Seasonal variability of fCO2 in the Angola-Benguela region

Progress in Oceanography 83 (2009) 124–133

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Seasonal variability of fCO2 in the Angola-Benguela region Melchor González-Dávila *, J. Magdalena Santana-Casiano, Ivan R. Ucha Faculty of Marine Science, Department of Chemistry, University of Las Palmas de Gran Canaria, 35017, Las Palmas de Gran Canaria, Spain

a r t i c l e

i n f o

Article history: Received 20 July 2008 Received in revised form 11 April 2009 Accepted 16 July 2009 Available online 29 July 2009

a b s t r a c t The data reported were collected on 28 cruises using volunteer observing ships (VOS) along the QUIMAVOS line to gauge the fugacity of CO2 (fCO2) over surface water in the south east Atlantic Ocean, over 3 years, from July 2005 to June 2008. The VOS line crosses the region from west to east, approaching the coast along the southern latitudes, and revealing the effects of offshore filaments in the northern areas, plus direct upwelled water in the southern areas. A complex pattern of distribution appears, with areas of super-saturation and low CO2 concentrations, associated with temperature variability and ocean circulation, with the highest variability associated to the most significant upwelling cells. Climatological seasonal cycles of sea surface temperature (SST) and fCO2 for the southern Benguela region presented increased monthly anomalies and a bi-modal seasonal SST cycle, related to the proximity to coastal waters and seasonal upwelled water between September and March, reducing the maximum summer temperature. The biological drawdown of fCO2, controlled by the seasonality thanks to the upwelling activity, contributed with values ranging from 150 to over 250 latm. From 2005 to 2008, the temperature sharply increased in the southern Benguela region, by 0.47 °C yr1 whereas the fCO2 in seawater decreased from 2.5 latm yr1 in the northern Namibia cell, to 0.4 latm yr1 in the southern Benguela area. The latter suggests higher biological activity related to an increase in the upwelling index together with the intrusion of lower inorganic carbon content seawater in the southern area from the Agulhas bank. south of 20°S, the east Atlantic Ocean acts as a net sink of CO2, with CO2 fluxes for the year 2007 increasing from 0.56 mol m2 yr1 (20–24°S) to 3.24 mol m2 yr1 (32–34°S). In the coastal area of South Africa, the constant biologically mediated CO2 drawdown reduces the fCO2 drastically, and maintains the partial CO2 pressure below the levels to be found in the atmosphere all year round. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The magnitude and direction of the air–sea CO2 exchange in the eastern boundary upwelling systems is largely unquantified, yet essential for an understanding of the role played by these productive regions in the global carbon cycle. Upwelling regions are sites of active physical and bio-geo-chemical processes of direct relevance to the global carbon cycle (Borges et al., 2005; Monteiro, in press). The coastal-ocean carbon transport and the air–sea CO2 exchange in these coastal margins are currently under-represented in the ocean carbon cycle models and the global partial pressure of CO2 surveys (Orr et al., 2001; Sarmiento and Gruber, 2002; Takahashi et al., 2002). Margins dominated by coastal upwelling are complex, with the ocean circulation controlled by wind forcing. The few studies carried out in world margins indicate that the low latitude shelves

* Corresponding author. Tel.: +34 928452914; fax: +34 928452922. E-mail addresses: [email protected] (M. González-Dávila), [email protected] (J.M. Santana-Casiano), [email protected] (I.R. Ucha). 0079-6611/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2009.07.033

act as a source of CO2 (Lefévre et al., 2002; Goyet et al., 1998), whereas those at mid to high latitudes act as CO2 sinks (Borges and Frankignoulle, 2002; van Geen et al., 2000). However, the precise annual fluxes are difficult to define for these areas due to both the heterogeneous nature of the ocean margins, and the lack of spatial and temporal coverage of fCO2 data (Cai and Dai, 2004). Revisions of air–seawater CO2 fluxes in coastal areas, including the upwelling areas, have been carried out (Borges et al., 2005; Chavez and Takahashi, 2007). However, most of the studies are located in the northern hemisphere, a few in the south Pacific, and others in the Peruvian and Chilean coastal upwelling system, (Copin Montégut and Raimbault, 1994; Lefévre et al., 2002) with very few in the south Atlantic Ocean (Bakker et al. 1999; Santana-Casiano et al., 2009). In the south Atlantic, the near-coastal south east Atlantic Ocean off Africa is a highly dynamic environment, influenced by the Angola coastal upwelling, the Benguela upwelling, and the western Agulhas Bank (Shannon, 1985; Hardman-Mountford et al., 2003). The Angola-Benguela front (ABFZ) sharply separates the warm northern nutrient-poor water of the Angola current from the cold nutrient-rich water of the Benguela current, and represents a transition zone between the more tropical ecosystem in the north, and

