Seasonal and Interannual Variability in Carbon Dioxide Fluxes in the Tropical Atlantic

Margaret Ojone Ogundare¹, Falilu Olaiwola Adekunbi²

¹Department of Marine Science and Technology, Federal University of Technology, Akure, Nigeria.
²Nigerian Institute for Oceanography and Marine Research, Victoria Island Lagos, Nigeria.

DOI: 10.4236/oalib.1112718                

Abstract

The seasonal and interannual variations of carbon dioxide flux in the tropical Atlantic were investigated from 1994-2023 based on 30 years of sea surface carbon dioxide fugacity (fCO₂ssw) data available in the latest version of the Surface Ocean CO₂ Atlas (SOCATv24). Using monthly mean global atmospheric CO₂ at Mauna Loa, CO₂ exchange between the ocean and atmosphere was calculated. The long-term trend across the tropical Atlantic investigated here showed the influence of sea surface temperature, salinity, and ocean surface CO₂ content on this region’s interannual variability of CO₂ flux. Correlation statistics revealed that ocean surface CO₂ content has the greatest influence, where 20 years of data show significant correlations (r-value ≥ 0.5), and ocean surface temperature has the lowest effect. This study showed salinity as the main factor controlling CO₂ uptake demonstrating a significantly positive correlation for eleven of the studied years, and a negative correlation for nine years. Thus, higher salinity decreases and increases CO₂ uptake for the different years respectively. This finding highlights biogeochemical complexity of the interaction between salinity and CO₂ uptake on long-term variability and carbon sink capacity in the tropical Atlantic Ocean.

Keywords

Seasonal and Interannual Variability, Carbon Dioxide Fluxes, Tropical Atlantic, Salinity

1. Introduction

As a source and sink of carbon dioxide (CO₂) [1] [2] through a variety of biological, physical, and chemical processes, the tropical Atlantic Ocean is vital to the global carbon cycle [3]-[5]. Climatologies of sea-air CO₂ flux [2] [6] [7] have estimated a net efflux of CO₂ to the atmosphere from the surface waters of the tropical Atlantic. The main surface water current influences the sea-air CO₂ flux distribution in the region [8]. The North Brazil Current (NBC) and North Equatorial Counter Current (NECC) were characterized by a permanent CO₂ oversaturation [9], and are attributed to the transport of cold CO₂-rich water from the equatorial upwelling system [10]. The region also acts as a net atmospheric CO₂ sink driven by river discharge for instance Amazon River and due to a seasonal distribution pattern identified in the North Equatorial Current (NEC) [9]. The occurrence of Atlantic Nino; a tropical climate interannual variability mode of sea surface temperature (SST) similar to the El Niño/Southern Oscillation (ENSO) in the Pacific also influences the CO₂ flux distribution in the region [11]. The Atlantic Nino enhances the CO₂ oversaturation in the central tropical Atlantic driven by sea surface temperature (SST) solubility changes in the partial pressure of CO₂ (pCO₂) and weakens it in the western region where freshwater-induced salinity changes drive the CO₂ flux variation [11].

Predicting future climatic trends and appreciating the tropical Atlantic Ocean’s contribution to the global carbon budget requires an understanding of the long-term variability of carbon dynamics in the region [12]-[14]. Seasonal fluctuations in carbon dioxide fluxes are largely driven by the equatorial upwelling system, which brings nutrient-rich waters to the surface [15]. The main factors influencing the seasonal variations include patterns of ocean circulation, SST, and biological activity [16] [17]. The upwelling increases biological productivity during boreal summer leading to greater uptake of CO₂ by phytoplankton [18]. Wintertime brings a decrease in phytoplankton productivity due to reduced sunlight and water column mixing [19]. This cyclical pattern of production affects the net exchange of CO₂ between the ocean and the atmosphere [20]. Inter-annual variability in CO₂ flux in the tropical Atlantic is influenced by two climatic phenomena: the Atlantic Meridional Mode (AMM) and ENSO [21]-[23]. The AMM is characterized by variations in air pressure gradients and SST influences CO₂ flux [24]. Biological output and upwelling increase the ocean’s capacity to absorb carbon, which is boosted by stronger trade winds during the La Niña phase [25] [26].

