Storm surges and the resulting extreme high sea levels are among the most dangerous natural disasters and are responsible for widespread social, economic and environmental consequences. Using a set ...of 220 tide gauges, this paper investigates the temporal variations in storm surges around the world and the spatial coherence of its variability. We compare results derived from two parameters used to represent storm surge: skew surge and the more traditional, non-tidal residual. We determine the extent of tide-surge interaction, at each study site, and find statistically significant (95% confidence) levels of tide-surge interaction at 59% of sites based on tidal level and 81% of sites based on tidal-phase. The tide-surge interaction was strongest in regions of shallow bathymetry such as the North Sea, north Australia and the Malay Peninsula. At most sites the trends in the skew surge time series were similar to those of non-tidal residuals, but where there were large differences in trends, the sites tended to have a large tidal range. Only 13% of sites had a statistically significant trend in skew surge, and of these approximately equal numbers were positive and negative. However, for trends in the non-tidal residual there are significantly more negative trends. We identified 8 regions where there were strong positive correlations in skew surge variability between sites, which meant that a regional index could be created to represent these groups of sites. Despite, strong correlations between some regional skew surge indices, none are significant at the 95% level, however, at the 80% level there was significant positive correlation between the north-west Atlantic - south and the North Sea. Correlations between the regional skew surge indices and climate indices only became significant at the 80% level, where Nińo 4 was positively correlated with the Gulf of Mexico skew surge index and negatively correlated with the east Australia skew surge index. The inclusion of auto-correlation in the calculation of correlation greatly reduced their significance, especially in the short time-series used for the regional skew surge indices.
The interaction between waves, surges, and astronomical tides can lead to high coastal total water level (TWL), which can in turn trigger coastal flooding. Here, a high-resolution (1.5 km) simulation ...from a UK-focused regional coupled environmental prediction system is used to investigate the extreme events of winter 2013/4 around the UK and Irish coasts. The aim is to analyse the spatial distribution of coastal TWL and its components during this period by assessing (1) the relative contribution of different TWL components around the coast; (2) how extreme waves, surges, and tide interacted and if they occurred simultaneously; and (3) if this has implications in defining the severity of coastal hazard conditions. The TWL components' coastal distribution in winter 2013/4 was not constant in space, impacting differently over different regions. High (>90th percentile) waves and high surges occurred simultaneously at any tidal stage, including high tide (7.7 % of cases), but more often over the flood tide. During periods of high flood risk, a hazard proxy, defined as the sum of the sea surface height and half the significant wave height, at least doubled from average over three-quarters of the coast. These results have important implications for the risk management sector.
In this paper, we show that over the next few decades, the natural variability of mid-latitude storm systems is likely to be a more important driver of coastal extreme sea levels than either mean sea ...level rise or climatically induced changes to storminess. Due to their episodic nature, the variability of local sea level response, and our short observational record, understanding the natural variability of storm surges is at least as important as understanding projected long-term mean sea level changes due to global warming. Using the December 2013 North Atlantic Storm Xaver as a baseline, we used a meteorological forecast modification tool to create “grey swan” events, whilst maintaining key physical properties of the storm system. Here we define “grey swan” to mean an event which is expected on the grounds of natural variability but is not within the observational record. For each of these synthesised storm events, we simulated storm tides and waves in the North Sea using hydrodynamic models that are routinely used in operational forecasting systems. The grey swan storms produced storm surges that were consistently higher than those experienced during the December 2013 event at all analysed tide gauge locations along the UK east coast. The additional storm surge elevations obtained in our simulations are comparable to high-end projected mean sea level rises for the year 2100 for the European coastline. Our results indicate strongly that mid-latitude storms, capable of generating more extreme storm surges and waves than ever observed, are likely due to natural variability. We confirmed previous observations that more extreme storm surges in semi-enclosed basins can be caused by slowing down the speed of movement of the storm, and we provide a novel explanation in terms of slower storm propagation allowing the dynamical response to approach equilibrium. We did not find any significant changes to maximum wave heights at the coast, with changes largely confined to deeper water. Many other regions of the world experience storm surges driven by mid-latitude weather systems. Our approach could therefore be adopted more widely to identify physically plausible, low probability, potentially catastrophic coastal flood events and to assist with major incident planning.