M. González-Dávila et al. / Progress in Oceanography 83 (2009) 124–133

the upwelling-driven ecosystem in the south. In these regions, the carbon-dioxide system is influenced by a variety of physical, chemical and biological processes, including upwelling, seasonal temperature changes, exchange of water on the northern and southern boundaries, the coastal shelf pump, the solubility pump, and the biological pump (Monteiro, in press; Santana-Casiano et al., 2009). In this paper, we report measurements of atmospheric fugacity of CO2 (fCO2) and seawater fCO2 from 28 cruises which followed the extremely constant track line, QUIMA, between 14°S and 33°450 S (Fig. 1). Fig. 1 shows the average position for the most representative features of the area considered, as well as the position of the ship track line on a MODIS SST satellite image for June 2008 (http://oceancolor.gsfc.nasa.gov/). A physical description of the area with special mention of the principal upwelling cells can be found in the Supplementary material. The objective of this work is to study the seasonal variability of fCO2 in surface seawater, and the seasonal air–sea CO2 exchange, taking into account the latitudinal transition of relative importance in the temperature and the biological effects on seawater fCO2 variability, and the dynamic of the upwelling that affects the air–sea fluxes of CO2. 2. Methods A VOS line was set up within the framework of the CARBOOCEAN Project from July 2005 onwards. The QUIMA-VOS line crosses the region between 5°S and 35°S, with all the cruises following the same line as indicated in Fig. 1. The automated underway xCO2 system used was developed by Craig Neill, following the system design demonstrated in the National Ocean and Atmospheric Administration (NOAA) VOS-xCO2 Workshop, Miami, October 2002 and described in detail in the Supplementary material. Data are available on request at http:// www.carboocean.org/. Three container cargo ships from the Mediterranean Shipping company were used in this study, the MSC-MARTINA, the MSCGINA and the MSC-BENEDETTA which paid monthly visits to the area of study. The air–sea flux, FCO2 was estimated as indicated in Santana-Casiano et al. (2009), using the daily winds and the exchange coefficient parameterisation of Wanninkhof (1992) and Nightingale et al. (2000). A positive value indicated that the area was acting as a source of CO2.

125

3. Results and discussion 3.1. Hydrographic situation Fig. 2 shows 8 of the 28 surface months’ temperature and salinity distribution over the QUIMA-VOS line from 14°S–1°300 E to Cape Town area at 33°500 S–8°150 E, from the western part of the ABFZ area to the coastal waters of the Cape Town area. The SST decreased smoothly from north to south, but showed the effect of the ABFZ and of the filaments of coastal upwelled water. The effect of the ABFZ is observed at its northernmost boundary, close to 14°S, in July, and through winter and early spring, later moving south in summer and early autumn to 18°S, particularly during February, a feature observed over the three years (Fig. 2a). A strong warm saline water signal, related to the ABFZ boundary, was observed at around 18°S in April 2006 (Fig. 2a) and also in June 2007, when the northern boundary was detected at around 17°S with a temperature change from 22.6 °C to 23.4 °C, and with a clear southern boundary located at around 20°S, together with temperature gradients of 0.6 °C. In May and June 2008 (Fig. 2a), the ABFZ affected the surface waters located at 15°S and 2°E, over 900 km off Angola, with a temperature gradient of close to 2 °C. The northern Namibia cell, south of Cape Frio (at around 19– 20°S), surface distribution are affected, mainly in salinity in the autumn and spring (between July and October) when the long-shore winds are strongest (Boyd et al., 1987). Our data show these influences in April, and at the beginning of July and November (Fig. 2a and b), also in March, June and at the end of September 2007, and March, 2008. Close to 24°S, the Central Namibia cell decreased the temperature distribution by 2 °C and salinity by around 0.3 units, with the strongest episodes in June and September 2007, and, in general, during the winter and spring. The main perennial centre of upwelling is the Lüderitz cell, with a secondary centre at the Cunene cell. The Lüderitz cell affected the temperature and salinity distribution at the position of the VOS line further north than 26°S, between the Austral summer and winter, moving to 27°S in spring, with temperature changes of around 1.7 °C and salinity decrease of 0.3–0.5. This effect is to be observed all year round. South of Lüderitz, the upwelling-favourable winds occur during the spring and summer in the southern Benguela (Hardman-Mountford et al., 2003). The Namaqua cell area (Fig. 1) may be considered a transi-

Fig. 1. The QUIMA VOS cruise track and the main upwelling cells in the Angola-Benguela region from a MODIS-color satellite image for June 2008. SEC refers to south Equatorial Current, cBC to the coastal Benguela Current, BC to the Benguela Current, and ABFZ to the Angola-Benguela Frontal zone.