This study examines the seasonal and inter-annual variability of CO₂ flux in the tropical Atlantic between 1994 and 2023 to understand the drivers of long-term changes in CO₂ uptake utilizing observational data of sea surface fugacity of CO₂ in the region from the latest version of Surface Ocean CO₂ Atlas (SOCATv24).

2. Methodology

Sea surface fugacity of CO₂ (fCO₂ssw) in the tropical Atlantic available in the latest version of Surface Ocean CO₂ Atlas (SOCATv24) was extracted from 1994 to 2023 (Figure 1). Using the monthly mean Mauna Loa global atmospheric CO₂ records in ppm, the fugacity in µatm was calculated using the SST in SOCATv24 following the method of [27].

Figure 1. Available Sea surface fugacity of CO₂ (fCO₂ssw) in the tropical Atlantic from 1994 to 2023.

fCO₂atm = pCO₂atm × exp [−1636.75 +12.0408 x SST − 0.0327957 × SST² + 0.0000316528 × SST³ + 2 (57.7 − 0.118 × SST)]/82.0578 × SST

where pCO₂atm is the partial pressure of CO₂ computed following [28].

The sea-air CO₂ exchange (dfCO₂) was calculated thus: fCO₂ssw-fCO₂atm

Where;

fCO₂ssw = sea surface fugacity of CO₂.

fCO₂atm = atmospheric fugacity of CO₂ fCO₂atm.

Monthly mean climatology of fCO₂ssw, dfCO₂, SST, and sea surface salinity (Sal) was computed to examine the spatial distribution, and the western region where more fCO₂ssw occur was analysed for seasonal variation and decadal trend.

3. Results and Discussion

3.1. Spatial, Inter-Annual, and Seasonal Distributions

The spatial distribution of fCO₂ssw (Figure 2(A)-(ZD)) shows fewer data in the first decade (figure 2(A)-(J); 1994-2003). Data density increases in the second decade (Figure 2(K)-(T); 2004-2013) through the third decade (Figure 2(U)-(ZD); 2014-2023) and concentrates more in the northwestern basin. Notably, the ocean time-series monitoring program in the tropical Atlantic include the CARIACO (CArbon Retention In A Colored Ocean) (10˚30'N, 64˚40'W) station and PIRATA (Prediction and Research moored Array in the Tropical Atlantic, [29]) mooring (6˚S, 10˚W). Also, the amount of CO₂ in the surface ocean increases over the years; ranging from 300 to 450 µatm. The higher values occur especially from 2019 to 2023 (Figure 2(Z)-(ZD)).

Figure 2. Spatial distribution of sea surface fugacity of CO₂ (fCO₂ssw) (1994-2023) using SOCATv24.

The sea-air CO₂ exchange (Figure 3(A)-(ZD)) generally shows increasing ocean uptake (negative values of dCO₂) over the years. This pattern corroborates the findings of [9] that reported certain regions of the eastern tropical Atlantic as weak CO₂ sinks.

The eastern tropical Atlantic basin demonstrates alternating behaviour, acting as a CO₂ sink in some years and a CO₂ source in others. Climate phenomena like the Atlantic Niño and Intertropical Convergence Zone (ITCZ) migration, as well as the Amazon River outflow, all contribute to these shifts [30]-[32]. The general increase in the region’s ocean CO₂ uptake over the years has been associated with some peculiarities about the ocean circulation in the region, river plumes formation with freshwater input and deep-water formation by high-saline sea-water parcels and the migration of the ITCZ in high precipitation regimes [33] [34]. [35] observed strong interannual sea-air CO₂ fluxes in the southwestern tropical Atlantic from 2006-2013, particularly in boreal spring which they attributed to dynamics in the Amazon River plume spatial extent. Furthermore, on interannual timescales, primary production fueled by the extensive river plume modulated and significantly resulted in a drawdown of CO₂.

