Project Synthesis. South Georgia is a small island surrounded by the swift and cold flow of the Antarctic Circumpolar Current (Fig. 1a-b). Although a small speck of land in the immensity of the Southern Ocean, South Georgia host some of its most exceptional chlorophyll blooms, which are the single and most important mechanism for carbon sequestration in the Southern Ocean. The goal of this project was to identify the physical processes sustaining South Georgia's primary productivity and assess their sensitivity to climate variability including climate change. To achieve this goal we conducted a series of experiments using a state-of-the-art computational model. Our study concluded that South Georgia's chlorophyll blooms are sustained by a continuous flow of nutrient rich waters that are drawn from the deep portions of the Antarctic Circumpolar Current and injected into the upper layers of South Georgia's shelf. This fertilization process is driven by winds, tides and shelf/open-ocean interactions (Fig. 1b). Our experiments also show that wind forcing generates an important (and hitherto unrecognized) upwelling regime in the northwestern portion of the island.

Our analysis also concluded that low-frequency fluctuations of South Georgia's circulation are dominated by inter-annual changes while annual and semi-annual changes are insignificant. A substantial portion of South Georgia's variability is related to the local wind forcing, where the term local refers to the winds over the shelf region. In fact, correlation analysis indicates that approximately 40% of the variability of the surface circulation and volume transport around the island is related to the local winds. With regard to the remote forcing, which encompasses the entire Southern Ocean region, ancillary calculations show no significant correlation between South Georgia's circulation and the Antarctic Oscillation Index but a relatively strong anti-correlation between sea surface temperatures (SSTs) over the South Georgia region and the Southern Oscillation Index. ENSO conditions are inversely correlated with South Georgia's SST anomalies, which are part of a large-scale dipole linking the polar and tropical climate variability. Prior studies show that during ENSO events SSTs changes in the southwestern Atlantic region are inversely correlated to sea ice concentration in the southeast Pacific. Warming over the equatorial Pacific strengthens and contracts the Hadley cell in the Pacific and weaken and expand the Hadley cell in the Atlantic. This causes an equatorward displacement of the subtropical jet stream in the Pacific and a poleward displacement in the Atlantic, while the Ferrel cell intensifies and expands in the Pacific and weakens and contract in the Atlantic. These changes triggers a decrease of the poleward surface heat flux in the South Georgia region. To assess the potential impact of climate driven changes on South Georgia's marine ecosystems we did an ancillary suite of process-oriented experiments focusing on the impact of changes of the wind stress magnitudes over South Georgia's circulation. These experiments predict that the climate change related increments in the intensity of the Southern Ocean winds would further strengthen South Georgia fertilization processes, thus resulting in a negative feedback for climate change.

Broader impacts. Most of the results of this project, which address the physical mechanisms underlying one of the most fertile marine ecosystems of the Southern Ocean, are of immediate relevance to a broader range of disciplines, including marine ecology, marine biology, biogeochemistry, paleoceonography, paleoclimatology and climate research. In addition, during the course of the project we had the opportunity to collaborate with several multi-disciplinary studies of the Patagonian marine ecosystem, which is another important marine ecosystems that is closely connected to South Georgia by the Antarctic Circumpolar Current. These collaborations are documented by publications detailed in the final project report and concern fertilization processes in the Burdwood Bank, studies of spawning of Patagonian Toothfish, migration of turtles, and climate change impacts on the southwestern Atlantic region. Most recently, our model simulations have also been requested by a group of geographers and anthropologists of Oregon State University (USA) and the UK to investigate the possible migration of pre-historic indigenous people from the continental portion of Patagonia to the neighboring islands.

Last Modified: 11/04/2021
Modified by: Ricardo P Matano