Mh. England et al., CHLOROFLUOROCARBON UPTAKE IN A WORLD OCEAN MODEL .1. SENSITIVITY TO THE SURFACE GAS FORCING, J GEO RES-O, 99(C12), 1994, pp. 25215-25233
The uptake and redistribution of chlorofluorocarbons (CFCs) CFC-11 and
CFC-12 are studied in a series of world, ocean model experiments. In
part 1 of this study the sensitivity of the simulated CFC distribution
s to the model parameterization of air-sea CFC fluxes is examined with
in a control experiment. The control experiment represents a low-resol
ution ocean model with global coverage and a proper seasonal cycling i
n surface thermohaline and wind stress conditions. The specification o
f a surface ocean CFC concentration that is instantaneously in saturat
ed equilibrium with the atmosphere is found to flux too much CFC into
the model. Signatures of CFC-11 are found to be grossly overestimated
in regions of deep and bottom water formation, both in the surface mix
ed layer and at depth. The use of a classical air-sea gas exchange for
mula (even one with a simplified gas transfer velocity that is indepen
dent of wind speed) is seen to greatly improve the CFC simulations at
depth. In addition, the model reproduces many of the observed trends i
n surface CFC concentrations; namely, undersaturation in regions of de
ep convective overturn and near-surface upwelling and supersaturation
in the summer mixed layer. In further sensitivity experiments, we cons
ider the effect of sea ice cover in limiting air-sea gas exchange in p
olar waters. It is found that bottom water in the Arctic Ocean and aro
und the Antarctic continent is significantly reduced in CFC content on
ce regions covered with sea ice are limited to fractional air-sea gas
exchange. This more physically meaningful framework is found to furthe
r reduce the spurious uptake of CFC-11 and CFC-12 found under a ''satu
rated surface'' assumption. In a final sensitivity experiment the gas
exchange rate is parameterized using a complete wind speed and Schmidt
number dependence. The wind speed dependent gas forcing increases the
surface CFC equilibration rate under the subpolar westerlies. On the
other hand, the polar and tropical oceans witness reduced CFC uptake u
nder a wind speed dependent flux regime. Simulated ocean CFC concentra
tions are compared directly with observational data in certain key are
as for deep and bottom water formation. It is found that a reasonable
representation of oceanic CFC is achieved in the convected water colum
n in the Weddell and Labrador Seas. In contrast, deep waters that have
left the convective area with the model ocean currents are found to b
e deficient in CFC-11 in the North Atlantic Ocean. This is because the
model advective timescale for North Atlantic Deep Water (NADW) outflo
w across the equator is too long compared with observed ocean estimate
s. The long timescale is not due to unrealistically sluggish deep curr
ents. Rather, the path of NADW outflow includes a loop eastward from t
he Labrador Sea into the Northeastern Atlantic Basin, effectively incr
easing the required outflow journey by around 4000 km. This ages the w
ater mass by almost 10 years, thereby yielding significantly lower CFC
concentrations in the NADW extension. In addition, the outflow signat
ure spreads too far into the eastern North Atlantic, presumably becaus
e the advective process is too broad and the horizontal diffusion too
strong at depth. Contrasting the North Atlantic, bottom water CFC vent
ilation in the Southern Ocean is found to be too strong, even when sig
nificant levels of surface undersaturation are simulated in polar wate
rs. CFC-tagged waters flowing into the deep South Atlantic basin (from
the Weddell Sea formation zone) are too enriched in CFC11, even when
the deep signatures adjacent to the Antarctic shelf remain close to ob
servations. This suggests that the advective timescale for bottom wate
r ventilation is too rapid in the Southern Ocean. In addition, too muc
h convective overturn persists in the Southern Ocean at 55 degrees S-7
0 degrees S, with unrealistically deep CFC-11 penetration noted at par
ticular longitudes. This is because not enough older (CFC-deprived) wa
ter recirculates and upwells into the Southern Ocean. For example, mor
e upwelled circumpolar deep water in the Southern Ocean would weaken t
he CFC-11 concentrations by contributing to a lower CFC mixture and by
suppressing the convective activity in the region. Bottom and deep le
vel CFC signatures are broad and diffuse compared with the real ocean.
The broadness of the CFC imprint is due, in part, to the model resolu
tion, which gives any convective event a spatial extent of at least 3.
75 degrees longitude by 4.5 degrees latitude and a bottom level CFC si
gnal thickness in excess of 800 m. An important finding of our study i
s that the vertical convection of unstable waters acts as the efficien
t tracer ventilator of the ocean system. This has significant implicat
ions for numerical studies of the world's climate, since the meridiona
l overturning has traditionally been considered the reason for the oce
an's moderating influence during global warming scenarios. Our study s
uggests that the vertical convection would play a much greater role ov
er the typical timescale for anthropogenic climate change.