A Lagrangian Method to Isolate the Impacts of Mixed Layer Subduction on the Meridional Overturning Circulation in a Numerical Model

• Published in: Journal of climate Vol. 28; no. 19; pp. 7503 - 7517
• Main Authors: Thomas, Matthew D., Tréguier, Anne-Marie, Blanke, Bruno, Deshayes, Julie, Voldoire, Aurore
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.10.2015
• Subjects: Atlantic Meridional Overturning Circulation (AMOC); Atmospheric circulation; Climate policy; Decomposition; Earth Sciences; Estimates; Geography; Geophysics; Gyres; Lagrangian function; Marine; Mathematical models; Mixed layer; Modelling; Numerical models; Ocean circulation; Ocean models; Oceans; Overflow; Particle trajectories; Sciences of the Universe; Sea water; Seas; Studies; Subduction; Trajectories; Irminger Sea; Scotland; United Kingdom > UK; Denmark Strait; Labrador Sea; Greenland; ANW, Labrador Sea; ANE, Irminger Sea; ANE, Norwegian Sea; ANE, North Atlantic, Subpolar Gyre; Meridional overturning circulation; General circulation models; Oceanic mixed layer; Circulation/ Dynamics; Atm/Ocean Structure/ Phenomena; Convection; Lagrangian circulation/transport; Models and modeling
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/jcli-d-14-00631.1

Large differences in the Atlantic meridional overturning circulation (AMOC) exhibited between the available ocean models pose problems as to how they can be interpreted for climate policy. A novel Lagrangian methodology has been developed for use with ocean models that enables a decomposition of the AMOC according to its source waters of subduction from the mixed layer of different geographical regions. The method is described here and used to decompose the AMOC of the Centre National de Recherches Météorologiques (CNRM) ocean model, which is approximately 4.5 Sv (1 Sv = 10⁶ m³ s−1) too weak at 26°N, compared to observations. Contributions from mixed layer subduction to the peak AMOC at 26°N in the model are dominated by the Labrador Sea, which contributes 7.51 Sv; but contributions from the Nordic seas, the Irminger Sea, and the Rockall basin are also important. These waters mostly originate where deep mixed layers border the topographic slopes of the Subpolar Gyre and Nordic seas. The too-weak model AMOC can be explained by weak model representations of the overflow and of Irminger Sea subduction. These are offset by the large Labrador Sea component, which is likely to be too strong as a result of unrealistically distributed and too-deep mixed layers near the shelf.