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Latitude -34 28

-32

-30

Southern Benguela

-28

-26

Lüderitz

-24

-22

-20

Central Namibia

-18

-16

-14

Northern Namibia

26 24

20 18 16

Jan 2007 Feb 2008 Apr 2006 May 2008 Jun 2007 Jul 2006 Sep 2007 Nov 2005

14 12

a 36.5 36.0 35.5 35.0

Salinity

SST, ºC

22

34.5

b

34.0

600 500

fCO2, μatm

400 360 320 280

c 240 -34

-32

-30

-28

-26

-24

-22

-20

-18

-16

-14

Latitude Fig. 2. Sea surface temperature (a), salinity (b) and fCO2 (c) along the VOS line for the seven selected cruises.

tion area, with strong temperature and salinity shifts in the shape of fronts at 29°200 S (1.3° in SST and 0.4 in salinity) and 30°300 S (1.5° in SST and 0.3 in salinity) in February (Fig. 2b). At the southernmost part of Benguela, two cells are located at Columbine (33°S) and Cape Peninsula (34°S), seasonally affected, strong during the Austral summer, and weak during the winter (Fig. 1). The VOS line approaches the coastal waters in this area. The cell produced an important front in both temperature and salinity, recorded in February 2006, 2007 and 2008, where the SST decreased by 4.5 °C from 31°300 S to 32°S. The effect of the Columbine cell was also observed in March, April and May, from 2006 to 2008, but with reduced temperature decreases of under 2 °C in May. The signal was not evident later in June, and was only to be observed in November, with temperature gradients of around 3 °C (Fig. 2a). A special event was observed in April, 2006 at 32°200 S, where a decrease of 2.7 °C in temperature was not followed by a decrease in salinity, but with a slight increase of 0.15 units. Bang (1973), and later Shannon et al.

(1990), noted that there are nearly always vestiges of Agulhas water west of the Cape Peninsula, as the result of a major perturbation in the retroflection of the Agulhas current. This perturbation resulted in 1987 (Shannon et al., 1989) in a substantial equatorward flow of Sub-Antarctic surface water, in the form of a cold filament, as far north as 33°S. In our study, temperatures at 32°S, as low as 12.2 °C and salinities of 34.22, can only be explained by considering this feature. The Cape Peninsula cell presented temperatures under 14 °C from September (2.5 °C and 0.4 in salinity lower than the surrounding waters) to around 13 °C in the Austral summer, in January and February. The sharpest decrease observed over the three years of study was observed in January, 2007, from 19.6 °C at 33°250 S to 12.8 °C at 33°480 S. In May, a decrease in 0.7 salinity units was determined close to 33°400 S, with a temperature of 15.11 °C, 1.8 °C lower than the surrounding water. Later, the upwelling of cold and fresher water is again stronger by October and throughout spring and summer.

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3.2. fCO2 variability

3.3. Seasonal cycles of SST and fugacity

The fugacity of CO2 presented a complex distribution with areas of super-saturation and low CO2 concentrations relating to temperature variability and ocean circulation, with the highest variability related to the position of the most significant upwelling cells (Fig. 2). Four main regions were defined in order to describe the observed behaviour: the northern Namibia area from 14°S to 19°S including the ABFZ, the central Namibia region (20–24°S) as a transition zone previous to, third, the Lüderitz area, from 25°S to 28°S, and the southern Benguela area from 29°S to 33°S, at Cape Town. In the northern Namibia region, the meeting of upwelled and tropical waters over the ABFZ increased the average value of 360–380 latm for the area to 420–440 latm in the 16°S to 19°S area (Fig. 2c). Strong over-saturations were observed at the Cunene cell area, 16–17°S, and close to the northern Namibia cell at 19°S. The over-saturation was strongest in winter from July to September, with values around 440 latm, decreasing to 400 latm in November, related to the upwelled waters, rich in carbon dioxide, not totally depleted by nutrient consumption, but was not observed in summer and autumn, when upwelling is at its weakest. A different situation was only observed in May and June 2008, with several abrupt decreases in the fCO2. Values as low as 285 latm, 295 latm and 315 latm were observed at 17°100 S, 18°S and 18°240 S, respectively in May (Fig. 2c). In this area, between 16° and 22°S, wind forcing is bimodal, with two peaks in October–November, and in March–April (Shannon and Nelson, 1996; Hardman-Mountford et al., 2003). In the Central Namibia region (20–24°S), values of fCO2 continued to decrease following the general decreasing trend in temperature from north to south. The presence of filaments of upwelled water around 20–21°S and 23–24°S produced increased fCO2 values around 400 latm, over the highest mean values for the area, over all the seasons. The relaxation in upwelling also produces the intrusions of warmer waters (Fig. 2a) characterised by higher fCO2 values (Fig. 2c). By January and February, during the Austral summer, the water arriving at the VOS line was characterised by depleted CO2 concentrations, reaching under-saturation levels at 21–22°S. The Lüderitz region is under-saturated in CO2 the whole year round, with values close to atmospheric levels of 370 latm in the summer. Around the prevalent upwelling filament, close to 26°S, oriented perpendicular to the coast (Shannon and Nelson, 1996), rich CO2 water was observed in winter and spring, with values close to 400 latm. However, after the end of summer, the system is under-saturated, indicating a significant decrease in the initial CO2 concentration of the upwelled water, due to the biological activity during the offshore transport of the water. The southern Benguela region presented the strongest influence on the fCO2 distribution. Over the seasons and years, the values were always below atmospheric values of 370–375 latm, acting as a sink of carbon dioxide. However, upwelling takes place during late spring to early autumn, with a maximum in summer. During late spring and summer, upwelled waters were characterised by very low CO2 concentrations, dropping to values of 200 latm in October–November 2006, and 138 latm in January 2007 when the temperature fell by over 7 °C. Values of 200 latm and 240 latm were also recorded in several sharp decreases in fCO2 at the position of Namaqua, Columbine, and the Cape Peninsula upwelling cells, during late summer, in February and March. Moreover, fCO2 values were recorded as high as 440 latm in March and April, 2006, and values of 490 latm, in February and March 2008, close to the Cape Peninsula area near Cape Town. Both opposed fCO2 behaviours are observed under the same upwelling hydrodynamic conditions at 100 km distance.

