Future sea-level rise on shallow continental shelves differs in one important aspect from open ocean sea-level rise: the local steric effect, that is the change in the water column height due to changes in sea water density, plays a minor role compared to the much deeper open ocean. Instead, the bulk of oceanic sea-level rise on continental shelves arises from an increase in ocean water mass that is being imported from the open ocean – the so-called shelf mass loading (SML). This redistribution is mainly driven by thermal expansion of water masses below shelf depth and magnifies as the subsurface ocean layers continue to warm. Few studies have tried to detect SML as the signal is only expected to become dominant over decadal to multidecadal periods given the large natural variability in shallow regions.Here, we combine hydrographic data from a section crossing the Norwegian shelf, with observations of total sea-level change from altimetry and estimates of mass changes from GRACE gravity missions to estimate the strength of SML over the past decades. We compare the residual of total sea level (from altimetry) and steric height (from hydrography) with GRACE estimates from three different solutions. Over the common period (2002 -2020), both estimates show a consistently higher trend over the shallow shelf area compared to the deep ocean. We estimate the shelf mass contribution in the order of 0.5 – 1.0 mm/yr, depending on the GRACE solution selected.

Studying ocean tides from satellite altimetry has traditionally been difficult in coastal regions, mainly due to the complexity of tides in these regions, limited spatial coverage, and land contamination of the radar returns. The Cal/Val phase and the science orbit phase of SWOT provide unique observations which can be exploited for tidal analysis. The nadir data provided by this mission complements other traditional altimetry missions and will serve the refinement of global ocean tide models well in future studies. The KaRIn data, however, is beneficial for evaluating the spatial variability of ocean tides at much smaller scales than previously possible from altimetry or in-situ measurements. In addition, areas very close to the shoreline can also be monitored. Analysing tides in complex coastal regions, such as fjords and inlets, is now also possible thanks to the increased spatial coverage of SWOT. This presentation evaluates the pixel cloud data of the hydrological product and the ocean product provided by SWOT in three regions. These regions are selected to provide examples of the usefulness of these data in very complex environments. The regions are as follows:The Bristol Channel, on the west coast of the UK. The Sognefjord along the west coast of Norway. The Long Island Sound on the east coast of the USA. These regions have relatively large tidal ranges and have been challenging for conventional altimetry, resulting in reduced accuracy in available ocean tide models. These regions are also well covered by in-situ measurements and are either covered by the Cal/Val phase or the nominal orbit of the SWOT mission. The resultant estimations will be contrasted with in-situ measurements and state-of-the-art global models.

Recent advances in Arctic observational capabilities have revealed that the Arctic Ocean is highly turbulent in all seasons and have improved temporal and spatial sampling of sea level retrievals from remote sensing, even above 80°N. Such data are expected to be increasingly valuable in the future when the extent of sea ice in the Arctic Ocean is reduced. Assimilating this new data into ocean models, together with in situ observations, provides an enriched representation of the mesoscale population that induces new eddy-driven contributions to local dynamics and thermodynamics. To quantify the content of the new information, we compare three-year-long assimilative experiments at ¼° resolution incorporating in situ-only data, in situ and standard altimetry, and in situ and high-latitude-enhanced altimetry, respectively. The enhanced altimetry data lead to an increase in three-dimensional eddy kinetic energy, generated by coherent vortexes, of up to 20% in several areas. Robust ocean warming is generated in the Arctic sector down to 800 m. Via heat budget analysis, this warming can be ascribed to a local enhancement of vertical mixing, as well as an increase in meridional heat transport. The assimilation of enhanced altimetry amplifies the transport, compared to standard altimetry, especially north of 70°N.

This study presents a comprehensive investigation of the complex dynamics of sea-level rise (SLR) and its multiple impacts on coastal regions in southern Africa. We meticulously analyse trends and patterns in SLR and subsidence rates using a wealth of data from 1993 to 2022, including observations from a network of 10 reliable tide gauges and XTRACK data processed using the Coastal Altimetry Approach to minimise the shortcomings of conventional coastal altimetry data. Our results show that sea level rise in coastal areas of South Africa, such as Cape Town (6.3 mm/yr), is almost double the global average (3.3 mm/yr). This alarming rate of SLR, coupled with a subsidence rate of more than 2.2 mm/yr, poses a significant and immediate threat to coastal communities, infrastructure and ecosystems. Our research also highlights the impact of seismic activity on coastal dynamics, further exacerbating the challenges posed by SLR. By incorporating the influence of earthquakes on subsidence, we provide a more nuanced understanding of the complex interplay of natural and anthropogenic factors contributing to SLR in the region. In addition, our research sheds light on the wider implications of SLR for some of Africa’s most iconic and culturally significant heritage sites, highlighting the urgent need for proactive coastal management and adaptation strategies.

