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Sensitivity of simulated CFC‐11 distributions in a global ocean model to the treatment of salt rejected during sea‐ice formation
Author(s) -
Caldeira Ken,
Duffy Philip B.
Publication year - 1998
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/98gl00336
Subject(s) - sea ice , geology , pycnocline , sink (geography) , climatology , convection , buoyancy , lead (geology) , ocean general circulation model , environmental science , oceanography , meteorology , general circulation model , climate change , geomorphology , mechanics , physics , cartography , geography
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.