Takamitsu Ito

Preformed phosphate, soft tissue pump and atmospheric CO 2

We develop a new theory relating atmospheric pCO2 and the efficiency of the soft tissue pump of CO2 in the ocean, measured by P*, a quasi-conservative tracer. P* is inversely correlated with preformed phosphate, and its global average represents the fraction of nutrients transported by the export and remineralization of organic material. This view is combined with global conservation constraints for carbon and nutrients leading to a theoretical prediction for the sensitivity of atmospheric pCO2 to changes in globally averaged P*. The theory is supported by sensitivity studies with a more complex, three-dimensional numerical simulations. The numerical experiments suggest that the ocean carbon cycle is unlikely to approach the theoretical limit where globally averaged P* 1 (complete depletion of preformed phosphate) because the localized dynamics of deep water formation, which may be associated with rapid vertical mixing timescales, preclude the ventilation of strongly nutrient-depleted waters. Hence, in the large volume of the deep waters of the ocean, it is difficult to significantly reduce preformed nutrient (or increase P*) by increasing the efficiency of export production. This mechanism could ultimately control the efficiency of biological pumps in a climate with increased aeolian iron sources to the Southern Ocean. Using these concepts we can reconcile qualitative differences in the response of atmospheric pCO2 to surface nutrient draw down in highly idealized box models and more complex, general circulation models. We suggest that studies of carbon cycle dynamics in regions of deep water formation are the key to understanding the sensitivity of atmospheric pCO2 to biological pumps in the ocean.

Is AOU a good measure of respiration in the oceans

AOU ¼ O2;satO2 ð1Þ where O2,sat depends on potential temperature, T, and salinity, S. Estimates of respiration inferred from AOU could contain errors due to several processes as pointed out by Shiller (1981), Broecker and Takahashi (1985) and Broecker et al. (1991), which include air-sea disequilibrium in the regions of water mass formation, non-linearity in the solubility of oxygen, and respiration involving denitrifica- tion. The impact of denitrification can be diagnosed from the distribution of N* (Gruber and Sarmiento, 1997; Sabine et al., 1999) whereas the impact of the other effects have not been well-understood nor quantified. In this study, we explicitly determine the decoupling of AOU from respira- tion in a 3-dimensional physical-biogeochemical model. (3) True Oxygen Utilization (TOU) (Broecker and Peng, 1982) is defined as the difference between the preformed oxygen content, O2,pre, and the observed oxygen content. TOU ¼ O2;preO2 ð2Þ DO2 ¼ TOUAOU ¼ O2;preO2;sat ð3Þ Preformed oxygen is set at the time of subduction and is transported into the interior ocean by physical circulation. Thus the preformed properties typically represent the mixed layer properties during winter seasons. Differences between AOU and TOU can arise from the surface disequilibrium at the time of subduction and water mass formation, and from mixing in the interior ocean combined with the nonlinearity in the solubility of oxygen. DO2 is exactly equal to the degree of saturation at the surface; see equation (3). DO2 is mapped into the interior ocean through advection and mixing. While O2,sat can be determined from observed temperature and salinity, it is impossible to determine DO2 from direct observation since the true preformed oxygen content remains unknown.

What controls the uptake of transient tracers in the Southern Ocean

CFC-11, bomb-D 14 C, and anthropogenic CO2. The model reproduces the observed relationship between CFC-11 and bomb-D 14 C and suggests that the upper branch of the residual overturning flow in the Southern Ocean is about 14 Sv, supporting previous inferences based on the observed buoyancy distribution and air-sea buoyancy fluxes. Scale analysis suggests that the limit of fast air-sea gas exchange is applicable to CFC-11, for which the surface concentration is close to equilibrium and cumulative ocean uptake is largely determined by physical transport processes. In the slow gas exchange limit, applicable to bomb-D 14 C, the surface concentration is far from equilibrium and the cumulative uptake is most sensitive to the parameterization of the gas transfer coefficient. Anthropogenic CO2 falls between those two limit cases and is sensitive to both transport processes and the gas transfer coefficient. Sensitivity studies using the streamlineaveraged model suggest that uncertainties in air-sea buoyancy fluxes in current climatologies result in significant uncertainty in estimates of Southern Ocean uptake of anthropogenic CO2 based on circulation and biogeochemistry models driven by, or brought into consistency with, the climatological fluxes. This uncertainty is sufficient to explain a significant amount of the spread in a recent model comparison study. INDEX TERMS: 4808 Oceanography: Biological and Chemical: Chemical tracers; 4806 Oceanography: Biological and Chemical: Carbon cycling; 4842 Oceanography: Biological and Chemical: Modeling; KEYWORDS: residual mean theory, Southern Ocean, transient tracers