A North Sea storm surge during 31 January-1 February 1953 caused Northwest Europe’s most severe coastal flood in living memory. This event killed more than 2,000 people on the coasts of England, the ...Netherlands and Belgium. In the UK, where this study focuses, this event was a pivotal influence for flood risk management. Subsequent progress included a national tide gauge network, a storm surge forecasting and warning service, and major defence upgrade such as the Thames Barrier. Almost 60-years later, on 5-6 December 2013 Storm “Xaver” generated a surge event of similar magnitude. This paper describes a detailed comparison of these two events in the UK in terms of: (1) the meteorological conditions; (2) the observed high sea levels; and (3) the coastal flooding and impacts. The 1953 storm had a more southerly track and generated bigger waves due to the north-northwesterly onshore winds off East Anglia. The 2013 storm had a more west-to-east path from the north Atlantic to Scandinavia. Consequently, the 1953 high waters were more extreme in the southern North Sea. However, the 2013 event coincided with larger astronomical tides, resulting in a larger spatial ‘footprint’ (the length of coastline impacted by extreme high waters and floods). The extreme sea levels impacted communities on the west, east and south coasts, with 2,800 properties flooded during the 2013 event, compared to 24,000 properties mainly between the Humber and Thames in 1953. The 1953 floods remain a modern benchmark in the UK of potential flood consequences in terms of failed defences, damaged property and infrastructure and loss of life. Measures taken after 1953 greatly reduced the consequences of the 5-6 December 2013 storm. However, the latter event is a reminder of the potential for national-scale coastal storm events and impacts. Continued monitoring of extreme sea levels and their consequences is important to inform a realistic perspective on future planning and resilience.
Anomalous atmosphere‐ocean conditions in the tropical Pacific associated with the El Niño‐Southern Oscillation (ENSO) drive interannual variations in mean and extreme sea levels. Climate change may ...lead to more frequent extreme ENSO events in the future. Therefore, it is important to enhance our understanding of ENSO's influence on coastal flood impacts. We assessed ENSO's influence on extreme sea levels using a global reanalysis of tides and storm surges. This allows for a full coverage of the global coastline from 1979 to 2014. A mean sea level component is added to account for steric effects. This results in a substantial improvement in the representation of the seasonal and interannual variability. Our results show significant correlations across the Pacific between ENSO and extreme sea levels (expressed as 95th annual percentiles), which is consistent with previous studies based on tide gauge observations. Average anomalies in the annual percentiles over El Niño years compared to neutral years show similar patterns. When examining total sea levels, results are largely statistically insignificant. This is because in many regions large tidal variability dominates over the other components. Combining sea levels with an inundation and impact model shows that ENSO has a significant but small effect on the number of people potentially exposed to flooding at the globally aggregated scale. Our results demonstrate that a model‐based approach allows for an assessment of the influence of ENSO on coastal flood impacts and could be used to assess impacts of future changes in ENSO.
Key Points
El Niño‐Southern Oscillation (ENSO) has a significant influence on mean and extreme sea levels across the tropical Pacific
Simulations show a significant but small influence of ENSO on flood impacts with wide uncertainties prohibiting a robust assessment
Model‐based approach enables the assessments of coastal flood impacts and could be used to assess impacts of future changes in ENSO
Billions of historical climatological observations remain
unavailable to science as they exist only on paper, stored in numerous
archives around the world. The conversion of these data from paper to
...digital could transform our understanding of historical climate variations,
including extreme weather events. Here we demonstrate how the rescue of such
paper observations has improved our understanding of a severe windstorm that
occurred in February 1903 and its significant impacts. By assimilating newly
rescued atmospheric pressure observations, the storm is now credibly
represented in an improved reanalysis of the event. In some locations this
storm produced stronger winds than any event during the modern period
(1950–2015) and it is in the top-4 storms for strongest winds anywhere over
land in England and Wales. As a result, estimates of risk from severe
storms, based on modern period data, may need to be revised. Examining the
atmospheric structure of the storm suggests that it is a classic
Shapiro–Keyser-type cyclone with “sting-jet” precursors and associated
extreme winds at locations and times of known significant damage. Comparison
with both independent observations and qualitative information, such as
photographs and written accounts, provides additional evidence of the
credibility of the atmospheric reconstruction, including sub-daily
rainfall variations. Simulations of the storm surge resulting from this
storm show a large coastal surge of around 2.5 m, comparing favourably with
newly rescued tide gauge observations and adding to our confidence in the
reconstruction. Combining historical rescued weather observations with
modern reanalysis techniques has allowed us to plausibly reconstruct a
severe windstorm and associated storm surge from more than 100 years ago,
establishing an invaluable end-to-end tool to improve assessments of risks
from extreme weather.
