The CO₂ system was studied in the Macaronesian region based on data collected on board the RV Poseidon during the POS533 cruise (February 28 to March 19, 2019). Both surface underway measurements and depth seawater samples were taken during the cruise mainly downwind in the archipelagos of Cape Verde (CV), Canary (CA) and Madeira (MA) and through the latitudinal transects that connect them ( Figure 1 ). A detailed cruise overview is given by Mehlmann et al. (2020) and Arnone et al. (2022).

The pH in total scale was underway monitored in surface waters along the vessel track by using a spectrophotometric pH sensor (SP101-SM) developed by the QUIMA group (IOCAG-ULPGC) and SensorLab (González-Dávila et al., 2014; González-Dávila et al., 2016) and based on the method described by Clayton and Byrne (1993). The SP101-SM pH sensor uses 4 wavelengths analysis for the m-cresol purple, includes auto-cleaning steps, performs a blank for pH calculation immediately after the dye injection and removes any dye effect in each pH reading (the accuracy concerning a TRIS seawater buffer was ±0.002 units). The spectrophotometric system was placed in the onboard lab and measured the pH of the seawater pumped from the main seawater intake of the vessel (at around 6 m depth) with a frequency of 5 minutes. The change in temperature between the seawater intake and the multiple seawater outlets in the onboard lab (0.3 ± 0.1°C) was considered to correct the total scale pH values to the in situ conditions (~0.015 ± 0.001 units °C^-1). A SeaCat SBE21 thermosalinograph placed at the same location of the seawater immersed pump was used to monitor at the main intake the sea surface temperature (SST) and salinity (SSS) with an accuracy of 0.01 °C and 0.001, respectively. The surface underway fluorescence was monitored by a not-calibrated Wetlabs ECO fluorometer and is given as chlorophyll a (Chl-a) data in arbitrary units.

The characterization of the water column was carried out by selecting 58 sampling stations ( Figure 1 ) ranging from the surface to around 250-300 meters deep. In the CV archipelago, 15 stations were selected leeward of the islands of Sao Antao, Sao Nicolau, Fogo, Santiago, Boa Vista and Sal. In the CA archipelago, 25 stations were selected around the islands of El Hierro, La Gomera, Tenerife and Gran Canaria, mainly on their lee side and followed eastward the coastlines. In the MA archipelago, 15 stations were selected leeward of Madeira at two sections and at the Ilhas Desertas. In addition, two windward stations were selected North of CV and CA at the CVOO and ESTOC sites, respectively.

Depth water sampling and in situ measurements were performed using a bottle rosette sampler containing twelve 10 L Niskin bottles with a CTD and additional sensors of pressure, temperature, salinity, dissolved oxygen and not-calibrated fluorescence data given as Chl-a (Mehlmann et al., 2020).

Latitudinal temperature differences were found in the upper water column up to 100 m depth, with maximum average values in the CV that decrease towards the North ( Table 1 ). The average salinity values in the first 100 m depth were highest in CA (36.75-36.85) followed by MA (36.66-36.70) and CV (35.94-36.41). The surface fluorescence signals (data not shown) reported that Chl-a was maximum in CA-SE and ranged similar in CV-N, CA and MA, showing local differences along the coastal areas. An area with local minimum values ​​of both SST (19.0-19.5°C) and SSS (36.0-36.2) and slightly higher surface fluorescence signals compared to adjacent waters was found at 21.5°N in the latitude of Cape Blanc along the CV-CA route ( Supplementary Figure 4 ).

Decreasing West-to-East gradients of SST were observed in CV-N from Sao Antao (22.5-23.0°C) to Sal (21.5-22°C) and in CV-SE from Fogo (23.5-24°C) to Santiago (22.5-23.0°C) and Maio (23.0-23.5°C) ( Figure 5A ). The SSS also decreased toward the East in CV-N from Sao Vicente (~36.5) to Sal (36.3-36.4) and even more in CV-SE from the southeastern coast of Fogo (36.2-36.3) to Maio (~35.9) ( Figure 5A.1 ). These longitudinal differences were also detected in CA ( Figure 5B.1 ), with SST and SSS values decreasing from El Hierro, La Gomera and the southwestern coast of Tenerife (19.5-20.5°C and 36.8-37.0, respectively) to the southeastern coast of Tenerife and the entire coastal area of Gran Canaria (18.5-19.5°C and 36.6-36.8, respectively). A decrease in SST and SSS was detected through both the coastal and open-ocean transects in the downwind areas Southwest of La Gomera, Tenerife and Gran Canaria ( Figure 5B.1 ). The physical properties in MA ( Figure 5C.1 ) were similar to those of CA-E, with SST and SSS values ​​of 18-19.5°C and 36.6-36.8 respectively. An eastward decrease was found from the South of Madeira at 17.1-17.2°W (19.0-19.5°C and 36.75-36.8, respectively) to the South of Ilhas Desertas (~18.5°C and 36.65-36.7, respectively) through both the coastal and open-ocean transects.

