Author Posting. © Macmillan Publishers, 2009. This is the author's version of the work. It is posted here by permission of Macmillan Publishers for personal use, not for redistribution. The definitive version was published in Nature Geoscience 2 (2009): 67-72, doi:10.1038/ngeo382. ; The process of open-ocean convection in the subpolar North Atlantic Ocean forms a dense water mass that impacts the meridional overturning circulation and heat flux, and sequesters atmospheric carbon. In recent years the convection has been shallow or nonexistent, which could be construed as a consequence of a warmer climate. However, in the winter of 2007-08 deep convection returned to the subpolar gyre in both the Labrador and Irminger Seas. Here we document this return and elucidate the reasons why it happened. Profiling float data from the Argo programme are used to document the deep mixing, and a variety of in-situ, satellite, and reanalysis products are analyzed to describe the conditions leading to the overturning. The transition to a convective state took place abruptly, without going through a preconditioning phase, which is contrary to general expectations. Changes in the hemispheric air temperature, tracks of storms, flux of freshwater to the Labrador Sea, and distribution of pack ice all conspired to enhance the air-sea heat flux, resulting in the deep overturning. This study illuminates the complexity of the North Atlantic convective system. ; Support for this work was provided by the Ocean Sciences Division of the National Science Foundation.