Sea Surface fCO2 Research Articles

Sea surface CO2 fugacity in the southwestern South Atlantic Ocean: An evaluation based on satellite-derived images

The South Atlantic Ocean is historically less sampled than the North Atlantic Ocean. Recent efforts have improved our understanding of the carbonate system variable distribution, mainly on sea surface CO2 fugacity (fCO2). However, these studies have been regionally and temporally restricted. Hence, in this research we developed seasonal algorithms of sea surface fCO2 to investigate the CO2 dynamics along the southwestern South Atlantic Ocean during spring-summer and fall-winter periods. The studied region includes the continental shelf areas of the Abrolhos-Campos Region (an area under the influence of central water upwelling), the South Brazil Bight (a large embayment affected by the mesoscale variability in a westward boundary current), the Southern Brazilian Shelf (a coastal zone influenced by freshwater discharge from continent and water mass entrainment), and offshore waters in the open ocean domain of the southwestern South Atlantic Ocean. Monthly satellite images of sea surface temperature, salinity, and chlorophyll-a, which were concomitantly available from August 2011 to June 2015, were used to reconstruct and evaluate the sea surface fCO2 seasonal field. The predicted fields of sea surface fCO2 enabled an investigation of the main drivers that change this variable over the distinct biogeochemical provinces in the region. As expected, the sea surface temperature was the main driver of seasonal changes in sea surface fCO2, but total dissolved inorganic carbon (DIC) and total alkalinity changes were also relevant, mainly in the biogeochemical provinces under the influence of continental freshwater input or central water upwelling. The latter can play an unpredictable role in CO2 dynamics due to nutrient- and DIC-rich water transport close to the surface. Finally, the use of satellite-derived images is a powerful tool to increase biogeochemical knowledge of relatively undersampled ocean regions, while the development of seasonal sea surface fCO2 algorithms allows a better spatiotemporal comprehension of the CO2 distribution, dynamics, and drivers in the southwestern South Atlantic Ocean – a key region for improving the understanding of the global carbon cycle.

The northern European shelf as an increasing net sink for CO<sub>2</sub>

Abstract. We developed a simple method to refine existing open-ocean maps and extend them towards different coastal seas. Using a multi-linear regression we produced monthly maps of surface ocean fCO2 in the northern European coastal seas (the North Sea, the Baltic Sea, the Norwegian Coast and the Barents Sea) covering a time period from 1998 to 2016. A comparison with gridded Surface Ocean CO2 Atlas (SOCAT) v5 data revealed mean biases and standard deviations of 0 ± 26 µatm in the North Sea, 0 ± 16 µatm along the Norwegian Coast, 0 ± 19 µatm in the Barents Sea and 2 ± 42 µatm in the Baltic Sea. We used these maps to investigate trends in fCO2, pH and air–sea CO2 flux. The surface ocean fCO2 trends are smaller than the atmospheric trend in most of the studied regions. The only exception to this is the western part of the North Sea, where sea surface fCO2 increases by 2 µatm yr−1, which is similar to the atmospheric trend. The Baltic Sea does not show a significant trend. Here, the variability was much larger than the expected trends. Consistently, the pH trends were smaller than expected for an increase in fCO2 in pace with the rise of atmospheric CO2 levels. The calculated air–sea CO2 fluxes revealed that most regions were net sinks for CO2. Only the southern North Sea and the Baltic Sea emitted CO2 to the atmosphere. Especially in the northern regions the sink strength increased during the studied period.

