Extreme Marine Events Research Articles

East Asian monsoon and Pacific basin dynamics jointly regulate boreal winter marine cold spells in the Taiwan Strait

Marine cold spells (MCSs), characterized by pronounced seawater temperature declines, are representative marine responses to atmospheric and climatic extremes. Notably, during the boreal winter, MCSs in the Taiwan Strait (TS) have caused substantial economic losses due to the concurrence of juvenile development stages in mariculture and labor shortages during China’s New Year vacation. Understanding the spatio-temporal features of MCSs and the underlying mechanisms is imperative for mitigating the associated economic detriments. This study, for the first time, delineates two MCS hotspots in the northeastern and western TS, based on various sea surface temperature datasets. Our findings reveal a notable frequency and intensity of MCSs in the 1980s, followed by a period of suppressed phase, reaching a historic low point in 2002. However, contrary to expectations of the overall trend of global warming, MCSs have experienced a resurgence since 2002, with a pronounced focus on the western coast of the TS. Diagnostic analyses suggest that the Winter East Asian Monsoon-driven cold current and the Pacific basin scale dynamic-forced warm current jointly regulated the spatio-temporal features of the MCSs. This study contributes a novel perspective on the impacts of climate change on extreme marine events along subtropical coastal areas.

Late Holocene evolution of a coastal barrier system at Bilene, Mozambique

AbstractThis research attempts to provide a contribution to the study on coastal barrier‐lagoons in the SE Africa generated by sea‐level fluctuations, and climate change during the Late Holocene. To achieve this goal, the grain‐size analysis and morphoscopic description helped to detect cyclic facie sequences with sediments sized over 200 μm mixed with silt, bioclasts and organic matter. Moreover, the magnetic susceptibility and X‐ray diffraction (XRD) analyses displayed over 90% of quartz and isolated mica, feldspar and heavy minerals. Additionally, the X‐ray fluorescence (XRF) results display the interplay between terrigenous (>Ti and Al) and marine influence (S, Cl and ~Ca) over the last ~3,500 cal. yrs. BP. The geochemical tracer fluctuations detected suggest five distinct deposits including beach, washover, lacustrine system, aeolian deflation that rest on top of the Pleistocene palaeodunes, which are tentatively associated with the sea‐level variations, extreme marine events or with local floods. Although the multiproxy data record refers to a study in Bilene‐Mozambique, it is interesting to understand the mechanisms and variables related to the evolutionary dynamics of coastal barrier lagoons generated by sea‐level fluctuations and climate change, especially in the SE Africa.

Battles beneath the Sea: Phoenician Votive Offerings as a Possible Religious Response to Extreme Marine Events in the Gulf of Cadiz

ABSTRACT This article reviews the possible cause for the underwater deposition of a series of Phoenician bronze figurines dated between the eighth and seventh centuries BC and discovered on the southwestern coast of the Iberian Peninsula, around the Islet of Sancti Petri (Cadiz) and on the coast near the city of Huelva. These figurines have been interpreted as votive offerings thrown into the waters near the ports of Cadiz and Huelva by Phoenician seafarers and merchants at the end of their voyages as an expression of gratitude to the god Melqart. Instead, I propose that these objects may have been thrown into the waters as part of religious rituals intended to appease the waters of the ocean following the occurrence of catastrophic marine floods, such as those that apparently affected the seaboard of the Gulf of Cadiz in the middle of the first millennium BC.

