Sea Level

Sea level is the height of the ocean surface. 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 damages built and natural infrastructure (Marra et al., 2012; Miles et al., 2020). Impacts of higher sea levels also occur because of saltwater intrusion and inundation of aquifers affects domestic water supplies, and salinization and flood damage affects agriculture.

Changes in mean sea level are indicative of overall warming of the ocean and melting of ice on land. Changes in absolute sea level relative to Earth’s center are measured by satellite altimeters (e.g., EU Copernicus and NASA/CNES). Changes in sea level relative to the elevation of the land at a particular location are measured by a tide gauges, like the one in Palau at Malakal operated by the University of Hawaii Sea Level Center. Differences between the two measurements can arise from vertical land motion (e.g., associated with some earthquakes), regional oceanographic conditions like currents, and changes to the earth’s gravitational field.

In addition to variability from place to place, sea level is highly variable over time. For Palau, the highest sea levels tend to occur in the late boreal summer to early fall, in association with the perigean spring or “king tides” that can result in minor flooding (SPC 2022, UHSLC 2025). Large sea level fluctuations in the western North Pacific are also associated with ENSO: Below-normal sea levels are typical during El Niño events and above-normal sea levels during La Niña (Marra et al., 2012; Widlansky et al., 2014).

In this section we look at four indicators of sea level change. For our first indicator, 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). Our second indicator looks at sea level anomalies on a regional and local (tide gauge) scale, using the same two datasets. Larger scale anomalies are common with ENSO-related sea level patterns.

For the third indicator, using observations form the Malakal tide gauge, we will look 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. Lastly, we will look at the top 10 high and low sea levels at the tide gauge for our fourth indicator. Ranking extreme sea levels helps us quickly put high and low water levels into historical context with relatively little coding effort.