Basin‐Scale Estimate of the Sea‐Air CO2 Flux During the 2010 Warm Event in the Tropical North Atlantic

AbstractFollowing the anomalous warming event occurring in the tropical North Atlantic in 2010, higher than usual surface fugacity of CO2 (fCO2) was observed. To evaluate the spatial extent of these anomalies and their drivers, and to quantify the sea‐air CO2 flux at basin scale, the Mercator‐Ocean model is used from 2006 to 2014 within the region 0–30°N, 70–15°W. Model outputs are generally in accordance with underway sea surface temperature, sea surface salinity, and surface fCO2 recorded by two merchant ships. The anomalous warming of 2010 is well reproduced by the model and is the main driver of fCO2 anomalies. The first coupled Empirical Orthogonal Function mode, between sea surface temperature and fCO2, captures more than 70% of the total variance and is characterized by a basin‐scale warming associated to positive fCO2 anomalies. The corresponding principal components are correlated to the Tropical North Atlantic Index and identify 2010 as the year with the highest positive anomaly over 2006–2014. Exceptions to this general pattern are located near the African coast, where the weakening of the coastal upwelling causes negative inorganic carbon anomalies, and close to the Amazon River plume, where fCO2 anomalies are primarily associated with sea surface salinity anomalies. Although the fCO2 anomalies of 2010 appear mostly in spring, they affect the annual CO2 budget and lead to an increased CO2 outgassing twice as large (46.2 Tg C per year) as the mean annual flux over the 2006–2014 period (23.3 Tg C per year).

Recent increase in surface fCO2 in the western subtropical North Pacific

We observed unusually high levels (> 440 μatm) of carbon dioxide fugacity (fCO2) in surface seawater in the western subtropical North Pacific, the area where Subtropical Mode Water is formed, during summer 2015. The NOAA Kuroshio Extension Observatory moored buoy located in this region also measured high CO2 values, up to 500 μatm during this period. These high sea surface fCO2 (fCO2SW) values are explained by much higher normalized total dissolved inorganic carbon and slightly higher normalized total alkalinity concentrations in this region compared to the equatorial Pacific. Moreover, these values are much higher than the climatological CO2 values, even considering increasing atmospheric CO2, indicating a recent large increase in sea surface CO2 concentrations. A large seasonal change in sea surface temperature contributed to higher surface fCO2SW in the summer of 2015.

Seasonal cycle of CO2 from the sea ice edge to island blooms in the Scotia Sea, Southern Ocean

The Scotia Sea region contains some of the most productive waters of the Southern Ocean. It is also a dynamic region through the interaction of deep water masses with the atmosphere. We present a first seasonally-resolved time series of the fugacity of CO2 (fCO2) from spring 2006, summer 2008, autumn 2009 and winter (potential temperature minimum) along a 1000km transect from the pack ice to the Polar Front to quantify the effects of biology and temperature on oceanic fCO2. Substantial spring and summer decreases in sea surface fCO2 occurred in phytoplankton blooms that developed in the naturally iron-fertilised waters downstream (north) of South Georgia island (54–55°S, 36–38°W) and following sea ice melt (in the seasonal ice zone). The largest seasonal fCO2 amplitude (ΔfCO2) of −159μatm was found in the South Georgia bloom. In this region, biological carbon uptake dominated the seasonal signal, reducing the winter maxima in oceanic fCO2 by 257μatm by the summer. In the Weddell–Scotia Confluence, the southern fringe of the Scotia Sea, the shift from wintertime CO2-rich conditions in ice covered waters to CO2 undersaturation in the spring blooms during and upon sea ice melt created strong seasonality in oceanic fCO2. Temperature effects on oceanic fCO2 ranged from ΔfCO2 sst of ~55μatm in the seasonal ice zone to almost double that downstream of South Georgia (98μatm). The seasonal cycle of surface water fCO2 in the high-nutrient low-chlorophyll region of the central Scotia Sea had the weakest biological control and lowest seasonality. Basin-wide biological processes dominated the seasonal control on oceanic fCO2 (ΔfCO2 bio of 159μatm), partially compensated (43%) by moderate temperature control (ΔfCO2 sst of 68μatm). The patchwork of productivity across the Scotia Sea creates regions of seasonally strong biological uptake of CO2 in the Southern Ocean.

