Two superimposed cold and fresh anomalies enhanced Irminger Sea deep convection in 2016-2018.

While Earth system models project a reduction, or even a shut-down, of deep convection in the North Atlantic Ocean in response to anthropogenic forcing, deep convection returned to the Irminger Sea in 2008 and occurred several times since then to reach exceptional depths > 1,500 m in 2015 and 2016. In this context, we used Argo data to show that deep convection persisted in the Irminger Sea during two additional years in 2017 and 2018 with maximum convection depth > 1,300 m. In this article, we investigate the respective roles of air-sea flux and preconditioning of the water column to explain this exceptional 4-year persistence of deep convection; we quantified them in terms of buoyancy and analyzed both the heat and freshwater components. Contrary to the very negative air-sea buoyancy flux that was observed during winter 2015, the buoyancy fluxes over the Irminger Sea during winters 2016, 2017 and 2018 were close to climatological average. We estimated the preconditioning of the water column as the buoyancy that needs to be removed (B) from the end of summer water column to homogenize the water column down to a given depth. B was lower for winters 2016-2018 than for the mean 2008-2015, including a vanishing stratification from 600 m down to ~1,300 m. It means that less air-sea buoyancy loss was necessary to reach a given convection depth than in the mean and once convection reached 600 m little additional buoyancy loss was needed to homogenize the water column down to 1,300 m. We showed that the decrease in B was due to the combined effects of a cooling of the intermediate water (200-800 m) and a decrease in salinity in the 1,200-1,400 m layer. This favorable preconditioning permitted the very deep convection observed in 2016-2018 despite the atmospheric forcing was close to the climatological average. [ABSTRACT FROM AUTHOR]