Sensitivity of the Ocean’s Climate to Diapycnal Diffusivity in an EMIC. Part I: Equilibrium State

Abstract

The diapycnal diffusivity in the ocean is one of the least known parameters in current climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggest as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity. In this paper the sensitivity of the current climate to the diapycnal diffusivity is studied, focusing on the changes occurring in the ocean circulation. To this end, a coupled model with a three-dimensional ocean with idealized geometry is used.The results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value based on scaling arguments for uncoupled models of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity in closer accordance with the theoretical value.The vertical heat balance in the global ocean is controlled by, in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) parameterized mesoscale eddy [Gent-McWilliams (GM)] advection. The size of the latter three fluxes increases with diapycnal diffusivity, because the thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence, enhanced isopycnal diffusion and GM advection. Thus larger diapycnal diffusion is compensated for by changes in isopycnal diffusion and GM advection. Little change is found for the advective flux because of compensation between downward and upward advection.Sensitivity results are presented for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result is consistent with the findings of two-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in this model.

Scaling of the Atlantic meridional overturning circulation in a global ocean model

“…Whether other ocean or ocean‐climate models will display the K 1/3 V scaling of the Atlantic MOC is to be seen, and this study hopefully motivates future GCM computations with these models. The results here are consistent with available model results in the literature (Dalan et al, 2005; Furue and Endoh, 2005); when K V is locally decreased in the Pacific pycnocline, the characteristic horizontal velocity decreases and hence the Atlantic MOC also. Note that in the model here, values of K V are relatively large, with a minimum value of 8.0 × 10 −5 m 2 s −1 , and for smaller K V , wind stress may be controlling the strength of the global MOC even more.…”

“…There has been quite some discussion in literature on the issue of the dependence of the strength of the Atlantic Meridional Overturning Circulation (AMOC) on forcing conditions and representations of internal processes such as vertical mixing (Gnanadesikan, 1999; Park and Bryan, 2000, 2001; Klinger et al, 2003; Levermann and Griesel, 2004; Dalan et al, 2005; Lucas et al, 2006). An adequate answer is important since it will lead to an understanding of the processes controlling the AMOC and the sensitivity of the associated heat transport to changes in external forcing conditions.…”

“…Although they mention a linear relationship between global deep overturning and K V , they have computed only a few equilibrium solutions and hence are not able to determine specific scaling relations. The scaling behaviour of the MOC was studied in such a two‐basin configuration by Dalan et al (2005) using an idealized coupled ocean‐atmosphere model. They find the interesting result that the AMOC scales with a power 0.44 of K V but that the Southern Pacific MOC scales with a power of 0.63.…”

“…They find the interesting result that the AMOC scales with a power 0.44 of K V but that the Southern Pacific MOC scales with a power of 0.63. This result in Dalan et al (2005) is based on only four equilibrium solutions. Such computations are indeed expensive as it takes a long time to integrate a global model to equilibrium for small values of K V .…”

The past and future ocean circulation from a contemporary perspective

“…Unhappily (Bryan 1987;Dalan et al, 2005;etc. ) it has been shown repeatedly that such properties as heat transport, and water mass distributions tend to depend directly upon the values chosen.…”