The effect of model complexity on the stability of the Atlantic Meridional Overturning Circulation.
University of Southampton, Faculty of Engineering Science and Mathematics, School of Ocean and Earth Science,
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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