Stability of the global thermohaline circulation in an intermediate complexity ocean model

Abstract

The work described in this thesis is an investigation of the stability of the global thermohaline circulation in an intermediate complexity ocean model.  It involves an exploration of the parameter space, to understand the sensitivity of the global THC to several factors under different freshwater forcing regimes.

I used the GOLDSTEIN (Global Ocean - Linear Drag Salt and Temperature INtegrator) ocean model (Edwards and Shepherd, 2002) as the main tool for numerical investigations.  The geometry used included the representation of the three main basins as well as of a simplified Bering Strait.  Because of the simplified physics and coarse resolution of the model.  I could afford to undertake numerous multi-millennial time integrations.  I was able therefore to carry out multifactorial experiments in which the Atlantic-to-Pacific atmospheric water vapour transport (AVT) and the global zonally homogeneous freshwater forcing amplitude (FFA) were varied progressively, in order to detect the presence of hysteresis.  The different factors considered for our exploration were the Bering Straits throughflow (On/Off), windstress (On/Off) and horizontal diffusivity (High/Low).

The analysis of the results have concentrated on the role of wind and horizontal mixing, as well as the presence of the Bering Straits throughflow on the bifurcation structure of the THC.  The main results are that multiple steady states and hysteresis occur, but that the influence of wind and horizontal mixing tend to suppress bifurcations of the system (under either AVT or FFA forcing).  The effect of the Bering Strait throughflow on the hysteresis is dependent on the control parameter used (AVT,FFA).  A complete two level, multi-way factorial design has been conducted (24 hysteresis experiments for a total of 7.136E6 years of integration).  All possible modes of circulation could be excited (including stable salinity dominated equatorial sinking in one of the basins).  Self-sustained oscillatory modes were observed, for restricted regions of parameter space, with periods ranging from interdecadal to millennial timescales.

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