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Abstract
Eight years of sea surface height data derived from the TOPEX/Poseidon altimeter, are analyzed in order to identify long- and a-periodic behavior of the North Atlantic sea level. For easy interpolation, sea surface height data are converted into sea surface topography data using the geoid derived from EGM96 to degree 360. Principal Component Analysis is used to identify the most dominant spatial and temporal variations. In order to separate dominant periodic signals, a yearly and a half-yearly oscillation, as well as alias effects from imperfect ocean tide corrections, are estimated independently by a Harmonic Analysis and subtracted. The residuals are smoothed by a 90-day moving average filter and examined once again by a PCA, which identifies a low-frequency variation with a period of approximately 6–7 years and an amplitude of about 1 dm, as well as a large sea level change of partially more than ±1 dm within only few months. This sea level change can also be seen in yearly and seasonal sea level residuals. Furthermore, the analysis shows a significant sea level change in 1998 occurring almost over the whole North Atlantic, which is not clearly identified by the PCA. Similar results are obtained by analyzing sea surface temperature and sea level pressure data.
Acknowledgments
M. Kuhn is currently at Western Australian Centre for Geodesy, Curtin University of Technology, Perth, WA, Australia. R. Kaniuth is currently at Institute of Geodesy and Navigation, University FAF Munich, Neubiberg, Germany.
This work was funded by the BMBF grant No. 03F0193A/0 for the Project EVAMARIA (Enterprise for Verification of Anomalies in the Mean Sea Level by Satellite Altimetry and Tide Gauge Records in the North Atlantic). The authors would like to thank two anonymous reviewers for their helpful comments on this article.