Publications

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2016
Resplandy, L, Keeling RF, Stephens BB, Bent JD, Jacobson A, Rodenbeck C, Khatiwala S.  2016.  Constraints on oceanic meridional heat transport from combined measurements of oxygen and carbon. Climate Dynamics. 47:3335-3357.   10.1007/s00382-016-3029-3   AbstractWebsite

Despite its importance to the climate system, the ocean meridional heat transport is still poorly quantified. We identify a strong link between the northern hemisphere deficit in atmospheric potential oxygen (APO = O + 1.1 CO) and the asymmetry in meridional heat transport between northern and southern hemispheres. The recent aircraft observations from the HIPPO campaign reveal a northern APO deficit in the tropospheric column of 10.4 1.0 per meg, double the value at the surface and more representative of large-scale air-sea fluxes. The global northward ocean heat transport asymmetry necessary to explain the observed APO deficit is about 0.7-1.1 PW, which corresponds to the upper range of estimates from hydrographic sections and atmospheric reanalyses.

2015
Rafelski, LE, Paplawsky B, Keeling RF.  2015.  Continuous measurements of dissolved O-2 and oxygen isotopes in the Southern California coastal ocean. Marine Chemistry. 174:94-102.   10.1016/j.marchem.2015.05.011   AbstractWebsite

Dissolved O-2/N-2, O-2/Ar, O-2 saturation and delta O-18 were measured continuously near the surface ocean at the Scripps Institution of Oceanography pier in La Jolla, California, for five weeks. The data showed diurnal cycles, in O-2 and delta O-18, with amplitudes of 19 mmol m(-3) and 1.1%., respectively. The diurnal cycles are well described by a box model that includes photosynthesis, respiration, air-sea gas exchange, and mixing. The timing of the cycles can be explained using a photosynthesis rate proportional to photosynthetically active radiation, and the shapes of the cycles can be explained by mixing with a subsurface layer of water that is supersaturated in O-2. Based on the diurnal cycles in O-2 and delta O-18, the average maximum daily photosynthesis rate was 3.7-4.7 mmol O-2 m(-3) h(-1), which is supported by the light-saturated photosynthesis rate estimated from the measured chlorophyll concentration. In the future, these continuous measurement techniques could be used at different locations and depths to improve the understanding of variability in oceanic primary production. (C) 2015 Elsevier B.V. All rights reserved.

2014
Rodgers, KB, Aumont O, Fletcher SEM, Plancherel Y, Bopp L, Montegut CD, Iudicone D, Keeling RF, Madec G, Wanninkhof R.  2014.  Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring. Biogeosciences. 11:4077-4098.   10.5194/bg-11-4077-2014   AbstractWebsite

Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model, where the objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000-2006, with a relative reduction with wind stirring on the order of 0.9 Pg C yr(-1) over the region south of 45 degrees S. This impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other ocean biogeochemical tracers. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong reduction on the order of 25-30% in the concentrations of NO3 exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry through its impact over the Southern Ocean.

2011
Keeling, RF, Manning AC, Dubey MK.  2011.  The atmospheric signature of carbon capture and storage. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 369:2113-2132.   10.1098/rsta.2011.0016   AbstractWebsite

Compared with other industrial processes, carbon capture and storage (CCS) will have an unusual impact on atmospheric composition by reducing the CO(2) released from fossil-fuel combustion plants, but not reducing the associated O(2) loss. CO(2) that leaks into the air from below-ground CCS sites will also be unusual in lacking the O(2) deficit normally associated with typical land CO(2) sources, such as from combustion or ecosystem exchanges. CCS may also produce distinct isotopic changes in atmospheric CO(2). Using simple models and calculations, we estimate the impact of CCS or leakage on regional atmospheric composition. We also estimate the possible impact on global atmospheric composition, assuming that the technology is widely adopted. Because of its unique signature, CCS may be especially amenable to monitoring, both regionally and globally, using atmospheric observing systems. Measurements of the O(2)/N(2) ratio and the CO(2) concentration in the proximity of a CCS site may allow detection of point leaks of the order of 1000 ton CO(2) yr(-1) from a CCS reservoir up to 1km from the source. Measurements of O(2)/N(2) and CO(2) in background air from a global network may allow quantification of global and hemispheric capture rates from CCS to the order of +/- 0.4 PgCyr(-1).

2001
Garcia, HE, Keeling RF.  2001.  On the global oxygen anomaly and air-sea flux. Journal of Geophysical Research-Oceans. 106:31155-31166.   10.1029/1999jc000200   AbstractWebsite

We present a new climatology of monthly air-sea oxygen fluxes throughout the ice-free surface global ocean. The climatology is based on weighted linear least squares regressions using heat flux monthly anomalies for spatial and temporal interpolation of historical O-2 data. The seasonal oceanic variations show that the tropical belt (20degreesS-20degreesN) is characterized by relatively small air-sea fluxes when compared to the middle to high latitudes (40degrees-70degrees). The largest and lowest seasonal fluxes occur during summer and winter in both hemispheres. By means of an atmospheric transport model we show that our climatology is in better agreement with the observed amplitude and phasing of the variations in atmospheric O-2/N-2 ratios because of seasonal air-sea exchanges at baseline stations in the Pacific Ocean than with previous air-sea O-2 climatologies. Our study indicates that the component of the air-sea O-2 flux that correlates with heat flux dominates the large-scale air-sea O-2 exchange on seasonal timescales. The contribution of each major oceanic basin to the atmospheric observations is described. The seasonal net thermal (SNOT) and biological (SNOB) outgassing components of the flux are examined in relation to latitudinal bands, basin-wide, and hemispheric contributions. The Southern Hemisphere's SNOB (similar to0.26 Pmol) and SNOT (similar to0.29 Pmol) values are larger than the Northern Hemisphere's SNOB (similar to0.15 Pmol) and SNOT (similar to0.16 Pmol) values (1 Pmol = 10(15) mol). We estimate a global extratropical carbon new production during the outgassing season of 3.7 Pg C (1 Pg = 10(15) g), lower than previous estimates with air-sea O-2 climatologies.

