Ocean

We are investigating the general ocean circulation in varied scales.

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.

The meridional wind over the Antarctic sea ice is less certain. We investigate the responses of the Southern Ocean circulation to the meridionl wind. Please refer to this page for this study.

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

Along the California coast, equatorward wind transports water to offshore, creating divergence and upwelling at the coast. In the open ocean, the divergence and upwelling can occur when the wind-driven offshore transport of water increases as we go offshore. The latter mechanism is referred to the Ekman pumping upwelling and depends on the wind field structure. Equatorward wind can have an abrupt change near the coast, but cliamte models cannot resovle a sharp drop-off of the wind. I wanted to investigate how the upwelling source waters change if we can resolve the sharp drop-off of the wind near the coast.

When equatorward wind stress has a gradual change (left panel in Figure 4), wind stress curl is weakly positive over the broad region near the coast, causing weak and shallow upwelling. Hence, the sources for the upwelling are local waters. In constrast, the sharp drop-off of the wind stress creates strong positive curl near the coast (right panel in Figure 4). The coast upwelling become stronger with the Ekman pumping upwelling, and the deep enough to have the poleward undercurrent as a source. Equatorward flow is located nearer to the coast, and it also supplies water to the upwelling. This result has applications for predicting how interannual and decadal changes in wind conditions alter the source and properties of upwelled waters and, consequently, the marine ecosystem in the California Current. Please refer Song et al. (2010) for more details.