Atmospheric and Climatic Drivers of Tide Gauge Sea Level Variability along the East and South Coast of South Africa

Bernardino J Nhantumbo,Björn C Backeberg,Jan Even Øie Nilsen,Chris J C Reason

doi:10.3390/jmse9090924

Abstract

Atmospheric forcing and climate modes of variability on various timescales are important drivers of sea level variability. However, the influence of such drivers on sea level variability along the South African east and south coast has not yet been adequately investigated. Here, we determine the timescales of sea level variability and their relationships with various drivers. Empirical Mode Decomposition (EMD) was applied to seven tide gauge records and potential forcing data for this purpose. The oscillatory modes identified by the EMD were summed to obtain physically more meaningful timescales—specifically, the sub-annual (less than 18 months) and interannual (greater than two years) scales. On the sub-annual scale, sea level responds to regional zonal and meridional winds associated with mesoscale and synoptic weather disturbances. Ekman dynamics resulting from variability in sea level pressure and alongshore winds are important for the coastal sea level on this timescale. On interannual timescales, there were connections with ENSO, the Indian Ocean Dipole (IOD) and the Southern Annular Mode (SAM), although the results are not consistent across all the tide gauge stations and are not particularly strong. In general, El Niño and positive IOD events are coincident with high coastal sea levels and vice versa, whereas there appears to be an inverse relationship between SAM phase and sea level.

Highlights

IntroductionStudies are needed in spite of this limitation in some regions to avoid the use of the global ocean mean estimate rates as a reference for future projections on regional and local spatial scales (e.g., [1])
Since sea level records were corrected for the inverse barometer (IB) effect and the linear trend as well as the seasonal cycle were removed in both datasets, the effect of static pressure is removed
The correlation with coastal sea level variability illustrated here may be explained in terms of its interaction with wind variations associated with the passing weather systems

Summary

Introduction

Studies are needed in spite of this limitation in some regions to avoid the use of the global ocean mean estimate rates as a reference for future projections on regional and local spatial scales (e.g., [1])

Methods

Results

Discussion

Conclusion