Complex oceanic circulation and air–sea interaction make the eastern tropical Pacific Ocean (ETPO) a highly variable source of CO2 to the atmosphere. Although the scientific community have amassed 70 ...000 surface fugacities of carbon dioxide (fCO2) data points within the ETPO region over the past 25 years, the spatial and temporal resolution of this data set is insufficient to fully quantify the seasonal to interannual variability of the region, a region where fCO2 has been observed to fluctuate by > 300 µatm.Upwelling and rainfall events dominate the surface physical and chemical characteristics of the ETPO, with both yielding unique signatures in sea surface temperature and salinity. Thus, we explore the potential of using a statistical description of fCO2 within sea-surface salinity–temperature space. These SSS/SST relationships are based on in situ surface ocean CO2 atlas (SOCAT) data collected within the ETPO. This statistical description is then applied to high-resolution (0.25∘) Soil Moisture and Ocean Salinity (SMOS) sea surface salinity (SSS) and Operational Sea Surface Temperature and Sea Ice Analysis (OSTIA) sea surface temperature (SST) in order to compute regional fCO2. As a result, we are able to resolve fCO2 at sufficiently high resolution to elucidate the influence that various physical processes have on the fCO2 of the surface ETPO.Normalised (to 2014) oceanic fCO2 between July 2010 and June 2014 within the entire ETPO was 39 (±10.7) µatm supersaturated with respect to 2014 atmospheric partial pressures, and featured a CO2 outgassing of 1.51 (±0.41) mmol m-2 d-1. Values of fCO2 within the ETPO were found to be broadly split between the Gulf of Panama region and the rest of the tropical eastern Pacific Ocean. The northwest, central and offshore regions were supersaturated, with wintertime wind-jet-driven upwelling found to constitute the first-order control onfCO2 values. This contrasts with the southeastern/Gulf of Panama region, where heavy rainfall combined with rapid stratification of the upper water column act to dilute dissolved inorganic carbon, and yield fCO2 values undersaturated with respect to atmospheric fugacities of CO2.
The considerable uncertainties in the carbon budget of the Southern Ocean are largely attributed to unresolved variability, in particular at a seasonal timescale and small spatial scale (~ 100 km). ...In this study, the variability of surface pCO2 and dissolved inorganic carbon (DIC) at seasonal and small spatial scales is examined using a data set of surface drifters including ~ 80 000 measurements at high spatiotemporal resolution. On spatial scales of 100 km, we find gradients ranging from 5 to 50 μatm for pCO2 and 2 to 30 μmol kg-1 for DIC, with highest values in energetic and frontal regions. This result is supported by a second estimate obtained with sea surface temperature (SST) satellite images and local DIC-SST relationships derived from drifter observations. We find that dynamical processes drive the variability of DIC at small spatial scale in most regions of the Southern Ocean and the cascade of large-scale gradients down to small spatial scales, leading to gradients up to 15 μmol kg-1 over 100 km. Although the role of biological activity is more localized, it enhances the variability up to 30 μmol kg-1 over 100 km. The seasonal cycle of surface DIC is reconstructed following Mahadevan et al. (2011), using an annual climatology of DIC and a monthly climatology of mixed layer depth. This method is evaluated using drifter observations and proves to be a reasonable first-order estimate of the seasonality in the Southern Ocean that could be used to validate model simulations. We find that small spatial-scale structures are a non-negligible source of variability for DIC, with amplitudes of about a third of the variations associated with the seasonality and up to 10 times the magnitude of large-scale gradients. The amplitude of small-scale variability reported here should be kept in mind when inferring temporal changes (seasonality, interannual variability, decadal trends) of the carbon budget from low-resolution observations and models.
