Mechanisms and Impacts of a Partial AMOC Recovery Under Enhanced Freshwater Forcing

• Published in: Geophysical research letters 2019-03, Vol.46 (6), p.3308-3316
• Main Authors: Thomas, M. D., Fedorov, A. V.
• Format: Article
• Language: English
• Subjects: 21st century; AMOC; AMOC recovery; Atlantic Meridional Overturning Circulation (AMOC); Climate change; Climate models; Convection; Deep water; Deep water formation; Density; Density gradients; Density stratification; Dynamics; Fresh water; Freshwater; Heat; Heat transport; Inland water environment; Mathematical models; Meridional heat transport; Negative feedback; Numerical models; Ocean currents; Ocean models; Oceans; Recovery; Reduction; Return flow; Stratification; Transport
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL080442

The Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken in the 21st century due to increased surface buoyancy. Such AMOC changes in ocean models are often accompanied by a subsurface reduction in density. Here we perform freshwater perturbation experiments with both a 1° coupled model and an idealized zonally averaged ocean‐only model to demonstrate that slow subsurface property changes (1) introduce a negative feedback that erodes the stratification and partially reinvigorates convection and the AMOC and (2) ensure the meridional heat transport weakens less than the AMOC. In the coupled model with a 0.1‐Sv net freshwater flux introduced around Greenland, an initial 22% AMOC reduction over 40 years is followed by a recovery of almost half the lost strength after 400 years. The final heat transport, however, is weakened by only 7%. Similar responses in the idealized model demonstrate that 2‐D ocean‐only dynamics control the changes. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) describes the northward upper‐ocean transport and subsequent deep return flow by the currents in the Atlantic, and its heat transport is an important element of the climate. The AMOC is predicted to weaken over the 21st century, and tentative evidence suggests that it may already be slowing down. Here we use freshwater experiments with both a comprehensive climate model and an idealized numerical model to identify and explain a North Atlantic oceanic mechanism that can induce a partial recovery of the AMOC and its heat transport under climate change forcing. This mechanism operates through gradual subsurface changes in ocean properties that reduce the high‐latitude vertical density gradient and allow a partial reinvigoration of the deep water formation that is necessary to sustain overturning. However, the mechanism requires that the AMOC remain active and therefore that climate change forcing remains limited. Key Points We isolate a feedback mechanism for a partial AMOC recovery following a simulated AMOC weakening The heat transported by the weakening AMOC becomes more efficient as its upper branch warms Similar results from a 2‐D model demonstrate the recovery arises through an ocean‐only 2‐D mechanism