Mechanisms of near-surface current and upwelling variability in the tropical Atlantic.

Hüttl, Sabine (2006) Mechanisms of near-surface current and upwelling variability in the tropical Atlantic. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Kiel, 111 pp.

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In the eastern equatorial oceans upwelling regions are found. The cold upwelling waters not only interact with the atmospheric circulation via changing the sea surface temperatures (SSTs) but also influence the biological activity via affecting the nutrient and oxygen contents. However, the sources of the upwelling waters are well known only for the sources of the equatorial upwelling but remained unclear for the off-equatorial upwelling regions. A main aspect of this work is to contribute to the understanding of the mean pathways into the Guinea and Angola Dome. The analysis bases on a high resolution model (1/12±) of the Atlantic and will focus on a discussion of the role of the various zonal current bands in supplying the offequatorial upwelling. Longer term variability of the equatorial currents and their possible influence on the equatorial upwelling will be discussed in a second part of this work. In the first part of the work trajectory calculations were used to investigate the sources of the equatorial zonal currents. It could be shown that they belong almost exclusively to the southern hemisphere and additionally, strong interaction between the different zonal currents has been found. This interaction is caused by tropical instability waves which are created by the shear between the zonal currents. The waves produce also eddies which cause a watermass transfer between the different currents. It was shown, that this eddy-transport is the main watermass pathway supplying the southern equatorial undercurrent (SEUC) and it was also found to play a major role for the sources of the northern equatorial undercurrent (NEUC). The analysis of the pathways between NEUC, SEUC and the eastern upwelling regions revealed that the NEUC feds into the Guinea Dome, into the equatorial upwelling (via retroflection into the equatorial undercurrent (EUC)) as well as into
the upwelling along the African coast, while most of the SEUC waters recirculate in the equatorial zonal currents. The second part of this work focused on interannual to decadal variability of the equatorial currents related to fluctuations of the subtropical-tropical cell (STC) and the conveyor belt (MOC) return flow. For the discussion of these phenomena different
sensitivity experiments have been performed using lower resolution models as a FLAME 1/3± Atlantic model and the 1/2± global ORCA configuration. The results from both models were found to be very similar and showed that the STC is not varying coherently, thus there is no obvious interaction between subtropical and tropical variability. In fact, the STCs are driven mainly by the local zonal wind stress. Despite the overriding importance of the wind-forced variability on interannual timescales, all model results showed also decadal variability signals. They were found to belong to variations of the deep water export across equator which is influenced by the formation rate of Labrador Sea Water. These MOC anomalies
revealed a basin-wide structure and were found to modulate the shallow transports in the tropical Atlantic on decadal timescales. However, the main signal of the MOC anomalies was found to be confined to the deep and intermediate waters and the amplitudes in vicinity of the surface are negligible.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Böning, Claus W.
Keywords: Tropical Atlantic; upwelling; STC; surface currents; Guinea Dome; Angola Dome; MOC variability; TIWs Atlantischer Ozean ; Tropen ; Meeresströmung
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
Refereed: No
Date Deposited: 03 Dec 2008 16:52
Last Modified: 30 Oct 2018 11:47

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