DOI: 10.17882/107013
title: Copernicus Marine In Situ TAC - Ocean Monitoring Indicator OMI_EXTREME_SL_NORTHWESTSHELF_slev_mean_and_anomaly_obs

description: 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, Tebaldi et al., 2021). 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 (Boumis et al., 2023). The increase in extreme sea levels over recent decades is, therefore, primarily due to the rise in mean sea level. Note, however, that the methodology used to compute this OMI removes the annual 50th percentile, thereby discarding the mean sea level trend to isolate changes in storminess.  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).
COPERNICUS MARINE SERVICE KEY FINDINGS The completeness index criteria is fulfilled in this region by 34 stations, eight more than in 2021 (26), most of them from Norway. The mean 99th percentiles present a large spatial variability related to the tidal pattern, with largest values found in East England and at the entrance of the English channel, and lowest values along the Danish and Swedish coasts, ranging from the 3.08 m above mean sea level in Immingan (East England)  to 0.57 m above mean sea level in Ringhals (Sweden) and Helgeroa (Norway). The standard deviation of annual 99th percentiles ranges between 2-3 cm in the western part of the region (e.g.: 2 cm in Harwich, 3 cm  in Dunkerke) and 7-8 cm in the eastern part and the Kattegat (e.g. 8 cm in Stenungsund, Sweden).. The 99th percentile anomalies for 2022 show positive values in Southeast England, with a maximum value of +8 cm in Lowestoft, and negative values in the eastern part of the Kattegat, reaching -8 cm in Oslo. The remaining stations exhibit minor positive or negative values.
 - References: Abram, N., Gattuso, J.-P., Prakash, A., Cheng, L., Chidichimo, M. P., Crate, S., Enomoto, H., Garschagen, M., Gruber, N., Harper, S., Holland, E., Kudela, R. M., Rice, J., Steffen, K., & von Schuckmann, K. (2019). Framing and Context of the Report. In H. O. Pörtner, D. C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, & N. M. Weyer (Eds.), IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (pp. 73–129). in press. https://www.ipcc.ch/srocc/.
 - Boumis, G., Moftakhari, H. R., & Moradkhani, H. 2023. Coevolution of extreme sea levels and sea-level rise under global warming. Earth's Future, 11, e2023EF003649. https://doi. org/10.1029/2023EF003649.
 - Dettmering D, Müller FL, Oelsmann  J, Passaro M, Schwatke  C, Restano M, Benveniste J, and Seitz F. 2021. North SEAL: A new dataset of sea level changes in the North Sea from satellite altimetry, Earth Syst Sci Data, 13, 3733–3753, https://doi.org/10.5194/ESSD-13-3733-2021. 
 - Legeais J-F, Llovel W, Melet A, and Meyssignac B. 2020. Evidence of the TOPEX-A Altimeter Instrumental Anomaly and Acceleration of the Global Mean Sea Level, In: Copernicus Marine Service Ocean State Report, Issue 4, Journal of Operational Oceanography, s77–s82, https://doi.org/10.1080/1755876X.2020.1785097.
 - Pérez-Gómez B, Álvarez-Fanjul E, She J, Pérez-González I, Manzano F. 2016. Extreme sea level events, Section 4.4, p:300. In: Von Schuckmann K, Le Traon PY, Alvarez-Fanjul E, Axell L, Balmaseda M, Breivik LA, Brewin RJW, Bricaud C, Drevillon M, Drillet Y, Dubois C , Embury O, Etienne H, García-Sotillo M, Garric G, Gasparin F, Gutknecht E, Guinehut S, Hernandez F, Juza M, Karlson B, Korres G, Legeais JF, Levier B, Lien VS, Morrow R, Notarstefano G, Parent L, Pascual A, Pérez-Gómez B, Perruche C, Pinardi N, Pisano A, Poulain PM , Pujol IM, Raj RP, Raudsepp U, Roquet H, Samuelsen A, Sathyendranath S, She J, Simoncelli S, Solidoro C, Tinker J, Tintoré J, Viktorsson L, Ablain M, Almroth-Rosell E, Bonaduce A, Clementi E, Cossarini G, Dagneaux Q, Desportes C, Dye S, Fratianni C, Good S, Greiner E, Gourrion J, Hamon M, Holt J, Hyder P, Kennedy J, Manzano-Muñoz F, Melet A, Meyssignac B, Mulet S, Nardelli BB, O’Dea E, Olason E, Paulmier A, Pérez-González I, Reid R, Racault MF, Raitsos DE, Ramos A, Sykes P, Szekely T, Verbrugge N. 2016. The Copernicus Marine Environment Monitoring Service Ocean State Report, Journal of Operational Oceanography. 9 (sup2):  235-320. https://doi.org/10.1080/1755876X.2016.1273446 
 - Pérez Gómez B, De Alfonso M, Zacharioudaki A, Pérez González I, Álvarez Fanjul E, Müller M, Marcos M, Manzano F, Korres G, Ravdas M, Tamm S. 2018. Sea level, SST and waves: extremes variability. In: Copernicus Marine Service Ocean State Report, Issue 2, Journal of Operational Oceanography, 11:sup1, Chap. 3.1, s79–s88, https://doi.org/10.1080/1755876X.2018.1489208.
 - Tebaldi, C., Ranasinghe, R., Vousdoukas, M. et al. 2021. Extreme sea levels at different global warming levels. Nat. Clim. Chang. 11, 746–751. https://doi.org/10.1038/s41558-021-01127-1. 
 - Tebaldi, C., Ranasinghe, R., Vousdoukas, M. et al. Author Correction: Extreme sea levels at different global warming levels. Nat. Clim. Chang. 13, 588 (2023). https://doi.org/10.1038/s41558-023-01665-w.
 - Vousdoukas MI, Mentaschi L, Hinkel J, et al. 2020. Economic motivation for raising coastal flood defenses in Europe. Nat Commun 11, 2119 (2020). https://doi.org/10.1038/s41467-020-15665-3. WCRP Global Sea Level Budget Group: Global sea-level budget 1993–present. 2018. Earth Syst. Sci. Data, 10, 1551-1590, https://doi.org/10.5194/essd-10-1551-2018.