What drives the latitudinal gradient in open ocean surface dissolved inorganic carbon concentration?

Abstract

Abstract. Previous work has not led to a clear understanding of the causes of spatial pattern in global surface ocean DIC, which generally increases polewards. Here, we revisit this question by investigating the drivers of observed latitudinal 10 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 15 higher values are introduced in upwelled waters. Temperature-driven effects explains the majority of the surface nDIC latitudinal gradient (182 out of 223 μmol kg -1 in 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 208 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 20 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.

Why are Surface Ocean pH and CaCO3 Saturation State Often out of Phase in Spatial Patterns and Seasonal Cycles?

“…Also, we find both Δ[TA-DIC] sea and ΔpH nontem-sea are positively correlated with ΔSST sea (Figure S4). This not only directly explains the opposite signs between ΔpH tem-sea and ΔpH nontem-sea , but also indicates that ΔpH nontem-sea or Δ[TA-DIC] sea is mainly controlled by SST associated processes such as air-sea exchange, vertical mixing and biological activity (e.g., Keppler et al, 2020;Wu et al, 2019) as analyzed in Section 3.1.1.…”

“…In the global surface ocean, the thermal and nonthermal components of pH spatial distribution, that is, ΔpH- Keppler et al, 2020;Wu et al, 2019).…”