However, the distribution shows a source of CO₂ (positive values of dCO₂; around 0 - 15 µatm) to the atmosphere in some regions in most of the years (Figure 3). This agrees with studies that have shown the tropical Atlantic as a source of CO₂ to the atmosphere [36]-[39]. A combination of physical and biological processes characterizes the temporal dynamics of dCO₂ flux in the tropical Atlantic. On an annual basis, the tropical Atlantic acts as a net source of CO₂ to the atmosphere with significant seasonal variability [40].

Figure 3. Distributions of sea-air CO₂ flux (dfCO₂) (sea - air CO₂ exchange) (1994-2023) using SOCATv24.

Figure 4 shows the box-whiskers plot of the of dCO₂ and generally shows a regular seasonal variation pattern for the CO₂ flux with higher CO₂ uptake for most of the years from January to March (boreal winter-early spring), which decreases and relapses into CO₂ source during July and August and sometimes also in September, i.e., during boreal spring-summer (July to September) and the uptake picking up again from October.

Figure 4. Box-Whisker plots showing the monthly mean of sea-air CO₂ flux (dfCO₂) (sea-air CO₂ exchange) (1994-2023) using SOCATv24 for seasonal variation pattern.

[11] has also shown a seasonal variability of high chlorophyll concentration during the cold season and low chlorophyll concentration during the warm season in the eastern tropical Atlantic (12˚W - 12˚E) especially in the coastal upwelling in the Gulf of Guinea and in the equatorial upwelling. This biological productivity could also potentially explain the winter uptake and summer source of CO₂ observed. When upwelling is strongest in the equatorial Atlantic, [39] reported a peak in CO₂ outgassing during boreal summer. These patterns are similar to observations for the period (1994-2023) in this study, as the tropical Atlantic acted as a CO₂ source to the atmosphere mainly in boreal summer (i.e., July-September) (Figure 4) when SST was warmest (Figure 5). In contrast, during boreal winter-early spring (January-March) the tropical Atlantic acts as a CO₂ sink (Figure 4).

The seasonal migration of the ITCZ over the tropical Atlantic also influences the CO₂ system and causes significant outgassing of CO₂ to the atmosphere in the western tropical Atlantic region relative to the eastern part during the boreal spring-summer. This pattern is associated with the effects of the ITCZ on surface currents and wind patterns [39]. In the eastern tropical Atlantic (6˚S - 6˚N, 10˚W - 10˚E) from 2006-2011 [39] reported significant interannual variations which tended from a CO₂ source from 2006-2009 to a net-zero state in 2010.

Generally, the tropical Atlantic exhibits warm temperatures with the SST climatology ranging between 21 and 30 degC (Figure 5) and the warmest temperatures occur mostly during boreal summer; July-August, and sometimes in September (Figure 5). It is also generally a region of high salinity with the Sal climatology ranging between 35 and 37 PSU (Figure not shown). The high SST and surface salinity are evidence of the South Equatorial Current (SEC) that carries relatively high temperature and high salinity waters (T~28 degC and S~36 PSU) in the region and the high salinity could also be associated with the high evapouration rate over precipitation occurring throughout the year in the northeastern Brazil region [41].

Figure 5. Box-Whisker plots with a monthly mean of SST (2004-2023) from SOCATv24 for seasonal variation pattern.

3.2. Decadal Trend Variability

Both fCO₂sww (Figure 6; upper panel) and fCO₂atm (Figure 6; middle panel) show increasing decadal trend of 0.002 - 0.006 µatmyr⁻¹ and 0.005 - 0.007 µatm yr⁻¹ respectively, over three decades with fCO₂atm increasing faster than fCO₂sww in each decade. The lower panel shows decreasing CO₂ uptake (−0.002 - 0.0005 µatmyr⁻¹) over the decades. All trends are significant at p-value < 0.00001.

