Kopte, Robert (2013) Seasonal variations of the Deep Western Boundary Circulation in the subpolar North Atlantic Ocean – Observations in comparison with model data. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 72 pp.

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Abstract

The western part of the subpolar North Atlantic Ocean is enclosed by a largely barotropic cyclonic boundary current system, hugging the continental slope. In this study the focus is on the seasonality of this current system. The analysis is based on current meter data from two different mooring arrays. Both the 53°N array near the southern exit of the Labrador Sea (1997-2012) and the Angmassalik array off the coast of East Greenland (1995-2010) provide data spanning a 15-year period. The analysis focusses on the deeper layers occupied by the Deep Western Boundary Current (DWBC). However, the instrument coverage of the 53°N array also allows the examination of the shelf-edge branch of the Shallow Labrador Current (SLC). Furthermore, the output of the high-resolution model VIKING20 is investigated with respect to seasonal cycles in order to allow for a comparison with the observations. Harmonic analysis is applied as the preferred tool for the detection and interpretation of seasonal signals, whereas spectral analysis is used to address the full range of variability. Within the DWBC only weak seasonal amplitudes usually smaller than 2 cm=s are detected both in the 53°N and the Angmassalik array. In particular, the DWBC does not show a pronounced response to the seasonal cycle of the large-scale wind stress curl, even though this has been suggested by previous (model) studies. However, in both the 53°N and the Angmassalik arrays considerable seasonal changes in the alongshore velocities are occasionally observed at the offshore end of the DWBC, suggesting a shifting of the DWBC sector during these periods. Three mechanisms are discussed as possible sources of these shifts: Meandering, expanding, and swinging. As the observed changes are quite barotropic, wind forcing is suspected as a potential driver, but a connection to anomalies in the large-scale wind stress curl cannot be established. The shelf-edge branch of the SLC is found to exhibit a significant seasonal cycle with maximum flow speed observed during the winter months, although seasonal amplitudes vary considerably between approx. 3 cm/s to more than 6 cm=s on interannual time scales. The time of maximmum flow in the shallow circulation roughly coincides with the wintertime maximum in basin-wide cyclonic wind stress curl. Hence, the seasonality of the shelf-edge branch of the SLC appears to be controlled by the wind forcing over the Labrador Sea. VIKING20 sucessfully reproduces the structure and magnitude of the mean circulation for both arrays. Furthermore, it reflects the general range of the observed seasonal amplitudes reasonably well, especially for the deep flow. Unlike previous model configurations but in very good agreement with the observations, VIKING20 does not suggest a deep-reaching barotropic response of the ocean to variations in the wind stress curl. Yet, the model seems to have difficulties to realistically simulate the seasonality of the shallow circulation for the 53°N section.

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