Swift currents can create robust, circular structures approximately 100 kilometers in diameter, known as "rings." The ocean is filled with these rings, which propagate according to their own dynamics. Observations and modeling studies have revealed several formation sites and pathways known for the prevalence of ocean rings. One of these sites is the northeast coast of Brazil. Each year, eight or nine rings form and propagate along the Brazilian coast. These rings trap warm water in their cores as well as nutrients and organisms, and are thus important to all aspects of oceanography. North Brazilian Rings, in particular, are thought to be important for climate variability by contributing half of the northward warm water transport of the global ocean circulation in the Atlantic Ocean. As these ocean rings circulate, they interact with the bottom topography but the ring-topographical interaction is still not well understood. Idealized studies have demonstrated that rings can accelerate or slow down when entering in contact with the topography. However it is unclear if these theories apply in realistic cases such as the North Brazilian Rings. This study will address this question in a combined theoretical and numerical study of the region. This research will clarify the ring-topographic interactions for realistic topographies and lead to a better understanding of the ocean dynamics in coastal areas. Each summer, a high school science teacher will be involved in the conduct of the research. The project team will visit the teacher's classes during the regular school year to give presentations on oceanography and update the class on the work done during the summer by their instructor.
North Brazil Current (NBC) rings are observed to accelerate suddenly upon departing the South American coast at 12°N (often doubling their propagation speed). This phenomenon has never been explained, but given that these rings are responsible for roughly 50% of the Meridional Overturning Circulation inter-hemispheric exchange, one needs to understand their pathways and evolution. The ring-topographic interaction will be examined in a combined modeling and analytical study. This study will proceed systematically from simple process-oriented work to regionally embedded, fine scale primitive equation modeling of the western tropical Atlantic. New ideas about the nature of mean flow-topography interaction will be explored and studied by solving a reduced equation set along with complementary process ocean circulation models. This combination should allow the project team to assess the validity of the theory and to test its limits of applicability. In addition, the mechanics of cyclonic vorticity generation, which is potentially a powerful effect on the separation of NBC rings from the coast, will be examined. Finally, a series of aggressive embedded, downscaling modeling experiments will be performed to provide detailed numerical statements of the regional dynamics. The possibility that the mean flow topographic interaction can be explicitly calculated, rather than having to rely on poorly known parameterizations and boundary conditions, could be a significant breakthrough.
