Sea Level Variations Research Articles

Seasonal and El Niño–Southern Oscillation-related ocean variability in the Panama Bight

Abstract. In the Panama Bight, two different seasonal surface circulation patterns coincide with a strong mean sea level variation, as observed from 27 years of absolute dynamic topography (ADT) and the use of self-organizing maps. From January to April, a cyclonic gyre with a strong southwestward Panama Jet Surface Current (PJSC) dominates the basin circulation, forced by the Panama surface wind jet that also produces upwelling, reducing sea surface temperature (SST), increasing sea surface salinity (SSS) and causing an ADT decrease. From June to December, the Choco surface wind jet enhances SST, precipitation and river runoff, which reduces SSS, causing an ADT rise, which in turn forces a weak circulation in the bight, vanishing the PJSC. Interannual variability in the region is strongly affected by El Niño–Southern Oscillation (ENSO); however this climatic variability does not modify the seasonal circulation patterns in the Panama Bight. In contrast, the positive (negative) ENSO phase increases (decreases) SST and ADT in the Panama Bight, with a mean annual difference of 0.9 ∘C and 9.6 cm, respectively, between the two conditions, while its effect on SSS is small. However, as the strong seasonal SST, SSS and ADT ranges are up to 2.2 ∘C, 2.59 g kg−1 and 28.3 cm, the seasonal signal dominates over interannual variations in the bight.

Late Quaternary palaeoenvironmental evolution and sea level oscillation of Santa Catarina Island (southern Brazil)

Abstract. Sea level oscillation during the Quaternary played a major role in the geomorphology and vegetation dynamics of coastal areas in southern Brazil, encompassing ecosystems that often have a unique biodiversity. Understanding the natural evolution of these areas is essential for decision making regarding land use regulations towards sustainable development, as well as to preserve the uniqueness of the coastal ecosystems. The southern Brazil coastal plain is formed by marine, transitional and continental Quaternary deposits controlled by past variations of the sea level. These variations shaped the coastal landscape and influenced the development of different Atlantic rainforest formations, such as mangroves and restingas. In particular, the restinga formation corresponds to a specific ecosystem that covers sandy soils of marine and fluvial–marine origin formed during the Quaternary on the Brazilian coastal plain. In this contribution, we present high-resolution palynological and stable isotope data from a Holocene core retrieved from the coastal plain of Santa Catarina Island (southern Brazil). We were able to identify four different environmental zones for the last 6520 yr BP. The first zone (6520–2920 cal yr BP) is characterized by a lagoon with large marine-water influence. Notably, the observed dinoflagellate cyst association suggests that marine waters entering the region had their origin in the relatively warm and saline Brazil Current waters. During the second zone (2920–1520 cal yr BP), marine-water contribution to the lagoon decreased until it became disconnected from the sea. The third zone (1520–550 cal yr BP) was marked by the decrease of the water level until it dried out and led to the colonization of herbaceous vegetation over the palaeo-lagoon. The last zone (550 cal yr BP–recent) is characterized by the consolidation of the coastal-plain Atlantic rainforest (restinga vegetation). Our results form an example of the strong sensitivity of southern Brazilian ecosystem change caused by relative sea level variations. As such, this study might contribute to the debate about the potential effects of current climate change induced by global sea level variations.

Isotopic evidence of environmental changes during the Devonian-Carboniferous transition in South China and its implications for the biotic crisis.

