DOI: 10.17882/107010
title: Copernicus Marine In Situ TAC - Ocean Monitoring Indicator OMI_EXTREME_SL_BALTIC_slev_mean_and_anomaly_obs

description: DEFINITION The OMI_EXTREME_SL_BALTIC_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 baltic_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 meter 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 Baltic Sea is affected by vertical land motion due to the Glacial Isostatic Adjustment (Ludwigsen et al., 2020) and consequently relative sea level trends (as measured by tide gauges) have been shown to be strongly negative, especially in the northern part of the basin. On the other hand, Baltic Sea absolute sea level trends (from altimetry-based observations) show statistically significant positive trends (Passaro et al., 2021).  
COPERNICUS MARINE SERVICE KEY FINDINGS Up to 45 stations fulfill the completeness index criteria in this region, a few less than in 2020 (51). The spatial variation of the mean 99th percentiles follow the tidal range pattern, reaching its highest values in the northern end of the Gulf of Bothnia (e.g.: 0.81 and 0.78 m above mean sea level at the Finnish stations Kemi and Oulu, respectively) and the inner part of the Gulf of Finland (e.g.: 0.83 m above mean sea level in St. Petersburg,  Russia). Smaller tides and therefore 99th percentiles are found along the southeastern coast of Sweden, between Stockholm and Gotland Island (e.g.: 0.42 m above mean sea level in Visby, Gotland Island-Sweden). Annual 99th percentiles standard deviation ranges between 3-5 cm in the South (e.g.: 3 cm in Korsor, Denmark) to 10-13 cm in the Gulf of Finland (e.g.: 12 cm in Hamina).  Negative anomalies of 2022 99th percentile are observed in the northern part of the basin, in the Gulf of Bothnia, in the inner part of the Gulf of Finland and in Lolland Island stations (Denmark) reaching maximum values of -12 cm in Kemi, -9 cm in St. Petersburg and -8 cm in Rodby, respectively.. Positive anomalies of 2022 99th percentile are however found in the central and southeastern parts of the basin, with maximum values reaching 7 cm in Paldisky (Estonia) and Slipshavn (Denmark).
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