Transportpfade und Zerfallsmechanismen von Nordbrasilstromringen im tropischen Nordatlantik - Eine Modellstudie
Final Report Abstract
The upper branch of the meridional overturning circulation in the North Atlantic is fed by cross-equatorial transport of various water masses from the Southern Hemisphere. One of the mechanisms of the interhemispheric exchange is the production of North Brazil Current Rings, which connect the equatorial gyre and the subtropical gyre of the northern hemisphere. The rings develop at the retroflection of the North Brazil Current into the North Equatorial Countercurrent at approx. 7°N and propagate along the South American shelf break towards the Caribbean Sea. The Lesser Antilles Islands arc constrains the propagation pathway of the rings and often leads to disintegration of the ring structure. The southern waters contained in the centers of the rings remain in the North Atlantic, mostly enter the Caribbean Sea and contribute to the Florida Current. In this project, an analysis of the spreading of south Atlantic water into the northern hemisphere was finalized. The analysis revealed a dominance of South Atlantic water in the Atlantic equatorial gyre. The transition zones from southern water masses to northern water masses show a different behavior for the shallow water masses of the upper water column. Antarctic Intermediate Water was found to spread farthest into the North Atlantic. The propagation and decay of North Brazil Current Rings was studied using the high resolution FLAME model (in cooperation with the University of Kiel, PI: Prof. C. Böning). Special emphasis was put on the interaction of the rings with the islands of the Lesser Antilles, and the modeling results were compared with observations and laboratory experiments. The study focused on the properties of the ring shedding for the different ring types, and the mean transport obtained by the rings. Up to 50% of the upper ocean flow related to the meridional overturning circulation can be achieved by NBC rings. An analysis of the ring pathways identified the Caribbean Sea as the dominant path for the rings in the model. Interaction with the islands of the Lesser Antilles prevents the rings to reach latitudes north of 15°N undisturbed in their structure. The trapping of the rings south of St. Lucia and Barbados can induce the formation of new vortices north of Barbados. K. Jochumsen supervised students in the analysis of the MITgcm model, which is run by the University of Hamburg (PI: Prof. D. Stammer, Dr. N. Serra). During this last part of the project, the formation and propagation of North Brazil Current Rings in MITgcm were investigated. Based on her work as a student helper, H. Paulsen carried on the analyses in her Bachelor thesis and examined possible mechanisms for the formation of the Rings. Her analysis pointed out, that transport fluctuations upstream of the retroflection of the North Brazil Current are the most likely trigger for the ring shedding.
Publications
- (2009): On the spreading of South Atlantic Water into the northern hemisphere. Journal of Geophysical Research, 114, C05019
Kirchner, K., M. Rhein, S. Hüttl-Kabus and C.W. Böning
(See online at https://doi.org/10.1029/2008JC005165) - (2010): On the propagation and decay of North Brazil Current Rings. Journal of Geophysical Research, 115, C10004
Jochumsen, K., M. Rhein, S. Hüttl-Kabus and C.W. Böning
(See online at https://doi.org/10.1029/2009JC006042)