The temperature and salinity signals in CV were approximately constant above the MLD located at around 50-60 m depth, with a minimum average variation range of ±0.16 °C and ±0.02 respectively ( Figure 2A ; Supplementary Figure 1A , respectively). The MLD increased northward and the temperature and salinity values ​​were approximately constant up to 70-100 m depth in CA ( Figure 3A ; Supplementary Figure 2A ) and MA ( Figure 4A ; Supplementary Figure 3A ), where the average variation range was ±0.27 and ±0.17 °C, respectively, for temperature and ±0.04 and ±0.03, respectively, for salinity. Below the MLD up to 250 m depth, a decrease in temperature and salinity of more than 4.0 °C and 0.4 units, respectively, were observed in the entire region. The fluorescence profiles ( Supplementary Figures 1C–3C ) reported a rapid decrease of Chl-a concentration in the first 50-100 m depth and reached the highest average values in CV, followed by CA and MA ( Table 1 ).

The surface values of fCO_2,sw ( Figure 5 ; Supplementary Figure 4 ) ranged similarly in CV (366.5-409.5 µatm) and CA (364.6-410.0 µatm), while lower values were found in the cooler MA region (352.3 and 378.4 µatm). The surface pH values (given in total scale) changed with fCO_2,sw by -0.001 units µatm^-1 (r²> 0.96) in the entire region and ranged between 8.04 and 8.08-8.09 units in both CV and CA and between 8.08 and 8.10 units in MA. The maximum fCO_2,sw and minimum pH surface values in the Macaronesian region were detected in a cold and saline filament crossed by the ship between 21.5 and 22.5 °N near Cape Blanc (~440.0-495.0 µatm and 7.95-8.00 units) ( Supplementary Figures 4C, D ) at a distance of 470 km away from the African coast.

In CV ( Figures 5A2, 5A3 ), the surface fCO_2,sw values were higher around CV-N (~375.0-400.0 µatm) and decreased with latitude from Sao Nicolau to Fogo and from Sal to Maio. Minimum surface values of fCO_2,sw in CV were obtained in CV-SE leeward of Santiago, Maio and Boa Vista (~365.0-380.0 µatm), leading pH values between 8.07 and 8.08. In CA ( Figures 5B2, B3 ), the surface fCO_2,sw values were higher in CA-W coastal areas (~380-410 µatm), followed by the CA-W open-ocean waters and East coast of Tenerife (~380-390 µatm) and the CA-E area (~365.0-390.0 µatm). The surface fCO_2,sw reached maximum values East of La Gomera (~405.0-410.0 µatm) and Southwest of Tenerife (~420.0-435.0 µatm) and decreased the pH to 8.035-8.040. Local decreases in the surface fCO_2,sw and increases in pH were observed in the cooler and fresher downwind coastal and open-ocean waters southwest of La Gomera, Tenerife and Gran Canaria. The minimum surface values of fCO_2,sw (~365-380 µatm) and maximum pH values (8.070-8.090 units) of the CA region were recorded along the inter-island transect between Tenerife and Gran Canaria. In MA ( Figures 5C.2, C.3 ), the surface fCO_2,sw values were higher to the South of Madeira through the more coastal transect (~370.0-380.0 µatm) followed by the Southwest of Madeira through the more open-ocean transect (~365.0-377.0 µatm), leading pH values of ~8.08. Outside the leeward area, the surface fCO_2,sw values fell below 365.0 µatm and drove pH values between 8.09 and 8.10.

The surface fCO_2,sw values decreased toward the East in CV-SE and CA following the West-to-East decrease of SST. The effect of SST over the changes in surface fCO_2,sw was removed by normalizing the fCO_2,sw to 21°C (fCO_2,T21). The longitudinal change of fCO_2,T21 was lower than those of fCO_2,sw in CV-SE, while was minimal in CA. An inverse longitudinal gradient was detected in CV-N, with a westward decrease of surface fCO_2,T21. These longitudinal differences were not observed in MA, where fCO_2,T21 remained approximately constant through both the coastal and open-ocean transects ( Figure 5 ).

Both fCO_2,sw and pH surface values kept approximately constant up to 40-50 and 50-60 m depth in CV-SE and CV-N respectively ( Figures 2B, D ), 60-75 m depth in CA ( Figures 3B, D ) and 80-95 m depth in MA ( Figures 4B, D ). In CV, the fCO_2,sw values at the base of the MLD (390-420 µatm) increased with depth and exceed 700 µatm around 80-150 m depth. Maximum fCO_2,sw values at these depths were found to the south of Fogo (773.2 and 819.4 µatm) and Maio (842.3 µatm) and at CVOO (685.2 µatm), which coincided with the lowest oxygen values encountered throughout the region (<80 µmol kg^-1) and drove the strongest decrease in pH values (from ~8.00-8.06 at the base of the MLD to 7.765-7.860). A weaker increase in fCO_2,sw with depth was obtained in CA and MA, where lower fCO_2,sw (~390-465 and ~380-440 µatm, respectively) and higher pH values (~7.99-8.06 and ~8.01-8.05, respectively) were obtained in deeper areas (150-250 m). Maximum fCO_2,sw and minimum pH values of CA and MA ​​(461.3 µatm and 7.996 units, respectively) were obtained at the ESTOC site at 150 m depth.

