South Georgia is a heavily glaciated sub-Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine-terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high-resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier. The addition of a postulated shallow inner sill in one fjord arm significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could accelerate both the subglacial plume-driven melt, and the melting of the wider submarine ice face, which is proposed as a possible explanation for the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub-Antarctic island fjord, highlighting the sensitivity of plume dynamics to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat.

We show that steric sea-level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6-year cycle do not always correspond to the equilibrium nodal tide.

South Georgia is a heavily glaciated sub-Antarctic island in the Southern Ocean. Cumberland Bay is the largest fjord on the island, split into two arms, each with a large marine-terminating glacier at the head. Although these glaciers have shown markedly different retreat rates over the past century, the underlying drivers of such differential retreat are not yet understood. This study uses observations and a new high-resolution oceanographic model to characterize oceanographic variability in Cumberland Bay and to explore its influence on glacier retreat. While observations indicate a strong seasonal cycle in temperature and salinity, they reveal no clear hydrographic differences that could explain the differential glacier retreat. Model simulations suggest the subglacial outflow plume dynamics and fjord circulation are sensitive to the bathymetry adjacent to the glacier, though this does not provide persuasive reasoning for the asymmetric glacier retreat. The addition of a postulated shallow inner sill in one fjord arm, however, significantly changes the water properties in the resultant inner basin by blocking the intrusion of colder, higher salinity waters at depth. This increase in temperature could significantly increase submarine melting, which is proposed as a possible contribution to the different rates of glacier retreat observed in the two fjord arms. This study represents the first detailed description of the oceanographic variability of a sub-Antarctic island fjord, highlighting the sensitivity of fjord oceanography to bathymetry. Notably, in fjords systems where temperature decreases with depth, the presence of a shallow sill has the potential to accelerate glacier retreat.

This paper reports a new and significantly enhanced analysis of US flood hazard at 30m spatial resolution. Specific improvements include updated hydrography data, new methods to determine channel depth, more rigorous flood frequency analysis, output downscaling to property tract level and inclusion of the impact of local interventions in the flooding system. For the first time we consider pluvial, fluvial and coastal flood hazards within the same framework and provide projections for both current (rather than historic average) conditions and for future time periods centred on 2035 and 2050 under the RCP4.5 emissions pathway. Validation against high quality local models and the entire catalogue of FEMA 1% annual probability flood maps yielded Critical Success Index values in the range 0.69-0.82. Significant improvements over a previous pluvial/fluvial model version are shown for high frequency events and coastal zones, along with minor improvements in areas where model performance was already good. The result is the first comprehensive and consistent national scale analysis of flood hazard for the conterminous US for both current and future conditions. Even though we consider a stabilization emissions scenario and a near future time horizon we project clear patterns of changing flood hazard (-3.8 to +16% changes in 100yr inundated area at 1° scale), that are significant when considered as a proportion of the land area where human use is possible or in terms of the currently protected land area where the standard of flood defence protection may become compromised by this time.

We demonstrate that long-term tidally-induced changes in extreme sea levels affect estimates of major flood hazard in a predictable way. Long-term variations in tides due to the nodal and perigean tidal cycles influence extreme sea levels at 234 global tide gauges out of a total of 344. Results show regions where the amplitudes of the modulations are particularly relevant in the 100-year return sea level; the eastern coast of China, the northwestern and northeastern coasts of Australia, the northeast coast of Pacific, and the eastern coast of Europe, reaching up to ~22 cm in western France. We identify locations that are currently in a positive phase of the modulation and therefore at a higher risk of flooding, as well as when (year) the next peak of the nodal/perigean modulations is expected to occur. The timing of the peak of the modulation is spatially coherent and influenced by the relative importance of each cycle (nodal or perigee) over the total amplitude. An evaluation of four locations suggests that the potentially flooded area in a 100-year event can vary up to ~45% (in Boston) as a result of the long-term tidal cycles; however, areal change is often smaller due to local topography and tidal characteristics (6-13%). We conclude that tidally-modulated changes in extreme sea levels can alter the potentially inundated area in a 100-year event and that the traditional, fixed 100-year floodplain is inadequate for describing coastal flood risk.

Low-lying coastal zones are prone to flooding from multiple drivers such as storm surge (oceanographic), excessive river discharge (fluvial), and/or surface runoff (pluvial). The flooding impacts can be exacerbated, depending on local characteristics, when flooding is intensified by concurrent (or successive) occurrence of multiple drivers known as ‘compound flooding’. Recently, compound flooding drivers are becoming more frequent and intense leading to more adverse impacts. In this study, we carry out a continental scale analysis for the CONUS coastline at locations with sufficiently long overlapping records to characterize the changes in dependence and co-occurrence between the compound flooding drivers over time. We also investigate the changes in dependence over time during tropical and extratropical seasons. Lastly, we assess how the dependence structure varies with time. We use observations (gauge records) for the analysis. Dependence between different pairs is assessed using co-occurrence counts and statistical measures for dependence (Kendall’s rank correlation coefficient, τ). The dependence structures (particularly the tails of bivariate distributions) are compared using Kullback–Leibler (KL) Divergence to assess if there are significant changes in tails of bivariate distributions over time. This analysis provides a comprehensive characterization of changes in compound flooding potential around the CONUS coastline. This will provide insights on where and how compound flooding potential has changed over time to be incorporated in flood risk assessments and planning.

In many places around the world, tide gauges have been measuring substantial non-astronomical changes. Here we document an exceptional large spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous, suggesting a superposition of local and large-scale processes at work within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines, which suggest movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which suggests an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.

Sea-level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientist and practitioners, builds on a framework of discussing physical evidence to quantify high-end global SLR for practice. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2 ˚C in 2100 (SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for +5 ˚C (SSP5-8.5) we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2 ˚C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while we emphasize the timing of ice-shelf collapse around Antarctica, which is highly uncertain due to low understanding of the driving processes.