The future evolution of the Southern Ocean CO 2 sink

We investigate the impact of century-scale climate changes on the Southern Ocean CO2 sink using an idealized ocean general circulation and biogeochemical model. The simulations are executed under both constant and changing wind stress, freshwater fluxes, and atmospheric pCO2, so as to separately analyze changes in natural and anthropogenic CO2 fluxes under increasing wind stress and stratification. We find that the Southern Ocean sink for total contemporary CO2 is weaker under increased wind stress and stratification by 2100, relative to a control run with no change in physical forcing, although the results are sensitive to the magnitude of the imposed physical changes and the rate of increase of atmospheric pCO2. The air-sea fluxes of both natural and anthropogenic CO2 are sensitive to the surface concentration of dissolved inorganic carbon (DIC) which responds to perturbations in wind stress and stratification differently. Spatially averaged surface DIC scales linearly with wind stress, primarily driven by changes in the Ekman transport. In contrast, changes in the stratification cause non-linear and more complex responses in spatially averaged surface DIC, involving shifts in the location of isopycnal outcrop for deep and thermocline waters. Thus, it is likely that both wind stress and stratification changes will influence the strength of the Southern Ocean CO2 sink in the coming century.

The transient response of the Southern Ocean pycnocline to changing atmospheric winds

[1] The vertical density structure of the Southern Ocean is dynamically linked to wind stress at the surface, but the nature of this coupling is not fully understood. Observations from the last several decades show a significant increase in the strength of westerly winds over the Southern Ocean, but an appreciable change in the tilt of constant density surfaces (isopycnals) has not yet been detected there. Using a combination of theory and idealized numerical simulations, we show that the response of the density structure occurs on centennial timescales, making it difficult to detect significant changes with a few decades of hydrographic observations. Dynamic coupling between the circumpolar current and northern basins regulates the slow adjustment of the density structure. Our results provide a new interpretation for recent observations and highlight the importance of the interaction between regional Southern Ocean dynamics and global ocean circulation. Citation: Jones, D. C., T. Ito, andN.S.Lovenduski(2011),Thetransientresponseofthe Southern Ocean pycnocline to changing atmospheric winds, Geophys. Res. Lett., 38, L15604, doi:10.1029/2011GL048145.

Upper ocean control on the solubility pump of CO 2

We develop and test a theory for the relationship of atmospheric pCO2 and the solubility pump of CO2 in an abiotic ocean. The solubility pump depends on the hydrographic structure of the ocean and the degree of saturation of the waters. The depth of thermocline sets the relative volume of warm and cold waters, which sets the mean solubility of CO2 in the ocean. The degree of saturation depends on the surface residence time of the waters. We develop a theory describing how atmospheric CO2 varies with diapycnal diffusivity and wind stress in a simple, coupled atmosphere-ocean carbon cycle, which builds on established thermocline theory. We consider two limit cases for thermocline circulation: the diffusive thermocline and the ventilated thermocline. In the limit of a purely diffusive thermocline (no wind-driven gyres), atmospheric pCO2 increases in proportion to the depth of thermocline which scales as 1/3 , where is the diapycnal mixing rate coefficient. In the wind-driven, ventilated thermocline limit, the ventilated thermocline theory suggests the thickness of the thermocline varies as wek . Moreover, surface residence times are shorter, and subducted waters are undersaturated. The degree of undersaturation is proportional to the Ekman pumping rate, wek, for moderate amplitudes of wek. Hence, atmospheric pCO2 varies as wek for moderate ranges of surface wind stress. Numerical experiments with an ocean circulation and abiotic carbon cycle model confirm these limit case scalings and illustrate their combined effect. The numerical experiments suggest that plausible variations in the wind forcing and diapycnal diffusivity could lead to changes in atmospheric pCO2 of as much as 30 ppmv. The deep ocean carbon reservoir is insensitive to changes in the wind, due to compensation between the degree of saturation and the equilibrium carbon concentration. Consequently, the sensitivity of atmospheric pCO2 to wind-stress forcing is dominated by the changes in the upper ocean, in direct contrast to the sensitivity to surface properties, such as temperature and alkalinity, which is controlled by the deep ocean reservoir.

