Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007e2008. Nature Geosci

• Main Authors: Kjetil Våge, Robert S. Pickart, Virginie Thierry, Gilles Reverdin, Craig M. Lee, Brian Petrie, Tom A. Agnew, Amy Wong, Mads H. Ribergaard
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2009
• Subjects: Labrador Sea; North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.420.1127; http://www.seas.harvard.edu/climate/seminars/pdfs/vage_etal_2009.pdf

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. The subpolar North Atlantic Ocean is a critical component of the global climate system. Deep convection in the Labrador and Irminger seas produces the water mass known as Labrador Sea Water (LSW (ref. 1)). This results in a net transfer of heat from the ocean to the atmosphere, which is balanced