Characterizing air–sea CO2 exchange dynamics during winter in the coastal water off the Hugli-Matla estuarine system in the northern Bay of Bengal, India

The distribution of the fugacity of CO2 (fCO2) and air-sea CO2 exchange were comprehensively investigated in the outer estuary to offshore shallow water region (lying adjacent to the Sundarban mangrove forest) covering an area of *2,000 km 2 in the northern Bay of Bengal during the winter. A total of ten sampling surveys were conducted between 1 December, 2011 and 21 February, 2012. Physico- chemical variables like sea surface temperature (SST), salinity, pH, total alkalinity (TAlk), dissolved inorganic carbon (DIC) and in vivo chlorophyll-a along with atmo- spheric variables were measured in order to study their role in controlling the CO2 flux. Surface water fCO2 ranged between 111 and 459 latm which correlated significantly with the SST (r = 0.71, p \ 0.001, n = 62). Neither DIC nor TAlk showed any linear relationship with varying salinity in the estuarine mixing zone, demonstrating the significant pre- sence of non-carbonate alkalinity. An overall net biological control on the surface fCO2 distribution was established during the study, although no significant correlation was found between chlorophyll-a and fCO2 (water). The shallow water region studied was mostly under-saturated with CO2 and acted as a sink for atmospheric CO2. The difference between surface water and atmospheric fCO2 (DfCO2) ranged from -274 to 69 latm, with an average seaward flux of - 10.5 ± 12.6 lmol m -2 h -1 . The DfCO2 and hence the air- sea CO2 exchange was primarily regulated by the variation in sea surface fCO2, since atmospheric fCO2 varied over a com- paratively narrow range of 361.23-399.05 latm.

Interannual variability in sea surface temperature and fCO2 changes in the Cariaco Basin

We examined the variability of sea surface carbon dioxide fugacity (fCO2sea) and its relation to temperature at the Cariaco Basin ocean time-series location (10°30′N, 64°40′W) for the period from 1996 through 2008. Periods of warm (positive) and cold (negative) anomalies at the station were related to variability in coastal upwelling intensity. A positive temporal trend in monthly-deseasonalized sea surface temperatures (SST) was observed, leading to an overall increase of 1.13°C over 13 years. Surface fCO2sea displayed significant short-term variation (month to month) with a range of 330–445μatm. In addition to a large seasonal range (58±17μatm), deseasonalized fCO2sea data showed an interannual positive trend of 1.77±0.43μatmyr−1. In the Cariaco Basin, positive and negative anomalies of temperature and fCO2sea are in phase. An increase/decrease of 1°C coincides with an increase/decrease of 16–20μatm of fCO2sea. Deseasonalized fCO2sea normalized to 26.05°C, the mean Cariaco SST, shows a lower rate of increase (0.51±0.49μatmyr−1). Based on these observations, 72% of the increase in fCO2sea in Cariaco Basin between 1996 and 2008 can be attributed to an increasing temperature trend of surface waters, making this the primary factor controlling fugacity at this location. During this period, a decrease in upwelling intensity was also observed. The phytoplankton community changed from large diatom-dominated blooms during upwelling in the late 1990's to blooms dominated by smaller cells in the first decade of the 21st century. The average net sea-air CO2 flux over the study period is 2.0±2.6molCm−2yr−1 employing the Wanninkhof parameterization, and 2.1±2.5molCm−2yr−1 based on Nightingale's model. To further understand the connection between the changes observed in the Cariaco Basin, the relationships between interannual variability in the temperature anomaly with three modes of climate variability (AMO, NAO and ENSO) were examined. The correlations between SSTA and two of these climate modes (AMO and ENSO) only show very weak relationships, although they were significant.

