Linking glacial-interglacial states to multiple equilibria of climate

Glacial-Interglacial cycles are often described as an amplified global response of the climate to perturbations in solar radiation caused by oscillations of Earth's orbit. However, it remains unclear whether internal feedbacks are large enough to account for the radically different Glacial and Interglacial states. Here we provide support for an alternative view: Glacial-Interglacial states are multiple equilibria of the climate system which exist for the same external forcing. We show that such multiple equilibria resembling Glacial and Interglacial states can be found in a complex coupled General Circulation Model of the ocean-atmosphere-sea ice system. The multiple states are sustained by ice-albedo feedback modified by ocean heat transport and are not caused by the bi-stability of the ocean's overturning circulation. In addition, expansion/contraction of the Southern Hemisphere ice pack over regions of upwelling, regulating outgassing of CO$_2$ to the atmosphere, is the primary mechanism behind a large pCO$_2$ change between states

Gaussian mixture modeling describes the geography of the surface ocean carbon budget.

Abstract—We use an unsupervised classification technique (i.e. Gaussian mixture modeling or GMM) to identify ocean regions with similar balances between processes that determine the surface budget of dissolved inorganic carbon. GMM objectively locates sub-populations in the distribution of carbon budget terms. We use a simple four-class description and find regimes that are broadly consistent with classical theoretical frameworks. Class 1 covers 24% of ocean surface area and corresponds to highly productive areas with strong vertical mixing, wind-driven open ocean upwelling, and absorption of atmospheric carbon dioxide. Class 2 covers 8% of ocean surface area and corresponds to regions of especially weak productivity. Class 3 covers 16% of ocean surface area and corresponds to wind-driven coastal and equatorial upwelling. Finally, class 4 covers the remaining 52% of ocean surface area and corresponds to the relatively unproductive subtropical gyres, which are typically characterized by downwelling and low surface nutrient concentrations. We argue that GMM may be a useful method for comparing biogeochemical regimes between climate models

Preformed phosphate, soft tissue pump and atmospheric CO2

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

The future evolution of the Southern Ocean CO2 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

Upper ocean control on the solubility pump of CO2

We develop and test a theory for the relationship of atmospheric p CO2 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 κ1/3, where κ 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 wek1/2. 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 wek3/2 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

Optimally Interpolated O2 anomalies based on World Ocean Database 2018

Dataset: OIO2-WOD2018OIO2 is a gridded data product of dissolved oxygen interpolated from shipboard observations archived in the World Ocean Database 2018 (WOD18). The quality-controlled WOD18 data are averaged for each bin at 1°x1° and monthly resolution where mean, variance, and sample size are recorded from 1965 to 2014 for the bottle data, and from 1987 to 2014 for the CTD-O2 data. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/886218NSF Division of Ocean Sciences (NSF OCE) OCE-212354

