One of the main consequences of mean sea level rise (SLR) on human settlements is an increase in flood risk due to an increase in the intensity and frequency of extreme sea levels (ESL). While substantial research efforts are directed towards quantifying projections and uncertainties of future global and regional SLR, corresponding uncertainties in contemporary ESL have not been assessed and projections are limited. Here we quantify, for the first time at global scale, the uncertainties in present-day ESL estimates, which have by default been ignored in broad-scale sea-level rise impact assessments to date. ESL uncertainties exceed those from global SLR projections and, assuming that we meet the Paris agreement goals, the projected SLR itself by the end of the century in many regions. Both uncertainties in SLR projections and ESL estimates need to be understood and combined to fully assess potential impacts and adaptation needs.

Ocean impact on decadal Atlantic climate variability revealed by sea-level observations. Nature, 521 (7553). 508-510. 10.1038/nature14491 Contact NOC NORA team at publications@noc.soton.ac.ukThe NERC and NOC trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. properties and offer timeseries of sufficient length (Ext. Data Fig. 1) to study decadal 46 ocean circulation variations. Investigating ocean circulation using tide gauges is not new: 47 the first attempt to estimate the Gulf Stream using tide gauges was made in 1938 14 . The 48 principle is based on geostrophic dynamics: on timescales longer than a few days, ocean 49 circulation is in geostrophic balance so, looking downstream, the sea level is seen to 50increase from left to right in the northern hemisphere. 51 52Estimates of the Gulf Stream using tide gauges have focused on the use of gauges on the 53American east coast with an offshore estimate of sea level from either an island gauge 15 54 or a reconstructed sea level 16 . A weakness of this method is that the offshore 55 measurement lies in the eddy-filled ocean where sea-level fluctuations at any one point 56 are influenced by the mesoscale 17 even on long timescales, increasing the difficulty of 57 making estimates of ocean circulation that is coherent on large spatial scales. This is the 58 case for sea level at Bermuda, whose decadal fluctuations can be reproduced by 59 considering a Rossby wave response to wind forcing 16 . To make estimates of ocean 60 circulation that capture the fluctuations in large-scale circulation and less eddy variability, 61 measurements close to or on the western boundary are necessary 18 . We account for this 62 by focusing on the gradient of sea level along the US east coast. The mean dynamic sea 63 level decreases to the north along the east coast of the US (Fig. 1a) (Fig. 1a), we can construct a single sea-level composite 82 representative of the subtropical (subpolar) circulation by averaging sea-level from 83 linearly detrended, deseasonalised tide gauges, with the inverse barometer effect removed, 84 south (north) of the Cape (Fig. 1b, c). The difference, south minus north (Fig. 1d), 85 represents our circulation index. This index projects onto observed surface velocities 86 during the satellite era in the intergyre region, with a positive index associated with more 87 northwards flow and a more northerly path of this circulation (Extended Data Fig. 4). captured by the accumulated sea-level index, observationally supporting the hypothesis 113 6 that circulation changes and not only air-sea fluxes were involved in these changes 28 . For 114 the purposes of statistical analyses, the timeseries have had a 7-year low-pass, Tukey 115 filter applied to them, which is referred to with the prefix '7-year' from here on. The 7-116 year sea-level index leads the 7-year rate of heat content change by 2 years with a 117 maximum correlation of ...

When river and coastal floods coincide, their impacts are often worse than when they occur in isolation; such floods are examples of 'compound events'. To better understand the impacts of these compound events, we require an improved understanding of the dependence between coastal and river flooding on a global scale. Therefore, in this letter, we: provide the first assessment and mapping of the dependence between observed high sea-levels and high river discharge for deltas and estuaries around the globe; and demonstrate how this dependence may influence the joint probability of floods exceeding both the design discharge and design sea-level. The research was carried out by analysing the statistical dependence between observed sea-levels (and skew surge) from the GESLA-2 dataset, and river discharge using gauged data from the Global Runoff Data Centre, for 187 combinations of stations across the globe. Dependence was assessed using Kendall's rank correlation coefficient ( ) and copula models. We find significant dependence for skew surge conditional on annual maximum discharge at 22% of the stations studied, and for discharge conditional on annual maximum skew surge at 36% of the stations studied. Allowing a time-lag between the two variables up to 5 days, we find significant dependence for skew surge conditional on annual maximum discharge at 56% of stations, and for discharge conditional on annual maximum skew surge at 54% of stations. Using copula models, we show that the joint exceedance probability of events in which both the design discharge and design sea-level are exceeded can be several magnitudes higher when the dependence is considered, compared to when independence is assumed. We discuss several implications, showing that flood risk assessments in these regions should correctly account for these joint exceedance probabilities.

