Sensitivity of simulated CFC‐11 distributions in a global ocean model to the treatment of salt rejected during sea‐ice formation

Abstract

Abstract. We show that simulated oceanic absorption of an atmospheric gas is very sensitive to the representation of a process that occurs beneath sea ice. As sea ice forms, salt is rejected, locally increasing surface sea-water density. This dense water can sink to the pycnocline at the base of the mixed-layer. Previous studies have not considered the impact of this subgrid-scale process on transient tracers in the ocean. To assess the potential importance of this process to the oceanic absorption of atmospheric gases, we performed two idealized simulations: a Control simulation in which salt rejected during sea-ice formation is placed in the model's 25 m thick surface layer; and a Test simulation in which salt rejected during sea-ice formation is distributed uniformly through the upper 160 m beneath the forming sea ice. Our treatment of rejected salt is highly idealized, and is intended to demonstrate the need for a physically-based parameterization of subgrid-scale convection for use in ocean general circulation models that takes into account the subgrid-scale heterogeneity of surface buoyancy forcing. Distributing rejected salt more deeply during periods of ice formation helps to maintain vertical density gradients, inhibiting grid-scale convection, especially in the Southern Ocean. This greatly diminishes simulated ocean uptake of CFC-11, and generally improves simulated CFC-11 and salinity fields. The modeled global ocean inventory of CFC-11 for year 1990 is about 30% lower, and modeled column inventories in the Southern Ocean are up to 90% lower, in our Test simulation relative to our Control simulation. We infer that a more detailed treatment of subgrid-scale processes occurring beneath sea ice may diminish simulated oceanic absorption of anthropogenic CO 2, especially in the Southern Ocean.

Effect of horizontal grid resolution on the near‐equilibrium solution of a global ocean–sea ice model

“…The problem is due in part to the absence of bottom boundary flows in our simulations; these flows bring very cold water (as well as other tracers) into the deep ocean, especially from Antarctic shelf regions. The absence of these flows in our model has been documented in previous simulations [ Caldeira and Duffy , 1998], which show that our model does not reproduce elevated concentrations of CFCs observed near the ocean floor; these elevated concentrations are thought to be produced by bottom boundary flows.…”

Comparison of water column [CO2aq] with sedimentary alkenone‐based estimates: A test of the alkenone‐CO2proxy

“…We estimated preindustrial water column [CO 2aq ] ([CO 2aq ] preind ) using two approaches: (1) reducing [CO 2aq ] NOPACCS values by 23% of the observed value across the entire transect (a 23% decrease from the modern value is approximately equivalent to a 30% increase from preindustrial values due to inputs of anthropogenic CO 2 ), and (2) correcting [CO 2aq ] NOPACCS at each site using anthropogenic [CO 2aq ] simulated from a modified version [ Duffy and Caldeira , 1997; Caldeira and Duffy , 1998] of the Geophysical Fluid Dynamics Laboratory's Modular Ocean Model [ Pacanowski et al , 1991] (Figure 5). Values of [CO 2aq ] preind reconstructed from our model simulation show smaller contributions of anthropogenic CO 2 across the low latitudes relative to a simple 23% reduction of water column [CO 2aq ] (Figure 5), conforming with expectations that surface waters influenced by upwelling are characterized by lower anthropogenic contributions.…”

Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models

“…In order to simulate concentrated brine plumes, the LLNL model transports salt, but not other tracers, downward during brine rejection (Duffy et al, 1997). Thus convection is reduced as is CFC-11 uptake in this region (Caldeira and Duffy, 1998). Still this parameterization is not the only mean for improving the representation of bottom water ventilation in these coarse resolution models.…”