Significant interest has been geared towards the long-term fCO₂ssw trends in the tropical Atlantic due to its role in global carbon cycling. Observations in the past few decades have shown a consistent increase in fCO₂ssw largely due to the uptake of anthropogenic CO₂. From 1970 to 2007, [8] recorded a mean rate of fCO₂ssw increase of about 1.5 μatmyr⁻¹ including the tropical Atlantic, indicating that this ocean basin is keeping pace with rising atmospheric CO₂ levels. Interestingly, over a three-decade period, this study also observed an increasing trend in seawater and atmospheric fCO₂ rate at 0.002 - 0.006 µatm yr⁻¹ and 0.005 - 0.007 µatm yr⁻¹ respectively in agreement with global trends and with fCO₂ increasing faster in the atmosphere than seawater in each decade.

Figure 6. Decadal trend using the monthly mean of; upper panel: sea surface fugacity of CO₂ (fCO₂ssw), middle panel: atmospheric fugacity of CO₂ (fCO₂atm) and lower panel: sea-air fCO₂ exchange (dfCO₂).

When compared to other ocean basins the tropical Atlantic is peculiar in its CO₂ flux trend as it shows regional dynamics and responses to global change [42]. On the one hand, in contrast to the Southern Ocean which went through a decline in CO₂ uptake during the 1990s followed by an increase in the 2000s, the tropical Atlantic has consistently showed outgassing trends. On the other hand, the Indian Ocean has experienced increased CO₂ uptake in its southern region with the tropical areas undergoing a stable flux pattern [43].

3.3. Drivers of Inter-Annual Variability

The annual regressions r-values for correlating the CO₂ uptake with fCO₂ssw, SST, and Sal (Figure 7) show positive correlation values for every year with fCO₂ssw (Figure 7(A)) indicating increasing fCO₂ssw leads to a source of CO₂ to the atmosphere from the surface ocean with significant r-values (r-values ≥ 0.5) for most of the years. With SST, both positive and negative correlations occur for different years (Figure 7(B)) indicating increasing SST leads to a source of CO₂ to the atmosphere in some years while in some other years it leads to an increase in CO₂ uptake, respectively. The increased uptake with increasing SST is observed for 1999, 2000, 2002, 2005, and 2011 with significant r-values only for 1999 and 2000. For the source of CO₂ to the atmosphere with increasing SST, the r-values are significant only for 1995, 1996, 2001, and 2003. The correlation with Sal (Figure 7(C)) also shows positive and negative correlations for different years indicating increasing Sal leads to a source of CO₂ to the atmosphere for some years and an increase in CO₂ uptake in some other years, respectively. Increased uptake with decreasing salinity (negative correlation) occurred for most of the years and with significant r-values (Figure 7(C)). For dCO₂ correlating both positively and negatively with SST and Sal annually is an indication of opposing mechanisms acting for the ocean CO₂ source and sink. Potentially, the increasing Sal could be associated with the surface equatorial current waters and possible upwelling of cool nutrient-rich high saline waters as occurs along the Central American coast [36].

Figure 7. r-values of regression correlation against sea-air fCO₂ exchange (dfCO₂) with (A) sea surface fugacity of CO₂ (fCO₂ssw) (B) SST, (C) Sal.

The year-to-year dCO₂ variability influenced by both SST and salinity (Figure 7(B)-(C), respectively) corroborates previous findings that SST variability and Sal are influenced by changes in the position of the ITCZ and are the main processes causing the interannual pattern in the region.