The surface distribution of C_T was linked to the SSS, with a higher concentration towards the north of Cape Blanc ( Supplementary Figure 4 ). Maximum C_T surface values were obtained in coastal CA-W areas (~2120-2140 µmol kg^-1) followed by CA-E (~2115-2130 µmol kg^-1) and MA (~2105-2120 µmol kg^-1), while the minimum values were found in CV (~2035-2095 µmol kg^-1) and especially leeward of Sao Antao and Sao Vicente (~2060-2075 µmol kg^-1) and throughout CV-SE (~2035-2075 µmol kg^-1) ( Figures 5A.4, B.4, C.4 ). The surface and upper water column C_T were normalized (NC_T) to constant salinities of 36.6 and 36.4, the average salinities of the Macaronesian region for surface and first 250 m depth, respectively, to remove the effect of evaporation/precipitation, the horizontal/vertical advection and the influence of different water masses.

The highest NC_T surface values ​​in the entire region were found around Cape Blanc (~2140-2165 µmol kg^-1) ( Supplementary Figure 4E ), while a strong NC_T depletion was observed south of Cape Blanc at CV ( Figure 5A.4 ). Minima NC_T surface values in CV were found southwest of Fogo and Maio (~2070-2080 µmol kg^-1), while maxima were observed through the Boa Vista-Sal-Sao Vicente northeast most transects (~2085-2105 µmol kg^-1). In CA ( Figure 5B.4 ), the surface NC_T-rich waters were found on the East coast of El Hierro (~2133-2136 µmol kg^-1) and La Gomera (~2134-2139 µmol kg^-1), in more open-ocean waters between both islands (~2130-2136 µmol kg^-1) and at certain locations along the southwest and southeast coast of Tenerife (~2127-2142 µmol kg^-1). The lowest NC_T surface values were obtained across the most oceanic transect south of La Gomera and El Hierro (~2106-2113 µmol kg^-1), in CA-E (~2105-2120 µmol kg^-1) and north of Gran Canaria around the ESTOC site (~2105-2112 µmol kg^-1). In MA, higher NC_T surface concentrations were measured through the coastal transect compared with those of the open-ocean transect ( Figure 5C.4 ). The maximum values were observed East of Madeira and along the West coast of Ilhas Desertas (~2113-2115 µmol kg^-1).

The C_T and NC_T remained depleted in the most productive euphotic layer and increased toward the remineralized deep waters below the MLD ( Table 1 ). The highest NC_T values in the upper water column of the Macaronesian region were collected in CV-N (~2187-2246 µmol kg^-1) and CV-SE (~2263-2281 µmol kg^-1) around 125-150 m depth ( Figure 2C ). These NC_T values were ~100-150 µmol kg^-1 greater than the maximums of CA (~2127-2169 µmol kg^-1; Figure 3C ) and MA (2110-2160 µmol kg^-1; Figure 4C ) in deeper areas between 150 and 250 m depth.

The distribution of fCO_2,sw and pH through the first 250 m depth was assessed in the Macaronesian region based on 3385 surface- ( Figure 5 ; Supplementary Figure 4 ) and 334 upper-ocean data ( Figures 2 – 4 ) collected during POS533. Their spatial variability was found to be strongly linked with differences in the intensity of the coastal upwelling along the African coast, the intense mesoscale activity acting in this region, the influence of different circulation patterns and water masses and the effect of trade winds (e. g. Pelegrí et al., 2005a; Pelegrí et al., 2005b; Valdés and Déniz-González, 2015; Pelegrí and Peña-Izquierdo, 2015a; Pelegrí and Benazzouz, 2015b; Bonino et al., 2021). These processes drove horizontal and vertical transports which modified the observed temperature, salinity and Chl-a patterns and biogeochemical cycles.

Minimal changes of surface fCO_2,sw and pH with latitude given by linear relationships were observed between CV and CA (-0.4 µatm (r² = 0.02) and 0.001 units (r² = 0.15) per degree of latitude, respectively) despite the strong measured decrease of SST with latitude between both archipelagos (-0.33 °C per degree of latitude; r² = 0.87) ( Supplementary Figures 4C, D ). The fCO_2,sw increased with SST by 7.93 µatm °C^-1 (r² = 0.31) between Cape Blanc and the south of CA along a permanent upwelling area (22.5-28.0°N), while an inverse relationship of -6.04 µatm °C^-1 (r² = 0.23) was encountered between the north of CV and Cape Blanc along a winter upwelling area (17.0-21.0°N). The highest decrease of fCO_2,sw with SST was observed at 21.5-22.5°N around Cape Blanc (-95.9 µatm °C^-1; r² = 0.58). Latitudinal gradients were stronger from CA to MA (-3.2 µatm (r² = 0.49) and 0.003 units (r² = 0.49) per degree of latitude, respectively), where the theoretical changes of fCO_2,sw with SST were observed (14.26 µatm °C^-1; r² = 0.35) ( Supplementary Figure 4 ). These findings agreed with previous studies along the African coastal transitional region through the latitudinal ranges of 10-28°N (González-Dávila et al., 2017) and 28-36°N (Curbelo-Hernández et al., 2021) and were explained by the seasonal and spatial variability of the Canary Upwelling System (Mittelstaedt, 1991; Cropper et al., 2014).