The biogeochemistry and residual mean circulation of the southern ocean

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2005.Includes bibliographical references (p. 233-244).I develop conceptual models of the biogeochemistry and physical circulation of the Southern Ocean in order to study the air-sea fluxes of trace gases and biological productivity and their potential changes over glacial-interglacial timescales. Mesoscale eddy transfers play a dominant role in the dynamical and tracer balances in the Antarctic Circumpolar Current, and the transport of tracers is driven by the residual mean circulation which is the net effect of the Eulerian mean circulation and the eddy-induced circulation. Using an idealized, zonally averaged model of the ACC, I illustrate the sensitivity of the uptake of transient tracers including CFC11, bomb-[Delta]¹⁴C and anthropogenic CO₂ to surface wind stress and buoyancy fluxes over the Southern Ocean. The model qualitatively reproduces observed distribution of CFC11 and bomb-[Delta]¹⁴C , and a suite of sensitivity experiments illustrate the physical processes controlling the rates of the oceanic uptake of these tracers. The sensitivities of the uptake of CFC11 and bomb-[Delta]¹⁴C are largely different because of the differences in their air-sea equilibration timescales. The uptake of CFC11 is mainly determined by the rates of physical transport in the ocean, and that of bomb-[Delta]¹⁴C is mainly controlled by the air-sea gas transfer velocity. Anthropogenic CO₂ falls in between these two cases, and the rate of anthropogenic CO₂ uptake is affected by both processes. Biological productivity in the Southern Ocean is characterized with the circum- polar belt of elevated biological productivity, "Antarctic Circumpolar Productivity Belt".(cont.) Annually and zonally averaged export of biogenic silica is estimated by fitting the zonally averaged tracer transport model to the climatology of silicic acid using the method of least squares. The pattern of export production inferred from the inverse calculation is qualitatively consistent with recent observations. The pattern of inferred export production has a maximum on the southern flank of the ACC. The advective transport by the residual mean circulation is the key process in the vertical supply of silicic acid to the euphotic layer where photosynthesis occurs. In order to illustrate what sets the position of the productivity belt, I examined simulated biological production in a physical-biogeochemical model which includes an explicit ecosystem model coupled to the phosphate, silica and iron cycle. Simulated patterns of surface nutrients and biological productivity suggest that the circumpolar belt of elevated biological productivity should coincide with the regime transition between the iron-limited Antarctic zone and the macro-nutrients limited Subantarctic zone. At the transition, organisms have relatively good access to both micro and macro-nutrients. Kohfeld (in Bopp et al.; 2003) suggested that there is a distinct, dipole pattern in the paleo-proxy of biological export in the Southern Ocean at the LGM. I hypothesize that observed paleo-productivity proxies reflect the changes in the position of the Antarctic Circumpolar Productivity Belt over glacial-interglacial timescales. Increased dust deposition during ice ages is unlikely to explain the equatorward shift in the position of the productivity belt due to the expansion of the oligotrophic region and the poleward shift of the transition between the iron-limited regime and the macro-nutrient limited regime.(cont.) I develop a simple dynamical model to evaluate the sensitivity of the meridional overturning circulation to the surface wind stress and the stratification. The theory suggest that stronger surface wind stress could intensify the surface residual flow and perturb the position of the productivity belt in the same sign as indicated by the paleo-productivity proxies. Finally, I examined the relationship between the surface macro-nutrients in the polar Southern Ocean and the atmospheric pCO₂. Simple box models developed in 1980s suggests that depleting surface macro-nutrients in high latitudes can explain the glacial pCO₂ drawdown inferred from polar ice cores. A suite of sensitivity experiments are carried out with an ocean-atmosphere carbon cycle model with a wide range of the rate of nutrient uptake in the surface ocean. These experiments suggest that the ocean carbon cycle is unlikely to approach the theoretical limit where "pre- formed" nutrient is completely depleted due to the dynamics of deep water formation. The rapid vertical mixing timescales of convection preclude the ventilation of strongly nutrient depleted waters. Thus it is difficult to completely deplete the "preformed" nutrients in the Southern Ocean even in a climate with elevated dust deposition in the region, suggesting some other mechanisms for the cause of lowered glacial pCO₂.by Takamitsu Ito.Ph.D

Feedback mechanism in the oceanic carbon cycle

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999.Includes bibliographical references (p. [84]-[87]).In this thesis, I designed and implemented a simple atmosphere-ocean coupled carbon cycle model which can be used as a tool to uncover the mechanisms of the interaction between the dynamics of the atmosphere-ocean system and the oceanic reservoir of CO 2 on the 101 to 103 years time scale. The atmosphere-ocean coupled model is originally developed by Marotzke (20,21), and the biogeochemical model is developed by Follows(personal communication). The atmosphere-ocean-carbon model makes the atmosphere-ocean dynamics and the carbon cycle fully interactive, and results in two stationary states characterized by two distinct patterns of the thermohaline circulation. The temperature driven, high latitudes sinking mode showed significantly lower atmospheric pCO2 than the salinity-driven, low latitudes sinking mode. The atmosphere-ocean dynamics dominates the system behavior of the model. The carbon cycle weakly feedbacks on the atmosphere-ocean system through the radiation balance. The model reveals two feedback mechanisms, the global warming feedback and the thermohaline pCO 2 feedback. The thermohaline pCO2 feedback has three sub-components, which are the biological pump feedback, the outgassing feedback and the DIC exporting feedback. The numerical experiments estimate the relative importance among them. The system becomes less stable when all the feedback mechanism is introduced. The model could be used to understand some basic mechanism of the situations similar to the anthropogenic global warming. The stability analysis is applied to evaluate the model runs. The current rate of 7 GTC yr - 1 can induce the spontaneous shutdown of thermohaline circulation after 550 years of constant emission. The stability of the thermohaline circulation rapidly decreases even before the system stops the thermohaline circulation. The model parameterized surface alkalinity as a simple function of sea surface salinity or as a constant, rather than solving the alkalinity cycle explicitly. The system is sensitive to the parameterization, in which different assumptions on alkalinity lead to different results both analytically and numerically.by Takamitsu Ito.S.M

