Interannual variability of primary production and air-sea CO2 flux in the Atlantic and Indian sectors of the Southern Ocean (original) (raw)
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Journal of Marine Systems, 2014
Keywords: Southern Ocean Surface pCO 2 seasonality Autonomous drifters Productivity Satellite chlorophyll Air-Sea CO 2 exchange Upper ocean and mixed layer processes On a mean annual basis, the Southern Ocean is a sink for atmospheric CO 2 . However the seasonality of the air-sea CO 2 flux in this region is poorly documented. We investigate processes regulating air-sea CO 2 flux in a large area of the Southern Ocean (38°S-55°S, 60°W-60°E) that represents nearly one third of the subantarctic zone. A seasonal budget of CO 2 partial pressure, pCO 2 and of dissolved inorganic carbon, DIC in the mixed layer is assessed by quantifying the impacts of biology, physics and thermodynamical effect on seawater pCO 2 . 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 130 mmol C m −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 ≈2-3, which is an underestimation previously reported for the Southern Ocean. Monthly satellite based NPP are computed over the 38°S-55°S, 60°W-60°E area. pCO 2 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 pCO 2 seasonal climatology of Takahashi (2009). On an annual timescale, mean NCP values, 4.4 to 4.9 mol C m −2 yr −1 are ≈4-5 times greater than air-sea CO 2 invasion, 1.0 mol C m −2 yr −1 . Our study based on in situ and satellite observations provides a quantitative estimate of both seasonal and mean annual uptake of CO 2 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 CO 2 fluxes.
Interannual sea–air CO2 flux variability from an observation-driven ocean mixed-layer scheme
Biogeosciences, 2014
Interannual anomalies in the sea-air carbon dioxide (CO 2) exchange have been estimated from surface-ocean CO 2 partial pressure measurements. Available data are sufficient to constrain these anomalies in large parts of the tropical and North Pacific and in the North Atlantic, in some areas covering the period from the mid 1980s to 2011. Global interannual variability is estimated as about 0.31 Pg C yr −1 (temporal standard deviation 1993-2008). The tropical Pacific accounts for a large fraction of this global variability, closely tied to El Niño-Southern Oscillation (ENSO). Anomalies occur more than 6 months later in the east than in the west. The estimated amplitude and ENSO response are roughly consistent with independent information from atmospheric oxygen data. This both supports the variability estimated from surface-ocean carbon data and demonstrates the potential of the atmospheric oxygen signal to constrain ocean biogeochemical processes. The ocean variability estimated from surface-ocean carbon data can be used to improve land CO 2 flux estimates from atmospheric inversions.
An empirical estimate of the Southern Ocean air-sea CO2 flux
Global Biogeochemical Cycles, 2007
1] Despite improvements in our understanding of the Southern Ocean air-sea flux of CO 2 , discrepancies still exist between a variety of differing ocean/atmosphere methodologies. Here we employ an independent method to estimate the Southern Ocean air-sea flux of CO 2 that exploits all available surface ocean measurements for dissolved inorganic carbon (DIC) and total alkalinity (ALK) beyond 1986. The DIC concentrations were normalized to the year 1995 using coinciding CFC measurements in order to account for the anthropogenic CO 2 signal. We show that independent of season, surface-normalized DIC and ALK can be empirically predicted to within $8 mmol/kg using standard hydrographic properties. The predictive equations were used in conjunction with World Ocean Atlas (2001) climatologies to give a first estimate of the annual cycle of DIC and ALK in the surface Southern Ocean. These seasonal distributions will be very useful in both validating biogeochemistry in general circulation models and for use in situ biological studies within the Southern Ocean. Using optimal CO 2 dissociation constants, we then estimate an annual cycle of pCO 2 and associated net air-sea CO 2 flux. Including the effects of sea ice, we estimate a Southern Ocean (>50°S) CO 2 sink of 0.4 ± 0.25 Pg C/yr. Our analysis also indicates a substantial CO 2 sink of 1.1 ± 0.6 Pg C/yr within the sub-Antarctic zone (40°S-50°S), associated with strong cooling and high winds. Our results imply the Southern Ocean CO 2 flux south of 50°S to be very similar to those found by , but on the higher end of a range of atmospheric/oceanic CO 2 inversion methodologies. This paper estimates for the first time basic seasonal carbon cycle parameters within the circumpolar Southern Ocean, which have up to now been extremely difficult to measure and sparse. The application of such an empirical technique using more widely available hydrographic parameters in the Southern Ocean provides an important independent estimate to not only CO 2 uptake, but also for other future biogeochemical studies. Refining and testing these empirical methods with new carbon measurements will be important to further reduce uncertainties and extend our understanding of Southern Ocean CO 2 dynamics.
