Recent trends and variability in the oceanic storage of dissolved
inorganic carbon
Abstract
\justify Several methods have been developed to quantify
the oceanic accumulation of anthropogenic carbon dioxide (CO$_2$) in
response to rising atmospheric CO$_2$. Yet, we still lack a
corresponding estimate of the changes in the total oceanic dissolved
inorganic carbon (DIC). In addition to the increase in anthropogenic
CO$_2$, changes in DIC also include alterations of natural CO$_2$.
Once integrated globally, changes in DIC reflect the net oceanic sink
for atmospheric CO$_2$, complementary to estimates of the air-sea
CO$_2$ exchange based on surface measurements. Here, we extend the
MOBO-DIC machine learning approach by
\citeA{keppler_mapped_2020} to estimate global
monthly fields of DIC at 1$^{\circ}$ resolution
over the top 1500 m from 2004 through 2019. We find that over these 16
years and extrapolated to cover the whole global ocean down to 4000 m,
the oceanic DIC pool increased close to linearly at an average rate of
3.2$\pm$0.7 Pg C yr$^{-1}$. This trend is
statistically indistinguishable from current estimates of the oceanic
uptake of anthropogenic CO$_2$ over the same period. Thus, our study
implies no detectable net loss or gain of natural CO$_2$ by the
ocean, albeit the large uncertainties could be masking it. Our
reconstructions suggest substantial internal redistributions of natural
oceanic CO$_2$, with a shift from the mid-latitudes to the tropics
and from the surface to below $\sim$200 m. Such
redistributions correspond with the Pacific Decadal Oscillation and the
Atlantic Multidecadal Oscillation. The interannual variability of DIC is
strongest in the tropical Western Pacific, consistent with the El
Ni$\tilde{n}$o Southern Oscillation.