Irina Marinov

The Southern Ocean biogeochemical divide

Modelling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air–sea balance of CO2 and global biological production. Box model studies first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure (pCO2). This early research led to two important ideas: high latitude regions are more important in determining atmospheric pCO2 than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric pCO2 are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling pre-industrial atmospheric CO2 because it serves as a lid to a larger volume of the deep ocean. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide and in controlling global biological production. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air–sea CO2 balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO2 and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.

Turbulent diffusion in the geostrophic inverse cascade

Motivated in part by the problem of large-scale lateral turbulent heat transport in the Earths atmosphere and oceans, and in part by the problem of turbulent transport itself, we seek to better understand the transport of a passive tracer advected by various types of fully developed two-dimensional turbulence. The types of turbulence considered correspond to various relationships between the streamfunction and the advected field. Each type of turbulence considered possesses two quadratic invariants and each can develop an inverse cascade. These cascades can be modified or halted, for example, by friction, a background vorticity gradient or a mean temperature gradient. We focus on three physically realizable cases: classical two-dimensional turbulence, surface quasi-geostrophic turbulence, and shallow-water quasi-geostrophic turbulence at scales large compared to the radius of deformation. In each model we assume that tracer variance is maintained by a large-scale mean tracer gradient while turbulent energy is produced at small scales via random forcing, and dissipated by linear drag. We predict the spectral shapes, eddy scales and equilibrated energies resulting from the inverse cascades, and use the expected velocity and length scales to predict integrated tracer fluxes. When linear drag halts the cascade, the resulting diffusivities are decreasing functions of the drag coefficient, but with different dependences for each case. When β is significant, we find a clear distinction between the tracer mixing scale, which depends on β but is nearly independent of drag, and the energy-containing (or jet) scale, set by a combination of the drag coefficient and β. Our predictions are tested via high- resolution spectral simulations. We find in all cases that the passive scalar is diffused down-gradient with a diffusion coefficient that is well-predicted from estimates of mixing length and velocity scale obtained from turbulence phenomenology.

Response of the Ocean Natural Carbon Storage to Projected Twenty-First-Century Climate Change

The separate impacts of wind stress, buoyancy fluxes, and CO2 solubility on the oceanic storage of natural carbon are assessed in an ensemble of twentieth- to twenty-first-century simulations, using a coupled atmosphere‐ocean‐carbon cycle model. Time-varying perturbations for surface wind stress, temperature, and salinity are calculated from the difference between climate change and preindustrial control simulations, and are imposed on the ocean in separate simulations. The response of the natural carbon storage to each perturbation is assessed with novel prognostic biogeochemical tracers, which can explicitly decompose dissolved inorganic carbon into biological, preformed, equilibrium, and disequilibrium components. Strong responses of these components to changes in buoyancy and winds are seen at high latitudes, reflecting the critical role of intermediate and deep waters. Overall, circulation-driven changes in carbon storage are mainly due to changes in buoyancy fluxes, with wind-driven changes playing an opposite but smaller role. Results suggest that climate-driven perturbations to the ocean natural carbon cycle will contribute 20PgC to the reduction of the ocean accumulated total carbon uptake over the period 1860‐2100. This reflects a strong compensation between a buildup of remineralized organic matter associated with reduced deep-water formation (196PgC) and a decrease of preformed carbon (2116PgC). The latter is due to a warming-induced decrease in CO2 solubility (252PgC) and a circulation-induced decrease in disequilibrium carbon storage (264PgC). Climate change gives rise to a large spatial redistribution of ocean carbon, with increasing concentrations at high latitudes and stronger vertical gradients at low latitudes.

