Influence of Ocean Diapycnal Diffusivity on the Amoc Stability and Variability

Abstract:

Spurious diapycnal mixing (κ_d) is often induced in the z-coordinate ocean model, which may be comparable in magnitude to the physical mixing. In order to investigate how the simulated climate may be affected by such spurious mixing, we use a fully coupled climate model, CM2G, in which the ocean module employs a density-coordinate in the vertical direction. The effect of spurious mixing in this model is much smaller than most z-coordinate models, allowing for a reliable specification of the values for κ_d. In a series of sensitivity tests, we increase κ_d by 1, 2, 4, 6 and 8 (unit: 10^-5 m² s^-1), respectively, relative to the default value. In the first set of experiments, we run the model to equilibrium for each of the six κ_d (including the default value) under pre-industrial conditions, and investigate how the variability of Atlantic Meridional Ocean Circulation (AMOC) changes with κ_d. For the run with default κ_d, there is a sharp peak between period 20 to 30 years in the spectrum of AMOC variability. When κ_d is increased, we find that this peak generally becomes lower and lower, from ~45 Sv²/Hz when κ_d has the default value to only ~15 Sv²/Hz when κ_dis increased by 8 units.

 In the second set of experiments, we apply freshwater forcing (0.1 Sv) over the North Atlantic (50°N - 70°N) to each of the six runs obtained in the first set, and study how AMOC responds to the forcing. We find that the response of AMOC to freshwater forcing is significantly different for different κ_d. It weakens less and the response time becomes shorter when κ_d is increased. For example, AMOC weakens from ~24 Sv to ~18 Sv for the default κ_d, but weakens from ~35 Sv to ~32 Sv when κ_d is increased by 8 units. Our study therefore demonstrates that the AMOC stability and variability are strongly dependent on the value of κ_d, reducing the spurious component of this parameter in climate models is critical for simulating realistic climate variability, either internally or externally forced.