The Norwegian Atlantic Current contributes fundamentally to the temperate climate of northwestern Europe and maintains an ice-free ocean well into the Barents Sea even in winter. The interaction between the two branches of the Norwegian Atlantic Current is not known in detail but is generally understood to be mediated by mesoscale eddies. In particular, the Lofoten Basin appears as a hot-spot of mesoscale activity in several studies in the literature, based on conventional altimetry. While satellite altimetry has made a fundamental contribution to our understanding of ocean circulation, the current constellation of nadir altimeters does not allow for resolving the spatial and temporal scales characterizing the intensification and dissipation of the mesoscale features. The Surface Water and Ocean Topography (SWOT) mission, based on Ka-band Radar Interferometry (KaRIn), extends the capability of existing nadir altimeters to two-dimensional mapping of the ocean surface at an unprecedented spatial resolution. The fast-sampling phase (1-day repeat) of the mission also allowed for resolving the temporal evolution of mesoscale eddies. The Lofoten Basin is located in an area where the SWOT tracks cross, which was sampled twice a day over 90 days in 2023. Building on this unique opportunity, the results presented in this work rely on both the fast-sampling and science (21-day repeat) phases of the SWOT mission to show a comparison of KaRIn retrievals with conventional altimeters and characterize the representation of the mesoscale field emerging from the different altimetry concepts.

The Gulf of Guinea (GoG) is highly vulnerable to sea level rise, with projections indicating a significant increase in permanently inundated land by 2100, ranging from 1,458.1 to 4,331.7 km2. This study evaluates the severity of potential coastal inundation in the GoG by comparing sea level rise projections from eight reliable CMIP6 models with historical sea surface height (SSH) data from 1993 to 2015 and current onshore topography. Eight model simulations were selected based on their accuracy in reproducing sea level variability in the Tropical Atlantic and the GoG, and their consistency in reflecting the one-month connection lag between equatorial-driven waves and Kelvin Coastal Trapped Waves (CTWs) along the GoG, critical for predicting regional ocean dynamics. Our findings indicate that this connection lag will remain consistent over time. Under high-emission scenarios, up to 95% of coastal areas could be inundated, potentially displacing 2 million people posing a socio-economic shock, given the region’s low GDP and heavy reliance on fisheries. The loss of cultural heritage and livelihoods further compounds the challenges. These findings emphasize the urgent need for targeted adaptation strategies and robust early warning systems, in line with the UN’s Sustainable Development Goals (SDGs), particularly SDG 13 (Climate Action) and SDG 14 (Life Below Water). This study offers a precise and regionally relevant assessment of future risks, providing a foundation for informed policy interventions to mitigate the impacts of climate change and protect vulnerable communities in the GoG.

