In the Atlantic Ocean, changes in the circulation of water are closely connected with multidecadal variations seen in ocean conditions and associated climate impacts. A recent paper in Reviews of Geophysics synthesizes modern observations, climate simulations, and paleo evidence to provide a comprehensive picture of our current understanding of this linkage, which is crucial for predicting future climate change and variability. This review paper is part of a special collection of journal articles which is jointly supported by the US AMOC Science Team and the UK RAPID program. Here, the authors of the paper give an overview of this topic and outline key challenges to be addressed in future research.
What is “Atlantic multidecadal variability” and what are the key characteristics?
Atlantic multidecadal variability (AMV) refers to large scale, slow variations in Atlantic Ocean conditions relative to the signal associated with global mean changes.
Scientists have observed key characteristics that indicate this variability.
One is opposite sea surface temperature patterns seen over the North and South Atlantic.
Another is co-occurring multidecadal variations in surface temperature, salinity, and heat fluxes over the mid- to high- latitude North Atlantic.
Further characteristics that have been observed include opposite multidecadal variations between upper and deep subpolar North Atlantic temperatures, as well as between surface and subsurface tropical North Atlantic temperatures.
What is “Atlantic meridional overturning circulation” and how is it connected with AMV?
The Atlantic meridional overturning circulation (AMOC) comprises a northward flow of warm salty water in the upper layers of the Atlantic Ocean and a southward flow of cold fresh water at depth. This circulation pattern transports a huge amount of heat northwards.
Multidecadal variations in the amount of heat transported by the AMOC can cause sea surface temperature anomalies in the subpolar North Atlantic, which is one of the key characteristics of AMV.
Coupled ocean-atmosphere interactions in response to AMOC-induced subpolar changes are important for these sea surface temperature anomalies to propagate from the subpolar North Atlantic into the tropical North Atlantic along a horseshoe-shaped pathway.
Many observational and modeling studies are consistent with the idea that multidecadal variability in the AMOC acts as an instigator of the observed AMV.
How do multidecadal AMOC variability and AMV impact climate?
Atlantic multidecadal variability has impacts on many regional and hemispheric-scale climate phenomena that have enormous societal and economic implications.
These include shifts in the Intertropical Convergence Zone; summer monsoon in the Sahel and India; hurricanes in the Atlantic; the El Niño Southern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; sea ice and surface air temperature over the Arctic; and mean surface temperature in the Northern Hemisphere.
Recent observational and modeling evidence suggests that the AMOC plays an essential role in driving many of these climate impacts, particularly through variations in surface heat released from the ocean into the atmosphere over the mid- to high- latitude North Atlantic.
Can the relationship between AMOC and AMV be used to predict future climate changes and impacts?
The relationship is very valuable to predict future climate changes and impacts on decadal timescales. Climate models can successfully predict how given initial conditions of AMOC anomalies at northern high latitudes cause decadal shifts in various Atlantic conditions and associated climate impacts. The subpolar North Atlantic emerges as a key region for predicting the tropical signal.
What are some of the unresolved questions where additional research, data or modeling is needed?
Most state-of-the-art climate models underestimate the amplitude of multidecadal AMOC variability. This leads to an underestimation of the linkage between the AMOC and AMV and associated climate impacts. Thus, there are both serious challenges and great opportunities for making substantial improvements in our understanding of these phenomena and predicting their behavior and impacts.
It would be valuable to employ a hierarchy of models, expand the number of high-resolution paleo records, and maintain long-term instrumental observations in the future.
—Rong Zhang (email: [email protected]), National Oceanic and Atmospheric Administration; Rowan Sutton, University of Reading; Gokhan Danabasoglu, National Center for Atmospheric Research; Young-Oh Kwon, Woods Hole Oceanographic Institution; Robert Marsh, University of Southampton; Stephen G. Yeager, National Center for Atmospheric Research; Daniel E. Amrhein, University of Washington; and Christopher M. Little, Atmospheric and Environmental Research, Inc.