DOI: 10.17882/107012
title: Copernicus Marine In Situ TAC - Ocean Monitoring Indicator OMI_EXTREME_SL_MEDSEA_slev_mean_and_anomaly_obs

description: DEFINITION The OMI_EXTREME_SL_MEDSEA_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 medsea_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 Mediterranean Sea shows statistically significant positive sea level trends over the whole basin. However, at sub-basin scale sea level trends show spatial variability arising from local circulation (Calafat et al., 2022; Meli et al., 2023).
COPERNICUS MARINE SERVICE KEY FINDINGS The completeness index criteria is fulfilled in this region by 38 stations, 26 more than in 2021,  significantly increasing spatial coverage with new in situ data in the central Mediterranean Sea, primarily from Italian stations. The mean 99th percentiles reflect the spatial variability of the tide, a microtidal regime, along the Spanish,  French and Italian coasts, ranging from around 0.20 m above mean sea level in Sicily and the Balearic Islands (e.g.: 0.22 m in Porto Empedocle, 0.23 m  in Ibiza)) to around 0.60 m above mean sea level in the Northern Adriatic Sea (e.g.: 0.63 m in Trieste, 0.61 m in Venice). . The annual 99th percentiles standard deviation ranges between 2 cm in Málaga and Motril (South of Spain) to 8 cm in Marseille. . The 2022 99th percentile anomalies present negative values mainly along the Spanish coast (as in 2021) and in the islands of Corsica and Sardinia (Western part of the region), while positive values are observed along the Eastern French Mediterranean coast and at most of the Italian stations (closer to the central part of the region), with values ranging from -4 cm in Málaga and Motril (Spain) to +5 cm in Ancona (Italy).
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