'''DEFINITION'''The OMI_EXTREME_SL_NORTHWESTSHELF_slev_mean_and_anomaly_obs indicator is based on the computation of the 99th and the 1st percentiles from in situ data (observations). It is computed for the variable sea level measured by tide gauges along the coast. The use of percentiles instead of annual maximum and minimum values, makes this extremes study less affected by individual data measurement errors. The annual percentiles referred to annual mean sea level are temporally averaged and their spatial evolution is displayed in the dataset northwestshelf_omi_sl_extreme_var_slev_mean_and_anomaly_obs, jointly with the anomaly in the target year. This study of extreme variability was first applied to sea level variable (Pérez Gómez et al 2016) and then extended to other essential variables, sea surface temperature and significant wave height (Pérez Gómez et al 2018).'''CONTEXT'''Sea level (SLEV) is one of the Essential Ocean Variables most affected by climate change. Global mean sea level rise has accelerated since the 1990’s (Abram et al., 2019, Legeais et al., 2020), due to the increase of ocean temperature and mass volume caused by land ice melting (WCRP, 2018). Basin scale oceanographic and meteorological features lead to regional variations of this trend that combined with changes in the frequency and intensity of storms could also rise extreme sea levels up to one metre by the end of the century (Vousdoukas et al., 2020). This will significantly increase coastal vulnerability to storms, with important consequences on the extent of flooding events, coastal erosion and damage to infrastructures caused by waves. The North West Shelf area presents positive sea level trends with higher trend estimates in the German Bight and around Denmark, and lower trends around the southern part of Great Britain (Dettmering et al., 2021).'''KEY FINDINGS'''The completeness index criteria is fulfilled in this region by 26 stations, three more than in 2020 (23). The mean 99th percentiles present a large spatial variability related to the tidal pattern, ranging from the 3.08 m above mean sea level in Immingan (East England) to 0.58 m above mean sea level in Ringhals (Sweden). The standard deviation ranges between 3 cm in the western part of the region (Sheerness, Lerwick or Dunkerke) and 8 cm in the eastern part and the Kattegat (e.g. Hornbaek, Denmark). All the stations show negative or close to zero anomalies in 2021, reaching larger negative values along the Swedish coast (up to -13 cm in Ringhals) and the North of Scotland (-10 cm in Ullapool). This negative anomaly is significantly smaller in the Southern part of the region (e.g. close to zero in Dover). '''DOI (product):''' https://doi.org/10.48670/moi-00272