On the stability of the Atlantic meridional overturning circulation

Abstract

One of the most important large-scale ocean current systems for Earth's climate is the Atlantic meridional overturning circulation (AMOC). Here we review its stability properties and present new model simulations to study the AMOC's hysteresis response to freshwater perturbations. We employ seven different versions of an Ocean General Circulation Model by using a highly accurate tracer advection scheme, which minimizes the problem of numerical diffusion. We find that a characteristic freshwater hysteresis also exists in the predominantly wind-driven, low-diffusion limit of the AMOC. However, the shape of the hysteresis changes, indicating that a convective instability rather than the advective Stommel feedback plays a dominant role. We show that model errors in the mean climate can make the hysteresis disappear, and we investigate how model innovations over the past two decades, like new parameterizations and mixing schemes, affect the AMOC stability. Finally, we discuss evidence that current climate models systematically overestimate the stability of the AMOC.

Fig. 1.

Maximum strength of the AMOC in Sv against the freshwater anomaly supplied to the NA between 30° and 50 °N for experiments STANDARD (A) and CLIMBER (black line) and BRIDGE (red line) (B). (A) Experiment STANDARD reveals a hysteresis curve with a stable “on” state and “off” state. Equilibrium experiments using an anomalous freshwater forcing F_h of 0.2 Sv provide two distinct values for the maximum NA overturning strength: 13.8 Sv and 4.5 Sv (green crosses). (B) Experiments CLIMBER and BRIDGE reveal a very narrow hysteresis curve for F_h values >0.2 Sv. Equilibrium runs with both models at F_h = 0.3 Sv and F_h = 0.45 Sv (green arrows and crosses) show that the upper and lower branch of the hysteresis loop for CLIMBER and BRIDGE converge against each other.

Fig. 2.

Hysteresis curves for the HISTORIC model set compared with STANDARD. Full red line, HISTORIC-1;dashed red line, HISTORIC-2; green line, HISTORIC-3; blue line, HISTORIC-4; black line, STANDARD. For details see Dependence on Parameterizations and Numerical Schemes.

Fig. 3.

Hysteresis curves for the maximum Atlantic overturning strength and NA mixed layer depths, meridional density gradients, and mean Atlantic pycnocline depth. (A) Maximum strength of Atlantic overturning (black) and maximum mixed layer depth in the Labrador and Nordic Seas between 60° and 80 °N (red) versus the anomalous freshwater flux over the NA for experiment STANDARD. (B and C) The same as in A but for CLIMBER and HISTORIC-1, respectively. (D) Zonal mean Atlantic potential density difference at depth of 750 m between 50 °N and 35 °S (black) and Atlantic mean pycnocline depth (red) (16) as a function of the maximum strength of the Atlantic overturning for experiment STANDARD. (E and F) The same as in D but for experiments CLIMBER and HISTORIC-1, respectively.

Fig. 4.

Turbulent air–sea surface freshwater fluxes (SFF) (in Sv), sea surface salinity [(SSS), in practical salinity units (PSU)], and Atlantic MFT (in Sv) for the climatological mean state. (A) The SFF distribution provided by STANDARD reveals a characteristic spatial pattern with net precipitation in the tropics and mid–high latitudes and net evaporation over the subtropical gyres. (B) The simulated STANDARD SSSs resemble the observational data (47) (see Fig. S1b). (C) Comparison of the northward Atlantic MFT from STANDARD (black curve) with observational data (black squares) compiled by Wijffels (48). (D and G) Difference of the SFF between simulations CLIMBER and STANDARD and between BRIDGE and STANDARD, respectively. Note that the SFF distributions reveal a much higher net evaporation over the North Atlantic Subtropical Gyre and a higher net precipitation over Atlantic equatorial regions compared with STANDARD. (E and H) Difference of SSSs between simulations CLIMBER and STANDARD and between BRIDGE and STANDARD, respectively. (F and I) The same as in C but for CLIMBER and BRIDGE, respectively.