Modeling the Impact of Ocean Circulation on Chlorophyll Blooms Around South Georgia, Southern Ocean.

The northeast periphery of the Scotia Sea hosts one of the largest chlorophyll‐a blooms of the Southern Ocean. This bloom peaks to the northwest of the island of South Georgia, extending eastward for hundreds of kilometers. Although the Southern Ocean has many islands of similar size, South Georgia is ecologically one of the most significant: It not only sustains one of the Southern Ocean's largest and most diverse ecosystems but also constitutes its single most important region for biological carbon sequestration. While the exceptional nature of South Georgia's blooms has been recognized widely, both the physical processes that contribute to their fertilization and the reasons why these blooms are larger than those of other similar regions (e.g., Kerguelen or Crozet Islands) are poorly understood. We use the results of a high‐resolution ocean model to investigate the physical processes that mediate the entrainment of deep, iron‐rich waters into the surface layers of the South Georgia region. We show that the Southern Antarctic Circumpolar Current Front, the southernmost jet of the Antarctic Circumpolar Current (ACC), pumps iron‐enriched waters from the deep ocean onto the bottom layers of South Georgia's shelf. These waters are upwelled along the northern coast of the island and are then exported into the Georgia Basin, where topographically steered circulation shields them from the dispersive effects of local currents and eddies, thus allowing the bloom development. Plain Language Summary: The growth of phytoplankton in the ocean is the base of the marine food web and is fueled by the supply of nutrients. In the Southern Ocean, this growth is anomalously strong downstream of islands and seamounts and very markedly so downstream of the island of South Georgia. We use a high‐resolution ocean model to explain how the ocean circulation supplies nutrients to the South Georgia bloom and find that the pumping of iron‐rich waters from depth onto the South Georgia shelf is key, as is upwelling of water on the northern side of the island and subsequent flow into the open ocean beyond. This marine growth supports a rich and diverse ecosystem, including some species which are commercially valuable. Key Points: The northwestern coast of South Georgia is an important upwelling regionThe SACCF generates strong obduction of deep waters around the South Georgia shelfLocal wind stress forcing is the primary driver of cross‐shelf mass exchanges [ABSTRACT FROM AUTHOR]