Overturning Response to a Surface Wind Stress Doubling in an Eddying and a Non-Eddying Ocean

In this paper, the overturning responses to wind stress changes of an eddying ocean and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic Ocean with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (similar to 25%; 1 Sv equivalent to 10(6) m(3) s(-1)). In contrast, the DWBC responds with a speedup in the non-eddying configuration and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models, which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies that are sourced from the upper ocean provide an excess input of relative vorticity that then actively forces the DWBC mean flow to slow down.

Automated summary

This paper compares the overturning responses to wind stress changes between an eddying ocean and a non-eddying ocean, finding differences in the deep overturning cell and the deep western boundary current in the North Atlantic Ocean. The study demonstrates that the slowdown of the DWBC in the eddying configuration is mainly balanced by eddy vorticity fluxes, which may not be captured in non-eddying ocean models, affecting the overall range of overturning responses. It also provides evidence that deep eddies sourced from the upper ocean can actively force the DWBC mean flow to slow down.