Abstract. The Intergovernmental Panel on Climate Change (IPCC) plays a pivotal role in delivering information and knowledge on sea level rise (SLR), a global threat impacting coastlines worldwide. However, considerable disparities still persist in Europe in understanding and applying sea level science, evaluating its repercussions, and devising effective adaptation strategies. These are influenced by local factors such as diverse environments, socioeconomic conditions, policy contexts, and diversity in stakeholder involvement, producing, in turn, varying knowledge gaps and information needs across European sea basins. In this context, this chapter presents the findings of a comprehensive scoping process carried out by the European Knowledge Hub on Sea Level Rise (KH SLR) to define the outline of the first KH SLR Assessment Report. It consists of the analysis of stakeholder responses to an online survey and insights shared during four regional workshops, aiming to pinpoint critical gaps in available information on SLR and its potential consequences in European sea basins. It considers viewpoints from both scientific and policy perspectives, engaging stakeholders from academia and research and government sectors. The analysis is divided into three categories: (i) SLR science and information, (ii) SLR impacts, and (iii) SLR adaptation policies and decision-making. Regarding SLR science and information, many respondents found that relevant gaps exist in regional SLR projections and uncertainties, particularly related to long-term (from multidecadal to end of century) SLR induced by potential melting of large ice sheets. Interestingly, the perspective on information gaps is different for scientists (emphasizing the need to increase regional projection capabilities) and government users (stressing the availability of accurate projections for their regions). Regarding impacts and hazards, shoreline erosion stands out as a dominant concern in all sea basins except the Arctic, while emerging issues like saltwater intrusion and the role of SLR in compound risks associated with extreme water levels and river flow were also given significant regional relevance. With regard to policy and decision-making, existing adaptation plans are perceived as ineffective and lacking adaptability, with gaps related to underestimated impacts and urban planning. Participants, especially end-users of sea level knowledge, emphasized the relevance of improved information dissemination and communication to support informed decision-making.

The paper presents the reconstruction of sea levels in the North Sea and Baltic Sea using Kalman filter approach. Based on the statistical characteristics of one year of daily maps of sea level from the Geesthacht COAstal model SysTem (GCOAST) and daily data at tide gauges along the coastline of two basins, the method can reconstruct effectively and accurately the multidecadal sea level anomalies. The high accuracy reconstruction data were then used to investigate the interannual variability in both basins and to estimate the difference between outflows and inflows (net flux) through the Danish Straits. The highest mean, standard deviation, and extreme values of sea level anomalies appear in winter and are well reproduced in different regions, such as the German Bight, the Southern North Sea, the Bothnian Bay, the Gulfs of Finland and Riga. The sea level variability is highly correlated with the mean sea level pressure and the zonal wind, particularly in the German Bight and in the winter months. The contributions of river runoff and net precipitation on the net flux are significant in the spring. The local wind has a greater influence on the net flux than the remote drivers.

Sea level rise (SLR) poses a significant threat to coastal regions worldwide, particularly affecting over 60 million people living below 10 m above sea level along the African coast. This study analyzes the spatio-temporal trends of sea level anomaly (SLA) and its components (thermosteric, halosteric and ocean mass) in the Eastern Tropical Atlantic Ocean (ETAO) from 1993 to 2022. The SLA trend for the ETAO, derived from satellite altimetry, is 3.52 ± 0.47 mm/year, similar to the global average of 3.56 ± 0.67 mm/year. Of the three upwelling regions, the Gulf of Guinea (GoG) shows the highest regional trend of 3.42 ± 0.12 mm/year. Using the ARMORD3D dataset, a positive thermosteric sea level trend of 0.88 ± 0.04 mm/year is observed, particularly in the equatorial and southern Atlantic regions. The steric component drives the interannual SLA variability, while the ocean mass component dominates the long-term trends, as confirmed by the GRACE and GRACE-FO missions for 2002–2022. For those two decades, the total SLR from altimetry amounts to 3.80 ± 0.8 mm/year, whilst the steric component is reduced to only 0.19 ± 0.05 mm/year, leaving a residual increase in the ETAO of 3.69 ± 0.5 mm/year. The independent mass change from GRACE amounts to 2.78 ± 0.6 mm/year for this region, which just closes the sea level budget within present uncertainty levels. Spatial analysis of the steric components indicates a warming along the equatorial African coast including the GoG and a freshening near Angola. Strong correlations with regional climate factors, particularly the Tropical South Atlantic Index, highlight the influence of persistent climate modes. These findings underscore the urgent need for mitigation and adaptation strategies to SLR in the ETAO, especially for densely populated coastal communities.