North-South asymmetry in the modeled phytoplankton community response to climate change over the 21st century

Here we analyze the impact of projected climate change on plankton ecology in all major ocean biomes over the 21st century, using a multidecade (1880–2090) experiment conducted with the Community Climate System Model (CCSM-3.1) coupled ocean-atmosphere-land-sea ice model. The climate response differs fundamentally in the Northern and Southern Hemispheres for diatom and small phytoplankton biomass and consequently for total biomass, primary, and export production. Increasing vertical stratification in the Northern Hemisphere oceans decreases the nutrient supply to the ocean surface. Resulting decreases in diatom and small phytoplankton biomass together with a relative shift from diatoms to small phytoplankton in the Northern Hemisphere result in decreases in the total primary and export production and export ratio, and a shift to a more oligotrophic, more efficiently recycled, lower biomass euphotic layer. By contrast, temperature and stratification increases are smaller in the Southern compared to the Northern Hemisphere. Additionally, a southward shift and increase in strength of the Southern Ocean westerlies act against increasing temperature and freshwater fluxes to destratify the water-column. The wind-driven, poleward shift in the Southern Ocean subpolar-subtropical boundary results in a poleward shift and increase in the frontal diatom bloom. This boundary shift, localized increases in iron supply, and the direct impact of warming temperatures on phytoplankton growth result in diatom increases in the Southern Hemisphere. An increase in diatoms and decrease in small phytoplankton partly compensate such that while total production and the efficiency of organic matter export to the deep ocean increase, total Southern Hemisphere biomass does not change substantially. The impact of ecological shifts on the global carbon cycle is complex and varies across ecological biomes, with Northern and Southern Hemisphere effects on the biological production and export partially compensating. The net result of climate change is a small Northern Hemisphere-driven decrease in total primary production and efficiency of organic matter export to the deep ocean.

Factors controlling interannual variability of vertical organic matter export and phytoplankton bloom dynamics – a numerical case-study for the NW Mediterranean Sea

Mid-latitude spring blooms of phytoplankton show considerable year-to-year variability in timing, spatial extent and intensity. It is still unclear to what degree the bloom variability is connected to the magnitude of the vertical flux of organic matter. A coupled three-dimensional hydrodynamic-biogeochemical model is used to relate interannual variability in phytoplankton spring-bloom dynamics to variability in the vertical export of organic matter in the NW Mediterranean Sea. Simulation results from 2001 to 2010, validated against remote-sensing chlorophyll, show marked interannual variability in both timing and shape of the bloom. Model results show a tendency for the bloom to start later after cold and windy winters. However, the onset of the bloom occurs often when the mixed layer is still several hundred metres deep while the heat flux is already approaching zero and turbulent mixing is low. Frequency and intensity of wind episodes control both the timing and development of the bloom and the consequent export flux of organic matter. The wintertime flux is greater than zero and shows relatively low interannual variability. The magnitude of the interannual variability is mainly determined in March when the frequency of windy days positively correlates with the export flux. Frequent wind-driven mixing episodes act to increase the export flux and, at the same time, to interrupt the bloom. Perhaps counterintuitively, our analysis shows that years with discontinuous, low-chlorophyll blooms are likely to have higher export flux than years with intense uninterrupted blooms. The NW Mediterranean shows strong analogy with the North Atlantic section within the same latitude range. Hence, our results may also be applicable to this quantitatively more important area of the world ocean.

Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model

A climate model is used to investigate the influence of Weddell Sea open ocean deep convection on anthropogenic and natural carbon uptake for the period 1860–2100. In a three-member ensemble climate change simulation, convection ceases on average by year 1981, weakening the net oceanic cumulative uptake of atmospheric CO2 by year 2100 (−4.3 Pg C) relative to an ocean that has continued convection. This net weakening results from a decrease in anthropogenic carbon uptake (−10.1 Pg C), partly offset by an increase in natural carbon storage (+5.8 Pg C). Despite representing only 4% of its area, the Weddell Sea is responsible for 22% of the Southern Ocean decrease in total climate-driven carbon uptake and 52% of the decrease in the anthropogenic component of oceanic uptake. Although this is a model-specific result, it illustrates the potential of deep convection to produce an intermodel spread in future projections of ocean carbon uptake.

