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Slowing of the Atlantic meridional overturning circulation at 25° N


The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator1. Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport2,3,4. Here we analyse a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

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Figure 1: Station positions for transatlantic hydrographic sections taken in 1957, 1981, 1992, 1998 and 2004.
Figure 2: Vertical distribution of mid-ocean meridional geostrophic flow across 25° N section.

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The 2004 transatlantic hydrographic section along 25° N was supported by the Natural Environment Research Council as part of the Core Strategic Research Programme ‘Ocean Variability and Climate’ at Southampton Oceanography Centre. Analysis of the five sections along 25° N was also supported by NERC as part of the Rapid Programme. Comments on an earlier draft by J. Hirschi, W. Johns, S. Josey, C. Meinen, G. Parrilla, P. Rhines, P. Saunders, J. Toole, P. Vélez and R. Wood led to substantial improvement. Author Contributions All authors contributed equally to this work.

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Correspondence to Harry L. Bryden.

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Supplementary information

Supplmentary Figure 1

Cumulative mid-ocean geostrophic transport for the 1957, 1981, 1992, 1998 and 2004 hydrographic sections along 25°N. (PDF 1962 kb)

Supplementary Figure 2

Cumulative mid-ocean geostrophic transport in the upper 1000 m for the 1957, 1981, 1992, 1998 and 2004 hydrographic sections along 25°N. (PDF 5827 kb)

Supplementary Figure 3

Accumulated meridional transport (Sv) above 1000 m depth as a function of zonal distance from the African coast for each of the 5 sections in 1957, 1981, 1992, 1998 and 2004. (PDF 3312 kb)

Supplementary Figure Legends

Text to accompany the above Supplementary Figures. (DOC 23 kb)

Supplementary Table 1

Mid-ocean meridional geostrophic water mass transports (Sv) across 26°N in potential temperature (θ) classes (DOC 32 kb)

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Bryden, H., Longworth, H. & Cunningham, S. Slowing of the Atlantic meridional overturning circulation at 25° N. Nature 438, 655–657 (2005).

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