Variability in Irminger Sea convection and hydrography from 2003 through 2020

The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system. Results from the OSNAP (Overturning in the Subpolar North Atlantic Program) moored array show that the largest contribution to both the total overturning and its variability originates from the Irminger Sea and Iceland Basin. Deep convection in the Irminger Sea strongly impacts the transformation of buoyant to dense waters. Additionally, its localization in the center of the basin directly affects the basin’s horizontal density gradients that drive transport. However, the strength of convection varies greatly from winter to winter and is expected to weaken as a result of strengthening stratification forced by climate change. How exactly the Irminger Sea convection responds to stratification versus forcing is not known.

The LOCO (Long-term Ocean Circulation Observations) mooring recorded convection in the central Irminger Sea from 2003 through 2018. This record is now continued by the OOI (Ocean Observatory Initiative) mooring, deployed nearby in 2014. The combined record of the two moorings showcase the variability of Irminger Sea convection through this 17-year period. This includes the deepest (>1600 m) convection observed in the basin, forced by the exceptionally strong winter of 2014-2015, as well as several winters (in 2010-2011 and 2019-2020) where convection was inhibited by strong upper ocean stratification. The Irminger Sea hydrography changed as a result. The basin warmed and became more saline and stratified during the initial period with weak convection. This trend was halted during the intermittent convection in the mid-2010s. After 2014-2015, the upper 1500 m of the basin cooled and became fresher as a result of stronger convection in the subsequent winter, which led to denser water classes and weaker upper to mid-ocean stratification in the center of the basin. These hydrographic changes and their impact on the cross-basin density gradients are reflected in the Irminger Current transport.

The long record of the Irminger Sea hydrography shows the respective influence of atmospheric buoyancy forcing versus stratification on deep convection. In terms of stratification, we see the effects of both ocean memory in the upper 1500 m of the water column, during prolonged periods of weak or strong convection, and more sudden changes in the uppermost (~100 m) ocean. These insights will help to better predict how Irminger Sea convection will respond to future stratification changes.