Recent research into rip currents has revealed the existence of multiple circulation patterns, meaning that no single escape strategy is appropriate in all situations. Rip circulation is influenced ...by surfzone morphology, which can be inferred from wave breaking patterns in video imagery. Wave breaking often occurs over the bars adjacent to the rip channel, with little breaking over the seaward end of the rip. However, under varying wave and tide conditions, breaking can also occur at the seaward extent of rip channels. Here, we use this difference as a novel wave dissipation parameter to classify a rip channel as either ‘open’ or ‘closed’ in terms of rip-head wave breaking. A 4-day field study provided Lagrangian rip current data at a macrotidal, dissipative beach monitored by a coastal imaging system. Using this new parameter, rip channels that were identified as closed exhibited a 31% increase in current speeds and 43% increase in horizontal vorticity compared to open channels. The transition between open and closed channels occurred over a single tidal cycle, which altered surfzone retention rates. Closed channels promoted surfzone retention, with <25% of drifters exiting the surfzone. In comparison, open channels were more conducive to exchange, with exit rates up to 91%. Analysis of the Royal National Lifeboat Institution lifeguard rip incident database showed that open rips were disproportionately represented in the occurrence of rescue events, and calculated here to be twice as dangerous as closed rips. The use of this new open/closed parameter could be used by surf lifesaving organisations, and may have implications for the cross-shore exchange of sediment and pollutants.
•A novel wave breaking parameter for investigation of rip currents•Rips classified as open or closed in terms of wave breaking at rip head•Wave breaking patterns shown to be a key driver of surfzone exit rate•Open rips shown to be twice as dangerous as closed rips
Coastal floods, driven by extreme sea levels, are one of the most dangerous
natural hazards. The people at highest risk are those living in low-lying
coastal areas exposed to tropical-cyclone-forced ...storm surges. Here we
apply a novel modelling framework to estimate past and/or present and future storm-surge-level and extreme-sea-level probabilities along the coastlines of southern
China, Vietnam, Cambodia, Thailand, and Malaysia. A regional hydrodynamic
model is configured to simulate 10 000 years of synthetic tropical cyclone
activity, representative of a past/present (1980–2017) and high-emission-scenario future (2015–2050) period. Results show that extreme storm surges, and therefore total water levels, will increase substantially in the coming decades, driven by an increase in the frequency of intense tropical
cyclones. Storm surges along the southern Chinese and northern and southern Vietnamese
coastlines increase by up to 1 m, significantly larger than expected changes in mean sea-level rise over the same period. The length of coastline that is presently exposed to storm surge levels of 2.5 m or greater will more than double by 2050. Sections of Cambodian, Thai, and Malaysian coastlines are projected to experience storm surges (at higher return periods) in the
future, not previously seen, due to a southward shift in tropical cyclone
tracks. Given these findings, coastal flood management and adaptation in
these areas should be reviewed for their resilience against future extreme
sea levels.
Future increases in flooding potential around the world's coastlines from extreme sea level events is heavily dependent on projections of future global mean sea level (GMSL) rise. Yet, the two main ...approaches for projecting 21st century GMSL rise—i.e., process‐based versus semi‐empirical—give inconsistent results. Here, a novel hybrid approach to GMSL projection, containing a process‐based thermosteric contribution and a semi‐empirical ice‐melt contribution, is embedded within a conceptual Earth system model (ESM). The ESM is run 10 million times with random perturbations to multiple parameters, and future projections are made only from the simulations that are historically consistent. The projections from our hybrid approach are found to be consistent with the dominant process‐based GMSL projections from the Climate Model Intercomparison Project phase 5 (CMIP5) ensemble, in that our future ensemble‐mean projections lie within ±2 cm of CMIP5 for the end of the 21st century when CMIP5‐simulated histories are used to constrain our approach. However, when observations are used to provide the historic constraints for our hybrid approach, we find higher ice‐melt sensitivity and additional ensemble‐mean GMSL rise of around 13–16 cm by the end of the century. We assess the impact of this additional GMSL rise, projected from observation‐consistency, on the increase in frequency of extreme sea level events for 220 coastal tide‐gauge sites. Accounting for regional effects, we infer a 1.5–8 times increase in the frequency of extreme sea‐level events for our higher GMSL projections relative to CMIP5.
Key Points
A novel hybrid approach to projecting sea‐level rise is presented, and embedded within an efficient conceptual Earth system model
Our computationally efficient approach generates probabilistic projections of future sea‐level rise, based on historic constraints
The frequency of extreme sea level events will increase by 1.5 to 8 times more than for previous sea level projections
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