Tropical Small Island Developing States (SIDS), such as those in the Caribbean, are among the most vulnerable to the impacts of climate change, most notably sea-level rise. The current sea-level rise in the Caribbean is 3.40 ± 0.3 mm/year (1993–2019), which is similar to the 3.25 ± 0.4 mm/year global mean sea-level (GMSL) rise (1993–2018). Throughout the year, Caribbean seasonal sea-level variability is found to respond to sea surface temperature variability. Over the past few decades, the trend in Caribbean Sea-level rise is also found to be variable. Satellite altimetry and steric sea-level records of the Caribbean region reveal a shift in the late 2003-early 2004, which separates two distinct periods of sea-level rise. Thermal expansion dominates the sea-level trend from 1993–2003. Following this period, there is an increased trend in sea-level rise, with a dominance of mass changes from 2004–2019, as confirmed by GRACE data. During this period, the sea-level trend is 6.15 ± 0.5 mm/year, which is 67% faster than the most recent estimates of global mean sea-level rise provided by the Intergovernmental Panel on Climate Change (3.69 ± 0.5 mm/year for the period 2006–2018). Despite its reduced importance, increasing temperatures contribute greatly to sea-level rise in the Caribbean region through thermal expansion of ocean water, hence there is a need to limit the current trend of global warming.

This study examines the impact of wave-induced processes (WIPs) in modulating thermosteric sea-level changes, highlighting the need to include these processes in future sea-level rise assessments and climate projections. The impact of wave-induced processes on thermosteric sea-level changes is investigated using coupled ocean-wave simulations. These simulations include the effects of Stokes-Coriolis forcing, sea-state dependent surface stress and energy fluxes, and wave-induced mixing. The experiments use a high-resolution configuration of the Geesthacht COAstal Model SysTem (GCOAST), covering the Northeast Atlantic, the North Sea and the Baltic Sea. The GCOAST system uses the Nucleus for European Modelling of the Ocean (NEMO) ocean model to account for wave-ocean interactions and ocean circulation. It is fully coupled with the WAM spectral wind wave model. The aim is to accurately quantify the sea state contribution to thermosteric sea level variability and trends over a 26-year period (1992–2017). The ability of wave-ocean coupled simulations to reveal the contribution of sea state to sea level variability and surge is demonstrated. It is clear that wave-induced processes (WIPs) play a significant role in sea surface dynamics, ocean mixing (mixed layer thickness) and modulation of air-sea fluxes (e.g. heat flux) in both winter (10–20

Coastal zones with dense populations, low elevations and/or inadequate adaptive capacity are on the frontline of unprecedented impacts from climate change. The Gulf of Guinea (GoG), stretching from Liberia to Gabon, is in particular vulnerable to coastal flooding caused by local and/or climate-induced sea level rise. In this region, interannual to decadal coastal sea level changes remain poorly understood, mainly due to a lack of tide gauge stations. Here we use nearly three decades (1993–2021) of satellite altimetry data to study the link between the Equatorial Atlantic and coastal GoG sea level variability. The rate of mean sea level rise increased from 3.47 to 3.89 ± 0.10 mm/yr from the Equatorial oceanic domain to the GoG coastal area, with an acceleration of 0.094 ± 0.050 mm/yr2. This corresponds to a mean sea level rise of about 8.9 cm over the entire altimetry period, 1993–2021. We focus on the (extreme) warm/cold events that occur in both the GoG during Atlantic Niños, and along the Angola-Namibia coast during Benguela Niños. Both events are driven by remote forcing via equatorial Kelvin waves and local forcing by local winds, freshwater fluxes and currents intensifications. Analysis of altimetry-based sea level, sea surface temperature anomalies, 20 °C isotherm based PIRATA moorings, and the Argo-based steric and thermometric sea level allows us to follow the coastal trapped waves (CTWs) along the GoG, and its link with major events observed along the strong Equatorial Atlantic warmings in 2010, 2012, 2019 and 2021. Both 2019 and 2021 warming have been identified as the warmest event ever reported in this region during the last 40 years. A lag of 1 month is observed between equatorial and West African coastal trapped wave propagation. This observation may help to better anticipate and manage the effects of extreme events on local ecosystems, fisheries, and socio-economic activities along the affected coastlines. In order to enable informed decision-making and guarantee the resilience of coastal communities in the face of climate change, it emphasises the significance of ongoing study in this field.

