Only eddy-resolving models capture the complete range of AMOC responses to surface winds

In a new paper, Veit Lüschow, Jin-Song von Storch and Jochem Marotzke from the Max Planck Institute for Meteorology show that the Atlantic meridional overturning circulation could respond unexpectedly to anticipated future changes in the winds near Antarctica: Besides increasing the northward transport of warm water near the surface, stronger winds in the Southern Ocean might also lead to a drastic weakening of the bottom circulation.

The eddying deep AMOC branch in the high-resolution configuration of the ocean model MPI-OM (we show a snapshot of the flow speed in 2000 m depth). © MPI-M/DKRZ

The Atlantic meridional overturning circulation (AMOC) is the most important carrier of heat in the ocean and a key part of our climate system. Unfortunately, future projections of AMOC remain difficult up to date. One reason is that we do not quite understand the link between AMOC and the westerly winds blowing around Antarctica, which we know will get stronger in a warming climate.

Veit Lüschow and co-authors ran a high-resolution configuration of the global ocean model MPI-OM driven with different wind scenarios at the ocean surface. Compared to the “present day” scenario, the “strong wind” scenario has a slightly stronger upper AMOC branch (in agreement with earlier studies), but it also shows an almost complete decline of the deep AMOC branch. The latter was unforeseen but can be explained with the help of mesoscale ocean eddies that can be explicitly resolved only in high-resolution models; the authors showed that stronger surface winds cause deep eddies to strengthen and decelerate the deep AMOC branch.

 

Applying the same wind scenarios in a non-eddy-resolving configuration of MPI-OM supports the ocean-eddy hypothesis: The decline in the deep branch cannot be reproduced because the non-eddy-resolving model captures eddies only indirectly via a parameterization. It is therefore likely that also other current-generation climate models, most of which are non-eddy-resolving, miss the complete range of AMOC responses. When interpreting (deep) AMOC changes in those models, one should keep in mind that the deep response might be biased.

 

Original publication
Lüschow, V., Marotzke, J., & von Storch, J. S. (2021). Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean. Journal of Physical Oceanography, 51, 1007-1020. doi:10.1175/JPO-D-20-0176.1.

 

Contact

Dr. Veit Lüschow
Max Planck Institute for Meteorology
Email: veit.lueschow@we dont want spammpimet.mpg.de  

Dr. Jin-Song von Storch
Max Planck Institute for Meteorology
Email: jin-song.von.storch@we dont want spammpimet.mpg.de