Spatial and seasonal variability of the air-sea equilibration timescale of carbon dioxide

The exchange of carbon dioxide between the ocean and the atmosphere tends to bring waters within the mixed layer toward equilibrium by reducing the partial pressure gradient across the air-water interface. However, the equilibration process is not instantaneous; in general, there is a lag between forcing and response. The timescale of air-sea equilibration depends on several factors involving the depth of the mixed layer, wind speed, and carbonate chemistry. We use a suite of observational data sets to generate climatological and seasonal composite maps of the air-sea equilibration timescale. The relaxation timescale exhibits considerable spatial and seasonal variations that are largely set by changes in mixed layer depth and wind speed. The net effect is dominated by the mixed layer depth; the gas exchange velocity and carbonate chemistry parameters only provide partial compensation. Broadly speaking, the adjustment timescale tends to increase with latitude. We compare the observationally derived air-sea gas exchange timescale with a model-derived surface residence time and a data-derived horizontal transport timescale, which allows us to define two nondimensional metrics of equilibration efficiency. These parameters highlight the tropics, subtropics, and northern North Atlantic as regions of inefficient air-sea equilibration where carbon anomalies are relatively likely to persist. The efficiency parameters presented here can serve as simple tools for understanding the large-scale persistence of air-sea disequilibrium of CO2 in both observations and models.

Terrestrial and marine perspectives on modeling organic matter degradation pathways

Organic matter (OM) plays a major role in both terrestrial and oceanic biogeochemical cycles. The amount of carbon stored in these systems is far greater than that of carbon dioxide (CO2 ) in the atmosphere, and annual fluxes of CO2 from these pools to the atmosphere exceed those from fossil fuel combustion. Understanding the processes that determine the fate of detrital material is important for predicting the effects that climate change will have on feedbacks to the global carbon cycle. However, Earth System Models (ESMs) typically utilize very simple formulations of processes affecting the mineralization and storage of detrital OM. Recent changes in our view of the nature of this material and the factors controlling its transformation have yet to find their way into models. In this review, we highlight the current understanding of the role and cycling of detrital OM in terrestrial and marine systems and examine how this pool of material is represented in ESMs. We include a discussion of the different mineralization pathways available as organic matter moves from soils, through inland waters to coastal systems and ultimately into open ocean environments. We argue that there is strong commonality between aspects of OM transformation in both terrestrial and marine systems and that our respective scientific communities would benefit from closer collaboration.

Sustained growth of the Southern Ocean carbon storage in a warming climate

We investigate the mechanisms controlling the evolution of Southern Ocean carbon storage under a future climate warming scenario. A subset of Coupled Model Intercomparison Project Phase 5 models predicts that the inventory of biologically sequestered carbon south of 40°S increases about 18–34 Pg C by 2100 relative to the preindustrial condition. Sensitivity experiments with an ocean circulation and biogeochemistry model illustrates the impacts of the wind and buoyancy forcings under a warming climate. Intensified and poleward shifted westerly wind strengthens the upper overturning circulation, not only leading to an increased uptake of anthropogenic CO2 but also releasing biologically regenerated carbon to the atmosphere. Freshening of Antarctic Surface Water causes a slowdown of the lower overturning circulation, leading to an increased Southern Ocean biological carbon storage. The rectified effect of these processes operating together is the sustained growth of the carbon storage in the Southern Ocean, even under the warming climate with a weaker global ocean carbon uptake.

The formation of the ocean's anthropogenic carbon reservoir

The shallow overturning circulation of the oceans transports heat from the tropics to the mid-latitudes. This overturning also influences the uptake and storage of anthropogenic carbon (Cant). We demonstrate this by quantifying the relative importance of ocean thermodynamics, circulation and biogeochemistry in a global biochemistry and circulation model. Almost 2/3 of the Cant ocean uptake enters via gas exchange in waters that are lighter than the base of the ventilated thermocline. However, almost 2/3 of the excess Cant is stored below the thermocline. Our analysis shows that subtropical waters are a dominant component in the formation of subpolar waters and that these water masses essentially form a common Cant reservoir. This new method developed and presented here is intrinsically Lagrangian, as it by construction only considers the velocity or transport of waters across isopycnals. More generally, our approach provides an integral framework for linking ocean thermodynamics with biogeochemistry.