Global Atmospheric Teleconnections and Multidecadal Climate Oscillations Driven by Southern Ocean Convection

AbstractA 1000-yr control simulation in a low-resolution coupled atmosphere–ocean model from the Geophysical Fluid Dynamics Laboratory (GFDL) family of climate models shows a natural, highly regular multidecadal oscillation between periods of Southern Ocean (SO) open-ocean convection and nonconvective periods. It is shown here that convective periods are associated with warming of the SO sea surface temperatures (SSTs), and more broadly of the Southern Hemisphere (SH) SSTs and atmospheric temperatures. This SO warming results in a decrease in the meridional gradient of SSTs in the SH, changing the large-scale pressure patterns, reducing the midlatitude baroclinicity and thus the magnitude of the southern Ferrel and Hadley cells, and weakening the SO westerly winds and the SH tropical trade winds. The rearrangement of the atmospheric circulation is consistent with the global energy balance. During convective decades, the increase in incoming top-of-the-atmosphere radiation in the SH is balanced by an incre...

A Coupled Mathematical Model to Predict the Influence of Nitrogen Fertilization on Crop, Soil and Groundwater Quality

Intensified use of nitrogen based fertilizers in agriculture has resulted in a significant increases in soils and groundwater nitrate concentrations all over the world. Here we propose a new coupled model which describes the physical transport and biogeochemical dynamics of the water and nitrogen compounds in a soil-water-plant-groundwater system. Our model takes into account water infiltration into an unsaturated porous medium, the complex biogeochemical cycle of nitrogen in soils, nitrate leaching from the agricultural system toward the aquifer’s water table, and the dispersion of nitrates in the groundwater. We calibrate our model and analyse the influence of soil type, precipitation or irrigation regime and fertilization schedules on leaching to groundwater as well as the temporal and spatial evolution of the nitrate pollutant plume in the aquifer. Simulations indicate that in order to achieve high crop yields while minimizing nitrogen loading to soils and groundwater we need to create an optimal balance between the amount of chemical fertilizers and water applied to crops on one hand, and the amount of nitrate and water used by plants on the other. We find that medium soils are more suitable for a sustainable corn production than coarse soils, ensuring both higher yield and less nitrate pollution of the aquifer. Regardless of soil type and irrigation schedule, a gradual fertilization throughout the plant life cycle reduces the potential for leaching and aquifer pollution. For medium soils, a more modest irrigation schedule results in more nitrate available for crops and less net leaching to the groundwater.

The Role of the Oceans in the Global Carbon Cycle: An Overview

Typical gases such as oxygen tend to be distributed mostly in the atmosphere (99.4%) and less so in the ocean (0.6%). Carbon dioxide is vastly different from such gases, with 98.5% of the pre-industrial CO2 in the ocean and only 1.5% in the atmosphere. The reason that CO2 is found preferentially in the ocean is because of its high solubility (thirty times that of oxygen) and because of the hydrolysis reaction it undergoes to form carbonate and bicarbonate ions. The oceanic carbon inventory of total dissolved inorganic carbon (DIC) is also influenced by a combination of biologically mediated processes (the biological pump) and physical and chemical processes (the solubility pump). Both the biological and solubility pumps contribute to a higher concentration of DIC in the deep ocean, which reduces atmospheric CO2 relative to what it would be otherwise.

Large-Scale, Persistent Nutrient Fronts of the World Ocean: Impacts on Biogeochemistry

This chapter identifies and describes the large-scale nutrient fronts that span the width of basins and explores the processes that maintain these fronts and those that act against them. In particular, we investigate the nutrient fronts that ring the subtropical gyres and propose that exchange across these fronts represents a critical pathway for nutrients to enter the gyres. However, these biogeochemical fronts most often coincide with dynamical fronts or jets, which are often considered barriers to exchange. Therefore, our view of ocean fronts as nutrient gateways must be reconciled with their tendency to act as barriers to exchange. Ekman transport is one mechanism that allows for nutrient transport across the surface of the fronts and is shown to be a leading term in the subtropical nutrient budgets. Ring formation and mixing beneath the core of jets are other mechanisms that can mediate cross-frontal exchange and have intriguing implications for nutrient budgets and their variability.