The effect of model complexity on the stability of the Atlantic Meridional Overturning Circulation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) represents a key component of the climate system. Previous studies indicate the present-day configuration of the circulation is highly sensitive to freshwater forcing, and appears able to exhibit bistability whereby contrasting states of circulation can exist under the same freshwater forcing, the choice of which is determined by the system’s history. This thesis presents an investigation into the dynamics and bistability of the AMOC in the context of the intermediate complexity Earth System Model, GENIE. Investigation of the dynamics of the AMOC is performed using diagnostic code implemented into the GENIE framework which allows decomposition of the dynamic and kinematic budgets of the ocean, in terms of density, pressure-gradient, velocity and overturning tendencies. Analysis of decomposition results from an optimal initial steady state demonstrates the complex and spatially heterogeneous nature of the underlying physical balances. Typically, simple dynamical balances are not representative, and a full compliment of advective, diffusive, convective and surface forcing components act to maintain a steady state. Implementation of a stratification dependent vertical diffusivity parameterisation into the GENIE framework allows assessment of the effect of mixing scheme complexity. A novel experimental approach based on factorial sampling of parameter space in combination with multi-objective optimisation techniques, provides a large database of AMOC stability metrics, which are analysed using general linear modelling techniques. The statistical conclusions suggest that whilst the stratification dependent diffusivity parameterisation can modify the initial strength of the AMOC, consequences for apparent bistability are minimal. The existence of hysteresis behaviour and apparent bistability is confirmed in a fully dynamic coupled model environment. However, experimental examination of the effect of rate of change of freshwater forcing on the apparent bistability using a simpler model configuration, indicates the magnitude of apparent bistability is strongly dependent on the rate of change of forcing. Decomposition of a quasi-equilibrium freshwater hosing experiment presents a complex picture of dynamic and kinematic changes which act to produce the overall hysteresis response. An initial analysis of these results indicates the importance of the role of convection for maintaining a deep zonal pressure gradient which is partly responsible for sustaining the AMOC.

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