4. Conclusion

The long-term variability of CO₂ flux in the tropical Atlantic remains poorly understood due to the sparseness of spatio-temporal CO₂ data. Monthly climatology of sea surface fugacity of CO₂ in the tropical Atlantic from 1994 to 2023 from the latest version of Surface Ocean CO₂ Atlas (SOCATv24) revealed the spatio-temporal sparseness of CO₂ data in the regions especially in the Gulf of Guinea within the eastern tropical Atlantic. Few data were available in the first decade and data density increased through the second decade to the third with the data concentrating in the western basin. The tropical Atlantic has been studied in regional isolation of portions in the eastern, western, and central tropical Atlantic. The long-term variability of the combined regions with more data density (entire fCO₂ssw observations in the western region) studied here shows the influence of sea surface temperature and salinity and the amount of CO₂ in the surface ocean on the year-to-year variability of CO₂ flux. Correlation statistics show the most influence from the amount of CO₂ in the surface ocean; with 20 years of data showing a significant correlation (r-values ≥ 0.5) and the least influence from surface ocean temperature; showing significant correlation values for only 6 years of the data. Salinity shows significant correlation values for 11 years of data out of which negative correlations occur for 9 years which means increasing salinity increases CO₂ uptake. This pattern negates the uptake of CO₂ hinged on the biological productivity enhanced by the effect of the river discharge in the region and is accompanied by a freshening effect (reduced salinity). This reveals the interaction between salinity and CO₂ uptake is biogeochemically complex, typically increased salinity is associated with reduced CO₂ uptake due to lower biological productivity however in the western tropical Atlantic discharge of the Amazon River carries high load of carbon dioxide from organic matter respiration that is as well associated with high nutrients that fuels increased primary production leading to enhanced CO₂ uptake. This mechanism has implications for long-term carbon uptake in the region.

Conflicts of Interest

The authors declare no conflicts of interest.