On a mean annual basis, the Southern Ocean is a sink for atmospheric CO2. However the seasonality of the air-sea CO2 flux in this region is poorly documented. We investigate processes regulating ...air-sea CO2 flux in a large area of the Southern Ocean (38 degree S-55 degree S, 60 degree W-60 degree E) that represents nearly one third of the subantarctic zone. A seasonal budget of CO2 partial pressure, pCO2 and of dissolved inorganic carbon, DIC in the mixed layer is assessed by quantifying the impacts of biology, physics and thermodynamical effect on seawater pCO2. A focus is made on the quantification at a monthly scale of the biological consumption as it is the dominant process removing carbon from surface waters. In situ biological carbon production rates are estimated from high frequency estimates of DIC along the trajectories of CARIOCA drifters in the Atlantic and Indian sector of the Southern Ocean during four spring-summer seasons over the 2006-2009 period. Net community production (NCP) integrated over the mixed layer is derived from the daily change of DIC, and mixed layer depth estimated from Argo profiles. Eleven values of NCP are estimated and range from 30 to 130mmolCm-2 d-1. They are used as a constraint for validating satellite net primary production (NPP). A satellite data-based global model is used to compute depth integrated net primary production, NPP, for the same periods along the trajectories of the buoys. Realistic NCP/NPP ratios are obtained under the condition that the SeaWiFS chlorophyll are corrected by a factor of approximately 2-3, which is an underestimation previously reported for the Southern Ocean. Monthly satellite based NPP are computed over the 38 degree S-55 degree S, 60 degree W-60 degree E area. pCO2 derived from these NPP combined with an export ratio, and taking into account the impact of physics and thermodynamics is in good agreement with the pCO2 seasonal climatology of Takahashi (2009). On an annual timescale, mean NCP values, 4.4 to 4.9molCm-2 yr-1 are approximately 4-5 times greater than air-sea CO2 invasion, 1.0molCm-2 yr-1. Our study based on in situ and satellite observations provides a quantitative estimate of both seasonal and mean annual uptake of CO2 in the subantarctic zone of the Southern Ocean. These results bring important constraints for ocean circulation and biogeochemical models investigating future changes in the Southern Ocean CO2 fluxes.
The Kerguelen Plateau region in the Indian sector of the Southern Ocean supports annually a large-scale phytoplankton bloom which is naturally fertilized with iron. As part of the second Kerguelen ...Ocean and Plateau compared Study expedition (KEOPS2) in austral spring (October–November 2011), one CARbon Interface OCean Atmosphere (CARIOCA) buoy was deployed east of the Kerguelen Plateau. It drifted eastward downstream along the Kerguelen plume. Hourly surface measurements of pCO2, O2 and ancillary observations were collected between 1 November 2011 and 12 February 2012 with the aim of characterizing the spatial and temporal variability of the biological net community production, NCP, downstream the Kerguelen Plateau, assessing the impact of iron-induced productivity on the biological inorganic carbon consumption and consequently on the CO2 flux exchanged at the air–sea interface. The trajectory of the buoy up to mid-December was within the longitude range 72–83° E, close to the polar front and then in the polar frontal zone, PFZ, up to 97° E. From 17 November to 16 December, the buoy drifted within the Kerguelen plume following a filament carrying dissolved iron, DFe, for a total distance of 700 km. In the first part of the trajectory of the buoy, within the iron plume, the ocean surface waters were always a sink for CO2 and a source for O2, with fluxes of respective mean values equal to −8 mmol CO2 and +38 mmol O2 m−2 d−1. To the east, as the buoy escaped the iron-enriched filament, the fluxes were in the opposite direction, with respective mean values of +5 mmol CO2 and −48 mmol O2 m−2 d−1. These numbers clearly indicate the strong impact of biological processes on the biogeochemistry in the surface waters within the Kerguelen plume in November–mid-December, while it is undetectable to the east in the PFZ from mid- December to mid-February. While the buoy follows the Fe-enriched filament, simultaneous observations of dissolved inorganic carbon (DIC) and dissolved oxygen (O2) highlight biological events lasting from 2 to 4 days. Stoichiometric ratios, O2 / C, between 1.1 and 1.4 are observed indicating new and regenerated production regimes. NCP estimates range from 60 to 140 mmol C m−2 d−1.
A CARbon Interface OCean Atmosphere (CARIOCA) surface buoy drifted from 2006 to 2007 in the polar regions of the South Atlantic and Indian Oceans. Derived values of the surface dissolved inorganic ...carbon (DIC) displayed conspicuous daily variations with a close to sunrise maximum and a close to sunset minimum. This decrease of carbon is a measurement of the Net Community Production (NCP) during daytime at 2 meters depth. NCP integrated over the mixed layer is computed from the daily change of the maxima of DIC. When combined with mixed layer depths estimated from Argo floats, we find that north of South Georgia Island, NCP ranges from 82 to 118 mmol m−2 d−1 in the fall and from 30 to 51 mmol m−2 d−1 close to 17°W in late spring. This study highlights the possibility of estimating biological carbon production rates by an in situ non‐intrusive method from unattended platforms.
