Abstract. Previous work has not led to a clear understanding of the
causes of spatial pattern in global surface ocean dissolved inorganic carbon (DIC), which generally
increases polewards. Here, we revisit this question by investigating the
drivers of observed latitudinal gradients in surface salinity-normalized DIC
(nDIC) using the Global Ocean Data Analysis Project version 2 (GLODAPv2)
database. We used the database to test three different hypotheses for the
driver producing the observed increase in surface nDIC from low to high
latitudes. These are (1) sea surface temperature, through its effect on the
CO2 system equilibrium constants, (2) salinity-related total alkalinity
(TA), and (3) high-latitude upwelling of DIC- and TA-rich deep waters. We
find that temperature and upwelling are the two major drivers. TA effects
generally oppose the observed gradient, except where higher values are
introduced in upwelled waters. Temperature-driven effects explain the
majority of the surface nDIC latitudinal gradient (182 of the 223 µmol kg−1 increase from the tropics to the high-latitude Southern Ocean).
Upwelling, which has not previously been considered as a major driver,
additionally drives a substantial latitudinal gradient. Its immediate
impact, prior to any induced air–sea CO2 exchange, is to raise Southern
Ocean nDIC by 220 µmol kg−1 above the average low-latitude value.
However, this immediate effect is transitory. The long-term impact of
upwelling (brought about by increasing TA), which would persist even if gas
exchange were to return the surface ocean to the same CO2 as without
upwelling, is to increase nDIC by 74 µmol kg−1 above the low-latitude average.
Southern Ocean carbon export efficiency in relation to temperature and primary productivity