The Agulhas Leakage: Role of Mesoscale Processes and Impact on the Atlantic Meridional Overturning Circulation

• Main Author: Biastoch, Arne
• Format: Thesis
• Language: English
• Published: 2008
• Subjects: Indian; Sea ice
• Online Access: https://oceanrep.geomar.de/id/eprint/5166/; https://oceanrep.geomar.de/id/eprint/5166/1/Habil_Biastoch_2008.pdf

The Agulhas region around South Africa is a key region of global climate and climate change. Under present climate conditions the Agulhas leakage from the Indian to the Atlantic Ocean feeds the bulk of the upper limb of the meridional overturning circulation (MOC) in the Atlantic Ocean, highly affected by the nonlinear constituents of the Agulhas Current system.To examine the role of the mesoscale processes in the mean flow in the Agulhas system, particularly in regard to the Agulhas leakage and its effect on the Atlantic MOC, an innovative ocean modeling program has been set up that utilizes new global model components and methodologies developed in international cooperation (DRAKKAR) based on a framework of the European model system NEMO. The model configuration involves a high-resolution grid of the greater Agulhas region nested into a coarse-resolution global ocean –sea-ice model forced by atmospheric conditions of the period 1958 –2004. Due to an effective “two-way” nesting approach this system for the first time allows to unravel, how the explicitly simulated mesoscale variability in the Agulhas dynamics feeds back to the global ocean.There is vast range of mesoscale –mean flow interactions in the Agulhas region. In the South East Madagascar Current offshore eddies do lead to different modes of the current extension, one favoring cyclonic flow into the Mozambique Channel, the other anticyclonic eddies drifting towards southwest. Eddies generated in the central Mozambique Channel introduce strong perturbations into the western boundary current systems off the African coast by triggering Natal Pulses, causing offshore displacements of the Agulhas Current which then lead to strong changes in the volume transport of the Agulhas Current and eventually to upstream retroflections of the current back into the Indian Ocean. The barotropic nature of the interplay with Mozambique eddies and Natal Pulses also affects the Agulhas Undercurrent leading to strong fluctuations similar to observed ones, raising the question ...