Abstract
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.
Highlights
The ocean absorbs about one-quarter of the anthropogenic CO2 emitted every year (Le Quéré et al, 2018)
We investigated the global surface dissolved inorganic carbon (DIC) and normalized DIC (nDIC) distributions in order to explain the large differences between high-latitude and low-latitude regions
We considered three drivers for how the phenomenon could be explained: (1) sea surface temperature variations through their effect on CO2 system equilibrium constants, (2) salinity-related total alkalinity (TA) variations through their effect on pressure of CO2 (pCO2), and (3) upwelling in the subpolar oceans
Summary
The ocean absorbs about one-quarter of the anthropogenic CO2 emitted every year (Le Quéré et al, 2018). It is the largest non-geological carbon reservoir (∼ 38 000 Gt C; Falkowski et al, 2000), containing 50 times as much carbon as the pre-industrial atmosphere, and thereby modulates the Earth’s climate system. Many previous studies focused on the vertical, rather than latitudinal, distribution of DIC. They investigated the contributions of the different “carbon pumps” – solubility pump, soft tissue pump, and carbonate pump
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