Kjetil Våge et al., Nature Geosci., Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008

Nature Geoscience, 2 (2008) 67-72, published online 30 November 2008; doi: 10.1038/ngeo382

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

Kjetil Våge¹, Robert S. Pickart¹, Virginie Thierry², Gilles Reverdin³, Craig M. Lee⁴, Brian Petrie⁵, Tom A. Agnew⁶, Amy Wong⁶ and Mads H. Ribergaard⁷

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.

1. Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
2. IFREMER, Laboratoire de Physique des Océans, UMR 6523 CNRS/IFREMER/IRD/UBO, 29280 Plouzané, France
3. Laboratoire d'Océanographie Dynamique et de Climatologie, FR-75252 Paris, France
4. Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA
5. Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada
6. Meteorological Service of Canada, Downsview, Ontario M3H 5T4, Canada
7. Danish Meteorological Institute, DK-2100 Copenhagen, Denmark

Correspondence to: Kjetil Våge¹ e-mail: kjetil@whoi.edu

Link to abstract: http://www.nature.com/ngeo/journal/v2/n1/abs/ngeo382.html