Herrford, Josefine (2021) Observing seasonal to decadal variability related to the Atlantic meridional overturning circulation at 11°S. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 141 pp.

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Abstract

The Atlantic Meridional Overturning Circulation (AMOC) plays a significant role in modulating the global climate by redistributing large amounts of heat, freshwater and carbon. It is projected to weaken under a continuously warming climate. Understanding the mechanisms that control its structure and variability on different timescales is, therefore, a key priority for oceanographers and climate scientists. This thesis presents the first estimate of AMOC variations for the tropical South Atlantic – based on bottom pressure observations from the TRACOS array at 11°S. Over the observed period (2013-2018), the AMOC and its components are dominated by seasonal variability, with peak-to-peak amplitudes of 12 Sv for the upper-ocean geostrophic transport, 7 Sv for the Ekman and 14 Sv for the AMOC transport. The observed seasonal variability of the geostrophic contribution to the AMOC in the upper 300 m is controlled by pressure variations at the eastern boundary, while at 500 m depth contributions from the western and eastern boundaries are similar. The respective mechanisms are investigated with the ocean general circulation model INALT01. In the model, seasonal AMOC variability at 11°S is governed, besides the Ekman transport, by the geostrophic transport variability in the eastern basin, which is modulated by oceanic adjustment to local and remote wind forcing. Seasonal transport variability in the western basin interior is instead mainly compensated by the Western Boundary Current. These analyses showed, that uncertainties in the wind forcing are particularly relevant for the resulting uncertainties of AMOC estimates at 11°S. A collection of different gridded surface wind products widely used within the oceanographic community is evaluated regarding the mean wind field and variability in the tropical Atlantic. While all of the wind products capture the mean, large-scale atmospheric circulation in this region, their ability to reproduce the wind variability observed with moored buoys depends on the considered timescale as well as wind regime. For example, the CORE II forcing dataset does not reproduce the observed intra-seasonal variability at any of the buoy locations, scatterometer winds show low correlations on interannual timescales and most of the products have problems to reproduce the wind variability in the eastern tropical South Atlantic. ....

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