C‐Glycosyltransferases catalyzing the formation of di‐C‐glucosyl flavonoids in citrus plants

Citrus plants accumulate many kinds of flavonoids, including di‐C‐glucosyl flavonoids, which have attracted considerable attention due to their health benefits. However, the biosynthesis of di‐C‐glucosyl flavonoids has not been elucidated at the molecular level. Here, we identified the C‐glycosyltransferases (CGTs) FcCGT (UGT708G1) and CuCGT (UGT708G2) as the primary enzymes involved in the biosynthesis of di‐C‐glucosyl flavonoids in the citrus plants kumquat (Fortunella crassifolia) and satsuma mandarin (Citrus unshiu), respectively. The amino acid sequences of these CGTs were 98% identical, indicating that CGT genes are highly conserved in the citrus family. The recombinant enzymes FcCGT and CuCGT utilized 2‐hydroxyflavanones, dihydrochalcone, and their mono‐C‐glucosides as sugar acceptors and produced corresponding di‐C‐glucosides. The Km and kcat values of FcCGT toward phloretin were <0.5 μm and 12.0 sec−1, and those toward nothofagin (3ʹ‐C‐glucosylphloretin) were 14.4 μm and 5.3 sec−1, respectively; these values are comparable with those of other glycosyltransferases reported to date. Transcripts of both CGT genes were found to concentrate in various plant organs, and particularly in leaves. Our results suggest that di‐C‐glucosyl flavonoid biosynthesis proceeds via a single enzyme using either 2‐hydroxyflavanones or phloretin as a substrate in citrus plants. In addition, Escherichia coli cells expressing CGT genes were found to be capable of producing di‐C‐glucosyl flavonoids, which is promising for commercial production of these valuable compounds.ArticlePlant Journal.91(2):187-198(2017)journal articl

Planetary-geometric constraints on isopycnal slope in the Southern Ocean

On planetary scales, surface wind stress and differential buoyancy forcing act together to produce isopycnal surfaces that are relatively flat in the tropics/subtropics and steep near the poles, where they tend to outcrop. Tilted isopycnals in a rapidly rotating fluid are subject to baroclinic instability. The turbulent, mesoscale eddies generated by this instability have a tendency to homogenize potential vorticity (PV) along density surfaces. In the Southern Ocean (SO), the tilt of isopycnals is largely maintained by competition between the steepening effect of surface forcing and the flattening effect of turbulent, spatially inhomogeneous eddy fluxes of PV. Here we use quasi-geostrophic theory to investigate the influence of a planetary-geometric constraint on the equilibrium slope of tilted density/buoyancy surfaces in the SO. If the meridional gradients of relative vorticity and PV are small relative to β, then quasi-geostrophic theory predicts ds/dz = β/ f0 = cot(ϕ0)/a, or equivalently r ≡ |∂zs/(β/ f0)| = 1, where s is the isopycnal slope, ϕ0 is a reference latitude, a is the planetary radius, and r is the depth-averaged criticality parameter. We find that the strict r = 1 condition holds over specific averaging volumes in a large-scale climatology. A weaker r = O(1) condition for depth-averaged quantities is generally satisfied away from large bathymetric features. We employ the r = O(1) constraint to derive a depth scale to characterize large-scale interior stratification, and we use an idealized sector model to test the sensitivity of this relationship to surface wind forcing. Finally, we discuss the possible implications for eddy flux parameterization and for the sensitivity of SO circulation/stratification to changes in forcing