The Southern Ocean carbon cycle 1985-2018: Mean, seasonal cycle, trends
and storage
Abstract
We assess the Southern Ocean CO2 uptake (1985-2018) using data sets
gathered in the REgional Carbon Cycle Assessment and Processes Project
phase 2 (RECCAP2). The Southern Ocean acted as a sink for CO2 with close
agreement between simulation results from global ocean biogeochemistry
models (GOBMs, 0.75±0.28 PgCyr-1) and pCO2-observation-based products
(0.73±0.07 PgCyr-1). This sink is only half that reported by RECCAP1.
The present-day net uptake is to first order a response to rising
atmospheric CO2, driving large amounts of anthropogenic CO2 (Cant) into
the ocean, thereby overcompensating the loss of natural CO2 to the
atmosphere. An apparent knowledge gap is the increase of the sink since
2000, with pCO2-products suggesting a growth that is more than twice as
strong and uncertain as that of GOBMs (0.26±0.06 and 0.11±0.03 PgCyr-1
decade-1 respectively). This is despite nearly identical pCO2 trends in
GOBMs and pCO2-products when both products are compared only at the
locations where pCO2 was measured. Seasonal analyses revealed agreement
in driving processes in winter with uncertainty in the magnitude of
outgassing, whereas discrepancies are more fundamental in summer, when
GOBMs exhibit difficulties in simulating the effects of the non-thermal
processes of biology and mixing/circulation. Ocean interior accumulation
of Cant points to an underestimate of Cant uptake and storage in GOBMs.
Future work needs to link surface fluxes and interior ocean transport,
build long overdue systematic observation networks and push towards
better process understanding of drivers of the carbon cycle.