The Devonian-Carboniferous (D-C) transition coincides with the Hangenberg Crisis, carbon isotope anomalies, and the enhanced preservation of organic matter associated with marine redox fluctuations. The proposed driving factors for the biotic extinction include variations in the eustatic sea level, paleoclimate fluctuation, climatic conditions, redox conditions, and the configuration of ocean basins. To investigate this phenomenon and obtain information on the paleo-ocean environment of different depositional facies, we studied a shallow-water carbonate section developed in the periplatform slope facies on the southern margin of South China, which includes a well-preserved succession spanning the D-C boundary. The integrated chemostratigraphic trends reveal distinct excursions in the isotopic compositions of bulk nitrogen, carbonate carbon, organic carbon, and total sulfur. A distinct negative δ15 N excursion (~-3.1‰) is recorded throughout the Middle Si. praesulcata Zone and the Upper Si. praesulcata Zone, when the Hangenberg mass extinction occurred. We attribute the nitrogen cycle anomaly to enhanced microbial nitrogen fixation, which was likely a consequence of intensified seawater anoxia associated with increased denitrification, as well as upwelling of anoxic ammonium-bearing waters. Negative excursions in the δ13 Ccarb and δ13 Corg values were identified in the Middle Si. praesulcata Zone and likely resulted from intense deep ocean upwelling that amplified nutrient fluxes and delivered 13 C-depleted anoxic water masses. Decreased δ34 S values during the Middle Si. praesulcata Zone suggests an increasing contribution of water-column sulfate reduction under euxinic conditions. Contributions of organic matter produced by anaerobic metabolisms to the deposition of shallow carbonate in the Upper Si. praesulcata Zone is recorded by the nadir of δ13 Corg values associated with maximal △13 C. The integrated δ15 N-δ13 C-δ34 S data suggest that significant ocean-redox variation was recorded in South China during the D-C transition; and that this prominent fluctuation was likely associated with intense upwelling of deep anoxic waters. The temporal synchrony between the development of euxinia/anoxia and the Hangenberg Event indicates that the redox oscillation was a key factor triggering manifestations of the biodiversity crisis.

The role of preconditioning for extreme storm surges in the western Baltic Sea

Abstract. When natural hazards interact in compound events, they may reinforce each other. This is a concern today and in light of climate change. In the case of coastal flooding, sea-level variability due to tides, seasonal to inter-annual salinity and temperature variations, or larger–scale wind conditions modify the development and ramifications of extreme sea levels. Here, we explore how various prior conditions could have influenced peak water levels for the devastating coastal flooding event in the western Baltic Sea in 1872. We design numerical experiments by imposing a range of precondition circumstances as boundary conditions to numerical ocean model simulations. This allows us to quantify the changes in peak water levels that arise due to alternative preconditioning of the sea level before the storm surge. Our results show that certain preconditioning could have generated even more catastrophic impacts. As an example, a simulated increase in the water level of 36 cm compared to the 1872 event occurred in Køge just south of Copenhagen (Denmark) and surrounding areas – a region that was already severely impacted. The increased water levels caused by the alternative sea-level patterns propagate as long waves until encountering shallow and narrow straits, and after that, the effect vastly decreases. Adding artificial increases in wind speeds to each study point location reveals a near-linear relationship with peak water levels for all western Baltic locations, highlighting the need for good assessments of future wind extremes. Our research indicates that a more hybrid approach to analysing compound events and readjusting our present warning system to a more contextualised framework might provide a firmer foundation for climate adaptation and disaster risk management. In particular, accentuating the importance of compound preconditioning effects on the outcome of natural hazards may avoid under- or overestimation of the associated risks.

Decadal coastal evolution spanning the 2010 Maule earthquake at Isla Santa Maria, Chile: Framing Darwin's accounts of uplift over a seismic cycle

AbstractCharles Darwin and Robert FitzRoy documented coseismic coastal uplift associated with the great 1835 Chile earthquake (M > 8.5) at Isla Santa María. In 2010, another similar earthquake (Mw 8.8) uplifted the island, ending the seismic cycle. The 2‐m uplift in 2010 caused major geomorphic and sedimentologic changes to the island's sandy beaches. Understanding the processes governing these changes requires pre‐ and post‐earthquake measurements to differentiate the effects of abrupt coseismic uplift from seasonal, annual, and decadal‐scale signals. Here, we combine spatial analysis of aerial imagery, field geophysics, wind and wave models to quantify geomorphic changes between 1941 and 2021 along the main beach. During the late interseismic phase (1941–2010), a ridge‐runnel system was formed and then buried by a frontal dune. Because of uplift in 2010, the shoreline prograded ~20 m, the uplifted berm was abandoned, and a new seaward berm was built. In the following decade, the abandoned berm was eroded by widening of the backshore as the shoreline and dune advanced seaward. Over the surveyed eight decades, the shoreline prograded continuously, increasing from <1 m/year to up to 3–5 m/year after the earthquake. We infer that these changes were caused by a sedimentary disequilibrium driven by variations in relative sea level, moving formerly passive sands from eroding cliffs and marine depths into the coastal sedimentary system, thus promoting long and cross‐shore sediment transport and, utterly, accretion. Our results have implications for studying beach evolution along tectonically‐active coasts associated with drastic changes in relative sea level.