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Ibánhez, J.S.P., Araujo, M. and Lefèvre, N. (2016) The Overlooked Tropical Oceanic CO2 Sink. Geophysical Research Letters, 43, 3804-3812. https://doi.org/10.1002/2016gl068020
[2]	Landschützer, P., Gruber, N., Bakker, D.C.E. and Schuster, U. (2014) Recent Variability of the Global Ocean Carbon Sink. Global Biogeochemical Cycles, 28, 927-949. https://doi.org/10.1002/2014gb004853
[3]	Chen, C.A. and Borges, A.V. (2009) Reconciling Opposing Views on Carbon Cycling in the Coastal Ocean: Continental Shelves as Sinks and Near-Shore Ecosystems as Sources of Atmospheric CO2. Deep Sea Research Part II: Topical Studies in Oceanography, 56, 578-590. https://doi.org/10.1016/j.dsr2.2009.01.001
[4]	Prentice, I.C., Farquhar, G.D., Fasham, M.J.R., Goulden, M.L., Heimann, M., Jaramillo, V.J., Kheshgi, H.S., Le Quéré, C., Scholes, R.J. and Wallace, D.W.R. (2001) The Carbon Cycle and Atmospheric Carbon Dioxide. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., Linden, P.J.V.D., Dai, X., Maskell, K. and Johnson, C.A., Eds., Climate Change 2001: The Scientific Basis, Cambridge University Press, 183-237.
[5]	Sabine, C.L. and Feely, R.A. (2007) The Oceanic Sink for Carbon Dioxide. In: Reay, D., Hewitt, N. and Smith, K.A., Eds., Greenhouse Gas Sinks, CABI, 31-49. https://doi.org/10.1079/9781845931896.0031
[6]	Bakker, D.C.E., Pfeil, B., Smith, K., Hankin, S., Olsen, A., Alin, S.R., et al. (2014) An Update to the Surface Ocean CO2 Atlas (SOCAT Version 2). Earth System Science Data, 6, 69-90. https://doi.org/10.5194/essd-6-69-2014
[7]	Takahashi, T., Sutherland, S.C., Wanninkhof, R., Sweeney, C., Feely, R.A., Chipman, D.W., et al. (2009) Climatological Mean and Decadal Change in Surface Ocean pCO2, and Net Sea-Air CO2 Flux over the Global Oceans. Deep Sea Research Part II: Topical Studies in Oceanography, 56, 554-577. https://doi.org/10.1016/j.dsr2.2008.12.009
[8]	Lefévre, N., Veleda, D., Araujo, M. and Caniaux, G. (2016) Variability and Trends of Carbon Parameters at a Time Series in the Eastern Tropical Atlantic. Tellus B: Chemical and Physical Meteorology, 68, 30305. https://doi.org/10.3402/tellusb.v68.30305
[9]	Ibánhez, J.S.P., Diverrès, D., Araujo, M. and Lefèvre, N. (2015) Seasonal and Interannual Variability of Sea-Air CO2 Fluxes in the Tropical Atlantic Affected by the Amazon River Plume. Global Biogeochemical Cycles, 29, 1640-1655. https://doi.org/10.1002/2015gb005110
[10]	Lefèvre, N., Mejia, C., Khvorostyanov, D., Beaumont, L. and Koffi, U. (2021) Ocean Circulation Drives the Variability of the Carbon System in the Eastern Tropical Atlantic. Oceans, 2, 126-148. https://doi.org/10.3390/oceans2010008
[11]	Koseki, S., Tjiputra, J., Fransner, F., Crespo, L.R. and Keenlyside, N.S. (2023) Disentangling the Impact of Atlantic Niño on Sea-Air CO2 Flux. Nature Communications, 14, Article No. 3649. https://doi.org/10.1038/s41467-023-38718-9
[12]	Legge, O., Johnson, M., Hicks, N., Jickells, T., Diesing, M., Aldridge, J., et al. (2020) Carbon on the Northwest European Shelf: Contemporary Budget and Future Influences. Frontiers in Marine Science, 7, Article 143. https://doi.org/10.3389/fmars.2020.00143
[13]	Nkrumah, T., Meiling, Z., Stephen, N. and Xingyu, W. (2022) Response of Carbon Budget to Climate Change of the Alpine Meadow in Gannan Using the Century Model. Journal of Water and Climate Change, 13, 2298-2318. https://doi.org/10.2166/wcc.2022.362
[14]	Patra, P.K., Canadell, J.G., Houghton, R.A., Piao, S.L., Oh, N., Ciais, P., et al. (2013) The Carbon Budget of South Asia. Biogeosciences, 10, 513-527. https://doi.org/10.5194/bg-10-513-2013
