Lankhorst, Matthias (2007) Analyses of the Circulation in Intermediate and Shallow Water Masses of the North Atlantic with Lagrangian and Profiling Methods. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 98 pp.

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Abstract

The Atlantic Meridional Overturning Circulation (MOC) transports warm water northwards in near-surface depth layers and cold water southwards at greater depths. This dissertation investigates aspects of the circulation within the context of the MOC in the northeastern and tropical Atlantic, based on observations of the past 20 years. The instrumentation mainly relies on submerged drifters called floats, which measure current velocities and—depending on instrument type—hydrographic profiles. In the northeastern Atlantic, the entire water column is analyzed. Volume transports by currents in the upper 1000 m and their temporal variabilities are derived via long-distance geostrophy. These currents are a part of the upper, warm branch of the MOC. The total transport by the North Atlantic Current in the study area is approximately 25 Sv with interannual variability up to 3 Sv on interannual time scales for individual current branches. At intermediate depths, various spreading paths within the deep, cold branch of the MOC are documented by selected float trajectories, and the properties of the mesoscale eddy field are quantified. Lagrangian eddy kinetic energy and time scales show a strong contrast between the subpolar and the subtropical basins. Especially in the subpolar basin, several regions of increased eddy activity emerge. An algorithm for automatic identification of coherent eddies reports particularly many and energetic cyclones in the subpolar northeastern Atlantic, which obviously play an important role in the mixing processes there. In the tropical Atlantic, depth layers in the intermediate and in the upper part of the deep waters are analyzed. The separation between northward- and southward-flowing branches of the MOC is between these layers. Both feature zonal bands of strong, alternating zonal currents near the equator. These bands are circa 200 km wide on average, and are in parts subject to a strong seasonal cycle. Boundary currents flow along the western boundary of the basin, northwards in the intermediate and southwards in the deep waters, which interact with the zonal current bands of the interior basin. Energy and time scales are described for the tropical region, too. Here, eddy activity generally increases towards the boundary. The Lagrangian eddy kinetic energies and time scales at intermediate depths both in the tropical and the northeastern Atlantic are found to be 1–150 cm2 s−2 and 2–10 days. Highest mean current velocities at these depths are approximately 15 cm s−1. New methodologies presented in this thesis are the observation of volume transport in the northeastern Atlantic by long-distance geostrophy and the algorithm to detect eddies in individual trajectories. Both are suited for future studies with similar instruments. The description of mean currents and the mesoscale eddy field is proper for validating numerical models of large-scale ocean circulation, which brings them back into the context of the MOC.

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