Impact of episodic vertical fluxes on sea surface pCO ₂

Abstract

Episodic events like hurricanes, storms and frontal-and eddy-driven upwelling can alter the partial pressure of CO 2 (pCO 2 ) at the sea surface by entraining subsurface waters into the surface mixed layer (ML) of the ocean. Since pCO 2 is a function of total dissolved inorganic carbon (DIC), temperature (T ), salinity and alkalinity, it responds to the combined impacts of physical, chemical and biological changes. Here, we present an analytical framework for assessing the relative magnitude and sign in the short-term perturbation of surface pCO 2 arising from vertical mixing events. Using global, monthly, climatological datasets, we assess the individual, as well as integrated, contribution of various properties to surface pCO 2 in response to episodic mixing. The response depends on the relative vertical gradients of properties beneath the ML. Many areas of the ocean exhibit very little sensitivity to mixing owing to the compensatory effects of DIC and T on pCO 2 , whereas others, such as the eastern upwelling margins, have the potential to generate large positive/negative anomalies in surface pCO 2 . The response varies seasonally and spatially and becomes more intense in subtropical and subpolar regions during summer. Regions showing a greater pCO 2 response to vertical mixing are likely to exhibit higher spatial variability in surface pCO 2 on time scales of days.

Interannual drivers of the seasonal cycle of CO<sub>2</sub> fluxes in the Southern Ocean

“…given that models and observations support this (Hoppema et al 1999;Bakker et al 2008;Mahadevan et al 2011;Wang et al 2012;Hauck et al 2013;Shetye et al 2015). However, our data show that the dominance of interannual Chl-a variability over ∆pCO 2 is largely limited to regions where Chl-a is high, such as the Atlantic, the Agulhas retroflection and south of Australia and New Zealand (Figure 9).…”

“…CC BY 4.0 License. (Mahadevan et al 2011;Thomalla et al 2011;Lenton et al 2013). The competition between light and iron limitation results in heterogeneous distribution of Chl-a in both space and time, with similar implications for pCO 2 (Thomalla et al 2011;Carranza and Gille, 2015).…”

Observed small spatial scale and seasonal variability of the CO<sub>2</sub> system in the Southern Ocean

“…11a and b). As in Mahadevan et al (2011), the seasonality is estimated by averaging the Glodap values within the mixed layer. In winter, the ML deepens and surface DIC concentrations increase when averaged with high-DIC subsurface concentrations.…”

“…Although the role of biological activity is more localized, it enhances the variability up to 30 µmol kg −1 over 100 km. The seasonal cycle of surface DIC is reconstructed following Mahadevan et al (2011), using an annual climatology of DIC and a monthly climatology of mixed layer depth. This method is evaluated using drifter observations and proves to be a reasonable first-order estimate of the seasonality in the Southern Ocean that could be used to validate model simulations.…”

Greenhouse gases in the Earth system: setting the agenda to 2030

“…Wolff [6] brings a palaeoclimate perspective from the ice core records. Ciais et al [7] consider Africa's poorly understood carbon balance, while Mahadevan et al [8], Gruber [9] and Watson et al [10] discuss the underappreciated changes taking place in the ocean, including deoxygenation and acidification. Rayner et al [11] and Keeling et al [12] discuss the general puzzles of where the carbon is going.…”