In order to further study the observed latitudinal variability in the fugacity of carbon dioxide in the area, the climatological seasonal cycles of SST and fCO2 were obtained by fitting monthly latitudinal degree averages with wave function as expressed by

y ¼ a þ bðx  2005Þ þ c sin 2px þ d cos 2px þ e sin 4px þ g cos 2px

ð1Þ

where y is either SST or fCO2, x is the date expressed as a year fraction in order to establish inter-annual changes, and a, b, c, d, e and g are the fitted constants, presented in Table 1. Fig. 3 presents the seasonal cycle of SST and fCO2 for the four selected regions, with the coefficients as given in Eq. (1), in Table 1. Maximum temperatures were observed at positions north of 24°S (Fig. 3a) by the end of March and April. At the Lüderitz region, the maximum temperatures shifted from March to February. The maximum SST decreased from 25.6 °C at 14°S to 20.9 °C at 28°S. The coolest month was in late winter, at the end of September, ranging from 19.7 °C at 14°S to 15.8 °C at 28°S. In the southern Benguela region, the proximity to coastal waters, and the seasonal upwelling-favourable winds between September and March, strongly reduced the maximum summer temperature. The result was a reduction in the correlation coefficient, an increase in the monthly anomalies, and a bi-modal seasonal SST cycle (Fig. 3a). The bimodality is expressed as two warm and two cold seasons. The first warm season is observed by the end of January at 29°S shifting to the end of December at 33°S, with temperatures of only 17.3. The second maximum takes place at the end of June, with temperatures around 17 °C at 33°S, and a minimum at the end of March, as the result of the arrival of cool, upwelled waters at the end of summer and the beginning of autumn. The second minimum is located in September, with temperatures of around 15 °C. Fig. 3b presents the seasonal cycle for fCO2 monthly averages. North of 19°S, maximum values of fCO2 are observed in February–April, during the warm season. A second maximum was also observed in August–October, coinciding with the highest upwelling intensity, and following the bi-modal seasonal cycle for sea surface height pattern, as described by Hardman-Mountford et al. (2003) for the coastal waters of South Africa. Minimum fCO2 values were observed in November–December and June–July when the low sea level seasons were also registered. In the Central Namibia region, the same bi-modal seasonal cycle is to be observed, whereas close to the area of influence of the Central Namibia upwelling cell, the maximum values observed during winter almost disappear, following the same pattern as the temperature, in line with the reduction in the bimodal amplitude for the sea surface height (Hardman-Mountford et al. 2003). Over the Lüderitz region, the distribution holds steady, with maximum values at 28°S of 365 latm in March, and minimum values in October, around 340 latm, whereas the temperature varies between 20.9 °C to 15.9 °C, respectively (Fig. 3a), indicating a significant biological CO2 drawdown. In the southern Benguela region, higher fCO2 values, from March–May to June-July, and low values by January, are observed. From 2005 to 2008, the temperature greatly increased (Table 1) while the fCO2 in seawater decreased from 2.49 latm yr1 in the northern Namibia cell to 0.41 latm yr1 in the southern Benguela area. Three years of data for a highly variable area can provide only a rough estimate of the trends. Hardman-Mountford et al. (2003) using satellite-derived anomalies for the area, from 1982 to 1999, found a general trend towards increase in temperature as was observed in our study. A special case is that found in the southern Benguela area, where the temperature increased from 2005 onwards, at a rate of 0.47 °C yr1. Hardman-Mountford