The intricate relationship between extreme river discharge and its consequential effects on coastal basins’ environment and dynamics remains a subject of profound significance. The impact of extreme river discharge on coastal basin dynamics and environment is a complex phenomenon, particularly relevant in the context of the German Bight. The inflow of freshwater from rivers into the German Bight plays a crucial role in driving the complex thermohaline circulation. Addressing the challenges posed by local hydro-meteorological extremes, compounded by strong wind waves, currents and tides is vital for comprehensive impact assessment. Utilizing the Geesthacht Coupled cOAstal model SysTem (GCOAST) with high-resolution configuration, we investigate the sensitivity of the thermohaline properties to the river forcings. By incorporating river discharge as lateral land forcing in a coupled hydrodynamic and wave model, we conducted five sensitivity experiments. Our findings reveal that the simulated temperatures closely match measurements in all experiments. The salinity, however, is remarkably sensitive to the variation of freshwater from the rivers Elbe and Ems in the German Bight, causing the haline stratification. The statistical evaluation, as demonstrated by the Taylor diagram at the Marnet DB station, underscores the skill of the Mesoscale Hydrologic Model (mHM) in generating the freshwater discharge that drives the thermohaline characteristics of the German Bight, especially during events like the June 2013 flooding. Significantly, the use of climatological runoff proves to be ineffective in simulating stratification during extreme flooding events. In essence, this investigation enhances our understanding of the pivotal role played by high-frequency river freshwater buoyancy. It emerges as a driving force behind salinity fluctuations during extreme floods, providing valuable insights into coastal dynamics within the German Bight.

Reliable sea-level observations in coastal regions are needed to assess the impact of sea level on coastal communities and ecosystems. This paper evaluates the ability of in-situ and remote sensing instruments to monitor and help explain the mass component of sea level along the coast of Norway. The general agreement between three different GRACE/GRACE-FO mascon solutions and a combination of satellite altimetry and hydrography gives us confidence to explore the mass component of sea level in coastal areas on intra-annual timescales. At first, the estimates reveal a large spatial-scale coherence of the sea-level mass component on the shelf, which agrees with Ekman theory. Then, they suggest a link between the mass component of sea level and the along-slope wind stress integrated along the eastern boundary of the North Atlantic, which agrees with the theory of poleward propagating coastal trapped waves. These results highlight the potential of the sea-level mass component from GRACE and GRACE-FO, satellite altimetry and the hydrographic stations over the Norwegian shelf. Moreover, they indicate that GRACE and GRACE-FO can be used to monitor and understand the intra-annual variability of the mass component of sea level in the coastal ocean, especially where in-situ measurements are sparse or absent.

<p>The effect of wave-induced processing on steric sea-level is investigated through ocean-wave coupled simulations. The experiments are performed with a high-resolution configuration of the Geestacht COAstal model SysTem (GCOAST), implemented in the Northeast Atlantic, the North Sea and the Baltic Sea which are considered as connected basins. The GCOAST system accounts for wave-ocean interactions and the ocean circulation relies on the NEMO (Nucleus for European Modelling of the Ocean) ocean model, while ocean-wave simulations are performed using the spectral wind wave model WAM. The objective is to quantify the sea-state contribution to steric sea-level variability and trend over a 26-year period (1992-2017). The ability of wave-ocean coupled simulations to disclose the sea-state contributions to sea-level variability and surge is demonstrated. The contribution of the wave-induced processes (WIPs) to the sea surface dynamics (e.g. temperature and salinity), ocean mixing (mixed layer thickness), and on the modulation of air-sea fluxes (e.g. heat flux) clearly appear both during winter (10-20 %) and summer (10 %), which in turn affect the steric sea-level variability. Investigating the components of steric sea-level signal, the thermosteric sea-level shows larger amplitudes compared to the halosteric component. Significant contributions to the thermosteric sea-level variability (40 %) due to wave-induced processes are observed in the North Atlantic (in summer) and along the Norwegian Trench (in winter). WIPs influence the thermosteric sea-level trends in the North Atlantic up to the order of 1 mm yr<sup>-1</sup>, both during winter and summer, in the open ocean and at the shelf break, while smaller contributions are observed over the shelf areas of the North Sea.</p>

