OS24A-01 INVITED
Sources and Sinks of Atmospheric CO2
Between 1960 and 2006, the ocean took up ~33% of the cumulative fossil fuel emissions of 251 Pg C and increases in atmospheric CO2 concentration accounted for ~56%. The remaining ~11% of fossil fuel emissions were taken up by the terrestrial biosphere despite emissions from deforestation occurring mostly in the tropics. Around 1990/91, the global carbon cycle appears to have undergone a major shift. The atmospheric growth rate decelerated from an average of 58.4 ' 1.8% of fossil fuel emissions prior to 1990, to 52.1% thereafter. Furthermore, ocean carbon model simulations suggest that the oceanic uptake leveled off after ~1990 instead of increasing as expected. Taken together, this implies an increase in the net uptake by the terrestrial biosphere of ~0.9 Pg C yr-1. While the impact of the 1991 Mt. Pinatubo eruption can account for about one-third of this increase, we cannot explain the remainder. Estimates of the regional distribution of sources and sinks over the oceans have improved greatly in the last decade, but remain a vexing challenge for the land biosphere. The difficulty in accounting for land sources and sinks and the associated uncertainty with regard to mechanisms leads to major uncertainties in the future behavior of the global carbon sinks, with major implications for climate policy.
OS24A-02
Observational and Model Estimates of Decadal-scale Changes in Anthropogenic Carbon in the Atlantic and Pacific Oceans
Dissolved inorganic carbon (DIC) in the upper ocean is increasing over time due to the invasion of anthropogenic CO2 from the atmosphere. The CLIVAR/CO2 Repeat Hydrography Program is attempting to quantifying these trends by reoccupying on approximately decadal time-scales ocean sections that were first sampled during the WOCE/JGOFS era in the late 1980s and 1990s. Direct point to point comparisons are strongly aliased by interannual variability, and we use multiple linear regression techniques to isolate the anthropogenic carbon signal. We show field-based estimates of decadal changes in DIC for meridional sections (about 60°N to 60°S) in the Atlantic and Pacific basins (A16 and P16). The field estimates are also compared with historical hindcast simulation results from the Community Climate System Model (CCSM) ocean biogeochemical model.
OS24A-03 INVITED
The Indian Ocean's Role in Ocean Carbon Uptake Over the Last Decade
An extensive survey of full water column inorganic carbon distributions in the Indian Ocean was conducted by
the US in 1995 as part of the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux
Study (JGOFS). These data provided a baseline of carbon distributions in the Indian Ocean. In an effort to
study how the ocean uptake of CO2 may be changing as a response to climate change, the US
CLIVAR/CO2 Repeat Hydrography Program has been reoccupying some of the cruises initially
surveyed as part of the WOCE/JGOFS effort. The first Indian Ocean Repeat Hydrography cruise, I8S/I9N
nominally along 90°E, was conducted in 2007. We will discuss approaches for separating the
anthropogenic CO2 signals from variations in local circulation and biology using an extended multiple
linear regression (eMLR) analysis on the I8S/I9N line. Column inventory changes along the section range
from 0.1 to 1 mol C m-2yr-1. We will examine the inventory change patterns and discuss the
mechanisms leading to these patterns. We will also discuss the Indian Ocean changes relative to changes
observed in the Atlantic and Pacific. Preliminary estimates suggest that the relative contributions of the three
ocean basins to the global uptake of atmospheric CO2 was different over this last decade compared to
the long-term carbon storage patterns.
http://ushydro.ucsd.edu/
OS24A-04 INVITED
Testing model estimated decrease in Southern Ocean carbon sink with data over the last 20 years
OS24A-05
Using CFCs and Sulfur Hexafluoride to Improve Estimates of Ventilation Rate Changes and Anthropogenic CO2 Uptake Along CLIVAR Repeat Hydrography Sections
A number of key hydrographic sections sampled in the 1990s as part of the World Ocean Circulation Experiment (WOCE) are being re-occupied at approximately decadal intervals as part of the CLIVAR Repeat Hydrography Program. Measurements of a number of physical and chemical properties are made at full depth, closely spaced (nominally 30 nautical mile) CTD/rosette stations, with water samples collected at between 24 and 36 depths per station. Among the central goals of the program are the detection of changes in ventilation, carbon uptake and storage, dissolved oxygen and water properties on decadal time scales. Repeat measurements of dissolved chlorofluorocarbon (CFC) CFC-11 and CFC-12 concentrations show significant decadal increases. Water mass ages derived from CFCs (pCFC ages) also show substantial changes (typically increases) with time along the repeat sections. Simple models indicate that much of the observed pCFC age increases are due to the impacts of mixing in the ocean interior. Measurements of sulfur hexafluoride (SF6), a transient tracer that has been rapidly increasing in the atmosphere during the past several decades, have been included along with CFCs on some recent CLIVAR repeat sections. Because the atmospheric history of SF6 differs substantially from that of the CFCs, concurrent SF6 and CFC measurements can be used to help diagnose the impacts of mixing on pCFC ages and on decadal changes in pCFC ages. We are exploiting this twin-tracer strategy in an attempt to improve estimates of ventilation rate changes and anthropogenic CO2 uptake rates along the CLIVAR repeat sections.