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Table 1 Fitting constants for Eq. (1) in one-degree latitude resolution for SST, and for SST and fCO2 for the four regions, the northern Namibia (14°S–19°S), the Central Namibia (20°S– 24°S), Lüderitz (25°S–28°S) and the southern Benguela (29°S–33°S). The standard error of estimate for SST is in °C and for fCO2 in latm. Degree

b

c

0.21 0.23 0.18 0.13 0.04 0.08 0.05 0.10 0.10 0.03 0.13 0.15 0.23 0.03 0.08 0.31 0.51 0.49 0.47 0.02

2.93 2.88 3.01 3.05 3.05 2.92 2.75 2.57 2.49 2.45 2.43 2.48 2.41 2.22 2.09 1.63 1.33 0.88 0.23 0.14

Temperature fitting parameters for each region 14–19°S 21.28 0.12 19.77 0.06 20–24°S 25–28°S 18.99 0.13 29–33°S 16.42 0.47

14°S 15°S 16°S 17°S 18°S 19°S 20°S 21°S 22°S 23°S 24°S 25°S 26°S 27°S 28°S 29°S 30°S 31°S 32°S 33°S

a 22.07 21.45 21.15 21.04 20.94 21.01 20.35 19.99 19.70 19.64 19.63 19.39 19.21 18.53 18.32 17.49 16.81 16.81 16.18 16.14

fCO2 fitting parameters for each region 14–19°S 380.95 20–24°S 365.76 25–28°S 357.90 29–33°S 336.98

2.49 0.58 0.89 0.41

d

g

0.15 0.35 0.52 0.4 0.46 0.44 0.65 0.67 0.74 0.82 0.79 0.82 0.82 1.14 1.27 1.16 1.37 1.48 0.75 0.18

0.04 0.27 0.23 0.25 0.08 0.09 0.16 0.13 0.07 0.09 0.11 0.10 0.15 0.15 0.16 0.14 0.37 0.19 0.11 0.31

0.14 0.08 0.06 0.01 0.01 0.03 0.03 0.09 0.11 0.05 0.07 0.01 0.08 0.27 0.24 0.70 0.44 0.75 0.83 0.94

0.528 0.482 0.427 0.415 0.462 0.522 0.430 0.410 0.346 0.440 0.319 0.426 0.522 0.539 0.512 0.732 0.747 1.109 1.319 1.650

0.96 0.97 0.98 0.98 0.97 0.96 0.97 0.97 0.98 0.96 0.98 0.96 0.94 0.94 0.95 0.87 0.86 0.68 0.36 0.18

2.97 2.50 2.27 0.65

0.39 0.77 1.01 0.92

0.16 0.15 0.09 0.04

0.04 0.03 0.07 0.81

0.393 0.337 0.419 0.976

0.98 0.98 0.96 0.61

1.37 6.09 13.10 11.05

0.22 0.85 3.84 11.09

5.89 0.80 1.25 3.76

7.96 3.72 1.17 0.96

10.945 7.304 6.898 18.755

0.44 0.53 0.82 0.30

et al. (2003) found that the SST increase in the southern Benguela appeared to correspond to the years of highest intrusions of the Agulhas waters, relating to changes in wind anomalies. This same behaviour may be responsible for the observed increase in temperature during our study in this area. This increase in SST should be followed by an increase in fCO2. The slight decrease (0.41 latm yr1) observed in our data may be the result of higher biological activity, relating to an increase in the upwelling index (http://www.pfeg.noaa.gov/products/), or the intrusion of lower inorganic carbon content seawater in the area, from the Agulhas bank. Seasonal amplitude, computed by taking the difference between the maximum and minimum mean monthly fCO2 values for the seasonal SST change at each degree in latitude, and for the area between 14°S and 30°S, is 50 latm. However, in the region from 17°S to 21°S, maximum fCO2 values were found during the period of SST below the annual mean, related to the upwelling activity, at its greatest between July and October. Outside this latitude range, seasonal changes in fCO2 and SST are in phase, including the area affected by the influence of the perennial upwelling Lüderitz region and crossed by the VOS line. However, the region south of 30°S presented seasonal amplitude as high as 180 latm in the Cape Town area, also related to the strongest upwelling with high fCO2 in summer and the lowest SST. In order to understand the relative importance of the temperature and biological effects on the seasonal changes of fCO2, the method of analysis proposed by Takahashi et al. (2002) is applied to the area studied, after averaging the values of SST and fCO2 for each 0.5° in latitude for each cruise. 0.5° in latitude was selected to get a better description of the high latitudinal variability. To remove the temperature effect from the observed fCO2 values (fCO2,obs), these were normalized to the average SST for the three studied years and for each 0.5° latitude (Tmean), following

f CO2

Standard error

R2

e

at T mean ¼ f CO2;obs  exp½0:0423  ðT mean  T obs Þ

ð2Þ

The effect of temperature changes on the observed fCO2 is computed by perturbing the mean average fCO2 for the three years of study and for each 0.5° latitude (mean annual fCO2) with the difference between the observed and the mean SST.