Co-occurrence of a marine heatwave and a reported tomato jellyfish (<i>Crambione mastigophora</i> Maas, 1903) bloom in March 2020 at El Nido, Palawan, Philippines

Globally, observations on marine species during marine heatwaves (MHWs) help outline the scope of the MHW’s possible biological effects. In line with this effort, this paper presents a 2020 MHW that coincided with a reported ‘tomato jellyfish’ (Crambione mastigophora Maas, 1903) bloom on 23 March 2020 in the Corong-Corong Bay of Palawan, Philippines. Detecting a moderate MHW from 21 March to 04 April 2020, the analysis of sea surface temperatures revealed that most areas surrounding the bloom site attained their peak positive anomalies on the same day as the reported bloom. Certain physical mechanisms present in the first quarter of 2020 may have played a role in the occurrence of both events: the presence of cyclonic eddies and parallel monsoonal winds alongshore can induce upwelling which promotes biological productivity in surface waters, while the observed weakening of winds have been associated with anomalous warming of the sea surface. Further studies are still highly recommended to determine the exact causes of the jellyfish bloom and what conditions make it more likely to happen during MHWs. However, if the C. mastigophora is hypothetically able to continually bloom amidst warming temperatures, the increasing trend of MHW frequency and intensity in the West Philippine Sea (where the reported bloom site is situated) may consequently yield more future co-occurrences. This paper aims to hopefully contribute to the existing knowledge of possible biological impacts associated with extreme marine events, especially in the Philippine context where both jellyfish blooms and MHWs are understudied.

The recent normalization of historical marine heat extremes

Climate change exposes marine ecosystems to extreme conditions with increasing frequency. Capitalizing on the global reconstruction of sea surface temperature (SST) records from 1870-present, we present a centennial-scale index of extreme marine heat within a coherent and comparable statistical framework. A spatially (1° × 1°) and temporally (monthly) resolved index of the normalized historical extreme marine heat events was expressed as a fraction of a year that exceeds a locally determined, monthly varying 98th percentile of SST gradients derived from the first 50 years of climatological records (1870–1919). For the year 2019, our index reports that 57% of the global ocean surface recorded extreme heat, which was comparatively rare (approximately 2%) during the period of the second industrial revolution. Significant increases in the extent of extreme marine events over the past century resulted in many local climates to have shifted out of their historical SST bounds across many economically and ecologically important marine regions. For the global ocean, 2014 was the first year to exceed the 50% threshold of extreme heat thereby becoming “normal”, with the South Atlantic (1998) and Indian (2007) basins crossing this barrier earlier. By focusing on heat extremes, we provide an alternative framework that may help better contextualize the dramatic changes currently occurring in marine systems.

Analysis of Sea Storm Events in the Mediterranean Sea: The Case Study of 28 December 2020 Sea Storm in the Gulf of Naples, Italy

The coastline of the Gulf of Naples, Italy, is characterized by a series of infrastructures of strategic importance, including touristic and commercial ports between Pozzuoli to Sorrento, main roads, railways, and urban areas. Furthermore, the Gulf of Naples hosts an intense traffic of touristic and commercial maritime routes. The risk associated with extreme marine events is hence very significant over this marine and coastal area. On 28 December 2020, the Gulf of Naples was hit by an extreme sea storm, with severe consequences. This study focuses on the waterfront area of Via Partenope, where the waves overrun the roadway, causing massive damage on coastal seawall, road edges, and touristic structures (primarily restaurants). Based on the analysis of the meteorological evolution of the sea storm and its effects on the waterfront, we suggest that reflective processes induced on the sea waves by the tuff cliffs at the base of Castel dell’Ovo had an impact in enhancing the local-scale waves magnitude. This caused in turn severe flooding of the roadway and produced widespread damage along the coast. The analysis of the event of 28 December 2020, also suggests the need of an effective mitigation policy in the management of coastal issues induced by extreme sea storm events. Wind-based analysis and prediction of the sea wave conditions are currently discussed in the literature; however, critical information on wave height is often missing or not sufficient for reliable forecasting. In order to improve our ability to forecast the effects of sea storm events on the coastline, it is necessary to analyze all the components of the coastal wave system, including wave diffraction and reflection phenomena and the tidal change. Our results suggest in fact that only an integrated approach to the analysis of all the physical and anthropic components of coastal system may provide a correct base of information for the stakeholders to address coastal zone planning and protection.