Trends in North Atlantic sea-surface fCO2 from 1990 to 2006

We examine observations from 1990 to 2006 from four voluntary observing ships and two time-series stations in the North Atlantic, fitting a sinusoidal annual cycle and linear year-on-year trend at all locations where there are sufficient data. Results show that in the subtropical regions, sea-surface fCO2 has closely followed the increasing trend in atmospheric fCO2. In contrast, farther north, sea-surface fCO2 has increased faster than fCO2 in the atmosphere. The resulting ?fCO2, driving air–sea flux of CO2, has therefore decreased in the North Atlantic, particularly at higher latitudes, as has the annual mean air–sea flux. Several underlying causes may have led to the observed changes in sea-surface fCO2. Low-frequency modes, such as the North Atlantic Oscillation, lead to changes in the sea-surface temperature, in sea-surface circulation and in vertical mixing, affecting sea-surface fCO2 through biogeochemical processes. A comparison with measurements covering a longer time period shows that the sea-surface fCO2 rise has accelerated since 1990 in the northern North Atlantic.

Carbon distribution and fluxes during the SERIES iron fertilization experiment with special reference to the fugacity of carbon dioxide (fCO2)

Abstract Surface seawater fugacity of carbon dioxide (fCO2) was measured during the Subarctic Ecosystem Response to Iron Enrichment Study (SERIES), July 9–August 5, 2002. Three ships sampled the iron-fertilized waters near Ocean Station P (50°N, 145°W): the Canadian CCGS J.P. Tully (July 9–23, 2002), the chartered Mexican M/V El Puma (July 9–28, 2002), and the Japanese fisheries research ship M/V Kaiyo Maru (July 24–August 5, 2002). Data used here are from the CCGS J.P. Tully and the M/V Kaiyo Maru. From the onset of the experiment to the peak of the iron-induced diatom bloom on day 19, sea-surface fCO2 decreased from 350 to 265 μatm and average DIC concentration in the upper 30 m decreased from 2030 to 1990 μmol kg−1. Changes in fCO2 in and near the iron patch as observed from the CCGS J.P. Tully and later from the M/V Kaiyo Maru were used to estimate CO2 drawdown and air-sea fluxes, and in generating a carbon budget during the growth phase (days 3–19) of the experiment. Without considering patch dilution, sources of dissolved inorganic carbon to the patch (1.6±0.25 mol m−2) were nearly double the sum (0.87±0.34 mol m−2) of the sinks: accumulations of dissolved organic and particulate carbon, and the flux of particulate carbon to sediment traps below the patch. However, the budget is balanced after considerations of the effects of patch expansion on property concentrations within the patch. A comparison with other iron fertilization experiments from 1995 to present was made to assess the CO2 drawdown values.

Modelled and observed sea surface fCO2 in the southern ocean: a comparative study

The results of an existing one-dimensional diagnostic model that calculates the fugacity of CO 2 (fCO 2 ) in the surface layer of the southern ocean were compared with in situ observations from different ocean sectors and seasons. Our model is based on the translation of monthly variations of constraints fields into surface water fCO 2 variations, and was used to assess the CO 2 uptake of the southern ocean. In situ observations are useful to verify the model results and were here applied to improve the estimation of the CO 2 uptake of the southern ocean south of 50 S. The model reproduces the fCO 2 distribution in both Pacific and Indian sectors of the southern ocean satisfactorily, the mean deviation being only 5 μatm. This diserepancy requires only a minor modification of the CO 2 uptake calculated by the model for that area. By contrast, the model strongly underestimates the fCO 2 levels in carly spring and early winter in the Weddell gyre. This indicates that the CO 2 uptake by the Atlantic sector of the southern ocean as calculated by the model, amounting to 0.47 GtC yr -1 . should be reduced, possibly by about half of this value. The reason for this mismatch lies in the use of climatological physical constraints by the model, that do not sufficiently well describe reality. Partly, the mismatch is also caused by a difference of seasonal stage between the model which reflects climatological conditions and the real ocean which is affected by interannual variability. Based on this study it is concluded that the CO 2 uptake of the southern ocean south of 50 S is likely to lie somewhere between 0.6 and 0.7 GtC yr -1 for the 1990s, which is a high value compared to estimates from other investigations.

