Determining nonlinear thermohaline precursors of the 2009 AMOC slow-down in an eddy permitting model

The year from 2009 to 2010 saw an observed 30% reduction in overturning at the RAPID 26°N array. This downturn preceded an unusually severe winter in western Europe, which had devastating effects on public health and the economy. It has previously been demonstrated that 27% of this change was associated with decreased Ekman transport, but questions remain regarding the mechanisms linked to the remaining thermohaline component. Here, we use an iterative method in an eddy-permitting ocean general circulation model (NEMO-ORCA025) to determine the buoyancy precursor of an efficient decreasing of the remaining transport of the AMOC. The method, of nonlinear optimal perturbations, seeks an optimal thermohaline perturbation targeting a particular AMOC strength at 26°N (we choose 0 Sv).

Using this novel method, we are able to reduce the annually-averaged strength of the AMOC by over 60% within 10 iterations, and over 90% within 20 iterations. We find that the nonlinear optimal perturbation simultaneously stimulates equatorial waves, has a large-scale zonal pattern spanning the subtropical Atlantic, and targets the pathways of North Atlantic Deep Water, particularly in the Labrador Basin. The described changes can be induced with a temperature and salinity perturbation whose respective magnitudes do not exceed 0.52 K or 0.13 psu globally, which has potential implications for the detectability of sudden AMOC events under the current global ocean observing system.