From 2008 to 2014, the MAREL-Iroise buoy, located in the Bay of Brest, collected high-frequency measurements of partial pressure of CO2 (pCO2) and ancillary hydrographic parameters, in conjunction ...with a comprehensive sampling regime of two additional carbonate system variables total alkalinity (AT), and dissolved inorganic carbon (DIC). Biological processes drive variations in AT and DIC throughout the year, except in winter, when primary production is negligible and large freshwater inputs occur. Annually, the Bay of Brest generally behaves as a source of CO2 to the atmosphere (0.14±0.20molCm−2yr−1), showing inter-annual variability significantly linked to annual net community production (NCP). The presence of a large community of benthic filter feeders leads to high levels of particulate organic matter (POM) and opal deposition during the spring diatom bloom. Over the following few months, benthic POM remineralisation reduces the spring CO2 deficit relative to the atmosphere, and remineralisation of biogenic silica supplies further late spring primary production. The result is an inverse spring NCP – air-sea CO2 flux relationship, whereby greater NCP in early spring results in lower fluxes of CO2 into the Bay in late spring. This recycling mechanism, or silicic acid pump, also links the spring and summer NCP values, which are both determined by the peak wintertime nutrient concentrations. The carbonate system is further affected by the benthic community in winter, when CaCO3 dissolution is evident from notable deviations in the ΔAT:ΔDIC ratio. This study highlights the necessity of individual study of coastal, temperate ecosystems and contributes to a better understanding of what determines coastal areas as sinks or sources of CO2 to the atmosphere.
•The Bay of Brest generally behaves as a source of CO2 to the atmosphere.•Inter-annual variability in air-sea CO2 exchange is linked to net community production.•Springtime net community production is determined by the winter, river dissolved silica supply.•Total alkalinity and dissolved inorganic carbon variability is driven by biology.
A high‐resolution ocean biogeochemical model is used to estimate oceanic pCO2 and air‐sea CO2 flux in the NE Atlantic. The model is validated against shipboard and Carioca drifting float data ...acquired during the POMME experiment. Between winter and spring, the seasonal variability is characterized by a rapid drawdown of pCO2 of ∼20 μatm associated with the phytoplankton bloom driving CO2 uptake by the ocean. The model reveals that this uptake propagates northward in response to the northward propagation of the bloom. More remarkably, this study demonstrates intense variability of the carbon system at the submesoscale. Our model predicts filamentary structures of pCO2 that show gradients of 25 μatm over 20 km, consistent with observations from Carioca drifting floats. This submesoscale variability is similar in magnitude to the mean seasonal drawdown. Lagrangian diagnostics suggest that pCO2 small‐scale structures are shaped by horizontal stirring of large‐scale gradients created by the bloom northward propagation. We compared air‐sea flux derived from model pCO2 and from observed pCO2 and estimated the error due to data undersampling to ∼15 to 30%. Results from a simulation at coarser resolution showed that the impact of model resolution on air‐sea CO2 flux is only ∼5%. This suggests that the submesoscale variability of pCO2, although large in amplitude, accounts for small modulation of the net air‐sea CO2 flux in this region.
The absolute calibration of the relationship between air‐sea CO2 transfer velocity, k, and wind speed, U, has been a topic of debate for some time, because k global average, 〈k〉, as deduced from ...Geochemical Ocean Sections Study oceanic 14C inventory has differed from that deduced from experimental k‐U relationships. Recently, new oceanic 14C inventories and inversions have lead to a lower 〈k〉. In addition, new measurements performed at sea in high–wind speed conditions have led to new k‐U relationship. Meanwhile, quality and sampling of satellite wind speeds has greatly improved. The QuikSCAT scatterometer has provided high‐quality wind speeds for more than 7 years. This allows us to estimate the global distributions of k computed using k‐U relationships and temperature‐dependent Schmidt numbers from 1999 to 2006. Given the difficulty of measuring in situ wind speed very accurately, we performed a sensitivity study of the 〈k〉 uncertainty which results from QuikSCAT U uncertainties. New QuikSCAT‐buoy U comparisons in the northern Atlantic Ocean and in the Southern Ocean confirm the excellent precision of QuikSCAT U (RMS difference of about 1 m s−1), but it is possible that QuikSCAT overestimates wind speeds by 5%, leading to a possible overestimation of k derived with quadratic relationships by 10%. The 〈k〉 values obtained with two recent experimental k‐U relationships are very close, between 15.9 and 17.9 cm h−1, and within the error bar of k average deduced from the new oceanic 14C inventory.