OS24A-06
Ocean ventilation as a driver of interannual variability in atmospheric potential oxygen
Atmospheric measurements of O2 and CO2 have shown great utility in separating terrestrial and oceanic sinks for anthropogenic CO2 on decadal timescales. However, unexplained variability at 2-5 year timescales has hampered efforts to constrain these sinks on shorter timescales. We present observations of interannual variations in atmospheric O2 that have no counterpart in atmospheric CO2 records, highlighted by the tracer atmospheric potential oxygen (APO~O2+CO2). We expect air-sea fluxes to be responsible for APO variations, because APO is insensitive to exchanges with the terrestrial biosphere since these affect O2 and CO2 in opposite ways. In records from Northern Hemisphere stations in the Scripps Institution of Oceanography global flask sampling network, increasing APO in the late 1990s was followed by an abrupt drawdown in 2000-01. The timing of the drawdown matches a renewal of deep-water formation in the North Atlantic in the winter of 1999-2000, followed the next year by a severe winter in the North Pacific. These ventilation events may have allowed deeper isopycnals than in previous years to contact the surface and create a strong flux of O2 from the atmosphere to the ocean, decreasing APO signals. Variations in the APO anomaly and in the interhemispheric APO difference show only weak correlations to El Niño indices, estimates of global primary productivity, or ocean heat content, suggesting that these processes are only a secondary influence over APO at interannual timescales. We conclude that the evidence points to variability in ocean ventilation as the main driver of interannual variability in APO and that this tracer may yield insight into the timing of oceanic O2 changes.
OS24A-07
The Changing North Atlantic Carbon Sink: 1992-2006
An ocean biogeochemical model is used to assess the impact of climate variability from 1992-2006 on air-sea
CO2 fluxes and ocean surface pCO2 in the North Atlantic, following evaluation against a variety of in situ
observations. Consistent with data analyses, the model suggests substantial changes in the magnitude of the
carbon sink throughout the basin. We investigate the mechanisms that dominate the spatial variability and
magnitude of the trends in the air-sea fluxes and pCO2. Subpolar trends are driven dynamically, primarily
through changing vertical supply of DIC. Subtropical trends are controlled primarily by changes in sea
surface temperature. Changing North Atlantic forcing and circulation sets the spatial pattern of the trend,
while rising atmospheric pCO2 drives the net increase in the CO2 sink over time.
http://www.aos.wisc.edu/~galen/research_natlantic.html
OS24A-08
The 2008 North Atlantic Spring Bloom Experiment I: Overview and Strategy
The 2008 North Atlantic Spring Bloom Experiment (NAB08) aimed to understand carbon export from this globally important event by combining a new generation of autonomous floats and gliders equipped with a new generation of sensors, and traditional and modern shipboard observational methods. Measurements were made from early April to late June 2008 in a region southeast of Iceland near the JGOFS and MLML sites. Although Sverdrup's classical explanation for the bloom is probably broadly correct, previous observations have revealed a large degree of spatial and temporal variability, often on scales of a few kilometers, which have made detailed tests of Sverdrup's hypothesis difficult. The experiment was designed to continuously sample the bloom and its temporal and spatial 'patchiness' from the pre-bloom, wintertime conditions through the Spring and early Summer. The spatial scales were sampled by 4 Seagliders operating together as a mobile array. Measurements were made in a Lagrangian, water-following coordinate system which minimized the effects of horizontal advection and most clearly separated temporal and spatial scales. The coordinate system was defined by two Lagrangian Floats, one of which was chosen as the center of the Seaglider array. Proper measurement of the bloom by the autonomous vehicles required a robust and redundant array of sensors measuring key physical, chemical and biological variables including temperature, salinity, spectral light, oxygen, multiple optical proxies for carbon (chlorophyll fluorescence, beam-c attenuation and optical backscatter coefficients) and nitrate. Redundant measurements were made whenever possible, with nearly identical sensors on many platforms and multiple sensors measuring similar quantities on the same platform. Such care is clearly necessary, since the current generation of biogeochemical sensor require considerable efforts in calibration and interpretation. The autonomous platforms provided good coverage in space and time, but could not sample the entire range of processes that control the bloom. More detailed measurements and multiple calibrations of the autonomous platforms were made on 4 cruises, particularly a 21-day Knorr cruise in May 2008 that included collaborators from five US and five international institutions. These measurements included nutrients; particulate organic carbon and nitrogen; characterization of plankton composition and physiology by size, imaging, genomics, HPLC pigments, absorption spectra, 14C-primary productivity, and variable fluorescence; particle flux from floating sediment traps; and ADCP and CTD measurements. The experiment clearly demonstrated the ability of autonomous platforms to make biogeochemically relevant measurements of blooms. Its success, however, required intensive shipboard support for sensor calibration and interpretation. Further development of sensor technology, validation protocols, and understanding is clearly required if these measurements are to made routinely and easily.