The depth scales of the AMOC on a decadal timescale

Abstract

Abstract. We use wind sensitivity experiments to understand the wind forcing dependencies of the level of no motion and the e-folding pycnocline scale as well as their relationship to northward transport of the mid-depth Atlantic meridional overturning circulation (AMOC) south and north of the equator. In contrast to previous studies, we investigate the interplay of nonlocal and local wind effects on a decadal timescale. We use 30-year simulations with a high-resolution ocean general circulation model (OGCM) which is an eddy-resolving version of the Max Planck Institute Ocean Model (MPIOM). Our findings deviate from the common perspective that the AMOC is a nonlocal phenomenon only, because northward transport in the inter-hemispheric cell can only be understood by analyzing nonlocal Southern Ocean wind effects and local wind effects in the northern hemisphere downwelling region where Ekman pumping takes place. Southern Ocean wind forcing predominantly determines the magnitude of the pycnocline scale throughout the basin, whereas northern hemisphere winds additionally influence the level of no motion locally. In that respect, the level of no motion is a better proxy for northward transport and mid-depth velocity profiles despite the Ekman return flow which is found to be baroclinic. We compare our results inferred from the wind experiments and a 100-year global warming experiment in which the atmospheric CO2 concentration is quadrupled, using MPIOM coupled to an atmospheric model. We find that the evolution of the level of no motion in response to global warming represents changes in vertical velocity profiles or northward transport, whereas the changes of the pycnocline scale are opposite to the changes of the level of no motion over time. Using the level of no motion as depth scale, the analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC. Furthermore, we put forward the idea that the ability of numerical models to capture the spatial and temporal variations of the level of no motion is crucial to reproduce the mid-depth cell in an appropriate way.

Nonlocal and local wind forcing dependence of the Atlantic meridional overturning circulation and its depth scale

“…Data will be published via MPG.PuRe (https://pure.mpg.de/pubman/faces/ViewItemOverviewPage.jsp? itemId=item_3279817, Rohrschneider et al, 2022). We publish the data associated with the present study during the external review process.…”

Response of Northern North Atlantic and Atlantic Meridional Overturning Circulation to Reduced and Enhanced Wind Stress Forcing

“…(2021) and Rohrschneider et al. (2021) find a strengthening of the AMOC at all latitudes in the North Atlantic. However, comparison with a simulation in which only the westerlies over the Southern Ocean are doubled suggests that a large part of the AMOC strengthening in the North Atlantic is caused by remote wind stress forcing over the Southern Ocean (Lueschow et al., 2021).…”

“…Using a similar model setup and doubling the wind stress forcing over the North Atlantic, Cessi (2018) finds an increase in the maximum AMOC strength in the subpolar North Atlantic, but a decrease in the AMOC strength at other latitudes in the North Atlantic. By globally doubling the wind stress forcing in a 30-year long eddy-resolving ocean model simulation, Lueschow et al (2021) and Rohrschneider et al (2021) find a strengthening of the AMOC at all latitudes in the North Atlantic. However, comparison with a simulation in which only the westerlies over the Southern Ocean are doubled suggests that a large part of the AMOC strengthening in the North Atlantic is caused by remote wind stress forcing over the Southern Ocean (Lueschow et al, 2021).…”