Review article: Extreme marine events revealed by lagoonal sedimentary records in Ghar El Melh during the last 2500 years in the northeast of Tunisia

Abstract. The Tunisian coast has been affected in the past by many events of extreme marine submersion (storms and tsunamis). A high-resolution study along two sediment cores taken from the lagoon of Ghar El Meleh was performed to identify the different paleoextreme events and to reconstruct the paleoenvironmental changes in the northeastern part of Tunisia during the Late Holocene. A very high-resolution sedimentological analysis (granulometric and geochemical) was applied to these cores. These cores were also dated with isotopic techniques (137Cs, 210Pbex, 14C), and the outcomes reveal five phases of paleoenvironmental changes in this lagoonal complex and identify two sediment layers that are in connection with two major historical marine submersion events. The first layer is mentioned as E1 and seems to fit with the great tsunami of 365 cal CE. This event was marked by an increase in the coarse sediment, and it is correlated for the first time with the immersed city of Neapolis in the northern Gulf of Hammamet discovered in 2017 by the same tsunamis of 365 cal CE. The other sandy layer, referred to as E2, was dated from 1690 to 1760 cal CE and is marked by one specific sedimentological layer attributed to a marine submersion event. This layer could be associated with the 1693 tsunami event in southern Italy or an increase in extreme storm events.

Insights on the origin of multiple tsunami events affected the archaeological site of Ognina (south-eastern Sicily, Italy)

South-eastern Sicily is one of the most seismically active areas of the Mediterranean Sea, marked by a high level of crustal seismicity, causing major earthquakes (up to Mw ∼7). As a consequence, this area is prone to earthquake-generated tsunamis, which affected the Ionian coast of Sicily in historical times. These tsunamis left geomorphic and sedimentary imprints, such as large boulders or high-energy deposits, along the coasts. One of these was reported by previous works along the coast of Ognina, a small residential area located 20 km south of Siracusa. The deposits fill the back edge of a ria incised into Miocene limestones, are composed of three main stratigraphic units and were attributed to several tsunami and storm events that occurred along the coasts of south-eastern Sicily since the IV century Common Era (CE). Here, we use numerical models to simulate the impact of these extreme marine events, at the time of their occurrence, along the Ognina coastal sector, with the aim to: i) better define the tsunamigenic sources responsible for the events found in the deposits, ii) verify if some units could be related to a storm event, iii) investigate constrains on the paleogeography of the studied area at the time of tsunami and storm occurrence. We reconstructed the morphology of ancient local landscapes using geological and historical information, together with a detailed topographic and geoelectrical survey. We implemented a modelling chain (composed of Delft Dashboard, Delft 3d-FLOW and XBeach) to simulate the tsunami and storm wave propagation upon the ancient landscapes. Our results demonstrate that the use of advanced modeling tools, combined with in situ geological evidence and geophysical survey, has the potential to support the attribution of coastal geomorphic imprints to specific tsunami or storm events, the better definition of the paleo-landscapes, and the identification of the most likely tsunamigenic sources. This last aspect plays a fundamental role in providing more reliable characteristics of the tsunami propagation as well as in the assessing of potential tsunami hazard and related coastal impacts.

Stronger Variability in the Arctic Ocean Induced by Sea Ice Decline in a Warming Climate: Freshwater Storage, Dynamic Sea Level and Surface Circulation

