Taka Ito

Climate-Forced Variability of Ocean Hypoxia

The spatial extent of ocean hypoxic zones, which are uninhabitable by many marine organisms, is very sensitive to dioxygen content. Oxygen (O2) is a critical constraint on marine ecosystems. As oceanic O2 falls to hypoxic concentrations, habitability for aerobic organisms decreases rapidly. We show that the spatial extent of hypoxia is highly sensitive to small changes in the ocean’s O2 content, with maximum responses at suboxic concentrations where anaerobic metabolisms predominate. In model-based reconstructions of historical oxygen changes, the world’s largest suboxic zone, in the Pacific Ocean, varies in size by a factor of 2. This is attributable to climate-driven changes in the depth of the tropical and subtropical thermocline that have multiplicative effects on respiration rates in low-O2 water. The same mechanism yields even larger fluctuations in the rate of nitrogen removal by denitrification, creating a link between decadal climate oscillations and the nutrient limitation of marine photosynthesis.

Anthropogenic carbon dioxide transport in the Southern Ocean driven by Ekman flow

The Southern Ocean, with its large surface area and vigorous overturning circulation, is potentially a substantial sink of anthropogenic CO2 (refs 1–4). Despite its importance, the mechanism and pathways of anthropogenic CO2 uptake and transport are poorly understood. Regulation of the Southern Ocean carbon sink by the wind-driven Ekman flow, mesoscale eddies and their interaction is under debate. Here we use a high-resolution ocean circulation and carbon cycle model to address the mechanisms controlling the Southern Ocean sink of anthropogenic CO2. The focus of our study is on the intra-annual variability in anthropogenic CO2 over a two-year time period. We show that the pattern of carbon uptake is correlated with the oceanic vertical exchange. Zonally integrated carbon uptake peaks at the Antarctic polar front. The carbon is then advected away from the uptake regions by the circulation of the Southern Ocean, which is controlled by the interplay among Ekman flow, ocean eddies and subduction of water masses. Although lateral carbon fluxes are locally dominated by the imprint of mesoscale eddies, the Ekman transport is the primary mechanism for the zonally integrated, cross-frontal transport of anthropogenic CO2. Intra-annual variability of the cross-frontal transport is dominated by the Ekman flow with little compensation from eddies. A budget analysis in the density coordinate highlights the importance of wind-driven transport across the polar front and subduction at the subtropical front. Our results suggest intimate connections between oceanic carbon uptake and climate variability through the temporal variability of Ekman transport.

Control of Lower-Limb Overturning Circulation in the Southern Ocean by Diapycnal Mixing and Mesoscale Eddy Transfer

A simple model is developed of the lower limb of the meridional overturning circulation in the Southern Ocean based on residual-mean theory. It is hypothesized that the strength of the lower-limb overturning () is strongly controlled by the magnitude of abyssal diapycnal mixing () and that of mesoscale eddy transfer (K ). In particular, it is argued that K. The scaling and associated theory find support in a suite of sensitivity experiments with an idealized ocean general circulation model. This study shows that intense diapycnal mixing is required to close the buoyancy budget of the lower-limb overturning circulation, in contrast to the upper limb, where air–sea buoyancy fluxes can provide the required diabatic forcing.

Lagrangian Analysis of the Meridional Overturning Circulation in an Idealized Ocean Basin

The Lagrangian ocean model is used as a tool to simulate the response of the basin-scale overturning circulation to spatially variable diapycnal mixing in an idealized ocean basin. The model explicitly calculates the positions, velocities, and tracer properties of water parcels. Owing to its Lagrangian formulation, numerical diffusion is completely eliminated and water parcel pathways and water mass ages can be quantified within the framework of the discrete, advective transit time distribution. To illustrate the ventilation pathways, simulated trajectories were tracked backward in time from the interior ocean to the surface mixed layer where the water parcel was last in contact with the atmosphere. This new diagnostic has been applied to examine the response of the meridional overturning circulation to highly localized diapycnal mixing through sensitivity experiments. In particular, the focus is on three simulations: the first holds vertical diffusivity uniform; in the second, the vertical diffusivity is confined within an equatorial box; and the third simulation has a diffusivity pattern based on idealized hurricane-induced mixing. Domain-integrated deep ventilation rates and heat transport are similar between the first two cases. However, locally enhanced mixing yields about 30% younger water mass age in the tropical thermocline due to intense localized upwelling. In the third simulation, a slower ventilation rate of deep waters is found to be due to the lack of abyssal mixing. These results are interpreted using the classical theories of abyssal circulation, highlighting the strong sensitivity of the ventilation pathways to the spatial distribution of diapycnal mixing.