Sea-level variations in coastal areas can differ significantly from those in the nearby open ocean. Monitoring coastal sea-level variations is therefore crucial to understand how climate variability can affect the densely populated coastal regions of the globe. In this paper, we study the sea-level variability along the coast of Norway by means of in situ records, satellite altimetry data, and a network of eight hydrographic stations over a period spanning 16 years (from 2003 to 2018). At first, we evaluate the performance of the ALES-reprocessed coastal altimetry dataset (1 Hz posting rate) by comparing it with the sea-level anomaly from tide gauges over a range of timescales, which include the long-term trend, the annual cycle, and the detrended and deseasoned sea-level anomaly. We find that coastal altimetry and conventional altimetry products perform similarly along the Norwegian coast. However, the agreement with tide gauges in terms of trends is on average 6 % better when we use the ALES coastal altimetry data. We later assess the steric contribution to the sea level along the Norwegian coast. While longer time series are necessary to evaluate the steric contribution to the sea-level trends, we find that the sea-level annual cycle is more affected by variations in temperature than in salinity and that both temperature and salinity give a comparable contribution to the detrended and deseasoned sea-level variability along the entire Norwegian coast. A conclusion from our study is that coastal regions poorly covered by tide gauges can benefit from our satellite-based approach to study and monitor sea-level change and variability.

Satellite altimetry measurements, complemented by in-situ records, have made a fundamental contribution to the understanding of global sea level variability for almost 30 years. Due to land contamination, it performs best over the open ocean. However, over the years, there has been a significant effort to improve the altimetry products in coastal regions. Indeed, altimetry observations could be fruitfully used in the coastal zone to complement the existing tide gauge network which, despite its relevance, does not represent the entire coast. Given the important role of coastal altimetry in oceanography, we have recently decided to check the quality of a new coastal altimetry dataset, ALES, along the coast of Norway. The Norwegian coast is well covered by tide gauges and, therefore, particularly suitable to validate a coastal altimetry dataset. Preliminary results show a good agreement between in-situ and remote sensing sea-level signals in terms of linear trend, seasonal cycle and inter-annual variability. For example, the linear correlation coefficient between the inter-annual sea level variability from altimetry and tide gauges exceeds 0.8. Likewise, the root mean square difference between the two is less than 2 cm at most tide gauge locations. A comparison with Breili et al. (2017) shows that ALES performs better than the standard satellite altimetry products at estimating sea level trends along the coast of Norway. Notably, in the Lofoten region, the difference between the sea level trends computed using ALES and the tide gauges range between 0.0 to 0.7 mm/year, compared to circa 1 to 3 mm/year found by Breili et al. (2017). These preliminary results go in the direction of obtaining an accurate characterization of coastal sea-level at the high latitudes based on coastal altimetry records, which can represent a valuable source of information to reconstruct coastal sea-level signals in areas where in-situ data are missing or inaccurate.

The mesoscale variability in the Mediterranean Sea is investigated through eddy detection techniques. The analysis is performed over 24 years (1993–2016) considering the three-dimensional (3D) fields from an ocean re-analysis of the Mediterranean Sea (MED-REA). The objective is to achieve a fit-for-purpose assessment of the 3D mesoscale eddy field. In particular, we focus on the contribution of eddy-driven anomalies to ocean dynamics and thermodynamics. The accuracy of the method used to disclose the 3D eddy contributions is assessed against pointwise in-situ measurements and observation-based data sets. Eddy lifetimes ≥ 2 weeks are representative of the 3D mesoscale field in the basin, showing a high probability (> 60%) of occurrence in the areas of the main quasi-stationary mesoscale features. The results show a dependence of the eddy size and thickness on polarity and lifetime: anticyclonic eddies (ACE) are significantly deeper than cyclonic eddies (CE), and their size tends to increase in long-lived structures which also show a seasonal variability. Mesoscale eddies result to be a significant contribution to the ocean dynamics in the Mediterranean Sea, as they account for a large portion of the sea-surface height variability at temporal scales longer than 1 month and for the kinetic energy (50–60%) both at the surface and at depth. Looking at the contributions to ocean thermodynamics, the results exhibit the existence of typical warm (cold) cores associated with ACEs (CEs) with exceptions in the Levantine basin (e.g., Shikmona gyre) where a structure close to a mode-water ACE eddy persists with a positive salinity anomaly. In this area, eddy-induced temperature anomalies can be affected by a strong summer stratification in the surface water, displaying an opposite sign of the anomaly whether looking at the surface or at depth. The results show also that temperature anomalies driven by long-lived eddies (≥ 4 weeks) can affect up to 15–25% of the monthly variability of the upper ocean heat content in the Mediterranean basin.