Time-varying mean sea level

Abstract

After a general theoretical consideration of basic mathematical aspects, we give numerical and physical details for the last two decades, where global and reliable data are available. The concept of a "mean sea level" is in itself rather artificial, because sea level varies at various temporal and spatial scales. This is because the sea is in constant motion, affected by the high- and low-pressure zones above it, the tides, local gravitational differences, and so forth. What is possible to do is to calculate the mean sea level at that point and use it as a reference datum. Traditionally, coastal tide gauges measure sea-level variability. Since 1993 satellite altimetry provides near-globalmaps of sea-level change with high spatial and temporal resolution. Sea-level change has therefore in altimetry its primary global source of information. Coastal and global sea-level variability is here considered. The coastal variability is estimated from tide gauges, from altimeter data colocated to the tide gauges and from altimeter data along the world coasts. The global variability is derived from altimeter data. We show that the sea level has in the interval 1993-2012 an average positive trend of 3:2 ± 0:4 mm/year, which is higher than the average trend of 1:8 ± 0:5 mm/year derived from tide gauge data over the twentieth century. The trends are not spatially uniform but regionally dependent. The variability at interannual time scale appears to be related to the variability of climatic indices, like NorthernAtlantic (NAO) and the El Nino-Southern Atlantic Oscillation (ENSO). At certain coastal locations, non-climatic components of relative sea-level change, mainly subsidence, are locally appreciable. The relative sea-level rise there is up to three to four times larger than the global mean sea-level rise.