Scientists and engineers have observed for some time that tidal amplitudes at many locations are shifting considerably due to nonastronomical factors. Here we review comprehensively these important changes in tidal properties, many of which remain poorly understood. Over long geological time scales, tectonic processes drive variations in basin size, depth, and shape and hence the resonant properties of ocean basins. On shorter geological time scales, changes in oceanic tidal properties are dominated by variations in water depth. A growing number of studies have identified widespread, sometimes regionally coherent, positive, and negative trends in tidal constituents and levels during the 19th, 20th, and early 21st centuries. Determining the causes is challenging because a tide measured at a coastal gauge integrates the effects of local, regional, and oceanic changes. Here, we highlight six main factors that can cause changes in measured tidal statistics on local scales and a further eight possible regional/global driving mechanisms. Since only a few studies have combined observations and models, or modeled at a temporal/spatial resolution capable of resolving both ultralocal and large‐scale global changes, the individual contributions from local and regional mechanisms remain uncertain. Nonetheless, modeling studies project that sea level rise and climate change will continue to alter tides over the next several centuries, with regionally coherent modes of change caused by alterations to coastal morphology and ice sheet extent. Hence, a better understanding of the causes and consequences of tidal variations is needed to help assess the implications for coastal defense, risk assessment, and ecological change.

[1] Periods of high astronomically generated tides contribute to the occurrence of extreme sea levels. Over interannual time scales, two precessions associated with the orbit of the Moon cause systematic variation of high tides. A global assessment of when these tidal modulations occur allows for the prediction of periods when the enhanced risk of coastal flooding is likely in different parts of the world. This paper uses modeled tides to assess the influence of the 18.61 year lunar nodal cycle and the 8.85 year cycle of lunar perigee (which affects high tidal levels as a quasi 4.4 year cycle) on high tidal levels on a global scale. Tidal constituents from the TPXO7.2 global tidal model are used, with satellite modulation corrections based on equilibrium tide expectations, to predict multidecadal hourly time series of tides on a one-quarter degree global grid. These time series are used to determine the amplitude and phase of tidal modulations using harmonic analysis fitted to 18.61, 9.305, 8.85, and 4.425 year sinusoidal signals. The spatial variations in the range and phase of the tidal modulations are related to the global distribution of the main tidal constituents and tidal characteristics (diurnal or semidiurnal and tidal range). Results indicate that the 18.61 year nodal cycle has the greatest influence in diurnal regions with tidal ranges of >4 m and that the 4.4 year cycle is largest in semidiurnal regions where the tidal range is >6 m. The phase of the interannual tidal modulations is shown to relate to the form of the tide.

This paper describes the assembly of an updated quasi-global dataset of higher-frequency sea level information obtained from tide gauges operated by many agencies around the world. We believe that the construction of such a dataset is fundamental to scientific research in sea level variability and also to practical aspects of coastal engineering. A first version of the dataset was used in approximately a dozen published studies, and this second version is about twice the size, containing longer and more geographically representative sea level records. The dataset has acquired a digital object identifier and may be obtained from several sources. The paper mentions some of the merits of and deficiencies with the present version and takes a forward look at how the dataset may be updated in the future.

In this paper we analyse the spatial footprint and temporal clustering of extreme sea level and skew surge events around the UK coast over the last 100 years (1915–2014). The vast majority of the extreme sea level events are generated by moderate, rather than extreme skew surges, combined with spring astronomical high tides. We distinguish four broad categories of spatial footprints of events and the distinct storm tracks that generated them. There have been rare events when extreme levels have occurred along two unconnected coastal regions during the same storm. The events that occur in closest succession (<4 days) typically impact different stretches of coastline. The spring/neap tidal cycle prevents successive extreme sea level events from happening within 4–8 days. Finally, the 2013/14 season was highly unusual in the context of the last 100 years from an extreme sea level perspective.

This paper assesses historic changes in mean sea level around the coastline of the North Sea, one of the most densely populated coasts in the world. Typically, such analyses have been conducted at a national level, and detailed geographically wider analyses have not been undertaken for about 20 years. We analyse long records (up to 200 years) from 30 tide gauge sites, which are reasonably uniformly distributed along the coastline, and: (1) calculate relative sea level trends; (2) examine the inter-annual and decadal variations; (3) estimate regional geocentric (sometimes also referred to as 'absolute') sea level rise throughout the 20 th century; and (4)