Séminaire

Deep convection is an important process for intermediate and deep-water masses formation and for deep ocean ventilation. Deep convection is particularly important in the North Atlantic Ocean because it contributes at setting the density of the lower limb of the Meridional Overturning Circulation. The Labrador Sea is the most famous place in the North Atlantic Ocean where deep convection occurs, forming the
intermediate Labrador Sea Water (LSW).

Recent studies suggested that the Irminger Sea is also a convection site where LSW could be formed locally. Since the early 2000s, Argo floats provide a regular sampling of the Irminger Sea, in particular during the winter season. This allowed us to investigate, through a study of the winter mixed layers, whether deep convection in the Irminger Sea is isolated or occurs at a broad spatial scale firstly and to investigate local formation of LSW in the Irminger Sea secondly.

The present analysis of Argo data highlights a deep convection event occurring in the Irminger Sea during the 2011-2012 winter. Intense convective activity at basin scale was observed for the first time in the Irminger Sea. From mid-January to mid-March, 41 Argo profiles exhibited a mixed layer depth exceeding 700 m over a wide area located east of the southern tip of Greenland. Among those 41 profiles, the mixed layer depth recorded by 4 different floats reached about 1000 m. Whereas, over the period 2002-2010, the deepest observed mixed layers in the Irminger Sea did not exceed 600 m, except for the winters 2007-2008 and 2008-2009 during which a few localized mixed layer depths reached 1000 m. In addition, the 2012 deepest mixed layers were deep enough to feed the pool of LSW located in the Irminger Sea. A lagrangian study along the trajectory of the 4 floats showed that there was a good agreement between heat loss at the air-sea interface and the heat content variations in the mixed layer. Those results clearly demonstrated that LSW was formed in the Irminger Sea in winter 2011 2012, which was further confirmed by oxygen data provided by one Argo float equipped with an oxygen sensor.

Recently, Argo data revealed two convection events that occurred during the 2013-2014 and 2014-2015 winters. These two last events were characterized by much deeper winter mixed layers (reaching 1300 and 1700 m respectively). Comparison of convective mechanisms among the three events will also be discussed.