The maximum fCO_2,sw and minimum pH values encountered around Cape Blanc at ~470 km away from the African coast ( Supplementary Figures 4C, D ) were explained by the highest-intense offshore transport of CO₂-rich upwelled water not compensated by biological uptake (González-Dávila et al., 2017) through the cold and fresh Cape Blanc giant filament (21.5-22.5 °N) (e. g. Bonino et al., 2020; Gabric et al., 1993; Lovecchio et al., 2017). The low intensity of the permanent annual upwelling in winter north of Cape Blanc weakened the injection in the euphotic zone of CO₂ and nutrient-rich water, which explained the increased relevance of SST fluctuations in the fCO_2,sw changes from Cape Blanc to Madeira ( Supplementary Figures 4A, C ). The highest average changes of fCO_2,sw and pH were explained by thermal processes in this area (-24.64 µatm and 0.023 units, respectively) compared with the area most influenced by non-thermal processes south of Cape Blanc (6.69 µatm and -0.007 units, respectively) ( Supplementary Figure 4C ). The less intense offshore transport through the Cape Ghir filament (Hagen et al., 1996; García-Muñoz et al., 2005; Pelegrí et al., 2005a; Pelegrí et al., 2005b; Laiz et al., 2012), together with the dominance of respiration over primary production which compensates the decrease in fCO_2,sw drove by the cooling of the water at this time of the year (Curbelo-Hernández et al., 2021), explained a lower increase in surface fCO_2,sw encountered at Cape Ghir latitude (30.7°N) at ~650 km away from the African coast ( Supplementary Figures 4A, C ).

The strong influence of the Canary Upwelling System through the easternmost parts of CV and CA introduced longitudinal differences in the surface distribution of fCO_2,sw ( Figures 5A.2, B.2 ) and pH ( Figures 5A.3, B.3 ). The low fCO_2,T21 values in CV-SE showed that the biological production actively reduced the excess of CO₂ in the upwelled waters. The fCO_2,sw and fCO_2,T21 decreasing eastward with the SST and SSS indicated that both the enhancement of the biological activity and the cooling of the water due to the influence of the African coastal upwelling accounted for the formation of the longitudinal physical and biogeochemical gradient in CV-SE. The minimal longitudinal changes in fCO_2,T21 and pH_T21 in CA indicated that the West-to-East decrease of fCO_2,sw and increase in pH was mainly controlled by the cooling of the upwelled waters around the closer islands to the African coast ( Figures 5B.2, B.3 ). The weaker influence of the coastal upwelling in MA due to its greater distance from the African coast and the low intensity of the offshore transport by the Cape Ghir filament at this time of the year explains the minimal observed longitudinal changes of fCO_2,sw and pH ( Figures 5C.2, C.3 ).

Several differences in the surface distribution of fCO_2,sw and pH were encountered between CV-N and CV-SE ( Figures 5A.2, A.3 ) and explained by the influence of the CVFZ, which reaches its southernmost position in winter (Zenk et al., 1991; Pérez-Rodríguez et al., 2001). The high surface fCO_2,sw values encountered in CV-N can be explained by the influence of cold but low-productive and CO₂-rich surface waters transported by the NEC. Although the upwelling-driven eastward decrease of SST was observed in CV-N, the fCO_2,sw and fCO_2,T21 values were higher in the eastern part ( Figures 5A.1, A.2 ). It indicated that the CO₂ enrichment of these waters through the injection of deep and remineralized waters in the closest area to the coastal upwelling was the main factor controlling the fCO_2,sw and pH distributions. In contrast, minimum fCO_2,sw values in the easternmost part of CV-SE around Santiago, Maio and Boa Vista ( Figure 5A.2 ) indicated that the CO₂ enrichment was weaker and that the fCO_2,sw distribution was controlled by the enhancement of the biological uptake in warmer but higher-productive surface water transported through the NECC and recirculated in the cyclonic Guinea Dome (Siedler et al., 1992; Faye et al., 2015).

The horizontal advection processes can be coupled with divergent/convergent movements of water in mesoscale eddies (Barton et al., 2004; Sangrà et al., 2009; Cardoso et al., 2020), which introduces local increases/decreases in surface fCO_2,sw, as it was observed leeward of the islands ( Figure 5 ) and through the CV-CA-MA track ( Supplementary Figure 4 ). Higher surface fCO_2,sw values compared with adjacent waters were encountered at 18.02°N (404.30 µatm), 18.37°N (417.70 µatm), 19.64°N (403.5 µatm) and 20.33°N (414.70 µatm) and coincided with lower signals of SST (21.65, 21.22, 20.67 and 19.87 °C, respectively) ( Supplementary Figures 4A, C ). The opposite occurred at 18.97°N (374.5 µatm and 21.55°C, respectively). These local changes were explained by the upwelling of cold and CO₂-rich deep-water in the core of cyclonic eddies and downwelling of surface waters in anticyclonic eddies, which has been previously detected south of Cape Blanc (e. g. Schütte et al., 2016a; Karstensen et al., 2017; Cardoso et al., 2020). Mesoscale islands-generated eddies were most frequently observed from Cape Blanc to the southern part of CA through The Canary Eddy Corridor (Sangrà et al., 2009) and introduced stronger local differences in the physical and biogeochemical seawater properties which modified the fCO_2,sw and pH through the CV-CA track ( Supplementary Figure 4 ). The highest local increase in surface fCO_2,sw occurred at 23.79°N ( Supplementary Figure 4C ) and coincided with a major low signal of SST and a high signal of Chl-a associated with a cyclonic eddy ( Supplementary Figures 4A, B ). The mesoscale variability and its effect on the local changes in surface fCO_2,sw weakened through the CA-MA track, where cyclonic eddies detected by Couvelard et al. (2012) south of Madeira were only observed in minimal decreases in SST and SSS between 31.3 and 32.8°N ( Supplementary Figures 4A, C ).