Impact of the Interdecadal Pacific Oscillation on tropical Indian Ocean sea surface height: An assessment from CMIP6 models

AbstractThis study examined the sea level (SL) changes on the interdecadal timescale in the tropical Indian Ocean (TIO) in response to the Interdecadal Pacific Oscillation (IPO) in 20 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The analysis suggests that the interdecadal sea level pattern variation in the Thermocline Ridge of the Indian Ocean (TRIO) area coincides with the IPO phases and is mainly caused by fluctuations of surface forcing over the TIO. Changes in tropical wind stress curl (WSC) mainly induce the SL variability on the interdecadal timescale associated with IPO in the TRIO region in both observations and models. The stochastic wind forcing and its interaction with the ocean's internal variability produce significant interdecadal SL fluctuation in the southeast Indian Ocean. We evaluated the ability of 20 CMIP6 models to capture interdecadal variations in sea level teleconnections. We found that about 75% of models reproduce decadal fluctuations related to IPO pretty well. The interdecadal variations of the SL in the western TIO emerge primarily in response to westward propagating downwelling Rossby waves induced by the WSC in the central‐south TIO in MMM, and models as in observations. Pacific IPO‐related anomalies primarily spread through the oceanic pathways of the Maritime Continent in the eastern Indian Ocean. Further, the IPO induces changes in the western Indian Ocean WSC through atmospheric pathways, which in turn causes Rossby waves to move westward, affecting the southwest Indian SL. Almost all the models and MMM show interdecadal SL variations in TIO associated with IPO. This study is beneficial as interdecadal prediction research on SL has a significant societal need.

Global ocean reanalysis CORA2 and its inter comparison with a set of other reanalysis products

We present the China Ocean ReAnalysis version 2 (CORA2) in this paper. We compare CORA2 with its predecessor, CORA1, and with other ocean reanalysis products created between 2004 and 2019 [GLORYS12v1 (Global Ocean reanalysis and Simulation), HYCOM (HYbrid Coordinate Ocean Model), GREP (Global ocean Reanalysis Ensemble Product), SODA3 (Simple Ocean Data Assimilation, version 3), and ECCO4 (Estimating the Circulation and Climate of the Ocean, version 4)], to demonstrate its improvements and reliability. In addition to providing tide and sea ice signals, the accuracy and eddy kinetic energy (EKE) of CORA2 are also improved owing to an enhanced resolution of 9 km and updated data assimilation scheme compared with CORA1. Error analysis shows that the root-mean-square error (RMSE) of CORA2 sea-surface temperature (SST) remains around 0.3°C, which is comparable to that of GREP and smaller than those of the other products studied. The subsurface temperature (salinity) RMSE of CORA2, at 0.87°C (0.15 psu), is comparable to that of SODA3, smaller than that of ECCO4, and larger than those of GLORYS12v1, HYCOM, and GREP. CORA2 and GLORYS12v1 can better represent sub-monthly-scale variations in subsurface temperature and salinity than the other products. Although the correlation coefficient of sea-level anomaly (SLA) in CORA2 does not exceed 0.8 in the whole region, as those of GREP and GLORYS12v1 do, it is more effective than ECCO4 and SODA3 in the Indian Ocean and Pacific Ocean. CORA2 can reproduce the variations in steric sea level and ocean heat content (OHC) on the multiple timescales as the other products. The linear trend of the steric sea level of CORA2 is closer to that of GREP than that of the other products, and the long-term warming trends of global OHC in the high-resolution CORA2 and GLORYS12v1 are greater than those in the low-resolution EN4 and GREP. Although CORA2 shows overall poorer performance in the Atlantic Ocean, it still achieves good results from 2009 onward. We plan to further improve CORA2 by assimilating the best available observation data using the incremental analysis update (IAU) procedure and improving the SLA assimilation method.

