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.
Key Points
Interannual tidal modulations influence extreme sea levels and coastal flooding
This paper maps the range and phase of the tidal modulations on a global scale
Determine where the modulations contribute largest to high tidal levels
The interaction between physical drivers from oceanographic, hydrological, and meteorological processes in coastal areas can result in compound flooding. Compound flood events, like Cyclone Idai and ...Hurricane Harvey, have revealed the devastating consequences of the co-occurrence of coastal and river floods. A number of studies have recently investigated the likelihood of compound flooding at the continental scale based on simulated variables of flood drivers, such as storm surge, precipitation, and river discharges. At the global scale, this has only been performed based on observations, thereby excluding a large extent of the global coastline. The purpose of this study is to fill this gap and identify regions with a high compound flooding potential from river discharge and storm surge extremes in river mouths globally. To do so, we use daily time series of river discharge and storm surge from state-of-the-art global models driven with consistent meteorological forcing from reanalysis datasets. We measure the compound flood potential by analysing both variables with respect to their timing, joint statistical dependence, and joint return period. Our analysis indicates many regions that deviate from statistical independence and could not be identified in previous global studies based on observations alone, such as Madagascar, northern Morocco, Vietnam, and Taiwan. We report possible causal mechanisms for the observed spatial patterns based on existing literature. Finally, we provide preliminary insights on the implications of the bivariate dependence behaviour on the flood hazard characterisation using Madagascar as a case study. Our global and local analyses show that the dependence structure between flood drivers can be complex and can significantly impact the joint probability of discharge and storm surge extremes. These emphasise the need to refine global flood risk assessments and emergency planning to account for these potential interactions.
In low-lying coastal regions, flooding arises from oceanographic (storm
surges plus tides and/or waves), fluvial (increased river discharge), and/or
pluvial (direct surface run-off) sources. The ...adverse consequences of a flood
can be disproportionately large when these different sources occur
concurrently or in close succession, a phenomenon that is known as
“compound flooding”. In this paper, we assess the potential for compound
flooding arising from the joint occurrence of high storm surge and high
river discharge around the coast of the UK. We hypothesise that there will be
spatial variation in compound flood frequency, with some coastal regions
experiencing a greater dependency between the two flooding sources than
others. We map the dependence between high skew surges and high river
discharge, considering 326 river stations linked to 33 tide gauge sites. We
find that the joint occurrence of high skew surges and high river discharge
occurs more frequently during the study period (15–50 years) at sites on the
south-western and western coasts of the UK (between three and six joint events per
decade) compared to sites along the eastern coast (between zero and one joint
events per decade). Second, we investigate the meteorological conditions
that drive compound and non-compound events across the UK. We show, for the
first time, that spatial variability in the dependence and number of joint
occurrences of high skew surges and high river discharge is driven by
meteorological differences in storm characteristics. On the western coast of
the UK, the storms that generate high skew surges and high river discharge
are typically similar in characteristics and track across the UK on
comparable pathways. In contrast, on the eastern coast, the storms that
typically generate high skew surges are mostly distinct from the types of
storms that tend to generate high river discharge. Third, we briefly examine
how the phase and strength of dependence between high skew surge and high
river discharge is influenced by the characteristics (i.e. flashiness, size,
and elevation gradient) of the corresponding river catchments. We find that high
skew surges tend to occur more frequently with high river discharge at
catchments with a lower base flow index, smaller catchment area, and steeper
elevation gradient. In catchments with a high base flow index, large
catchment area, and shallow elevation gradient, the peak river flow tends to
occur several days after the high skew surge. The previous lack of
consideration of compound flooding means that flood risk has likely been
underestimated around UK coasts, particularly along the south-western and western
coasts. It is crucial that this be addressed in future assessments of flood
risk and flood management approaches.
We introduce a novel approach to statistically assess the non-linear interaction of tide and non-tidal residual in order to quantify its contribution to extreme sea levels and hence its role in ...modulating coastal protection levels, globally. We demonstrate that extreme sea levels are up to 30% (or 70 cm) higher if non-linear interactions are not accounted for (e.g., by independently adding astronomical and non-astronomical components, as is often done in impact case studies). These overestimates are similar to recent sea-level rise projections to 2100 at some locations. Furthermore, we further find evidence for changes in this non-linear interaction over time, which has the potential for counteracting the increasing flood risk associated with sea-level rise and tidal and/or meteorological changes alone. Finally, we show how accounting for non-linearity in coastal impact assessment modulates coastal exposure, reducing recent estimates of global coastal flood costs by ~16%, and population affected by ~8%.
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 (τ) 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.
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.
Plain Language Summary
The orbital position of the moon and the gravity pull it exerts on the earth varies with a period of 18.6 years. This cycle is called the lunar nodal cycle and it results in small variations of yearly averaged sea level (∼1–2 cm). Understanding this variability is important because it allows, for example, to quickly detect an acceleration in local sea‐level rise due to global warming. Here we show that the lunar nodal cycle also has an influence on the temperature and salinity in the surface 400m of the ocean. As a result, the ocean density changes and amplifies sea level variations along the western European coast. We make the hypothesis that since the lunar nodal cycle also influences the amplitude of the semidiurnal tides, and since those tides are known to be responsible for a large part of ocean mixing, a change in ocean mixing could be the cause of the ocean density variability that we observe.