[15]	Chakraborty, K., Joshi, A.P., Ghoshal, P.K., Ghosh, J., Akhand, A., Bhattacharya, T., et al. (2023) Mechanisms and Drivers Controlling Spatio-Temporal Evolution of P CO2 and Air-Sea CO2 Fluxes in the Southern Java Coastal Upwelling System. Estuarine, Coastal and Shelf Science, 293, Article ID: 108509. https://doi.org/10.1016/j.ecss.2023.108509
[16]	Mu, L., Gomes, H.D.R., Burns, S.M., Goes, J.I., Coles, V.J., Rezende, C.E., et al. (2021) Temporal Variability of Air-Sea CO2 Flux in the Western Tropical North Atlantic Influenced by the Amazon River Plume. Global Biogeochemical Cycles, 35, e2020GB006798. https://doi.org/10.1029/2020gb006798
[17]	Ríos, A.F., Pérez, F.F., Álvarez, M., Mintrop, L., González-Dávila, M., Santana Casiano, J.M., et al. (2005) Seasonal Sea-Surface Carbon Dioxide in the Azores Area. Marine Chemistry, 96, 35-51. https://doi.org/10.1016/j.marchem.2004.11.001
[18]	Ardyna, M. and Arrigo, K.R. (2020) Phytoplankton Dynamics in a Changing Arctic Ocean. Nature Climate Change, 10, 892-903. https://doi.org/10.1038/s41558-020-0905-y
[19]	Häder, D.P. and Gao, K. (2017) The Impacts of Climate Change on Marine Phytoplankton. In: Phillips, B.F. and Pérez-Ramírez, M., Eds., Climate Change Impacts on Fisheries and Aquaculture: A Global Analysis, Wiley-Blackwell, 897-924.
[20]	Czerny, J., Barcelos e Ramos, J. and Riebesell, U. (2009) Influence of Elevated CO2 Concentrations on Cell Division and Nitrogen Fixation Rates in the Bloom-Forming Cyanobacterium Nodularia spumigena. Biogeosciences, 6, 1865-1875. https://doi.org/10.5194/bg-6-1865-2009
[21]	Bastos, A.F.F. (2015) Impacts of Climate Variability and Extremes on Carbon Uptake by Land Ecosystems. Master’s Thesis, Universidad de Lisboa.
[22]	Foltz, G.R., Brandt, P., Richter, I., Rodríguez-Fonseca, B., Hernandez, F., Dengler, M., Rodrigues, R.R., Schmidt, J.O., Yu, L., Lefèvre, N. and Da Cunha, L.C. (2019) The Tropical Atlantic Observing System. Frontiers in Marine Science, 6, Article 206.
[23]	Nicoli, D. (2019) Global Climate Impacts of the Atlantic Multidecadal Variability: A Model-Based Approach. Master’s Thesis, Università Ca’ Foscari Venezia.
[24]	Lefèvre, N., Veleda, D. and Beaumont, L. (2024) Trends and Drivers of CO2 Parameters, from 2006 to 2021, at a Time-Series Station in the Eastern Tropical Atlantic (6˚S, 10˚w). Frontiers in Marine Science, 11, Article 1299071. https://doi.org/10.3389/fmars.2024.1299071
[25]	Espinoza-Morriberón, D., Echevin, V., Colas, F., Tam, J., Ledesma, J., Vásquez, L., et al. (2017) Impacts of el niño Events on the peruvian Upwelling System Productivity. Journal of Geophysical Research: Oceans, 122, 5423-5444. https://doi.org/10.1002/2016jc012439
[26]	Pittman, N.A., Strutton, P.G., Johnson, R., Matear, R.J. and Sutton, A.J. (2022) Relationships between Air-Sea CO2 Flux and New Production in the Equatorial Pacific. Global Biogeochemical Cycles, 36, e2021GB007121. https://doi.org/10.1029/2021gb007121
[27]	Lencina-Avila, J.M., Ito, R.G., Garcia, C.A.E. and Tavano, V.M. (2016) Sea-Air Carbon Dioxide Fluxes along 35°S in the South Atlantic Ocean. Deep Sea Research Part I: Oceanographic Research Papers, 115, 175-187. https://doi.org/10.1016/j.dsr.2016.06.004
[28]	Pierrot, D., Neill, C., Sullivan, K., Castle, R., Wanninkhof, R., Lüger, H., et al. (2009) Recommendations for Autonomous Underway pCO2 Measuring Systems and Data-Reduction Routines. Deep Sea Research Part II: Topical Studies in Oceanography, 56, 512-522. https://doi.org/10.1016/j.dsr2.2008.12.005
