Fast mechanisms linking the Labrador Sea with subtropical Atlantic overturning

• Published in: Climate Dynamics
• Main Authors: Kostov, Yavor, Messias, Marie-José, Mercier, Herlé, Johnson, Helen, L, Marshall, David, P
• Other Authors: University of Exeter, Laboratoire d'Océanographie Physique et Spatiale (LOPS), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut national des sciences de l'Univers (INSU - CNRS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Department of Earth Sciences Oxford, University of Oxford, Department of Physics Oxford
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2022
• Subjects: NAC; North Atlantic Current; Surface heat fuxes; Water mass transformation; OSNAP; RAPID-MOCHA; s AMOC; Labrador Sea; LNADW; Lower North Atlantic Deep Water; Atlantic Meridional Overturning Circulation; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; north atlantic current; North Atlantic Deep Water; North Atlantic
• Online Access: https://hal.science/hal-03814779; https://hal.science/hal-03814779/document; https://hal.science/hal-03814779/file/s00382-022-06459-y%281%29.pdf; https://doi.org/10.1007/s00382-022-06459-y

International audience We use an ocean general circulation model and its adjoint to analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea to variability in the deep branch of the Atlantic meridional overturning circulation (AMOC) on inter-annual timescales. Our study highlights the importance of the North Atlantic Current (NAC) for the north-to-south connectivity in the AMOC and for the meridional transport of Lower North Atlantic Deep Water (LNADW). We identify two mechanisms that allow the Labrador Sea to impact velocities in the LNADW layer. The first mechanism involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background NAC. The second mechanism plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea; the NAC adjustment modifies the northward transport of salt and heat and exerts a strong positive feedback, amplifying the upper ocean buoyancy anomalies. The two mechanisms spin up/down the subpolar gyre on a timescale of years, while boundary trapped waves rapidly communicate this signal to the subtropics and trigger an adjustment of LNADW transport on a timescale of months. The NAC and the eastern subpolar gyre play an essential role in both mechanisms linking the Labrador Sea with LNADW transport variability and the subtropical AMOC. We thus reconcile two apparently contradictory paradigms about AMOC connectivity: (1) Labrador Sea buoyancy anomalies drive AMOC variability; (2) water mass transformation is largest in the eastern subpolar gyre.