Interannual sea–air CO2 flux variability from an observation-driven ocean mixed-layer scheme
Interannual anomalies in the sea-air carbon dioxide (CO 2 ) exchange have been estimated from surface-ocean CO 2 partial pressure measurements. Available data are sufficient to constrain these anomalies in large parts of the tropical and North Pacific and in the North Atlantic, in some areas covering the period from the mid 1980s to 2011. Global interannual variability is estimated as about 0.31 Pg C yr −1 (temporal standard deviation 1993-2008). The tropical Pacific accounts for a large fraction of this global variability, closely tied to El Niño-Southern Oscillation (ENSO). Anomalies occur more than 6 months later in the east than in the west. The estimated amplitude and ENSO response are roughly consistent with independent information from atmospheric oxygen data. This both supports the variability estimated from surface-ocean carbon data and demonstrates the potential of the atmospheric oxygen signal to constrain ocean biogeochemical processes. The ocean variability estimated from surface-ocean carbon data can be used to improve land CO 2 flux estimates from atmospheric inversions. C. Rödenbeck et al.: Interannual sea-air CO 2 flux variations r) r)
Quantifying the drivers of ocean-atmosphere CO2 fluxes
A mechanistic framework for quantitatively mapping the regional drivers of air-sea CO 2 fluxes at a global scale is developed. The framework evaluates the interplay between (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon-and nutrient-rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse-resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO 2 fluxes driven by surface heat fluxes and a combination of biologically driven carbon uptake and disequilibrium-driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real-world air-sea flux of CO 2 .
Ocean Science, 2013
A temporally and spatially resolved estimate of the global surface-ocean CO 2 partial pressure field and the sea-air CO 2 flux is presented, obtained by fitting a simple data-driven diagnostic model of ocean mixed-layer biogeochemistry to surface-ocean CO 2 partial pressure data from the SOCAT v1.5 database. Results include seasonal, interannual, and short-term (daily) variations. In most regions, estimated seasonality is well constrained from the data, and compares well to the widely used monthly climatology by Takahashi et al. (2009). Comparison to independent data tentatively supports the slightly higher seasonal variations in our estimates in some areas. We also fitted the diagnostic model to atmospheric CO 2 data. The results of this are less robust, but in those areas where atmospheric signals are not strongly influenced by land flux variability, their seasonality is nevertheless consistent with the results based on surface-ocean data. From a comparison with an independent seasonal climatology of surface-ocean nutrient concentration, the diagnostic model is shown to capture relevant surface-ocean biogeochemical processes reasonably well. Estimated interannual variations will be presented and discussed in a companion paper. Ocean Sci., 9, 193-216, 2013 www.ocean-sci.net/9/193/2013/ C. Rödenbeck et al.: Global surface-ocean p CO 2 and sea-air CO 2 flux variability 195 22 C. Rödenbeck et al.: Sea-air CO 2 flux estimated from CO 2 partial pressure data Atm. mixing ratio [ppm]:
Effect on atmospheric CO2 from seasonal variations in the high latitude ocean
Advances in Space Research, 1989
Data from the North Pacific gyre, Bering Sea, and North Atlantic show large seasonal fluctuations in the pCO2 of surface waters. The seasonal variation in these high latitudes apparentlyhas a generic pattern: higher surface water pCO2 in winter and lower in summer. Satellite data will eventually help decipher the relative effects of temperature and biological production in the seasonal carbon cycle, but as yet little work has been done on what possible role the seasonality ofpCO~in the high latitudes might have on the average value of atmospheric pCO2. Here I develop a model that shows the average value for atmospheric pCO2 depends upon the ratio of the rates at which the ocean/atmosphere system moves toward equilibrium values during the summer and winter conditions of the high latitude ocean. SEAS ONAL1TY OF PCO2 IN THE HIGH LATITUDE OCEAN SURFACES Data for two sites off Iceland in the north Atlantic shows an important seasonal pattern in many properties of the surface waters /1/. The pCO2 is lower in the summer than in the winter, which is the opposite the trend expected if the surface temperature, being relatively high in summer, were the determining factor. The nutrients, such as phosphate and nitrate, are relatively low in the summer months, apparently a result of photosynthesis, which removes C02 from the surface water as well as nutrients. Higher nutrients in winter are due to a lack of photosynthesis and increased exchange between surface and deep waters. This cycle of nutrients corresponds to a cycle in the organic matter, which controls the seasonality of surface pCO2. This cycle has been modeled by Peng etal.12/.