupled general circulation model is used to incorporate observations to reconstruct the AMOC historical states and make multi-decadal climate predictions.
Highlights
The Atlantic meridional overturning circulation (AMOC), the core of the thermohaline circulation, is an essential component of the World Ocean circulations (e.g., Delworth and Greatbatch, 2000)
Given that the discrepancy exists between the influencing factors of the AMOC in the real world and the corresponding parameters of models, parameter estimation is used to estimate the model parameters
The observational information from the “truth” is assimilated into the assimilation models, and the transition path of the AMOC is optimized by parameter estimation, so that regime transitions of the AMOC are captured correctly
Summary
The Atlantic meridional overturning circulation (AMOC), the core of the thermohaline circulation, is an essential component of the World Ocean circulations (e.g., Delworth and Greatbatch, 2000). One of its important characteristics is the existence of multiple equilibria (Mu et al, 2004) The research addressing this characteristic originates from Stommel (1961), who used two boxes with uniform temperature and salinity to simulate the equatorial ocean and the polar ocean, respectively. This box model simulates multiple equilibria of thermohaline circulation, including three steady solutions: a stable thermal mode, an unstable thermal mode (mainly driven by heat), and a stable haline mode (mainly controlled by salinity). The idealized box model, of limited applicability in simulating the entire Atlantic circulation or even the global circulation, provides the most basic explanation for some of the important characteristics of the AMOC (Scott et al, 1999). Besides Stommel’s box model, which places two boxes side by side, Welander (1982) placed one box on top of the other to simulate the vertical structure of the Published by Copernicus Publications on behalf of the European Geosciences Union & the American Geophysical Union
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