AbstractArctic liquid freshwater content (FWC) influences both regional and large‐scale ocean dynamics and climate. In this study the responses of Arctic FWC, sea surface height (SSH) and surface circulation to different atmospheric circulation modes and the impact of sea ice decline on these responses are investigated by using wind perturbation simulations. The responses are intensified by sea ice decline through its resulting enhancement in ocean surface stress, indicating stronger decadal variability in the Arctic liquid FWC, SSH and ocean circulation in a warming climate. The Arctic Oscillation (AO) and Beaufort High (BH) wind forcing can significantly change Arctic regional and total FWC. Compared to the sea ice condition in the 1980s, the amplitudes of the ocean responses to the same AO forcing are much larger under the sea ice condition in the 2010s for both Arctic total FWC (by up to 50%) and regional SSH and velocity (doubled in some places). Sea ice decline intensifies ocean responses to the BH forcing in the Canada Basin with a similar strength. The Arctic Dipole Anomaly (DA) causes opposite changes in FWC between the Eurasian and Amerasian sectors in the cold decade, with the impact through changing sea ice thermodynamics being nonnegligible compared with that through changing ocean surface stress. Sea ice decline makes the ocean response to DA forcing less regular spatially. This study indicates an increasing vulnerability of the Arctic Ocean to winds in a warming world, which implies that extreme marine events may occur more often in the future.

Morphological controls and statistical modelling of boulder transport by extreme storms

The study of coastal boulder accumulations generated by extreme marine events, and of the energy and frequency involved in boulder transport, is of paramount importance in understanding the risk associated with extreme marine inundations. One of the frequently asked questions is whether the deposits are storm or tsunami-related, both events being characterized by different return periods. Boulder transport by storms was monitored on the west coast of Portugal. Significant changes were detected in boulders' position as a result of extreme inundation by the 2013/2014 winter storms. Results presented in this work indicate that the wave power associated with the “Christina” and “Nadja” storms occur once every three years. However, this interval is not supported by field observations of boulder displacement, which suggests that wave power over-predicts boulder movement in the study area. Furthermore, wave parameters from the “Christina” and “Nadja” storms were very similar, but have generated different impacts in the boulder accumulation described herein. Differences include the magnitude and direction of boulder movement, and are most likely associated with distinct tidal levels during the events. Higher tide levels generated an increase in the sea surface level and thus in the reach of waves, which generated displacement of larger boulders and consequent cross-shore contribution in boulder transport. Regardless, the combination of monitoring campaigns, wave data, and statistical modelling of extreme values indicate that boulder transport by storms is more frequent than initially expected. Based on recorded boulder movements, we present a conceptual model for boulder ridge formation and development and identify significant control of incoming flow by local geomorphological/topographical features. Storm events, not less frequent tsunamis, are identified as the events responsible for modulating this rocky coastline. These results question a direct attribution of coastal boulder deposits to tsunamis in coastal regions with a high risk of tsunami inundation.

OSL over-dispersion: A pilot study for the characterisation of extreme events in the shallow marine realm

Identification of extreme events in the sedimentary record relies on the correct characterisation of the deposit by means of multiple parameters and different diagnostic criteria. Multiple proxies based on well-tested geological, biological, and chemical analyses have been used to distinguish tsunami layers from other marine extreme events. However, few proxies distinguish or compare the “degree of sedimentary chaos” of an event relative to background sediments and between other extreme events. In this pilot study, the potential of Optically Stimulated Luminescence (OSL), normally used as a dating method, is demonstrated and a new potential sedimentological proxy is proposed when analysing in depth the OSL signal from single grain measurements to distinguish between underwater deposits derived from marine extreme events embedded in the texturally and compositionally homogeneous shallow shelf archive of the eastern Mediterranean. Newly and previously collected sediment samples from underwater excavations and a sediment core offshore Caesarea Maritima (Israel) were used in this experiment. Each sample was associated with either tsunami, storm, or fair weather (normal) marine conditions based on multiple proxies. In this study, single grain over-dispersion (SG-OD) values and ranges are used as a potentially novel way to quantify the degree of sedimentary chaos, thus the degree of scatter of the dose distribution as a function of the efficiency of exposure to sunlight (zeroing) of the quartz grains entrained during transport and deposition. Given that turbidity of a fluid should be associated with lower light penetration, and higher energy flow with greater sediment density (as can occur in tsunamis relative to storms), these conditions should be reflected in variations in single grain over-dispersion values, hence identifying luminescence distribution patterns related to known depositional conditions. This quantification is coupled with detailed statistical analysis of the resulting single grain equivalent dose distributions, minimally used in OSL-dating. Three types of underwater deposits were categorised based on three distribution typologies, to which, preliminary, conservative single grain over-dispersion ranges were assigned: <25% for Gaussian-like sands deposited under normal marine conditions; 25–60% for storm-laid sediments depicting bimodal distributions; and >60% for tsunami-related deposits showing asymmetric distributions with strong positive skewness. This new tool provides a means to study offshore extreme marine events, without potential dismissal of other identification criteria and proxies. Based on these results, single grain over-dispersion OSL could be considered as a potential new sedimentological proxy for identification, comparison and characterisation of marine-derived extreme events in the shallow shelf (such as tsunami deposits), as it is intrinsic to the sediment grain, and the degree of sedimentary chaos of the event itself.

