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Ocean Dynamics

, Volume 52, Issue 1, pp 36–49 | Cite as

A simple model of seasonal open ocean convection

Part II: Labrador Sea stability and stochastic forcing
  • Till Kuhlbrodt
  • Sven Titz
  • Ulrike Feudel
  • Stefan Rahmstorf

Abstract

 Aspects of open ocean deep convection variability are explored with a two-box model. In order to place the model in a region of parameter space relevant to the real ocean, it is fitted to observational data from the Labrador Sea. A systematic fit to OWS Bravo data allows us to determine the model parameters and to locate the position of the Labrador Sea on a stability diagram. The model suggests that the Labrador Sea is in a bistable regime where winter convection can be either “on” or “off ”, with both these possibilities being stable climate states. When shifting the surface buoyancy forcing slightly to warmer or fresher conditions, the only steady solution is one without winter convection.  We then introduce short-term variability by adding a noise term to the surface temperature forcing, turning the box model into a stochastic climate model. The surface forcing anomalies generated in this way induce jumps between the two model states. These state transitions occur on the interannual to decadal time scale. Changing the average surface forcing towards more buoyant conditions lowers the frequency of convection. However, convection becomes more frequent with stronger variability in the surface forcing. As part of the natural variability, there is a non-negligible probability for decadal interruptions of convection. The results highlight the role of surface forcing variability for the persistence of convection in the ocean.

Keywords North Atlantic deep convection Labrador Sea Bistability Stochastic climate model Decadal variability 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2001

Authors and Affiliations

  • Till Kuhlbrodt
    • 1
  • Sven Titz
    • 1
  • Ulrike Feudel
    • 2
  • Stefan Rahmstorf
    • 1
  1. 1.Institute of Physics, University of Potsdam, Postfach 60 15 53, 14415 Potsdam, GermanyDE
  2. 2.Institute for Chemistry and Biology of the Marine Environment, University of Oldenburg, Postfach 2503, 26111 Oldenburg, GermanyDE

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