Carbon distribution, fluxes, and budgets in the subtropical North Atlantic Ocean (24.5°N)

Abstract

[1] Hydrographic CO 2 system data obtained from World Ocean Circulation Experiment (WOCE) transatlantic zonal section A5 across 24.5°N and Florida Straits are described. By combining CO 2 measurements with hydrographic velocity calculations, the zonal and vertical variability of meridional fluxes of total alkalinity (TA), total dissolved inorganic carbon (TIC), and anthropogenic CO 2 (dC) are estimated. The resulting CO 2 fluxes are examined in four geostrophic mid-ocean zones as well as in the Gulf Stream flow through Florida Straits and in the surface ageostrophic Ekman flow. This method allows an estimate of the net budgets of these chemical species in the system considering together the Arctic and Atlantic Oceans north of 24.5°N. Taking into account the net flux contribution through Bering Strait, total meridional transports of chemical properties across 24.5°N latitude are also estimated. The slightly divergent net TA budget (À460 ± 200 kmol s À1 ) suggests that the North Atlantic is a small alkalinity source. The divergent TIC budget (À2430 ± 200 kmol s À1 or À0.92 ± 0.08 GtC yr À1 ) suggests that the North Atlantic is a source of TIC and therefore a net sink for atmospheric CO 2 . This value is twice as large as a previous estimation made from a poorly sampled section. Surprisingly, the North Atlantic Ocean appears to act as a net sink of anthropogenic CO 2 (+630 ± 200 kmol s À1 or +0.24 ± 0.08 GtC yr À1 ) and therefore a weak source of anthropogenic CO 2 to the atmosphere. Its main contributor is the intense northward flux in the Florida Current (+1280 ± 100 kmol s À1 ). The calculations imply a divergent inorganic carbon budget of À3060 ± 200 kmol s À1 or À1.16 ± 0.08 GtC yr À1 in preindustrial times (TIC 278 , when the molar fraction of CO 2 in the atmosphere was 278.2 ppm). This means that the North Atlantic would have had a 25% stronger divergence of TIC prior to the beginning of anthropogenic CO 2 penetration.

Global ocean storage of anthropogenic carbon

“…which the calculation of the Green function and C ant was repeated by randomly sampling the various parameters used in the inversion from a uniform distribution centered about its observed value and width equal to the reported uncertainty. This approach combined with the entropic uncertainty leads to a (spatially variable) uncertainty between ±2 and (Holfort et al, 1998;Rosón et al, 2003;Macdonald et al, 2003;Lundberg and Haugan, 1996). Errors in individual C ant estimates propagate into uncertainty in column and regional inventories.…”

“…Transport of anthropogenic CO 2 along ship transects can also be estimated from hydrographic data and data-based anthropogenic CO 2 estimates (e.g., Lundberg and Haugan, 1996;Holfort et al, 1998;Álvarez et al, 2003;Macdonald et al, 2003;Rosón et al, 2003;Álvarez and Gourcuff, 2010). This approach has been widely used in the Atlantic, and a number of these transect-based estimates have been included in Fig.…”

Trends in anthropogenic CO2 in water masses of the Subtropical North Atlantic Ocean

“…Procedures for CO 2 system parameters analyses and their adjustments are described in Rosón et al (2003) and Guallart et al (2013). The 1998 measurements are reported in Peltola et al (2001) and Macdonald et al (2003).…”

“…The entrance of C ant into the ocean interior takes place in the NASPG through deep convection, significantly contributing to the efficiency of the North Atlantic sink. This C ant entrance is sustained up to 65±13% due to lateral transports that carry C ant -loaded subtropical waters to these northern latitudes through the upper limb of the Meridional Overturning Circulation (MOC) (Álvarez et al, 2003;Macdonald et al, 2003;Rosón et al, 2003;Pérez et al, 2013). At 24.5ºN, the MOC is responsible for almost 90% of the meridional heat flux (Johns et al, 2011) and it also transports up to 0.17-0.20 PgC·y -1…”

“…and presented the first C ant calculations in the late 1970s, which attempted to separate the C ant signal from the background CO 2 distribution by correcting the measured C T for changes due to biological activity and by removing an estimate of the preindustrial preformed C T . Several authors have tried to improve those first back-calculation (also called carbon-based) methods, leading to a number of methodologies: ΔC*(Gruber et al, of C ant (Macdonald et al, 2003;Rosón et al, 2003). Due to the importance of the North Atlantic Subtropical Gyre (NASTG) in the uptake of C ant from the atmosphere, the WOCE A05 hydrographic line, situated at 24.5°N, is suitable for the evaluation and quantification of the North Atlantic C ant sink.…”