The shallowest position of the MLD in the tropical CV area enhanced the vertical mixing processes and deep-water injection in surface layers, which contributed to change strongly the fCO_2,sw and pH in the first 250 m depth ( Figures 2B, D ). These changes were lower in CA ( Figures 3B, D ) and MA ( Figures 4B, D ) due to the deepening of the MLD. Several heterogeneities in the upper-ocean distribution were found between the leeward coastal areas, the inter-island waters and the windward open-ocean waters. The trade winds drove intense vertical mixing processes and raised the MLD in the windward areas of each archipelago, while the suppression of the wind effect in the leeward areas weakened the vertical mixing and decreased the MLD. These differences were observed at inter-islands windward stations compared to CV and CA downwind stations ( Figures 2 , 3 ). The effect of trade winds cools the upper water column in the windward areas, while the leeward coast show higher temperature values. However, the increase in surface fCO_2,sw with temperature in leeward coastal upper-ocean waters was compensated by the enhancement of biological production, as was observed with higher Chl-a values (data not shown).

The high spatial variability of fCO_2,sw in the study area has a great influence on the distribution of ocean CO₂ along the African coastal transitional zone and should be studied on a regional scale. Thus, the fCO_2,sw data during POS533 were compared and compilated with 52328 surface fCO_2,sw winter data in the Macaronesian region available in the Surface Ocean CO₂ Atlas database (SOCAT v2022; Bakker et al., 2016) and 2120 fCO_2,sw upper water-column data (first 250 m depth) in the Northeast Atlantic available in the GLODAP database (GLODAP v2022; Key et al., 2015; Olsen et al., 2016). The location of the sampled stations for SOCAT and GLODAP data is shown in Supplementary Figure 5 .

The spatio-temporal changes in surface fCO_2,sw were analysed in CV, CA, MA and for the entire Macaronesian region using POS533 and SOCAT data. These changes were also assessed from 10 to 100 m depth and compared with the changes observed between 100 and 250 m depth using POS533 and GLODAP data. Multiparametric regressions were applied to the compilated datasets (POS533+GLODAP datasets and POS533+SOCAT datasets) to analyse the distribution of fCO_2,sw based on changes in the seawater physical properties (SST and SSS). The combination of the POS533 with the SOCAT and GLODAP datasets provided a set of equations statistically significant at the 95% level (ρ value< 0.05) that can be used to estimate the fCO_2,sw in the upper layers of the entire region by considering temperature and salinity fluctuations and both temporal (seasonal and interannual) and longitudinal changes ( Table 2 ). The interannual increase of surface fCO_2,sw in CA (2.01 ± 0.01 µatm yr^-1) and MA (2.11 ± 0.01 µatm yr^-1) was similar to the observed rate at the ESTOC site (1.92 µatm yr^-1; Bates et al., 2014). The highest interannual increase around CV (2.36 ± 0.02 µatm yr^-1) coincided with those obtained at the subpolar Irminguer Sea sites (2.37 µatm yr^-1; Bates et al., 2014), where vertical mixing processes were enhanced and contributed to the increased observed trend. The lower change of surface fCO_2,sw with SST in CV compared to CA and MA ( Table 2 ) was indicative of the relevance of thermal processes in the change of fCO_2,sw north of Cape Blanc while non-thermal processes (mainly the horizontal and vertical advection of CO₂-rich water) played a key role in the distribution of surface fCO_2,sw south of Cape Blanc. This set of equations improved the understanding of the carbon cycle along the African coastal transition area in the Northeast Atlantic.

The shallow ocean C_T content was studied leeward of each archipelago by considering the non-conservative factors controlling its vertical distribution. The variations of NC_T from preformed NC_T values (NC_Tpre) were explained by fluctuations in the organic carbon pump (biological production/respiration processes), the carbonate pump (carbonate dissolution/formation) and the uptake of anthropogenic carbon (Chen and Millero, 1979; Feely et al., 2002; Chou et al., 2007; Andreev et al., 2009) and were described at any given depth by Eq. 4. The results allow to quantify the relative contribution of each term involved in the temporal evolution of NC_T from NC_T ^pre values above and below the MLD (up to 250 m depth) ( Figure 6 ; Table 3 ).

