Kemena, Tronje P. (2011) Analyse von zonalen Oberflächenströmungen im äquatorialen Atlantik aus Lagrange’schen Driftern und aus Altimetrie. (Bachelor thesis), Christian-Albrechts-Universität, Kiel, Germany, 38 pp.

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Abstract

The surface current of the equatorial Atlantic can be calculated from drifter or altimetry. With drifter you have in-situ measurements of the current velocity and from altimetry data you get the anomaly of the geostrophic current.
An important part of this work is the comparison of zonal geostrophic currents from altimetry- and drifter data for a 32°W to 15°W range and for different latitude bins between 5°N and 5°S. The altimetry data consists of a weekly dataset of surface anomalies from AVISO on a 0.33°x0.33° Mercator-grid. The drifter data contains observed surface velocities of the ARGO Floats and velocities of the Surface Velocity Program (SVP) Drifter. For further investigations satellite altimetry is converted in geostrophic velocity with a modified model of the equatorial geostrophy from Lagerloef et al. (1999). Also Drifter data is converted into geostrophic velocity by subtracting the Ekman current and correcting the „down-wind slip“. This is generated by the effect of wind and waves. Due to the “down-wind slip” the measured in-situ velocity differs from the current velocity. Further statistical analysis shows a widely significant positive correlation of altimetry- and drifter data and an increased variance of the drifter time series.
The second part of this work deals with the investigation of interannual variability of the zonal geostrophic current system. A mode observed by altimetry with a period of 1670 days and an amplitude of 5.9 (+/- 1.9) cm/s shall be verified (Brandt, et al. 2011). A similar mode can be detected in the drifter data with a period of 1627 days for the latitude bins (chapter 3.2,table 4) and of 1780 days for the interpolated grid (chapter 4). The amplitude for the 1780 days period was determinate with 4.4 cm/s and 4.1 cm/s (chapter 4, Abbildung 19,c respectively 19,d ). Differences are a result of different methods of calculation.

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