Relative sea‐level changes in southeastern Australia during the 19th and 20th centuries

ABSTRACTRates of global and regional sea‐level rise between ~1850 and 1950 were high compared to those in preceding centuries. The cause of this sea‐level acceleration remains uncertain, but it appears to be pronounced in a small set of relative sea‐level proxy records from the Southern Hemisphere. Here we generate three new proxy‐based relative sea‐level reconstructions for southeastern Australia to investigate spatial patterns and causes of historical sea‐level changes in the Tasman Sea. Palaeo sea‐level estimates were determined using salt‐marsh foraminifera as sea‐level indicators. Records are underpinned by chronologies based on accelerator mass spectrometry 14C, radiogenic lead (210Pb), stable lead isotopes and palynological analyses. Our reconstructions show that relative sea level rose by ~0.2–0.3 m over the last 200 years in southeastern Australia, and rates of sea‐level rise were especially high over the first half of the 20th century. Based on modelled estimates of the contributing components to sea‐level rise, we suggest that the episode of rapid sea‐level rise was driven by barystatic contributions, but sterodynamic contributions were dominant by the mid‐20th century. Significant spatial variability in relative sea level indicates that local to sub‐regional drivers of sea level are also prominent. Our reconstructions significantly enhance our understanding of the spatiotemporal pattern of early 20th century sea‐level rise in the region.

Atmospheric and advective forcing of upwelling on South Africa's central Agulhas Bank

Current and temperature structures on South Africa's central Agulhas Bank are described from six months of moored observations and ancillary data collected during the seasonal transition from austral spring to summer (October 2018–March 2019). The occurrence of an intermittend mid-shelf upwelling ridge, associated with increased productivity, is an important part of the shelf's thermal structure. However, the subsurface evolution of this cold ridge has not been documented to date. A mooring array that transected the wide central shelf captured the seasonal increase in stratification that culminated in the formation of a cold ridge in late summer. We show that the cold ridge originates in the wind-driven upwelling zone on the eastern Agulhas Bank and emphasise the importance of oceanic forcing in maintaining the subsurface thermal structure through advective steering of wind-driven coastal upwelling plumes and through dynamic shelf edge upwelling. The main source of the cold basal layer at the mooring transect originated from upwelling further east, but persistent near-seabed temperatures <9 °C on the outer shelf during the encroachment of the Agulhas Current confirm the contribution of shelf edge upwelling, offshore of the mooring transect, to the cold bottom layer on the Agulhas Bank. Of particular interest is the strengthening of the south-westward shelf current, inshore of cyclonic flow in the Agulhas Bight that accelerated the offshore advection of productive coastal water in the shape of the cold ridge. We show the shelf circulation to be highly coherent across and along the shelf. Shelf-wide barotropic pulses were driven by increased zonal wind stress that was in turn associated with variation in coastal sea level. In the light of global climate change, this work highlights the importance of long term in-situ monitoring in understanding ecosystem functioning on the highly dynamic Agulhas Bank.

Regime Shift of the Sea Level Trend in the South China Sea Modulated by the Tropical Pacific Decadal Variability

AbstractDuring the altimeter era, the sea level in the South China Sea (SCS) and western tropical Pacific (WTP) experienced significant decadal variability. The sea level rose during 1993–2009 and fell during 2010–2019. The decadal variability of Walker Circulation associated with the Pacific Decadal Oscillation can explain the sea level variability in the WTP to a great extent. The wind forced westward propagating Rossby waves increased (decreased) the sea level in the WTP during 1993–2009 (2010–2019). However, the interior wind forcing has a negligible contribution to the decadal variability of the sea level in the SCS. The remote forcing from WTP through the oceanic bridge was supposed to play a dominant role. The sensitive experiments of a 1½‐layer model and Regional Oceanic Modeling System suggested that the sea level signals via the Sibutu Passage and Mindoro Strait accounted for the decadal variability of sea level in the central basin of SCS.

