An observational study of the western boundary currents in the Indian and South Atlantic Oceans
Ponsoni, L. (2016). An observational study of the western boundary currents in the Indian and South Atlantic Oceans. PhD Thesis. Universiteit Utrecht: Utrecht. ISBN 978-94-6299-374-7. xix, 171 pp. https://hdl.handle.net/1874/335229
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Document type: Dissertation
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Western Boundary Currents, East Madagascar Current, Agulhas Retroflection, Brazil Current |
| Abstract |
In this thesis we have investigated different aspects of the WBCs in the Indian and South Atlantic Oceans, based on observational data sampled both in situ and from satellites. In October 2010 an array of five moorings were deployed off eastern Madagascar, nominally at 23S, as part of the “INdian-ATlantic EXchange in present and past climate” (INATEX) observational program. The instruments remained in the water for about 2.5 years, until March 2013. Based on the INATEX data, we study the East Madagascar Current (EMC) in terms of its observed velocities, estimated volume transport and variability. The EMC is dominated by a nearly bi-monthly (45–85 days) period band. Satellite data show that such variability is explained by westward-propagating eddies impinging on the EMC. Anticyclonic eddies strengthen the flow while cyclonic eddies attenuate the EMC transport. Besides the surface patterns of the western boundary currents, the presence of an undercurrent flowing in opposite direction and beneath the surface cur rent is a recurring feature observed in different WBC systems. This thesis is also dedicated to the study of this counter flow, observed below the EMC: the equatorward East Madagascar Undercurrent (EMUC). Also supported by data from the INATEX moorings, we show that the EMUC is hugging the continental slope, with a core at a depth of 1200 m. Its mean equatorward volume transport amounts to nearly 10% of the mean transport estimated for the surface poleward EMC. On approaching Madagascar, the bifurcation of the SEC also generates the equatorward directed North Madagascar Current (NMC). We here provide the first evidence of the existence of a poleward counter current below the NMC, which we named North Madagascar Undercurrent (NMUC). Two interocean teleconnections, which take place downstream and upstream of the EMC system, are also investigated in this study. First, we explore the connection between the WBCs in the Indian and South Atlantic Oceans. The EMC, as well as the flow in the Mozambique Channel, are sources for the Agulhas Current (AC). In turn, the AC flows downstream until the southern tip of Africa, where part of the water transported by this current turns back into a loop to the Indian Ocean, generating an important feature known as Agulhas Retroflection. Anticyclonic eddies are created and are released into the Atlantic Ocean by this retroflection process, establishing the so-called Agulhas Leakage. We reveal that this mechanism of retroflection also represents a pulse of energy that crosses the Atlantic, imposing a nearly trimonthly period of variability on the Brazil Current, at the Santos Basin. Second, we focus on the teleconnection between Pacific and Indian Oceans by means of both Indonesian Throughflow and South Indian tropical gyre. The first represents the waters passing through the Indonesian islands, while the latter is a clockwise gyre near the equatorial region. The SEC, which is the northern branch of the subtropical gyre, can also be interpreted as the southern branch of the tropical gyre, and it has an important role in redistributing waters from the Indonesian Throughflow into the Indian Ocean. We show that the tropical gyre has a marked seasonal cycle. An analysis of major forcing mechanisms suggests the interaction of basin-scale wind stress curl, local-scale wind stress forcing, remote forcing driven by the Indonesian Throughflow, as well as westward-propagating Rossby waves |
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