f CO2

at T obs ¼ mean annual f CO2  exp½0:0423  ðT obs  T mean Þ

ð3Þ

The biological effect on the fCO2 distribution, (DfCO2)bio, was calculated using the equation:

ðDf CO2 Þbio ¼ ðf COK 2

at T mean Þmax  ðf CO2

at T mean Þmin

ð4Þ

where max and min indicate the maximum and minimum seasonal values. This biology effect also includes the air–sea exchange of CO2 and the addition of CO2 (also alkalinity) by upwelled waters and vertical mixing. The air–sea exchange from winter to summer is considered small, since the air–sea fCO2 difference changes from positive to negative during this period at a similar magnitude. Vertical mixing is also assumed to be small because the mixed layer depth tends to become shallower from winter to summer. Upwelled waters tend both to increase the seasonal temperature amplitude (in particular the lowest temperature values), and the fCO2 seasonal amplitude in the initial stages, if the biological CO2 removing processes are delayed (Takahashi et al., 2002). The temperature effect on the fCO2 distribution, (DfCO2)temp, was calculated using the equation:

ðDf CO2 Þtemp ¼ ðf CO2 atT obs Þmax  ðf CO2 atT obs Þmin

ð5Þ

The relative importance of the effect presented in Eqs. (4) and (5) was calculated by applying the two following expressions

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28

a

14ºS - 19ºS 20ºS - 24ºS 25ºS - 28ºS 29ºS - 33ºS

26 24

SST, ºC

22 20 18 16 14 12 Jul Sep Nov Jan MarMay Jul Sep Nov Jan MarMay Jul Sep Nov Jan MarMay Jul

2005 ---------><------------ 2006-----------><------------2007------------><----------2008 410 390

fCO2, μatm

370 350 330 310 290

14ºS - 19ºS 20ºS - 24ºS 25ºS - 28ºS 29ºS - 33ºS atm

270 250

b

Jul Sep Nov Jan MarMay Jul Sep Nov Jan MarMay Jul Sep Nov Jan MarMay Jul

2005 ---------><------------ 2006-----------><------------2007------------><----------2008 Fig. 3. Three-year climatological seasonal cycles of sea surface temperature (a) and fCO2 (b) in the northern Namibia (14°S–19°S), Central Namibia (20°S–24°S), Lüderitz (25°S–28°S) and southern Benguela (29°S–33°S) regions. fCO2 values for the mean atmospheric value are also included.

ðT=BÞ ¼ ðDfCO2 Þtemp =ðDfCO2 Þbio

ð6Þ

ðT  BÞ ¼ ðDfCO2 Þtemp  ðDfCO2 Þbio

ð7Þ

where T refers to the temperature effect, and B to the biological effect on the fCO2 distribution. Fig. 4 shows the seasonal biological drawdown and temperature effect on seawater fCO2 (Fig. 4a), together with the ratio T/B between the effects on fCO2 of seasonal temperature changes and biology (Fig. 4b), for 0.5° latitude. The temperature effect on the fCO2 presented an average value of 95 latm. Marked temperature effects, exceeding 100 latm, are to be observed in the north Namibia area, due to the presence of filaments of seasonal upwelled water, affecting the temperature seasonal cycle. The area around Lüderitz showed slightly increased values (90 latm) with respect to neighbouring values of 80 latm, due to its relative upwelling constancy (although there is a slight minimum during winter). In

the southernmost part of the Benguela area, the summer seasonal upwelling affects the coldest water, and contributes to the temperature effect on the fCO2, with values around 100 latm. Very significant biological drawdown effects on the fCO2 in excess of 250 latm (Fig. 4a) are to be observed at the area south of 32°S, due to the aforementioned seasonal induced upwelling and chlorophyll-induced effects (see Supplementary material). Marked biological effects are also shown by Takahashi et al. (2002) for the coast of Mexico, Peru and Chile, whereas the coastal upwelling areas off west Africa had not been characterised, due to insufficient seasonal data. In the northern Namibia area, and south of Cape Frio, the biological drawdown of fCO2, controlled by the seasonality in the upwelling activity, contributed with values in the range of 150–200 latm, Fig. 4b indicates the areas dominated by the biological effect are located in the regions with strong seasonal upwelling activity. Values of T/B as low as 0.3 south of 32°S (values

