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Project OC1

Changes in large-scale overturning circulation: present and past

A. Paul, M. Losch, M. Rhein, S. Mulitza, G. Lohmann
J. Burrows, S. Kasemann, K. Zonneveld

Different processes affect the strength of the large-scale overturning circulation, for example, ventilation of the thermocline and of the deep ocean, and interactions between the subpolar and subtropical gyres. Their relative importance and underlying forcing factors on timescales from 10 to 1000 years are evaluated, using numerical models, recent observations, and paleodata.


The meridional heat transport associated with the large-scale overturning circulation controls the maritime climate conditions in and around the North Atlantic Ocean, including northwestern Europe. According to climate model projections, this overturning circulation is very likely to become weaker during the 21st century, possibly accompanied by significantly reduced formation of Labrador Sea Water (IPCC 2007). Details of the development of the AMOC, however, are uncertain according to IPCC-type coupled climate models (IPCC 2007).The same is true for the strength of the AMOC during the LGM (Otto-Bliesner et al. 2007), for which different climate models simulate very different overturning rates. Thus, the LGM provides a prime target for assessing model behavior. We connect a wide range of timescales from decades to tens of millennia by numerical climate modeling, physical oceanography and paleoceanography, and clarify the reasons for the above ambiguities and improve our understanding of the sensitivity of the large-scale ocean circulation to future perturbations.

Key Hypotheses

  • The strength of the large-scale overturning circulation is primarily controlled by the highlatitude surface heat and freshwater fluxes, and less so by changes in the wind field. During the LGM and for about the last 2000 years, high-latitude precipitation has been the dominant forcing factor.
  • More inflow of warm water into the North Atlantic subpolar gyre is related to increased deep-water ventilation and a stronger AMOC, a decreased ventilation of the thermocline and a weaker subtropical gyre.

Specific Methods

  • Data assimilation by the adjoint method
  • Paired measurements of oxygen isotopes and Mg/Ca ratios on benthic foraminifera to reconstruct deep-water temperature and salinity
  • Neodymium isotopes and selective degradation of particulate organic matter to estimate deep-water oxygen content
  • Analysis of long-term observational time series including remote-sensing data, temperature, salinity, transient tracers, sea-surface height from altimetry
  • High-resolution numerical ocean modeling