Chlorofluorocarbon uptake in a World Ocean model: 2. Sensitivity to surface thermohaline forcing and subsurface mixing parameterizations

Abstract

Part 1 of this study [ England et al., 1994] examined the sensitivity of simulated oceanic Chlorofluorocarbon (CFC) to changes in the way the air‐sea gas exchange rate is parameterized in a World Ocean model. In part 2 we consider more closely the role of surface thermohaline forcing and subsurface mixing parameterizations in redistributing CFC‐11 and CFC‐12 in the ocean. In particular, a series of five different model ocean experiments are forced with the same air‐sea CFC flux parameterization. The five cases include (1) a control run with a standard seasonal cycle in surface forcing and traditional Cartesian mixing, (2) a run in which the production rate and salinity of Antarctic Bottom Water (AABW) is increased, (3) a run in which the production, outflow rates, and density of North Atlantic Deep Water (NADW) is increased, (4) a run with enhanced isopycnal mixing of passive tracers, and finally (5) a run in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams [1990]. The simulated CFC uptake in the Southern Ocean far exceeds observations in the first four experiments. The excessive uptake is linked to the poor model simulations of Southern Ocean deep water masses, where, for example, model Circumpolar Deep Water is typically 0.2 to 0.4 kg m3 too buoyant. The insufficient density of the deep water allows for extensive penetration of convective adjustment to great depth during winter, in contrast to observations, and this results in excessive downward mixing of the CFC‐enriched surface waters. Compared with the control experiment, the Southern Ocean CFC uptake is reduced in the cases with increased AABW salinity and NADW density, as a result of slightly higher deep water density and reduced wintertime convection in those experiments. Nevertheless, CFC uptake in the Southern Ocean still substantially exceeds observed ocean CFC content in the adjusted surface forcing cases. The most extreme uptake occurs in case 4, where, in addition to deep convective mixing of CFC, there is also mixing into the ocean interior along isopycnal surfaces having an unrealistic orientation. The Southern Ocean CFC uptake in case 5, using the mixing scheme of Gent and McWilliams [1990], is dramatically reduced over that in the other runs. Only in this run do deep densities approach the observed values, and wintertime convection is largely suppressed south of the Antarctic Circumpolar Current. Deep penetration of CFC‐rich water occurs only in the western Weddell and Ross Seas. This run yields CFC sections in the Southern Ocean which compare most favourably with observations, although substantial differences still exist between observed and simulated CFC. The simulation of NADW production is problematic in all runs, with the CFC signature indicating primary source regions in the Labrador Sea and immediately to the southeast of Greenland, while the Norwegian‐Greenland Sea overflow water (which is dominant in reality) plays only a minor role. Lower NADW is insufficiently dense in all runs. Only in the run with surface forcing designed to enhance NADW production does the CFC signal penetrate down the western Atlantic boundary in a realistic manner. However, this case exhibits an unrealistic net ocean surface heat loss adjacent to Greenland and so cannot be advocated as a technique to improve model NADW production. Conventional depth sections and volumetric maps of CFC concentration indicate that on the decadal timescales resolved by CFC uptake the dominant determining factor in overall model ventilation is the choice of subsurface mixing scheme. The surface thermohaline forcing only determines more subtle aspects of the subsurface CFC content. This means that the choice of subgridscale mixing scheme plays a key role in determining ocean model ventilation over decadal to centennial timescales. This has important implications for climate model studies.