Interior pathways of the North Atlantic meridional overturning circulation

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Abstract

To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales1,2,3. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC)4,5,6,7,8,9. These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator10,11. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003–2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated ‘e-floats’ released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.

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Figure 1: Study area and RAFOS float trajectories at the LSW level in the western North Atlantic.
Figure 2: Loss of floats from the DWBC.
Figure 3: Simulated trajectories from FLAME.
Figure 4: Transport map for 1,338 e-floats released in the layer 703–1,548 m at 50° N that crossed the latitude 32° N within 15 years.

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Acknowledgements

We acknowledge the captains and crews of the R/V Oceanus and numerous Canadian fisheries research vessels, and scientists from the Northwest Atlantic Fisheries Centre in St. John’s, Newfoundland, for their assistance in the deployment of the floats and sound sources used in this study. In particular, we thank E. Colbourne for his support of the float release programme. We also acknowledge J. Valdes and B. Guest for their expert technical support in the preparation of the floats, and H. Furey for providing float tracking. J. Fischer of IFM-GEOMAR provided several days of ship time to replace sound sources. We dedicate this work to the memory of F. Schott. The work was supported by the US National Science Foundation.

Author Contributions A.S.B. and M.S.L. contributed equally to this work. A.S.B. led the RAFOS float field programme, and analysed the float and altimetric data. M.S.L. led the modelling study and with S.F.G. analysed the simulated trajectories. C.W.B. provided the model output and assisted with the calculation of the simulated trajectories from FLAME.

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Correspondence to Amy S. Bower or M. Susan Lozier.

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This file contains Supplementary Tables S1 and S2, Supplementary Data, Supplementary Figure S1 with Legend and Supplementary References. (PDF 197 kb)

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Bower, A., Lozier, M., Gary, S. et al. Interior pathways of the North Atlantic meridional overturning circulation. Nature 459, 243–247 (2009) doi:10.1038/nature07979

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