Sources and Sinks of Interannual Steric Sea Level Variability

AbstractUnderstanding the mechanisms of regional steric sea level variability is fundamental to understand the regional sea level variability recorded by satellite altimetry for years and to insure that future projections made by climate models are realistic. Here, we first develop a novel method based on steric sea level variance budget that allows to detect the sources and sinks of the variability. Using the “Estimating the Circulation and Climate of the Ocean” (ECCO V4) state estimate, we then show that interannual steric sea level variability is mainly sustained by interannual fluctuating winds via Ekman transport almost everywhere. The damping of the variability is made by both the interannual fluctuating net heat flux from the atmosphere, that largely dominates the atmospheric freshwater fluxes, and the parametrized effect of eddies. It is also found that the parametrized effect of diffusion on the variability is weak in most regions and that, although globally weak, the fluctuations of atmospheric freshwater fluxes are a source of variance close to the Equator in the Pacific Ocean.

INFLUENCE OF POST‐DANIAN SEA‐LEVEL CHANGES AND VARIATIONS IN SEDIMENTATION RATE ON OVERPRESSURE BUILD UP IN THE CLAY‐RICH OVERBURDEN IN THE DANISH SECTOR OF THE NORTH SEA CENTRAL GRABEN

Overpressure build up in the clay‐rich succession between sea floor and the top of the Chalk Group in the area around wells North Jens‐1 and Fasan‐1 in the Danish sector of the Central Graben, North Sea was examined by forward modelling. “Overpressure”, i.e. fluid pressure higher than hydrostatic pressure, is expressed here in terms of both the difference between pore pressure and hydrostatic pressure at a given depth and the ratio between these pressures. Pore pressure changes over time were estimated by numerical simulation of post‐Danian depositional processes, incorporating sea level changes and variations in sedimentation rate. Results show that the deposition of the post‐Danian (“overburden”) succession led to overpressure build up both in the overburden itself and in the underlying sediments (the so‐called “underburden”). The largest estimated present‐day overpressures (4.9‐5.6 MPa, 23‐26% above hydrostatic) occur at the base of the overburden, while an overpressure of up to 5.5 MPa was calculated to occur in the underburden. Variations in sedimentation rate appeared to have influenced the build‐up of overpressure in the overburden, although no significant effect was found in the underburden.The results indicate that more than 50% of the present‐day overpressure in the overburden was generated in the last 5.3 million years, i.e. during the Pliocene and the Quaternary. When variations in sedimentation rate during the Miocene were included in the modelling calculation, this proportion increased to nearly 70%. A decrease in sedimentation rate in the mid‐Miocene (Serravallian, 15‐11.2 Ma) and the late Miocene (Messinian, 7.5‐5.3 Ma) resulted in the dissipation of overpressures generated previously when the sedimentation rate was higher. About 60% of the overpressure generated in the Miocene developed during the Tortonian but only 14% during the Messinian.Water depth appears to influence the overpressure magnitude. Sea level changes played a minor and short‐lived role in overpressure build up. The influence of water depth was most pronounced when it was significantly greater than the thickness of the deposited sediments.The method of overpressure estimation used in this paper may be a valuable alternative to methods based on porosity trend analysis which are widely used in the oil and gas industry. Both the methods used here and the results may be useful in subsurface evaluations related to carbon storage in the Danish Central Graben (e.g. project Green Sand).

