Anthropogenic Carbon Transport Variability in the Atlantic Ocean Over Three Decades

Abstract

The change in anthropogenic CO2 (Canth) in the Atlantic Ocean is linked to the Atlantic Meridional Overturning Circulation (AMOC), that redistributes Canth meridionally and in depth. We have employed direct biogeochemical measurements and hydrographic data from the last 30 years, adjusted using inverse models for each decade with both physical and biogeochemical constraints. We then have computed the meridional transports and the vertical transports between two sections at the interphases by advection and diffusion. We have focused on the repeated sections at three latitudes—30°S, 24, and 55°N, dividing the Atlantic into two boxes. We have divided the net transport into upper, deep and abyssal layers, with an upper and abyssal northward transport of Canth and a southward component in deep layers. The change in time in the net transports of Canth appears to be mainly due to modifications in the transport of upper layers. The lower layer of the AMOC, a combination of deep and abyssal waters, maintain more consistent transports in time. Vertical advection plays an important role in the North Atlantic, exporting Canth from upper to deep layers. In the South Atlantic, the newly formed Antarctic Bottom Water exports Canth from abyssal to deep layers. The strong gradient in Canth concentration at the interphase of upper and deep layers results in a strong vertical diffusion.

An Assessment of CO2 Storage and Sea‐Air Fluxes for the Atlantic Ocean and Mediterranean Sea Between 1985 and 2018

“…(2021) using the RAPID long time series (2004–2012), with an assessment of C ant transports at 10‐day timescale, confirm a strong C ant transport at 26.5°N of 0.191 ± 0.013 PgC yr −1 , which is in the middle of the range (0.128 ± 0.032 to 0.25 ± 0.05 PgC yr −1 ) of the eight estimates obtained from five sections between 1992 and 2011 (collected in Cainzos et al. (2022)). The ensemble average C ant transport over 26°N obtained for the nine GOBMs used here is 0.053 ± 0.037 PgC yr −1 , which is almost four times lower than the C ant transport of Brown et al.…”

“…These C ant transports are fully compatible with the AMOC, in which the upper branch transports more C ant northward than the southward lower branch, and also with the decrease of the vertical gradient of C ant northward such that in the NA SPSS biome the vertical gradient of C ant is small (Figure S10 in Supporting Information S1). With these results, the net transports of 0.163 ± 0.057 PgC yr −1 at the South Atlantic boundary obtained from the GOBM results are consistent with recent transports estimated from ocean sections at 30°S of 0.186 ± 0.019 PgC yr −1 (Cainzos et al., 2022). This suggests that the weak anthropogenic sea‐air CO 2 fluxes are the primary cause of low ΔC ant in the South Atlantic.…”

“…The likely underestimation of F ant by the GOBMs in the NA STPS biome is further supported by their ∆C ant that is only about half as large as the observation‐based estimate from Gruber et al. (2019) (Table 1), as well as the lower northward C ant transport compared to two observation‐based estimates (Brown et al., 2021; Cainzos et al., 2022).…”

“…In fact, Cainzos et al. (2022) show that the contribution of vertical mixing is somewhat larger than the southward horizontal advection of ΔC ant in the lower limb of AMOC. Therefore, the insufficient incorporation of the RCO in the GOBMs may also result in a lower CO 2 uptake, and at the same time also generates an excess CO 2 uptake in the NA SPSS (Aumont et al., 2001; Gao et al., 2023).…”

Global Surface Ocean Acidification Indicators From 1750 to 2100

“…By 2100, the difference between Arctic and tropical surface f CO 2 is projected to be reduced to 19–61 μatm (2%–4%). This is because: (a) the loss of sea ice cover allows more CO 2 to enter the ocean through air‐sea gas exchange (Qi et al., 2022; Steinacher et al., 2009; Yamamoto et al., 2012); (b) there is substantial lateral transport of anthropogenic DIC from outside the Arctic (Anderson & Olsen, 2002; Olsen et al., 2015; Terhaar, Orr, Gehlen, et al., 2019; Tjiputra et al., 2010), which could enhance surface ocean f CO 2 in the Arctic and even lead to outgassing of anthropogenic DIC (Caínzos et al., 2022; Terhaar et al., 2020); (c) the loss of sea‐ice cover also drives enhanced warming of Arctic waters in summer, which contributes to the rise in summer time seawater f CO 2 through thermodynamics (Carton et al., 2015; Orr et al., 2022; Ouyang et al., 2021); (d) waters outside the Arctic lag the atmospheric CO 2 rise such that there is an increase in the global air‐to‐sea Δ f CO 2 . As in the Arctic Ocean, f CO 2 in the North Atlantic also increases due to cooling of the northward flowing waters and increased anthropogenic DIC in these waters (Caínzos et al., 2022; Völker et al., 2002) (Figure 5).…”