Spatial analysis of extreme sea states affecting Atlantic France: a critical assessment of the RFA approach

Spatial extremes assessment of geophysical variables represents a complex field largely because of the great variety of existing methodologies employed at different complexity levels, generally making use of multivariate techniques. This manuscript investigates the feasibility of a different approach, namely “Regional Frequency Analysis” (RFA), consisting into dividing the area of study into homogeneous regions whose normalized data are clustered and treated with univariate techniques. RFA is employed to analyse the spatial assessment of extreme significant wave heights within the north-east Atlantic Ocean. This expanse provides the physical basis for the determination of “homogeneous” regions, identified by means of typical storm footprints which allow the clustering of similar extreme events. Furthermore, a spatio-temporal criterion for storm tracking is implemented and incorporated into the analysis, since it is assumed that extreme marine events on a basin scale may be influenced by the same low pressure systems following similar trajectories. A 23-year wave hindcast database at high spatial resolution provided by the WWIII model on unstructured mesh grid covering the Celtic Sea, the English Channel and the Bay of Biscay was employed to perform the analysis. Due to the large size of the dataset (more than 110, 000 spatial locations), a limited number of points were selected, either uniformly or guided by the bathymetry). In both situations, RFA proves effective in clustering homogeneous regions physically and statistically. Its reliability is based on the varying impact of major meteorological events on different areas at different scales. The spatial assessment of return levels of significant wave heights achieved by means of RFA are consistent with results of a pointwise model, with findings generally more smoothed in proximity to steep gradients and when ranging from offshore to coastal areas. The adoption of a “spatio-temporal declustering” allows more detailed wave patterns to be reproduced on a smaller spatial scale, especially in the vicinity of channel systems. Effects of swell are also described accurately.

Assessment of hydrodynamic competence in extreme marine events through application of Boussinesq–Green–Naghdi models

Here in present work, rotational Boussinesq–Green–Naghdi models were applied to assess the hydrodynamic intensity through the study of the boulder transport in east coast of Philippines during typhoon Haiyan and damage to coastal residences in New Jersey coast due to hurricane Sandy. The hydrodynamic forces were quantitatively analyzed and correlated to both boulder transport distance and the structural damage state in the two cases. The boulder transport was found initiated at vicinity of infragravity swash bores. Inertial force generated by the acceleration in front of the bore was found increasingly large as boulder sizes increased therefore far from negligible as in some other literatures. Besides, transport distances were highly sensitive to wave-heights and boulder sizes, so that onshore positions might be a viable approach of identifying rough magnitudes of paleostorm before other information is available. Fragility functions to predict the damage state of coastal residences due to runups was derived and preliminary validated. Water velocity and the shielding parameter were identified as major predictors of damage while free board and water depth are relatively insignificant. Due to the relative lack of wind damage observed, nearshore hydrodynamics featuring instantaneous nonhydrostatic impact might be the persistent cause of massive littoral processes and low-level structural failure in coastal regions during extreme marine events. Nonhydrostatic phase-resolving models such as Boussinesq-type models would be necessary complements for the intermediate-scale assessment of marine hazards in coastal ocean.