The increase in NC_T from NC_T ^pre above the MLD was around 75-90 µmol kg^-1 throughout the region. The increment below the MLD was approximately twice in CV than in CA and MA ( Table 3 ). The highest increase with depth of NC_T below the MLD occurred in CV-N and CV-SE (1.4 ± 0.8 and 1.9 ± 1.3 µmol kg^-1 m^-1, respectively) and was explained by the maximum NC_T ^pre values (2051.1 ± 25.3 and 2050.4 ± 29.2 µmol kg^-1, respectively) and organic carbon pump contribution (52.3% and 57.43%, respectively) encountered throughout the study region. Minor changes of NC_T with depth below the MLD were observed in CA-W (0.5 ± 0.3 µmol kg^-1 m^-1), CA-E (0.4 ± 0.2 µmol kg^-1 m^-1) and MA (0.4 ± 0.1 µmol kg^-1 m^-1) due to the lower NC_T ^pre values (2025.1 ± 3.8, 2028.2 ± 3.1 and 2036.3 ± 7.4 µmol kg^-1, respectively) and the weakened of the biological contribution (16.2%, 18.93% and 16.9%, respectively). These differences were mainly explained by spatial heterogeneities in the ventilation and remineralization rates through the Northeast Atlantic ITCZ. The longer residence time of tropical waters south of Cape Blanc originated in the South Atlantic compared with the well-ventilated subtropical waters that originated in the high-latitude of the North Atlantic enhanced remineralization (Pelegrí and Peña-Izquierdo, 2015a) and led a higher NC_T increase in (sub)surface waters around CV compared with CA and MA. The high primary production through the coastal Mauritania-Senegalese upwelling at this time of the year and the upwelling of water in the Guinea Dome also accounted for the increase of remineralization rates south of Cape Blanc (Karstensen et al., 2008; Stramma et al., 2008b). This fact was observed in a slightly higher contribution of the organic carbon pump above the MLD in CV-N and CV-SE (6.11% and 12.75%, respectively) compared with CA-W, CA-E and MA (4.4%, 2.6% and 1.9%, respectively).

The change in NC_T from NC_T ^pre in the entire region was mainly controlled by the anthropogenic carbon input and storage, which increased the NC_T content throughout the region around 72-80 and 68-74 µmol kg^-1 above and below the MLD, respectively. The uptake of C_ant in surface waters contributed more than 90% of the NC_T change above the MLD. The C_ant storage below the MLD account for 79.1% in CA-W, 78.2% in CA-E and 80.8% in MA of the NC_T increase, while its contribution fell to 47.4% in CV-N and 42.35% in CV-SE. It is related to the weakening of mixed-down processes due to higher vertical stratification and the enhancement of remineralization processes in the ITCZ (Mayer and Weisberg, 1993; Stramma and Schott, 1999).

The relevance of the carbonate pump in the change of NC_T through the upper layers was considerably lower in the entire region, coinciding with previous observations in the first 500 depths at the ESTOC site (González-Dávila et al., 2010). The dissolution of CaCO₃ in (sub)surface waters increased the NC_T by 3.7-4.8% and 1.4-3.1% in CA and MA, respectively. The decrease in CaCO₃ solubility toward warm tropical waters (Feely et al., 2004) explained the minimal contribution of NC_T ^carb on the increase of NC_T below the MLD in CV-N (0.3%) and CV-SE (0.2%). Negative ΔNC_T ^carb values above the MLD in CV ( Table 3 ) indicated that the formation processes of CaCO₃ exceed those of dissolution and accounted for the depletion of NC_T by 2.9-3.0%.

The coastal regions and marginal seas have been found to store more C_ant than the open oceans in proportion to their areas (Park et al., 2006; Olsen et al., 2010; Sabine and Tanhua, 2010; Schneider et al., 2010), but have been largely ignored in global ocean studies of the C_ant storage due to the limited data available (Khatiwala et al., 2013). This study provides a new analysis of the vertical distribution and inventory of C_ant in shallow coastal and inter-islands waters of the Macaronesian region ( Figure 7A ). The upper-ocean profiles of C_ant were evaluated in CV, CA and MA by applying a smoothing spline with a smoothing parameter p=0.999 to the total of C_ant values calculated from POS533 data in each archipelago ( Figure 7A ). The smoothing spline provided vertical profiles well-fitted to the calculated C_ant values (r²>0.98) and with standard error of estimate of 0.12 µmol kg^-1 for CV, 0.03 µmol kg^-1 for CA and 0.06 µmol kg^-1 for MA. The entry of C_ant into the ocean through air-sea exchange and the large time scales of mixing processes explained the highest C_ant values in near-surface waters and its decrement with depth (Sabine et al., 2004). The range of variation of C_ant in the first 250 depths was lower in CA and MA (between 65 and 83 µmol kg^-1) compared with CV (40-87 µmol kg^-1). These differences in the upper water column distribution of C_ant were related to differences in the exposition time of waters to the atmosphere and to the Revelle factor (Revelle and Suess, 1957; Takahashi et al., 1993), which value is inversely proportional to the capacity of oceans to uptake atmospheric CO₂. The lower ventilation rates of the ocean interior in the tropical Northeast Atlantic (Karstensen et al., 2008; Stramma et al., 2008b) and slightly higher (in average) Revelle factors in CV-N and CV-SE (10.77 ± 1.76 and 11.06 ± 2.02, respectively) compared to CA-W, CA-E and MA (10.14 ± 0.23, 10.28 ± 0.42 and 10.34 ± 0.42, respectively) explained the strongest decrease of C_ant with depth in CV ( Figure 7A ).

