Fig. 3. Conceptual model to explain observed synoptic variability in pCO_2-DIC′.

a Vertical profiles of total alkalinity (A_T, black) and dissolved inorganic carbon (DIC, green) for different cruises at the location of the Wave Glider (WG) (54˚S, 0˚E) taken from GLODAPv2.2020^{82, 83}. b Lateral distribution of surface A_T and DIC from the same cruises. Shown on both a and b are the average A_T and DIC from the WG observations (blue outlined circles) (see Methods) and from the summer (Dec–Feb) gridded climatology (dashed lines)^{36, 37}. c The range of WG estimated observations of surface A_T (y-axis) vs DIC (x-axis) with the partial pressure of CO₂ in the surface ocean (pCO_2sea) as contours (µatm). Overlaid are the scatter plots of modelled lateral Ekman displacement of DIC vs A_T, the modelled entrainment of DIC vs A_T and the corresponding slopes (m) are shown, and the time-averaged A_T and DIC value estimated from the WG (black circle). d The black line shows the model estimate of the synoptic variability of the non-thermal component of pCO₂ (pCO_2-DIC′) through combined Ekman advection and entrainment. The dashed line is the actual WG observed pCO_2-DIC′. A rolling mean of the local inertial period is applied to both and their corresponding coefficient of determination (r²) is shown. The grey shading shows the separate estimated contribution due to Ekman advection and the blue shading of entrainment to estimated pCO_2-DIC′.