Climate change effects on the marine characteristics of the Aegean and Ionian Seas

This paper addresses the effects of estimated climate change on the sea-surface dynamics of the Aegean and Ionian Seas (AIS). The main aim is the identification of climate change impacts on the severity and frequency of extreme storm surges and waves in areas of the AIS prone to flooding. An attempt is made to define design levels for future research on coastal protection in Greece. Extreme value analysis is implemented through a nonstationary generalized extreme value distribution function, incorporating time harmonics in its parameters, by means of statistically defined criteria. A 50-year time span analysis is adopted and changes of means and extremes are determined. A Regional Climate Model (RegCM3) is implemented with dynamical downscaling, forced by ECHAM5 fields under 20C3M historical data for the twentieth century and the SRES-A1B scenario for the twenty-first century. Storm surge and wave models (GreCSSM and SWAN, respectively) are used for marine climate simulations. Comparisons of model results with reanalysis and field data of atmospheric and hydrodynamic characteristics, respectively, are in good agreement. Our findings indicate that the dynamically downscaled RegCM3 simulation adequately reproduces the present general circulation patterns over the Mediterranean and Greece. Future changes in sea level pressure and mean wind fields are estimated to be small, yet significant for marine extremes. In general, we estimate a projected intensification of severe wave and storm surge events during the first half of the twenty-first century and a subsequent storminess attenuation leading to the resettlement of milder extreme marine events with increased prediction uncertainty in the second half of the twenty-first century.

Communicating natural hazards: marine extreme events and the importance of variability and forecast errors

Abstract Scientific knowledge has to meet some necessary conditions. Among these, it has to be properly communicated. Usually, scientists (mis)understand this requirement and believe that publishing their results in peer-reviewed journals is enough. Society demands the availability of information in other formats. This is particularly relevant when dealing with natural hazards, which affect the life and fate of human beings. Natural hazards and extreme events are becoming more frequent and energetic, and future scenarios sustain that this tendency will remain or even increase. Furthermore, in trying to explain scientific results to untrained members of society, scientists usually focus more on forecasts than on the prediction errors. Talking about our forecast limitations outside of academia seems shameful, rather than being a logical limitation of sciences and regardless of the fact that discussing limitations is not something of which to be ashamed. Using numerical modelling combined with field observations to predict extreme marine events we hope to show here that it is possible to be less cryptic. Also, the general public needs to know enough to properly deal with variability and uncertainties. Scientists have the ethical responsibility of communicating the uncertainties of our results to society because these limitations are neither obvious nor irrelevant.

Modeling intersite dependence for regional frequency analysis of extreme marine events

AbstractThe duration of observation at a site of interest is generally too low to reliably estimate marine extremes. Regional frequency analysis (RFA), by exploiting the similarity between sites, can help to reduce uncertainties inherent to local analyses. Extreme observations in a homogeneous region are especially assumed to follow a common regional distribution, up to a local index. The regional pooling method, by gathering observations from different sites into a regional sample, can be employed to estimate the regional distribution. However, such a procedure may be highly affected by intersite dependence in the regional sample. This paper derives a theoretical model of intersite dependence, dedicated to the regional pooling method in a “peaks over threshold” framework. This model expresses the tendency of sites to display a similar behavior during a storm generating extreme observations, by describing both the storm propagation in the region and the storm intensity. The proposed model allows the assessment of (i) the regional effective duration of the regional sample and (ii) different regional hazards, e.g., return periods of storms. An application to the estimation of extreme significant wave heights from the numerical sea‐state database ANEMOC‐2 is provided, where different patterns of regional dependence are highlighted.

