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Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008

Abstract

In the process of open-ocean convection in the subpolar North Atlantic Ocean, surface water sinks to depth as a distinct water mass, the characteristics of which affect the meridional overturning circulation and oceanic heat flux. In addition, carbon is sequestered from the atmosphere in the process. In recent years, this convection has been shallow or non-existent, which could be construed as a consequence of a warmer climate. Here we document the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008. We use profiling float data from the Argo programme to document deep mixing. Analysis of a variety of in situ, satellite and reanalysis data shows that contrary to expectations the transition to a convective state took place abruptly, without going through a phase of preconditioning. Changes in hemispheric air temperature, storm tracks, the flux of fresh water to the Labrador Sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection. Given this complexity, we conclude that it will be difficult to predict when deep mixing may occur again.

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Figure 1: Potential density and buoyancy frequency squared for three selected Argo floats drifting in the Labrador and Irminger seas from 2006–2008.
Figure 2: Changes in wintertime mixed layer depth distribution.
Figure 3: Interannual variability of wintertime air–sea heat flux.
Figure 4: Relationship between air temperature and ice concentration in the winter of 2007–2008.
Figure 5: Wind anomalies and storm properties for the two high-NAO winters of 2006–2007 and 2007–2008.
Figure 6: Hemispheric air temperature decrease in the winter of 2007–2008.

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Acknowledgements

The authors wish to thank T. Haine, J. Morison, L. Yu, S. Häkkinen, A. Sarafanov, I. Rigor, R. Kwok, T. Mitchell, H. Stern and O. Martius for valuable suggestions. We thank H. Wernli for providing the cyclone tracking algorithm and R. Goldsmith for developing a Matlab tool to compute and analyse the tracks. J. Hurrell kindly provided the NAO index time series. Argo data were obtained from the GODAE (www.usgodae.org) and Coriolis (www.coriolis.eu.org) data centres. The Sea Winds, OI SST, and NARR and NCEP reanalysis data sets were obtained from the NOAA National Climatic Data Center (www.ncdc.noaa.gov). The AMSR-E data were obtained from the National Snow and Ice Data Center (www.nsidc.org). We thank J. Wang and H. Adiwidjaja for assistance with the reanalysis data. Support for this work was provided by the Ocean Sciences Division of the National Science Foundation.

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Våge, K., Pickart, R., Thierry, V. et al. Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008. Nature Geosci 2, 67–72 (2009). https://doi.org/10.1038/ngeo382

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