The upper water-column inventory of C_ant in the sampled stations was calculated by integrating its vertical distribution through the first 250 m depth. The C_ant storage in the subsurface waters of CV, CA and MA was 7.57 x 10³, 9.26 x 10³ and 8.86 x 10³ µmol kg^-1, respectively. The C_ant storage downwind of each archipelago was calculated by considering the leeward area that encloses all the stations of CV (8,570 Km²), CA (7,960 Km²) and MA (1,250 Km²) and the neutral density of 26.0 kg m^-3 for (sub)surface waters (Sabine et al., 2004). The total amount of C_ant stored in the upper water column leeward of CV, CA and MA was 0.51, 0.58 and 0.09 Tg C (1 Tg = 10¹² g), respectively.

These results were compared with C_ant values calculated from 9,870 GLODAP data available for the Macaronesian region during the period 1989-2020. The vertical distribution in subsurface waters of both POS533 and GLODAP C_ant values was analysed by averaging the data every 25 m depth ( Figure 7B ). The GLODAP C_ant values in the first 250 m depth of the Macaronesian region increased on average by 10.47 ± 3.38 µmol kg^-1 from 1989-1999 to 2000-2009 and by 8.08 ± 1.68 µmol kg^-1 from 2000-2009 to 2010-2020, which were in agreement with those of C_T and C_ant in the subtropical North Atlantic during the periods 1992-1998 (8.8 ± 0.5 and 8.0 ± 0.2 µmol kg^-1, respectively) and 1998-2004 (8.6 ± 0.5 and 6.8 ± 0.3 µmol kg^-1, respectively) (Brown et al., 2010) and in the ESTOC site between 1995 and 2004 (9.4 and 8.1 µmol kg^-1 over a decade in surface and 200 m depth waters, respectively) (González-Dávila et al., 2010). It indicated that the physical and biological conditions could be assumed as constants and that the increase in C_T in the Macaronesian region is mainly driven by the rate of C_ant uptake by the ocean (González-Dávila et al., 2010).

The average C_ant inventory in the first 250 m depth of the Macaronesian open-ocean waters based on GLODAP data was 5.78 x 10³ µmol kg^-1 for 1989-1999, 7.03 x 10³ µmol kg^-1 for 2000-2009 and 7.99 x 10³ µmol kg^-1 for 2010-2020. A higher C_ant inventory was encountered during POS533 around the CV, CA and MA archipelagos (8.74 x 10³ µmol kg^-1), which proves the relevance of shallow coastal waters in C_ant storing. The interannual change of C_ant in the subsurface Macaronesian region was assessed at any given depth by compilating the POS533 (2019) and the GLODAP (1989-2020) databases and applying a multiparametric regression statistically significance at 95% level (ρ value< 0.05) with a standard error of estimate of 6.60 µmol kg^-1 (Eq. 11; r^{2 =} 0.68). An increase in C_ant of ~57% is expected for the year 2050 ( Figure 7B ), with total storage between 10 and 250 m depth in the Macaronesian region of 1.25 x 10³ µmol kg^-1.

An area in the subtropical Northeast Atlantic of 5.4 x 10³ km² was considered to calculate the total amount of C_ant in the first 250 m depth of the entire Macaronesian region. The C_ant inventories in this parcel of water were 0.24 Pg C (1 Pg = 10¹⁵ g) for the period 1989-1999, 0.30 Pg C for the period 2000-2009 and 0.34 Pg C for the period 2010-2020. The C_ant inventory estimated for the year 2050 was 0.53 Pg C.

The spatial distribution of FCO₂ was analysed based on changes in the ΔfCO₂, the wind speed and the solubility (linked with temperature) to evaluate the air-sea CO₂ exchange in the Macaronesian region during winter ( Figures 8 , 9 ). The atmospheric CO₂ uptake was found to dominate over outgassing processes at this time of the year, coinciding with the CO₂ sink behaviour encountered in the Northeast Atlantic during the cold months (González-Dávila et al., 2003; Curbelo-Hernández et al., 2021).

The wind forcing has an important role in the variability of the surface physical and biogeochemical properties by controlling the spatio-temporal changes in the intensity of the African coastal upwelling and contributing to the formation of mesoscale eddies. The intense trade winds run the injection of cold deep-water in the surface layers favouring the solubility of the atmospheric CO₂. It explained the high ingassing rates encountered in areas most exposed to wind stress between Sao Antao and Sao Vicente in CV-N, in inter-island waters of CA and southwest of Madeira ( Figure 8 ), where the wind speed reached 15-20 m s^-1. It shows the relevance of the climatology on the air-sea exchange. However, the wind speed is only directly involved in the CO₂ gas transfer and influences the intensity of the ingassing/outgassing processes, while the formation of CO₂ source/sink is controlled by changes in ΔfCO₂ throughout the Macaronesian region ( Figures 8 , 9 ).