High‐Tide Flooding Along the China Coastline: Past and Future

AbstractCoastal flooding has become a major issue for low‐lying coastal cities in China, and a lot of research has focused on assessing flood risk from storms and associated extreme sea levels. High‐tide flooding (HTF), however, which leads to minor inundation and occurs more frequently as sea level continues to rise has not been assessed comprehensively. Here, we analyze HTF along the China coastline using tide gauge records. We show that the frequency of HTF has increased (Xiamen doubled HTF frequency every 11.5–37.4 years, with the median being 17.5 years). As a result, the cumulative loss ratios of HTF are higher than those of major (or extreme) flooding events in several locations. To gain insights into the processes driving HTF changes, we decompose still water levels that occurred during HTF into five components, including non‐linear trend, interannual to decadal mean sea level (MSL) variability, seasonal MSL cycle, tidal anomaly, and nontidal residuals. It is evident that due to sea‐level rise (SLR) fewer components need to combine to raise the water levels above HTF thresholds. We show that the South China Sea coast already experiences HTF purely driven by high spring tides, and will also see the fastest future increase in the number of tide‐only HTF events. In general, China will experience more HTF days under all warming scenarios as SLRs. This long‐term trend will be modulated by the nodal cycle of ocean tides leading to more rapid increases in HTF in the 2030s and 2050s.

Climate changes affect to The Water avaibility in Sausu Watershed Catchment Area (Case Study)

Climate change in Indonesia is marked by changes in temperature, rainfall patterns, sea level and climate variability. The climate condition of the Sausu watershed is a natural process that occurs in a rainfall patterns, magnitude and changes in air temperature. This study was conducted to identify the dangers of climate change to the water sector in the Sausu watershed. The analytical method used is Modified Penman evapotranspiration, water balance analysis model F.J. Mock, Makesens, Cumulative Distribution Frequency (CDF) statistical analysis and hazard weighting. This study uses secondary data, namely rainfall sourced from the Sausu rain station in 2002-2021 and climate data (climatology) sourced from BMKG in 2002-2021. Results from this study is that climate change occurs in the Sausu watershed which is characterized by a positive Z value (an increasing trend) and negative (a decreasing trend). A positive Z value (increasing trend) occurs in June, and a negative Z value (declining trend) occurs in December. The danger of decreasing water availability (PKA) occurs in the period 2052-2061 with a low level of danger. Flood hazard occurs in the period 2052-2061 with a high level of danger. Drought hazards occur in the period 2042-2051 and the period 2052-2061 with a low level of danger.

Dynamic sea-level changes and potential implications for storm surges in the UK: a storylines perspective

Global sea-level rise caused by a warming climate increases flood risk from storm surge events for those who live in coastal and low-lying areas. Estimates of global thermosteric sea-level rises are well constrained by model projections, but local variability in dynamic sea-level arising from seasonal and interannual changes is less well characterised. In this paper we use satellite altimetry observations coupled with CMIP6 model projections to understand drivers of change in dynamic sea-level over the UK shelf seas. We find a northward shift in the atmospheric jet stream and a weakening of the Atlantic meridional overturning circulation to be the key drivers of local dynamic sea-level variability. Using a storyline approach to constrain climate system responses to changes in atmospheric greenhouse gas concentrations, we find that dynamic sea-level is predicted to rise between 15–39 cm by 2080–2099 along the east coast of England (ECE). Under a worst-case scenario, assuming maximum variability as seen in the CMIP6 projections, ECE dynamic sea-level rise could reach 58 cm by 2100. We illustrate the impact of this dynamic sea-level rise in addition to non-dynamic components on the risks posed by storm surge events in ECE using an idealised example. If a storm surge event of the magnitude of the one experienced in ECE on the 5th of December 2013 was to occur in 2100, an additional 1414 km2 of land would potentially be affected in our worst-case idealised example, 22.4% of which can be attributed to dynamic sea-level rise.

Sea-level trend variability in the Mediterranean during the 1993–2019 period

Sea-level change is one of the most concerning climate change and global warming consequences, especially impacting coastal societies and environments. The spatial and temporal variability of sea level is neither linear nor globally uniform, especially in semi-enclosed basins such as the Mediterranean Sea, which is considered a hot spot regarding expected impacts related to climate change. This study investigates sea-level trends and their variability over the Mediterranean Sea from 1993 to 2019. We use gridded sea-level anomaly products from satellite altimetry for the total observed sea level, whereas ocean temperature and salinity profiles from reanalysis were used to compute the thermosteric and halosteric effects, respectively, and the steric component of the sea level. We perform a statistical change point detection to assess the spatial and temporal significance of each trend change. The linear trend provides a clear indication of the non-steric effects as the dominant drivers over the entire period at the Mediterranean Sea scale, except for the Levantine and Aegean sub-basins, where the steric component explains the majority of the sea-level trend. The main changes in sea-level trends are detected around 1997, 2006, 2010, and 2016, associated with Northern Ionian Gyre reversal episodes, which changed the thermohaline properties and water mass redistribution over the sub-basins.

