Restratification Structure and Processes in the Irminger Sea

The Irminger Sea is one of the few regions in the ocean where deep (>1,000 m) convection occurs. Convection is followed by restratification during summer, when the stratification of the water column is reestablished and the convectively formed water is exported at depth. There are currently no descriptions of interannual variability and physical drivers of restratification in the Irminger Sea. We investigate restratification in the upper 600 m of the central Irminger Sea using reanalysis data for the years 1993-2019. We find distinctly different restratification processes in the upper 100 m (the upper layer) and the water below it (the lower layer). In the upper layer, the stratification is dominated by a seasonal cycle that matches the cycle of the surface heat flux. In 2010 and 2019, there were peaks in upper layer restratification, which could partly be related to strong atmospheric heat and freshwater fluxes. Greenland runoff likely also contributed to the high restratification, although this contribution could not be quantified in the present study. In the lower layer there is strong interannual variability in stratification, caused by variability both in the convection and the restratification strength. The restratification strength is strongly correlated with the eddy kinetic energy in the eastern Irminger Sea, suggesting that lower layer restratification is driven by lateral advection of warm, saline waters through Irminger Current eddies. In the future, surface warming and freshening of the Irminger Sea due to anthropogenic climate change are expected to increase upper layer stratification, potentially inhibiting convection. Plain Language Summary In the Irminger Sea, cold winters can cause the ocean's surface waters to cool enough that they start mixing downward. The mixing can continue for months and eventually form a dense water mass of more than 1 km deep. This process is called deep convection. During summer, the dense water gets exported at depth, and the water column will go back to its original structure with lighter waters over denser waters. This is called restratification. The formation and export of dense waters in the Irminger Sea plays an important role in the global ocean circulation. Here we study what drives the restratification in the Irminger Sea, and how it varies in time. We find that in the upper 100 m the restratification is influenced by heating from the atmosphere, which warms the surface waters and makes them lighter. Additionally, rainfall or ice melt which make the surface waters fresher can contribute to restratification. Below the surface layer, restratification happens through transport of warm and saline waters from the eastern Irminger Sea. With rising ocean temperatures due to climate change, the Irminger Sea might see more restratification in the future, which might slow down global oceanic heat transport.