The average winter ingassing rate in MA (-8.34 mmol m^-2d^-1) was twice that of CV (-4.74 mmol m^-2d^-1) and CA (-3.90 mmol m^-2d^-1), which could be explained by its colder surface waters favouring CO₂ solubility and by the lower influence of both the coastal upwelling and the arrival of CO₂-rich surface filaments due to its furthest location from the African coast. However, the leeward side of the MA region showed the strongest spatial variability in the air-sea CO₂ exchange ( Figure 8C ) due to the highly heterogeneous interaction between the atmosphere and the surface ocean in this area (Pullen et al., 2017), with FCO₂ ranged between 0 and -28 mmol m^-2d^-1. These spatial differences were weaker south of CA (between 3 and -15 mmol m^-2d^-1; Figure 8B ) and CV (between 0 and -15 mmol m^-2d^-1; Figure 8A ), where the vertical movement of water in the core of the island-induced eddies influenced the changes of the ingassing/outgassing rates. The upwelling of deep CO₂-rich water in cyclonic eddies decreases the absolute value of ΔfCO₂ and weakened the ingassing, while the opposite occurs in anticyclonic eddies through the Canary Eddy Corridor (Sangrà et al., 2009) and downwind of Cape Verde (Cardoso et al., 2020). These deep-water rise movements caused surface fCO_2,sw to exceed fCO_2,atm (the ΔfCO₂ became positive), which indicate in several locations between CV and Cape Blanc and south of CA that behaved as a CO₂ source. The maximum outgassing was detected at the Cape Blanc latitude (20-25 mmol m^-2d^-1) ( Figure 9 ) and was explained by the offshore transport of cold but high CO₂-saturated waters through the giant Cape Blanc filament (Bonino et al., 2020; Gabric et al., 1993; Lovecchio et al., 2017).

The FCO₂ coastal values during POS533 were compared with FCO₂ calculated for the entire Macaronesian region (13-36°N, 6-30°W) by using winter SOCAT fCO_2,sw data referred to 2019 (an interannual increase of 1.8 µatm per elapsed year was considered; Bates et al., 2014). The fCO_2,atm data used in this calculation were collected at the Izaña Atmospheric Research Center (Tenerife, Canary Islands) and provided by the Agencia Estatal de Meteorología (AEMET). The satellite surface ocean 6-hourly and 0.25° grid wind speed data are derived from the Advanced Scatterometer (ASCAT) and provided by the IFREMER-CERSAT. The average FCO₂ and ΔfCO₂ through the monitored vessel track during POS533 were -4.40 mmol m^-2 d^-1 and -19.5 µatm, respectively. This ingassing rate was considerably higher than that obtained throughout the entire Macaronesian region based on SOCAT data (-0.53 mmol m^-2 d^-1 and -5.6 µatm, respectively). It is indicative of a stronger winter CO₂ sink in island shelves compared to open-ocean areas in the Northeast Atlantic ( Figures 8 , 9 ). Similar behaviour was determined in previous studies of the air-sea exchange in other coastal regions and continental shelves (e. g. Borges et al., 2005; Borges et al., 2006; Cai et al., 2006; Chen and Borges, 2009; Laruelle et al., 2010; Cai, 2011; Chen et al., 2013; Laruelle et al., 2013; Laruelle et al., 2018). The POS533 and SOCAT databases were compilated to analyse the air-sea CO₂ exchange in each archipelago and in the entire Macaronesian region. The total FCO₂ data available was well-fitted (r^{2 =} 0.76) to a multiparametric regression statistically significance at 95% level (ρ value< 0.05) given by Eq. 12 to assess the variability of the air-sea CO₂ exchange and the different factors involved in it. The latitudinal and longitudinal distribution of FCO₂ and the changes in the physical properties of the surface ocean (SST and SSS), in the climatology (wind speed) and in the ΔfCO₂ were considered. This equation fits the winter air-sea exchange in the Macaronesian region and can be used to estimate the FCO₂ with a standard error of estimate of 1.80 mmol m^-2 d^-1.

The winter average FCO₂ for the ocean area of CV (186,000 Km²), CA (145,000 Km²) and MA (40,250 Km²) were -11.42, -12.40 and -4.45 Gg CO₂ d^-1 (1 Gg = 10⁹ g), respectively. The entire ocean area of the Macaronesian region (4,190,000 Km²) acted as a CO₂ sink during the cold months, with an average FCO₂ of -107.30 Gg CO₂ d^-1. Thus, the winter ingassing rate of the archipelagic ocean waters of CV, CA and MA represented 26.4% of the atmospheric CO₂ absorbed by the ocean in the Macaronesian region despite occupying only 8.9% of its total ocean area. These results highlighted the relevance of coastal regions and inter-island waters on atmospheric CO₂ uptake as well as the importance of their incorporation in regional and global-scale biogeochemical studies. The assessment of the FCO₂ in this study improved the knowledge of the air-sea exchange in the Northeast Atlantic.