2000
Najjar, RG, Keeling RF.  2000.  Mean annual cycle of the air-sea oxygen flux: A global view. Global Biogeochemical Cycles. 14:573-584.   10.1029/1999gb900086   AbstractWebsite

A global monthly-mean climatology of the air-sea oxygen flux is presented and discussed. The climatology is based on the ocean oxygen climatology of Najjar and Keeling [1997] and wind speeds derived from a meteorological analysis center. Seasonal variations are characterized by outgassing of oxygen during spring and summer and ingassing of oxygen during fall and winter, a pattern consistent with thermal and biological forcing of the air-sea oxygen flux. The annual mean flux pattern is characterized by ingassing at high latitudes and the tropics and outgassing in middle latitudes. The air-sea oxygen flux is shown to exhibit patterns that agree well with patterns seen in a marine primary productivity climatology, in model generated air-sea O-2 fluxes, in estimates of remineralization in the shallow aphotic zone based on seasonal oxygen variations, in observed seasonal nutrient-temperature relationships, and in independent estimates of meridional oxygen transport in the Atlantic ocean. We also find that extratropical mixed layer new production during the spring-summer period, computed from biological seasonal net outgassing of oxygen, is equivalent to the production of 4.5-5.6 Gt C, much lower than previous estimates based on atmospheric O-2/N-2 measurements.

1997
Najjar, RG, Keeling RF.  1997.  Analysis of the mean annual cycle of the dissolved oxygen anomaly in the World Ocean. Journal of Marine Research. 55:117-151.   10.1357/0022240973224481   AbstractWebsite

A global climatology of the dissolved oxygen anomaly (the excess over saturation) is created with monthly resolution in the upper 500 m of the ocean. The climatology is based on dissolved oxygen, temperature and salinity data archived at the National Oceanographic Data Center. Examination of this climatology reveals statistically significant annual cycles throughout the upper 500 m of the World Ocean, though seasonal variations are most coherent in the North Atlantic, where data density is greatest. Vertical trends in the phase and amplitude of the annual cycle are noted. The cycle in surface waters is characterized by a summer maximum and a winter minimum, consistent with warming and high rates of photosynthesis during the summer, and cooling and entrainment of oxygen-depleted water during the winter. In low and middle latitudes, the amplitude increases with depth and the maximum occurs later in the year, a trend consistent with the seasonal accumulation of oxygen associated with the shallow oxygen maximum. At a depth that varies between about 30 and 130 m, the phase of the annual cycle undergoes an abrupt shift. We call this depth the oxygen nodal depth. Below the nodal depth, the annual cycle is characterized by an early-spring maximum and a late-fall minimum, consistent with a cycle dominated by respiration during the spring and summer and replenishment of oxygen from the atmosphere by ventilation during the fall and winter. Below the nodal depth, the amplitude of the annual cycle generally decreases with depth, indicative of decreasing respiration and ventilation rates, or less seasonality in both processes. We postulate that the nodal depth in middle and high latitudes corresponds closely to the summertime compensation depth, where photosynthesis and net community respiration are equal. With this interpretation of the nodal depth and a simple model of the penetration of light in the water column, a compensation light intensity of 1 W m(-2) (4 mu E m(-2) s(-1)) is deduced, at the low end of independent estimates. Horizontal trends in the phase and amplitude of the annual cycle are also noted. We find that the nodal depth decreases toward the poles in both hemispheres and is generally greater in the Southern Hemisphere, patterns found to be consistent with light-based estimates of the compensation depth. The amplitude of the annual cycle in the oxygen anomaly increases monotonically with latitude, and higher latitudes lag lower latitudes. In the North Atlantic and North Pacific, the amplitude of the annual cycle tends to increase from east to west at all depths and latitudes, as expected considering that physical forcing has greater seasonal variability in the west. The tropics and the North Indian Ocean have features that distinguish them from other regions. Below about 75 m, these waters have pronounced annual cycles of the oxygen anomaly that areshown to be caused mainly by wind-driven adiabatic displacements of the thermocline. A semiannual cycle of the oxygen anomaly is found in the surface waters of the North Indian Ocean, consistent with the known semiannual cycle of surface heat flux in this region.

1995
Keeling, RF, Peng TH.  1995.  Transport of heat, CO2 and O2 by the Atlantic's thermohaline circulation. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences. 348:133-142.   10.1098/rstb.1995.0055   AbstractWebsite

We estimate transport of heat, CO2 and O-2 by the Atlantic's thermohaline circulation using an approach based on differences in the chemical and physical characteristics of North Atlantic Deep Water (NADW), Antarctic Intermediate Water (AAIW), and the northward return flow across the equator. The characteristics of the return-flow waters are constrained by imposing conservation of phosphate in the North Atlantic as a whole. Based on a total equatorial return flow of 13 x 10(6) m(3) s(-1), we find that the Atlantic north of the equator is a source of 7.7 +/- 1.4 x 10(14) W to the atmosphere, a sink of 0.51 +/- 0.21 x 10(14) mol of O-2, and preindustrially was a sink of 0.33 +/- 0.15 x 10(14) mol of CO2. Uptake of O-2 and CO2 by the North Atlantic is driven mainly by thermal, as opposed to biological processes.