Seasonal Patterns, Inter‐Annual Variability, and Long‐Term Trends of Mean Sea Level along the Western Iberian Coast and the North Atlantic Islands

Sea level rise is challenging for coastal communities and land management decision makers. Understanding the patterns of regional variations at different temporal and spatial scales is key to implement adaptation plans that mitigate the local impacts of sea level rise. In this study, in situ observations from 14 tide gauges were complemented with satellite altimetry data to assess seasonality, multidecadal variability and long-term trends in mean sea level around the Western Iberian Coast (WIC) and the Portuguese archipelagos (Azores and Madeira). Results show varying spatial seasonal patterns between regions, with minimum (maximum) sea level observed in April (September) at the islands and minimum (maximum) observed in July (November) at the WIC. The influence of coastal upwelling on the seasonal mean sea level variations was detected over mainland. Although the influence of atmospheric patterns was observed on sea level inter-annual variability, the Atlantic Multidecadal Oscillation (AMO) showed a greater correlation with the sea level inter-decadal patterns. Finally, the trend analysis confirmed widespread sea level rise along the mainland and around the islands, which has intensified in recent decades. The regions of La Coru & ntilde;a and Cascais showed trends that were similar to the global average sea level rise since 1993, but the mainland regional average pointed to lower rates of rise (2.00 +/- 0.06 mm/year). This work reinforces the need for long-term monitoring networks of sea level, ensuring the vertical stability of instruments and platforms. The implementation of regional adaptation plans to sea level rise is deeply dependent on high quality information. Sea level is rising worldwide, and coastal communities are starting to have to deal with the effects of such change. However, the rate of sea level rise changes geographically and with time, so it is important to understand regional sea level variations, as it is what local communities directly experience. Important knowledge gaps remained regarding sea level variability along the Western Iberian Coast (WIC) and the Portuguese Islands. To overcome these, 14 long time series of sea level observations were used along with satellite data to study: (a) seasonal variations; (b) inter-decadal patterns; and (c) long-term trends in mean sea level across these regions. Different seasonal patterns were obtained: minimum and maximum sea level was reached 2/3-month later at the WIC than along the islands. The influence of coastal upwelling on the seasonality over the WIC was confirmed. The multidecadal variation in ocean temperature had a strong influence on the inter-annual variation in sea level. Lastly, sea level has been rising in almost all locations, but the WIC average rate was lower than the global average. This study highlights the need to monitor instrument stability and measurement quality as this will guarantee the implementation of effective regional adaptation plans. Western Iberian Coast and islands with differences in phase and amplitude of the mean seasonal cycle, in part due to upwelling Multidecadal variation in ocean temperature with the strongest influence on the inter-annual variation in sea level over the region Mainland regional average trend of 2.00 +/- 0.06 mm/year lower than the global rise observed over the past 30 years