[29]	Bourlès, B., Lumpkin, R., McPhaden, M.J., Hernandez, F., Nobre, P., Campos, E., et al. (2008) The Pirata Program: History, Accomplishments, and Future Directions. Bulletin of the American Meteorological Society, 89, 1111-1126. https://doi.org/10.1175/2008bams2462.1
[30]	Lefèvre, N., Guillot, A., Beaumont, L. and Danguy, T. (2008) Variability of fCO2 in the Eastern Tropical Atlantic from a Moored Buoy. Journal of Geophysical Research: Oceans, 113, C01015. https://doi.org/10.1029/2007jc004146
[31]	Araujo, M., Noriega, C. and Lefãvre, N. (2014) Nutrients and Carbon Fluxes in the Estuaries of Major Rivers Flowing into the Tropical Atlantic. Frontiers in Marine Science, 1, Article 10. https://doi.org/10.3389/fmars.2014.00010
[32]	Lefèvre, N., Flores Montes, M., Gaspar, F.L., Rocha, C., Jiang, S., De Araújo, M.C., et al. (2017) Net Heterotrophy in the Amazon Continental Shelf Changes Rapidly to a Sink of CO2 in the Outer Amazon Plume. Frontiers in Marine Science, 4, Article 278. https://doi.org/10.3389/fmars.2017.00278
[33]	Bonou, F.K., Noriega, C., Lefèvre, N. and Araujo, M. (2016) Distribution of CO2 Parameters in the Western Tropical Atlantic Ocean. Dynamics of Atmospheres and Oceans, 73, 47-60. https://doi.org/10.1016/j.dynatmoce.2015.12.001
[34]	Lefèvre, N., Diverrès, D. and Gallois, F. (2010) Origin of CO2 Undersaturation in the Western Tropical Atlantic. Tellus B: Chemical and Physical Meteorology, 62, 595-607. https://doi.org/10.1111/j.1600-0889.2010.00475.x
[35]	Ibánhez, J.S.P., Flores, M. and Lefèvre, N. (2017) Collapse of the Tropical and Subtropical North Atlantic CO2 Sink in Boreal Spring of 2010. Scientific Reports, 7, 41694. https://doi.org/10.1038/srep41694
[36]	Andrié, C., Oudot, C., Genthon, C. and Merlivat, L. (1986) CO2 Fluxes in the Tropical Atlantic during FOCAL Cruises. Journal of Geophysical Research: Oceans, 91, 11741-11755. https://doi.org/10.1029/jc091ic10p11741
[37]	Oudot, C., Ternon, J.F. and Lecomte, J. (1995) Measurements of Atmospheric and Oceanic CO2 in the Tropical Atlantic: 10 Years after the 1982-1984 FOCAL Cruises. Tellus B: Chemical and Physical Meteorology, 47, 70-85. https://doi.org/10.3402/tellusb.v47i1-2.16032
[38]	Koffi, U., Lefèvre, N., Kouadio, G. and Boutin, J. (2010) Surface CO2 Parameters and Air–sea CO2 Flux Distribution in the Eastern Equatorial Atlantic Ocean. Journal of Marine Systems, 82, 135-144. https://doi.org/10.1016/j.jmarsys.2010.04.010
[39]	Lefèvre, N., Caniaux, G., Janicot, S. and Gueye, A.K. (2013) Increased CO2 Outgassing in February-May 2010 in the Tropical Atlantic Following the 2009 Pacific El Niño. Journal of Geophysical Research: Oceans, 118, 1645-1657. https://doi.org/10.1002/jgrc.20107
[40]	Lefèvre, N., Veleda, D. and Hartman, S.E. (2023) Outgassing of CO2 Dominates in the Coastal Upwelling off the Northwest African Coast. Deep Sea Research Part I: Oceanographic Research Papers, 200, Article ID: 104130. https://doi.org/10.1016/j.dsr.2023.104130
[41]	Bertini, L. and Braga, E.D.S. (2022) The Contribution of Nutrients and Water Properties to the Carbonate System in Three Particular Areas of the Tropical Atlantic (Nebrazil). Journal of Geoscience and Environment Protection, 10, 135-161. https://doi.org/10.4236/gep.2022.102009
[42]	Landschützer, P., Gruber, N. and Bakker, D.C.E. (2016) Decadal Variations and Trends of the Global Ocean Carbon Sink. Global Biogeochemical Cycles, 30, 1396-1417. https://doi.org/10.1002/2015gb005359
[43]	Sarma, V.V.S.S., Lenton, A., Law, R.M., Metzl, N., Patra, P.K., Doney, S., et al. (2013) Sea-Air CO2 Fluxes in the Indian Ocean between 1990 and 2009. Biogeosciences, 10, 7035-7052. https://doi.org/10.5194/bg-10-7035-2013