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OSNAP: Overturning in the Subpolar North Atlantic


OSNAP is an international program designed to provide a continuous record of the full-water column, trans-basin fluxes of heat, mass and freshwater in the subpolar North Atlantic. The OSNAP observing system consists of two legs: one extending from southern Labrador to the southwestern tip of Greenland across the mouth of the Labrador Sea (OSNAP West), and the second from the southeastern tip of Greenland to Scotland (OSNAP East).

The observing system also includes subsurface floats (OSNAP Floats) in order to trace the pathways of overflow waters in the basin and to assess the connectivity of currents crossing the OSNAP line.

OSNAP: Floats

Deep RAFOS floats were used to track the pathways of the lower limb of the overturning circulation through the subpolar North Atlantic.  These passive drifters were ballasted to drift at depths ranging from 1800 to 2800 meters, and deployed in the deep boundary currents: along the western side of the Reykjanes Ridge in the Iceland Basin, Read More

in the Charlie-Gibbs Fracture Zone which is a deep channel connecting the east and west North Atlantic Basins just south of the Rekjanes Ridge, along the eastern side of the Reykjanes Ridge, and, finally, along the eastern side of Greenland in the Irminger Sea.  The floats seeded the components of the deep AMOC limb: the Iceland-Scotland Overflow Water (ISOW) in the Iceland Basin, through the CGFZ, and along the east side of the Irminger Sea, and the combination of ISOW and Denmark Strait Overflow Water (DSOW) that makes up the deep limb on the west side of the Irminger Sea (east of Greenland).  Using this tool allowed us to connect the deep limb of the AMOC between and beyond the mooring arrays.

OSNAP: Moorings

The Bower Lab is responsible for the Greenland Deep Boundary Current mooring array located offshore of the shelf break east of Greenland, bracketing the OOI Irminger Node.


OSNAP: Gliders

EKE across supolar norht atlantic w/ glider track loc

A WHOI-OUC collaboration took place from 2014-2016 where TWR gliders were deployed and maintained west of the Rockall Plateau along the OSNAP line.  The glider array was used to measure the highly variable branch of the NAC which travels over the Charlie-Gibbs Fracture Zone and continues into the Iceland Basin, meandering northward towards the Iceland-Scotland sill.



Ramsey, A., H. Furey, and A. Bower, 2020. Overturning of the Subpolar North Atlantic Program (OSNAP): RAFOS Float Data Report, June 2014 - January 2019.Woods Hole Oceanographic Institution Technical Report, WHOI-2020-06; doi:10.1575/1912/26515.

Zou, S., A. Bower, H. Furey, S. Lozier, and X. Xu, 2020.  Redrawing the Iceland-Scotland Overflow Water Pathways in the North Atlantic. Nature Communications, 11(1), pp.1-8.

Bower, A., Lozier, S., Biastoch, A., Drouin, K., Foukal, N., Furey, H., Lankhorst, M., Rühs, S. and Zou, S., 2019. Lagrangian views of the pathways of the Atlantic Meridional Overturning Circulation. Journal of Geophysical Research: Oceans, 124(8), pp.5313-5335.

Lozier, M.S., F. Li, S. Bacon, F. Bahr, A. Bower, S. Cunningham, F. de Jong, L. de Steur, B. DeYoung, J. Fischer, S. Gary, B. Greenan, N.P. Holliday, A. Houk, L. Hupert, M. Inall, W. Johns, H. Johnson, J. Karsetensen, G. Koman, I. LeBras, X. Lin, N. Mcackay, D. Marshall, H. Mercier, M. Oltmanns, R.S. Pickart, A. Ramsey, D. Rayner, F. Straneo, V. Tierry, D.J. Torres, R. Williams, C. Wilson, J. Yang, I. Yashayaev, J. Zhao, 2018. A sea change in our view of overturning--first results from the Overturning in the Subpolar North Atlantic Program. Science, 363(6426), 516-521, doi:10.1126/science.aau6592.

Ramsey, A., H. Furey, and A. Bower, 2018. Deep floats reveal ocean circulation patterns. Eos, 99,

Xu, X., A. Bower, H. Furey, and E. Chassignet, 2018. Variability of the Iceland-Scotland overflow water transport through the Charlie-Gibbs Fracture Zone: Results from an eddying simulation and observations. Journal of Geophysical Research – Oceans, 123, 5808-5823,

Zhao, J., A. Bower, J. Yang, and X. Lin, 2018.  Meridional heat transport variability induced by mesoscale processes in the subpolar North Atlantic.  Nature Communications, 9(1), 1124, doi:10.1038/s41467-018-03134-x.

Zou, S., M. S. Lozier, W. Zenk, A. Bower, and W. Johns, 2017.  Observed and modeled pathways of the Iceland Scotland Overflow Water in the eastern North Atlantic.  Progress in  Oceanography, 159, 211-222,

Bower, A., and H. Furey, 2017. Iceland-Scotland overflow water transport variability through the Charlie-Gibbs Fracture Zone and the impact of the North Atlantic CurrentJournal of Geophysical Research – Oceans,  doi:10.1002/2017JC012698.

Lozier, M.S., S. Bacon, A.S. Bower, S.A. Cunningham, M.F. de Jong, L. de Steur, B. deYoung, J. Fischer, S.F. Gary, B.J.W. Greenan, P. Heimbach, N.P. Holliday, L. Houpert, M.E. Inall, W.E. Johns, H.L. Johnson, J. Karstensen, F. Li, X. Lin, N. Mackay, D. P. Marshall, H. Mercier, P.G. Myers, R. S. Pickart, H.R. Pillar, F. Straneo, V. Thierry, R.A. Weller, R.G. Williams, C. Wilson, J. Yang, J. Zhao, and J.D. Zika, 2017. Overturning in the Subpolar North Atlantic Program: a new international ocean observing systemBulletin of the American Meteorological Society, 98, 737-752, doi:


This project is generously funded by the National Science Foundation.

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