Key Points
Steric sea level changes are influenced by the 18.6‐year lunar nodal cycle along the western European coast
This influence could result from the modulation of semidiurnal tides by the lunar nodal cycle and the associated change in ocean mixing
This finding is a step toward resolving the long‐standing discrepancy between the theoretical long‐period nodal tide and observed signal
This study reports a new and significantly enhanced analysis of US flood hazard at 30 m 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 centered on 2035 and 2050 under the RCP4.5 emissions pathway. Validation against high‐quality local models and the entire catalog 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 (3σ changes in 100 years inundated area of −3.8 to +16% 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 defense protection may become compromised by this time.
Plain Language Summary
We develop a method to estimate past, present, and future flood risk for all properties in the conterminous United States whether affected by river, coastal or rainfall flooding. The analysis accounts for variability within environmental factors including changes in sea level rise, hurricane intensity and landfall locations, precipitation patterns, and river discharge. We show that even for a conservative climate change trajectory we can expect locally significant changes in the land area at risk from floods by 2050, and by this time defenses protecting 2,200 km2 of land may be compromised. The complete dataset has been made available via a website (https://floodfactor.com/) created by the First Street Foundation in order to increase public awareness of the threat posed by flooding to safety and livelihoods.
Key Points
First complete high‐resolution flood hazard analysis of conterminous US flood risk from all major sources (fluvial, pluvial, and coastal)
In validation tests the model achieved Critical Success Index scores of 0.69–0.82, similar to many local custom‐built 2D models
By 2050, flood hazard increases for the Eastern seaboard and Western states, but decreases or changes little for the center and South‐West
Although regional studies and projections suggest the deterioration of estuaries as a consequence of climate change, it is still difficult to fully understand the importance of such changes in ...estuarine systems. This limitation is particularly important considering their high dynamism and the lack of temporally extended in situ databases with a good spatial coverage for these systems worldwide. Furthermore, contradictory patterns have been observed across the globe. Motivated by these issues, in this study we question the availability of in situ observational evidence of climate change in estuarine systems through a detailed meta-analysis of existing publications. A topic-related search considering the outputs of the Web of Science library was conducted in order to obtain a characterization of the existing studies on climate change in estuarine systems.
Results confirmed that climate change has increasingly been studied since 2000 and that marine climate change constituted the focus of 9.69 % of those studies. From these, only 9.30 % encompassed estuarine studies and just 1.13 % used in situ observations from estuarine systems (i.e., 0.11 % of the total climate change publications). Reanalysis products were the most used tools to assess changes in estuarine systems and sea temperature was the most analyzed variable. These results highlight the need to further address such questions using in situ observational data and to implement long-term observatories to fully identify evidence of climate change in estuarine systems, supporting modelling approaches and promoting the development of effective mitigation plans.
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•Data is lacking to fully understand the importance of climate change in estuaries.•Only 0.11 % of climate change studies focus observational changes in estuaries.•Water temperature is the most studied variable in the literature focusing estuaries.•Numerical models are the most used approach to assess these type of changes.•We highlight the urgent need to implement long-term observatories in estuaries.
Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea ...surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall, European summer precipitation, Atlantic hurricanes and variations in global temperatures. It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source. Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres--the intergyre region. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO. The Atlantic overturning circulation is declining and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States.
Coastal flooding is a major global hazard, yet few
studies have examined the spatial and temporal characteristics of extreme
sea level and associated coastal flooding. Here we analyse sea-level ...records
around the coast of New Zealand (NZ) to quantify extreme storm-tide and
skew-surge frequency and magnitude. We identify the relative magnitude of
sea-level components contributing to 85 extreme sea level and 135 extreme
skew-surge events recorded in NZ since 1900. We then examine the spatial and
temporal clustering of these extreme storm-tide and skew-surge events and
identify typical storm tracks and weather types associated with the spatial
clusters of extreme events. We find that most extreme storm tides were
driven by moderate skew surges combined with high perigean spring tides. The
spring–neap tidal cycle, coupled with a moderate surge climatology,
prevents successive extreme storm-tide events from happening within 4–10 d of each other, and generally there are at least 10 d between extreme
storm-tide events. This is similar to findings from the UK (Haigh et al.,
2016), despite NZ having smaller tides. Extreme events more commonly
impacted the east coast of the North Island of NZ during blocking weather
types, and the South Island and west coast of the North Island during trough
weather types. The seasonal distribution of both extreme storm-tide and
skew-surge events closely follows the seasonal pattern of mean sea-level
anomaly (MSLA) – MSLA was positive in 92 % of all extreme storm-tide events and
in 88 % of all extreme skew-surge events. The strong influence of
low-amplitude (−0.06 to 0.28 m) MSLA on the timing of extreme events shows
that mean sea-level rise (SLR) of similarly small height will drive rapid
increases in the frequency of presently rare extreme sea levels. These
findings have important implications for flood management, emergency
response and the insurance sector, because impacts and losses may be
correlated in space and time.
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