Effects of Offsets and Outliers on the Sea Level Trend at Antalya 2 Tide Gauge within the Eastern Mediterranean Sea

Abstract

Antalya 2 tide gauge (TG) station is located on the coast of Turkey within the Eastern Mediterranean Sea. Relative sea level trends 6.0 ± 1.5 and 6.44 ± 0.45 mm/year over 1985–2009 at Antalya 2 TG are different from the trend (1.6 ± 1.5 mm/year over 1935–1977) at Antalya TG within 10 km. In order to investigate this trend discrepancy, the monthly mean series at Antalya 2 TG is re-analyzed for offsets, outliers and trend estimation. The Zivot-Andrews method and the $Q_p$ outlier test result in one offset at 1994.0417 year with magnitude of 71.24 ± 13.48 mm and nine outliers. The series, corrected for the offset and outliers, de-seasonalized and filled for missed points, is identified as trend-stationary process and analyzed for trend estimation by various models. The optimal model providing the lowest Akaike Information Criteria is polynomial linear trend with multiplicative seasonal Autoregressive Moving Average (ARMA(2,0)x(1,0)₁₂). The estimated relative sea level trend by the optimal model is 1.77 ± 0.65 mm/year. The large trend discrepancy at Antalya 2 TG is accounted for by one offset primarily (∼71%) and nine outliers (∼3%).

Keywords:

• Tide Gauge
• offset
• outlier
• sea level trend
• Eastern Mediterranean Sea

Acknowledgements

The author appreciates to PSMSL for gathering and releasing the monthly mean sea level data at Antalya 2 TG station. Thanks to the geodetic staff at the General Directorate of Mapping to share their knowledge on Antalya 2 TG. I am grateful to Prof. Rob J. Hyndman and Dr. JQ Veenstra for their helps to understand the usage of the packages forecast and arfima respectively. Thanks to the two anonymous reviewers for their constructive comments. Finally my profound thanks to the RStudio (https://rstudio.com/) which is an open-source integrated development environment for R and to the maintainers of the packages used in this study. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Disclosure statement

The author reports no declarations of interest.

Data availability statement

The monthly mean sea level series at Antalya 2 TG underlying in this article are available on PSMSL Web site https://www.psmsl.org/.

Author’s contribution

The author M.E.A. achieved all required works and wrote the paper.