Ocean Circulation

The Southern Ocean is unique. Consistent eastward wind results in upwelling of the water from the interior of the ocean, providing a passage for the exchange between the atmosphere and deep ocean. Unlike other major ocean basins, it does not have boundaries so that water circulates all around Antarctica. This wind-driven current is called Antarctic Circumpolar Current (ACC) and flows eastward connecting Atlantic, Pacific and Indian Oceans. The Southern Ocean plays an important role in earth’s climate. It stores up to 40% of anthropogenic carbon dioxide (CO2) that enters the ocean. The connection between the atmosphere and subsurface ocean makes the Southern Ocean the area where the ocean breathes. The Southern Ocean is also biologically interesting area. The biological activity has a strong seasonal variability (Fig. 1) and is generally limited by light in winter and micronutrient in summer. Although intensive biological activities in the SO (Fig. 1) promote carbon dioxide (CO2) uptake, the overturning circulation and the intensive vertical mixing in winter allow CO2 in the deep ocean to escape to the atmosphere at the same time. The Southern Ocean has high eddy activities. When the upwelling brings dense water to the surface, it makes the ocean high in potential energy and eddies convert it to kinetic energy. My research interest lies on how these eddies modify the air-sea CO2 flux and other oceanic biogeochemical states.

A chlorofluorocarbon-11 (CFC-11) is an anthropogenic gas that was first introduced to the atmosphere in 1940s. CFC-11 is inert in the ocean with known atmospheric source, so it is useful to evaluate the ocean ventilation rate in ocean circulation models. Using an eddy-resolving model, we find that warm anticyclones have lower CFC-11 concentration at the surface, leading to more uptake of CFC-11 by the ocean (Fig. 2(e)). In cold cyclones, in contrast, the anomaly of CFC-11 concentration is positive, suggesting that cyclones take up less CFC-11 (Fig. 2(f)). The opposite sign of CFC-11 anomaly is attributed to the anomalous vertical mixing. Eddy centric analysis reveals more intense vertical mixing in anticyclones, but reduced mixing in cyclones (Fig. 2(a,b)). With strong vertical mixing, the surface ocean, usually saturated with CFC-11, is well mixed with low CFC-11 water at the subsurface, resulting in reduction of CFC-11 concentration at the surface and more uptaking of CFC-11. The opposite occurs in cyclones: less vertical mixing leads to higher CFC-11 concentration at the surface and less uptake. The anomalies in anticyclones and cyclones do not completely cancel out, leaving a net positive effect of CFC-11 uptake by mesoscale eddies. Please refer to Song et al. (2015) for more details.

While investigating the role of mesoscale eddies using both observations and numerical models, we find quite complicated, yet very interesting impacts of eddies in the Southern Ocean. The air-sea CO2 exchange depends on the difference in partial pressure of CO2 (pCO2) between air and ocean. Since atmospheric pCO2 is nearly uniform and well-known, oceanic pCO2 is a key factor for the air-sea CO2 exchange. In contrast to CFC-11, not only physical but also biogeochemical states of the ocean incluence pCO2, which requires more comprehensive analysis. In summer, oceanic pCO2 is lower in anticyclones while cyclonic eddies have higher oceanic pCO2. Interestingly, this relationship is flipped in winter and anticyclones/cyclones have higher/lower pCO2. The seasonality of the relationship between eddies and pCO2 is resulted from the time-varying balance between two opposing thermal and DIC effects. Please refer to our paper, Song et al. (2016), for more details.

Patagonian shelf, located in the southwest Atlantic ocean, is one of the regions of the biggest CO2 uptake due to the high productivity. I investigate the nutrient sources that support the ecosystem in the Patagonian shelf area using an adjoint sensitivity analysis with passive tracers. This computationally efficient method identifies three major nutrient sources: local area, Chilean coastal area and deep southeast Pacific. It also shows that the wintertime vertical mixing is one of the key processes that deliver nutrients to the Patagonian shelf region as shown in Figure 3. Guided by the adjoint experiments, I also performed a series of forward model with nutrient perturbations at the source regions. The forward biogeochemical integrations support the source water regions for the Patagonian shelf are also the sources of the nutrients. The manuscript that describes this study is under review, and I will provide it when it is published. Check out the story at mitgcm.org