After dissolving in surface water masses, anthropogenicCO2 is then transported at depth by intermediate and deep currents. Mapping the spread of anthropogenicCO2 shows maximum penetration down to around 1.5 km at the level of intermediate water plunge zones. Contamination of the North Atlantic Ocean extends as far as 3 km, due to the North Atlantic Deep Water (NADW), an important component of the global circulation known as thermohaline (or meridional overturning circulation or MOC). This contamination of the North Atlantic depths is well illustrated by the high levels of transient anthropogenic tracers such as carbon-14 and tritium of thermonuclear origin, or industrial chlorofluorocarbons.
Warming and decreasing salinity of surface waters combine to reduce their density, leading to a reduction in deep convection. Numerical general circulation models with ocean-atmosphere coupling generally show a slow decline in the intensity of MOC over the next century in response to global warming (a drop of around 30 % for a medium scenario leading to a pCO2 of 540 ppm in 2100).
For the past ten years, the mean flux of JI has been measured directly by various sensors along an instrumented section at 26° N in the Atlantic, between Florida and Morocco. The various transport components at 26° N show high-frequency temporal variability, and seasonal variability since 2004 (RAPID/MOCHA program). Statistical analysis of the data series suggests a long-term decrease of around 15 % over this decade. However, other instrumented sections at 41° N and 16° N indicate that this trend is still difficult to detect amid interannual fluctuations. What's more, a possible decrease of 15 % over 10 years is five times greater than that expected from numerical simulations of MOC disturbed by global warming due to anthropogenicCO2.