Modelled and obserModelled and observed sea surface fCO2 in the southern ocean: a comparative study

The results of an existing one-dimensional diagnostic model that calculates the fugacity of CO2 (fCO2) in the surface layer of the southern ocean were compared with in situ observations from diVerent ocean sectors and seasons. Our model is based on the translation of monthly variations of constraints fields into surface water fCO2 variations, and was used to assess the CO2 uptake of the southern ocean. In situ observations are useful to verify the model results and were here applied to improve the estimation of the CO2 uptake of the southern ocean south of 50°S. The model reproduces the fCO2 distribution in both Pacific and Indian sectors of the southern ocean satisfactorily, the mean deviation being only 5 μatm. This discrepancy requires only a minor modification of the CO2 uptake calculated by the model for that area. By contrast, the model strongly underestimates the fCO2 levels in early spring and early winter in the Weddell gyre. This indicates that the CO2uptake by the Atlantic sector of the southern ocean as calculated by the model, amounting to 0.47 GtC yr-1, should be reduced, possibly by about half of this value. The reason for this mismatch lies in the use of climatological physical constraints by the model, that do not sufficiently well describe reality. Partly, the mismatch is also caused by a difference of seasonal stage between the model which reflects climatological conditions and the real ocean which is affected by interannual variability. Based on this study it is concluded that the CO2 uptake of the southern ocean south of 50°S is likely to lie somewhere between 0.6 and 0.7 GtC yr-1 for the 1990s, which is a high value compared to estimates from other investigations.

Interannual variability of fCO2 in the Greenland and Norwegian Seas

The fCO2 in the Greenland and Norwegian Seas surface water varied significantly during the period from 1995 to 1997. Comparison of fCO2 data from winter 1995 with data from winter 1997 showed that sea surface fCO2 decreased between these winters by 20–30 μatm in the central Greenland Sea, and the potential CO2 uptake during the winters of 1995 and 1997 was 3.9·10-3 Gt C month-1 and 5.9·10-3 Gt C month-1(based on Wanninkhof’s relationship for the gas transfer coeYcient), respectively. This difference in CO2 fluxes can be attributed to lower sea surface temperatures and more extensive sea ice cover in 1997, and these observations were related to increased convection in the Greenland Sea during winter 1997. Larger amplitudes in the seasonal variations of CO2 flux were also seen during the other seasons in the period 1996–97, compared to 1995. Over the years of investigation in the Greenland Sea, the carbon flux showed an increasing trend of 9·10-4 Gt C yr-1 into the ocean, which may be related to the anthropogenic input of carbon to the atmosphere. The Greenland and Norwegian Seas appear to be sinks for atmospheric CO2 and together absorb approximately 0.12 ± 0.015 Gt C yr-1.

Modelling the monthly sea surface fCO2 fields in the Indian Ocean

In order to construct monthly fields of sea surface fugacity of carbon dioxide (fCO2) on a large scale in the Indian Ocean, we use a one-dimensional model which takes into account the main physical and biogeochemical processes controlling fCO2 variations in the ocean. Physical and biogeochemical processes are constrained by the monthly variations of sea surface temperature, salinity, chlorophyll concentration, wind speed and mixed-layer depth. The model is applied to four locations in the Indian Ocean and it well predicts observed temporal variations in fCO2 at these locations. Regarding to monthly fCO2 observations, the model also well simulates the fCO2 distribution and its temporal variations along a track located between 20 ° and 50 °S with a maximal error of + 10 μatm. The model is also used to predict fCO2 for 2 ° × 2 ° grids over the entire Indian Ocean and simulates seasonal cycles that are consistent with observations. The monthly fCO2 fields derived from the model are used to estimate a global air-sea CO2 flux over the Indian Ocean basin. We estimate a net sink of 0.5 Gtyr C for the Indian Ocean (20 °N-50 °S), with the main sink located between 20 ° and 50 °S.