Instabilities and multiple equilibria of the thermohaline circulation.

Marotzke, Jochem (1990) Instabilities and multiple equilibria of the thermohaline circulation. (Doctoral thesis/PhD), Christian-Albrechts-Universität Kiel, Kiel, Germany, 126 pp.

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Abstract

In order to understand the causes of cross-equatorial heat transport in the ocean I investigate the existence of multiple equilibria of the thermohaline circulation and instabilities leading to transitions between different steady states. Idealized ocean models are used, the crucial feature common to all models is the different coupling of temperature and salinity to the atmosphere. A feedback mechanism which under infinitesimal perturbations leads from a state symmetric about the equator to cross-equatorial flow is demonstrated to exist in a box model of one ocean basin. The reaction of a global box model to finite-amplitude anomalies of the salinity field shows that states asymmetric about the equator are preferred to symmetric ones.
A two-dimensional model of one ocean basin also shows a transition from a symmetric to an asymmetric steady state, under very small salinity perturbations. Introduction of convective adjustment (removal of static instability) in the 2-D model greatly enhances the sensitivity to perturbations in the salinity field. The three-dimensional GFDL model of one idealized hemisphere, with thermohaline and wind forcing, has two steady states, one of which is stable. Both states have poleward surface flow and sinking at high latitudes, but deep water formation is confined to smaller areas in the stable equilibrium. Without wind stress, no stable steady state has been found, for most of the time, the model is in a state with sinking at low latitudes and sluggish, on zonal average equatorward surface flow. The global GFDL model with idealized geometry (two ocean basins of equal size) has three fundamentally different equilibria, under identical boundary conditions which for each state are equal in the two oceans. Sinking occurs in both Northern Hemispheres, or in both Southern Hemispheres, or in either one of the Northern Hemispheres but not in the other. The latter state corresponds to the observed global thermohaline circulation pattern. Preference of the three different steady states depends strongly on small variations of the surface freshwater fluxes. A transition from one state to another can be accomplished by anomalies in the freshwater fluxes that have the observed order of magnitude of the differences between North Atlantic and North Pacific.

Document Type: Thesis (Doctoral thesis/PhD)
Thesis Advisors: Willebrand, Jürgen
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
Date Deposited: 04 Jun 2018 09:37
Last Modified: 04 Jun 2018 09:38
URI: http://oceanrep.geomar.de/id/eprint/43244

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