High-frequency pCO2 and ancillary data were recorded for seven years during the first deployment of a CARbon Interface OCean Atmosphere (CARIOCA) sensor in the surface waters of a temperate coastal ...ecosystem, the Bay of Brest, which is impacted by both coastal (via estuaries) and oceanic (North Atlantic via the Iroise Sea) water inputs. The CARIOCA sensor proved to be an excellent tool to constrain the high pCO2 variability in such dynamic coastal ecosystem. Biological processes (e.g. pelagic photosynthesis/respiration) were the main drivers of the seasonal and diurnal pCO2 dynamics throughout seven years of observations. Autotrophic processes were responsible for abrupt pCO2 drawdown of 100 to 200 mu atm in spring. During the spring bloom, diurnal variations were driven by diel biological cycle. The average daily drawdown due to autotrophy (observed during highest daily PAR) was equivalent to 10 to 60% of the total pCO2 drawdown observed every year during the spring season. From late summer to fall, heterotrophic processes increased pCO2 in the surface water of the Bay back to the pre-bloom level. The average daily increase due to heterotrophy (observed during lowest daily PAR) corresponded to 10 to 70% of the total pCO2 increase observed every year during the late summer to fall period. Air-sea CO2 fluxes estimates based on hourly, daily and monthly calculations showed that careful consideration of the diurnal variability was needed to accurately estimate air-sea CO2 fluxes in the Bay of Brest. Sampling only during daytime or night-time would induce 8 to 36% error on monthly air-sea CO2 fluxes. This would in turn reverse the direction of the fluxes at annual level for the Bay. The annual emissions of CO2 from the surface waters of the Bay to the atmosphere showed relatively low inter-annual variations with an average of +0.7+/-0.4molCm-2yr super(-1) computed for the study period. Further, air-sea CO2 fluxes computed for the adjacent inner-estuaries and Iroise Sea for an annual cycle were +17+/-3molCm-2yr super(-1) and -0.2+/-0.2molCm-2yr super(-1), respectively. The spatial gradient showed a clear pattern from strong source to sink of CO2, from the inner-estuaries to the open oceanic waters of the North Atlantic. We suggest that semi-enclosed Bays act as buffers for sea to air emissions of CO2 from inner estuaries to adjacent costal seas.
We determine the distribution of oceanic CO2 partial pressure (pCO2) with respect to remotely sensed parameters (sea surface temperature (SST) and chlorophyll (Chl)) in order to gain an understanding ...of the small‐scale (10–100 km) pCO2 variability and to estimate the net air–sea CO2 flux in the region (125°E–205°E; 45°S–60°S), which represents 22% of the Southern Ocean area between 45°S and 60°S. We split the study area into several biogeochemical provinces. In chlorophyll‐poor regions, pCO2 is negatively correlated with SST, indicating that pCO2 is mostly controlled by mixing processes. For Chl > 0.37 mg m−3, pCO2 is negatively correlated with Chl, indicating that pCO2 variability is mostly controlled by carbon fixation by biological activity. We deduce fields of pCO2 and of air–sea CO2 fluxes from satellite parameters using pCO2‐SST, pCO2‐chlorophyll relationships and air–sea gas exchange coefficient, K, from satellite wind speed. We estimate an oceanic CO2 sink from December 1997 to December 1998 of −0.08 GtC yr−1 with an error of 0.03 GtC yr−1. This sink is approximately 38% smaller than that computed from the Takahashi et al. (2002) climatological distribution of ΔpCO2 for the 1995 year but with the same K (−0.13 GtC yr−1). When we correct ocean pCO2 for the interannual variability between 1995 and 1998, the difference is even larger, and we cannot reconcile both estimates in February–March and from June to November. This strengthens the need of new in situ measurements for validating extrapolation methods and for improving knowledge of interannual pCO2 variability.
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