350

130

a Biology Temperature

300

120

250 110 200 100 150 90 100 80

50 0

70 -34

-32

-30

-28

-26

-24

-22

-20

-18

-16

-14

ΔfCO2,tem = (fCO2 at Tobs)max-(fCO2 at Tobs)min

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ΔfCO2,bio = (fCO2 at Tmean)max-(fCO2 at Tmean)min

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Latitude 1.6

b 1.4 1.2

T/B

1.0 0.8 0.6 0.4 0.2 -34

-32

-30

-28

-26

-24

-22

-20

-18

-16

-14

Latitude Fig. 4. (a) The effect of the biological use of CO2 calculated using Eq. (4) and the temperature effect on the fCO2 distribution, calculated using Eq. (5), (b) The relative importance of the temperature and biological effects on the surface-water fCO2 defined as the ratio T/B, using Eq. (6). (T/B). Ratios under 1 correspond to areas where the biological effect exceeds the temperature effect.

of T  B as negative as 220 latm) and under 0.7 from 20°S to 17°S (T  B values of 65 latm), indicated areas dominated by the biological effect on the fCO2. The coastal Lüderitz upwelling region presents high chlorophyll concentration throughout the year, with minima in winter (July and August). However, the extension of the offshore filaments from the Benguela area are higher from June to November (Supplementary material), reaching the VOS line. From December to February, the upwelling is stronger, but more restricted, to the marginal continental areas. As a result, T/ B values for this area ranged from 0.8 to 1.4, indicating that the effect of temperature over-compensates the effect of biology in waters not strongly affected by offshore filaments, while the area around 26°S presented biological effects exceeding the temperature effect, by 25 latm (T/B of 0.8). The thermodynamic effect of the SST changes on the seasonal amplitude of fCO2 is less effective in the Angola-Benguela area than in open ocean waters (Lefévre and Taylor, 2002; Santana-Casiano et al., 2007) due to the greater influence of the hydrodynamic conditions and the biological activity.

3.4. Seasonal air–sea CO2 fluxes variability Fig. 5 shows the CO2 gradients, with the negative values indicative of the CO2 uptake by the ocean. The northern Namibia region acts as a source of CO2, with a mean gradient of 9 latm, and maximum values to be observed in winter and summer, following the bi-modal seasonal cycle described for the fCO2 seasonal variability. A strong negative gradient of 25 and 30 latm, was observed during May and June 2008, respectively. The Central Namibia region presents a transition between the CO2 source areas in the north, and the sink area in the south. The mean DfCO2 value for the 3-year period was – 5 latm, with a maximum of – 16 latm in winter and spring. Only during summer did the arrival of upwelled water from the area favour the outgassing of CO2. The Lüderitz area is characterised by a mean DfCO2 of around – 14latm, with peaks of – 36 latm in winter and at the beginning of spring, when the sea surface temperature is lower, and almost in equilibrium in autumn. The southern Benguela region presented the highest DfCO2 with a mean value of – 37 latm. During the

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2005---------><-----------2006-----------><----------2007-----------><---------2008 Jul

Nov

Mar

Jul

Nov

Mar

Jul

Nov

Mar

Jul

60 40

14ºS - 19ºS

20 0 -20 -40

20 15 10 5 0 -5 -10 -15 -20

20ºS - 24ºS

ΔfCO2, μatm

10

25ºS - 28ºS

0 -10 -20 -30 -40

0 -25 -50 29ºS - 33ºS

-75

10 0 -10 -20 total 14ºS - 33ºS

-30 Jul

Nov

Mar

Jul

Nov

Mar

Jul

Nov

Mar

Jul

2005---------><-----------2006-----------><----------2007-----------><---------2008 Fig. 5. The monthly average DfCO2 = fCO2,sw  fCO2,atm for the four regions in Figure 5.

