Imperfections of the three-dimensional thermohaline circulation: Hysteresis and unique-state regimes

Henk A. Dijkstra, Wilbert Weijer and J. David Neelin
J. Phys. Oceanog., 33, 2003.

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© Copyright 2003 by the American Meteorological Society.

Abstract. Different equilibria of oceanic thermohaline circulation may exist under the same forcing conditions. One of the reasons for the existence of these multiple equilibria is a feedback between the overturning circulation and the advective transport of salt and heat. In an equatorially symmetric configuration, the multiple equilibria arise through symmetry-breaking pitchfork bifurcations when the strength of the freshwater forcing is increased. Here, continuation methods are used to track the fate of the different equilibria under equatorially asymmetric conditions in a three-dimensional, low-resolution ocean general circulation model (GCM) in an Atlantic-like basin coupled to an energy-balance atmosphere model. The effect of the continental geometry, the presence of the Antarctic Circumpolar Current (ACC) and asymmetric air-sea interaction on the preference of equilibria is considered. Although all asymmetry inducing mechanisms favor northern Atlantic sinking states, the open southern ocean and ACC are shown to be substantial contributors. The origin of the hysteresis behavior between strong and weak overturning states is clarified in terms of the overall bifurcation picture. The disappearance of a class of southern sinking equilibria due to the combined effects of all asymmetry mechanisms leads to a substantial regime with a unique steady state. The relationship between the hysteresis regime and the unique-state regime provides a larger context for quantitative determination of the relevance of each to climate.

Citation. Dijkstra, H. A., W. Weijer and J. D. Neelin, 2003: Imperfections of the three-dimensional thermohaline ocean circulation: Hysteresis and unique-state regimes. J. Phys. Oceanog., 33, 2796-2814.

Acknowledgements. This work was supported by the National Science Foundation grant ATM-0082529 and by the Institute for Geophysics and Planetary Physics (IGPP). It was partially supported by the Netherlands Organization for Scientific Research (N.W.O.) under a PIONIER grant to the first author. Use of the computing facilities was sponsored by the National Computing Facilities Foundation (N.C.F.) with financial support from N.W.O.

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