Formation of homogeneous regions for regional frequency analysis of extreme significant wave heights

Regional frequency analysis (RFA) can reduce uncertainties in the estimations of return levels, provided that homogeneous regions can be delineated. In the framework of extreme marine events, a physically based method to form homogeneous regions by identifying typical storms footprints is proposed. First, a spatiotemporal declustering procedure is employed to detect storms generating marine extremes. Second, the identification of the most typical storms footprints relies on a clustering algorithm based on a criterion of storm propagation. The resulting regions are readily explicable: sites from a given region are likely to be impacted by the same storms, and any storm impacting a region is likely to remain enclosed in this region. This procedure is fairly simple to implement, as the only information required is the time of occurrence of the observed extremes. An application to the estimation of extreme significant wave heights from the numerical sea-state database ANEMOC-2 is given. Six regions, both physically and statistically homogeneous, are delineated in the North-East part of the Atlantic Ocean. It is shown that the identification of storms footprints allows the increase of the overall statistical homogeneity. Combined with RFA, the proposed method highlights regional differences in the spatial extent and intensity of storms.

Dune behavior and aeolian transport on decadal timescales

Dunes provide protection for low lying hinterland against flooding due to storms. Due to the dynamic nature of dunes, safety levels provided vary in time. This paper aims to gain insight in how dune building processes govern dune behavior on the yearly to decadal timescale. Dunes grow mainly due to aeolian transport.Most sediment transport formulations suggest that wind velocity is the most important governing parameter for aeolian dune growth. However, in several coastal studies, wind-driven sediment transport has been shown to reach limiting conditions regardless of the wind velocity. Factors that limit the wind-driven sediment transport, such as sediment properties, moisture and beach geometry, can be more important than the wind velocity.The Dutch JARKUS dataset is used to analyze yearly to decadal dune behavior where dune volume changes are taken as a measurable parameter of dune behavior. Measurements of dune volume changes over these timescales mainly show a positive linear trend. Therefore a constant dune growth could be assumed for these specific profiles, which simplifies predictive possibilities. However, the magnitude of the linear dune behavior varies in space (alongshore). It is aimed to find spatial and temporal variations in dune behavior and how these correlate with wind conditions and beach slope. The dune volume changes are found to depend on beach slope which is a transport limiting parameter but no relation between dune behavior and wind forcing is found on the yearly to decadal timescale. Annually averaged erosion volumes as a result of extreme marine events are found to be of similar order as the aeolian growth. In future modeling of decadal dune behavior, we suggest that the variability of transport limiting parameters is of interest rather than time varying forcing conditions such as varying wind speeds and drift potentials.

Sea surges in Camargue: Trends over the 20th century

The vulnerability to short-term and long-term sea-level rises is particularly high in subsiding deltaic areas, especially in microtidal seas, when surges (the differences between the observed sea heights and the simultaneous astronomical tide) are frequent. At the Grau-de-la-Dent tide-gauge in the Camargue (Rhone delta, France), daily sea-level records are available since 1905. Hourly tide data spanning the period 1979–1995 were obtained through the digitisation of the original paper records: the local harmonic constants and the surges for the whole 20th century have been computed from these hourly observations. It appears that the annual maximum observed sea-level height increases by 4 mm/yr at a rate that is two times faster than the average observed relative sea level. The increasing trend of the annual maximum positive sea surges (+1.9 mm/yr), which is equal to the average relative sea-level rise, is thus responsible for this difference. The most important meteorological factor associated with local sea-surge occurrences is wind blowing from 100° to 120° sectors, which tends to push the water toward the coasts. Since 1961, the frequency and the speed of wind from this sector increased, although with some variability, thus contributing in part to the increase in the frequency and intensity of the surges. Due to the changing hydrodynamics phenomenon in the Camargue, a positive feedback mechanism between extreme marine events and shoreline regression is another factor to explain the sea-surge rise over the long term. The increase in sea-surge frequency and height during the last century is especially of concern in the deltaic area if the near-future global sea-level rise predicted by climate models is also taken into account.