strong upwelling season in summer, the area is still under-saturated but with a lower gradient. In order to study the net air–sea CO2 fluxes, FCO2 was computed considering the Wannikhof (wind speed)2 gas transfer velocity dependence, while the Nightingale et al. (2000) parameterisation was also considered for comparative purposes and included in brackets for each FCO2 value presented below for the years 2006 and 2007 (Fig. 6). Fluxes for the year 2006 and 2007 were similar, except in the areas where seasonal upwelling is present, indicating an important inter-annual variability in the upwelling intensity. North of 20°S the area acts as a slight source of CO2, with values of 0.40 (0.28) mol m2 yr1 in 2006 and 0.26 (0.21) mol m2 yr1 in 2007. South of 20°S, the area is a net sink of CO2, with fluxes increasing to the south. From 20°S to 24°S, the ocean is ingassing 0.34 (0.30) and 0.56 (0.45) mol m2 yr1 in 2006 and 2007, respectively. In the Lüderitz region from 25°S to 28°S, the FCO2 values increased to 0.97 (0.80) mol m2 yr1 in 2006 and to 1.22 (0.97) mol m2 yr1 in 2007. An increase in both DfCO2 and wind speed controlled the differences between the two years considered. South of 29°S, two sub-areas are clearly observed. From 29°S to 32°S, both years presented similar ingassing values of 1.80 (1.50) mol m2 yr1. South of 32°S, the area is acting as a sink of CO2 but with a value of only 1.17 (1.03) mol m2 yr1 in 2006, as compared to a value of 3.24 (2.65) mol m2 yr1 in 2007. The inter-annual variability observed between 2006 and

2007 in sea surface temperature and salinity (Fig. 2) and in chlorophyll concentration (http://oceancolor.gsfc.nasa.gov) related to changes in the upwelling intensity and in the Agulhas ring behaviour, strongly influenced the distribution of fCO2 and FCO2. These results agree with previous studies for the area in winter and spring (Santana-Casiano et al., 2009) confirming that coastal upwelling regions may act as sinks of CO2, due to carbon consumption by photosynthesis, counteracting the physical processes. 4. Conclusion The area covered by the QUIMA-VOS line from 14°S to Cape Town and considering the oblique section that approaches coastal waters at the end, showed a complex surface hydrodynamic system, with upwelling, filaments and fronts affecting the physical, chemical and biological parameters that control the surface distribution of the fCO2. South of 20°S, the ocean acts as an active sink of CO2. In the coastal area of South Africa, the arrival of upwelled water, that decreases the SST by over 5 °C, with high CO2 concentrations should strongly increase the fCO2 in the area. However, high biologically mediated CO2 drawdown strongly reduces the fCO2 in the area, keeping the partial CO2 pressure below that of the atmosphere, ingassing -1.17 mol m2 yr1 in 2006, and 3.24 mol m2 yr1 in 2007, relating to changes in the seawater properties and the wind fields. Moreover, north of 20°S in the

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2

FCO2 (mol m-2 yr-1)

1 0 -1 -2 -3

Year 2006 Year 2007

-4 -5 -6 -34

-32

-30

-28

-26

-24

-22

-20

-18

-16

-14

Latitude Fig. 6. Fluxes of CO2 (mol m2 yr1) computed the Wanninkhof (1992) parameterisation for the year 2006 and 2007, at 0.5° latitude.

northern Namibia area, characterised by seasonal upwelling, at its greatest from July to October, acted as a slight source of CO2 with a value of 0.40 and 0.26 mol m2 yr1 in 2006 and 2007, respectively. The strong fCO2 anomalies cannot be correlated with anomalies in the SST. The results confirm that the areas dominated by the biological effect are located in the regions with strong seasonal upwelling activity, overcompensating the thermodynamic control in the seasonal fCO2 variability. Very significant biological drawdown effects on the fCO2 over 250 latm are determined south of 32°S, and in the northern Namibia area with values within the 150–200 latm range. As a result, significant DfCO2 variability is also to be observed in this area. More studies should be carried out, both to establish the inter-annual variability along the VOS line, and to study how the fCO2 changes longitudinally from the coastal upwelled waters to the oceanic areas, for the strongest upwelling cells in the Benguela area from Namibia to South Africa. Acknowledgements This work has been supported by the European Project CARBOOCEAN 2005-2009, CN 511176-GOCE. Special thanks go to the MEDITERRANEAN SHIPPING COMPANY (MSC) who provided the ship platforms and scientific facilities, and M. Hart for correcting the English version. We thank the invited editors and the three anonymous reviewers for their helpful discussions and comments which have greatly improved upon the original manuscript. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.pocean.2009.07.033. References Bakker, C.E., de Baar, H.J.W., de Jong, E., 1999. The dependence of temperature and salinity of dissolved inorganic carbon in east Atlantic surface waters. Marine Chemistry 65, 263–280. Bang, N.D., 1973. Characteristics of an intense ocean frontal system in the upwell regime west of Cape Town. Tellus 25 (3), 256–265. Borges, A.V., Frankignoulle, M., 2002. Aspect of dissolved carbon dynamics in the upwelling system off the Galician coast. Journal of Marine Systems 32, 181–198. Borges, A.V., Delille, B., Frankignoulle, M., 2005. Budgeting sinks and sources of CO2 in the coastal ocean: diversity of ecosystems counts. Geophysical Research Letters 32, L14601. doi:10.1029/2005GL023053.

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