Sea Level

Changes in mean sea level are indicative of overall warming of the ocean and melting of ice on land. Rising sea level is a crucial issue for many islands in the Pacific. It causes ocean inundation, increased coastal flooding when coupled with waves, and shoreline erosion that leads to building and infrastructure damage. Impacts of higher sea levels also occur as a result of saltwater intrusion and inundation of aquifers and other domestic water supplies, and salinization and flood damage to agriculture.

In this section we look at two indicators of sea level change. We’ll be creating a table, a map, and a time series plot of absolute and regional sea level change at the Malakal, Palau tide gauge station from 1993-2022. Absolute sea level, typically measured by satellite altimetry, refers to the height of the sea surface relative to a reference ellipsoid. Here, we’ll use the global ocean gridded L4 Sea Level Anomalies (SLA) available from Copernicus, which is the sea surface height (SSH) minus the mean sea surface (MSS), where the MSS is the temporal mean of SSH over a given period. Relative sea level is measured by a tide gauge, and is the sea level relative to land at that location. We will be using observations form the Malakal tide gauge to assess relative sea level. Differences between the two measurements can arise from vertical land motion, regional oceanographic conditions like currents, and changes to the gravitational field (affecting the geoid). Using observations form the Malakal tide gauge, we will also be looking at change in the frequency of minor flooding. Minor flooding (also commonly referred to as high or king tide flooding, nuisance flooding) can occur when exceptionally high tides combine with large waves and/or other oceanic and atmospheric phenomena that raise the coastal water level. Here, a minor flood day is defined as a day in which the sea level at the tide gauge reaches or exceeds 30 cm above MHHW for at least one hour.