Surface circulation in the Gulf of Thailand from remotely sensed observations: seasonal and interannual timescales

Abstract. The Gulf of Thailand (GoT), a shallow semi-enclosed basin located in the western equatorial Pacific, undergoes much wind variabilities on both seasonal and interannual timescales that produce complex surface circulation. The local Ekman pumping modifies sea level in the northern GoT, while remote wind forcing influences sea level variability at the GoT western boundary, potentially through the coastal trapped Kelvin waves. The importance of the Ekman current on ageostrophic current is also important; the stronger influence of the Ekman current is found toward the southern part of the GoT. The GoT circulation reverses its direction seasonally following the monsoon wind reversal which is well-captured by the most dominant complex empirical orthogonal function explaining 28 % of the total circulation variance. During the monsoon transition, a strong meridional current along the western boundary that connects to the flow at the GoT southeastern entrance is observed. This implies high exchange between the GoT and the South China Sea and thus modification of the GoT water. On the interannual timescale, the GoT circulation is directly impacted by both the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). Interestingly, the two climate modes have different spatial influences on the GoT circulation. The IOD dominates the interannual current along the GoT western boundary and the southern boundary of the observing domain (8∘ N), while the ENSO correlates with that in the interior. The results highlight the complex circulation pattern as being contributed by different dynamics over each region of the GoT.

A framework for estuarine future sea-level scenarios: Response of the industrialised Elbe estuary to projected mean sea level rise and internal variability

In this study, we apply probabilistic estimates of mean sea level (MSL) rise and a sub-set of regional climate model ensemble simulations to force a numerical model of the southern North Sea, downscaling projected sea level variability to the Elbe estuary that serves as a prototype for an industrialised meso-tidal estuary. The specific forcing combination enables a localised projection of future estuarine hydrodynamics accounting for the spread of projected global sea level rise and the spread of the regional climate projection due to internal variability. Under the applied high-emission scenario, the Elbe estuary shows high decadal rates of mean water level (MWL) rise beyond 19 mm y-1, increase in the tidal range of up to 14 mm y-1 and increase in extreme water levels of up to 18 mm y-1. The bandwidth of the estuarine response is also high. For example, the range of average monthly extreme water levels is up to 0.57 m due to the spread of projected global sea level rise, up to 0.58 m due to internal variability whereas seasonal range attains 1.99 m locally. In the lower estuary, the spread of projected global sea level rise dominates over internal variability. Internal variability, represented by ensemble spread, notably impacts the range of estuarine water levels and tidal current asymmetry in the shallow upper estuary. This area demonstrates large seasonal fluctuations of MWLs, the M2 tidal amplitude and monthly extreme water levels. On the monthly and inter-annual time scales, the MWL and M2 amplitude reveal opposite trends, indicative of a locally non-linear response to the decadal MSL rise enforced at the open boundary. Overall, imposed by the climate projections decadal change and MSL rise enhance the horizontal currents and turbulent diffusivities whereas internal variability locally mitigates sea level rise–driven changes in the water column. This work establishes a framework for providing consistent regionalised scenario-based climate change projections for the estuarine environment to support sustainable adaptation development.

Multi-temporal high-resolution marsh vegetation mapping using unoccupied aircraft system remote sensing and machine learning

Coastal wetlands are among the most productive ecosystems in the world and provide important ecosystem services related to improved water quality, carbon sequestration, and biodiversity. In many locations, wetlands are threatened by coastal development and rising sea levels, prompting an era of tidal wetland restoration. The creation and restoration of tidal marshes necessitate the need for ecosystem monitoring. While satellite remote sensing is a valuable monitoring tool; the spatial and temporal resolution of imagery often places operational constraints, especially in small or spatially complex environments. Unoccupied aircraft systems (UAS) are an emerging remote sensing platform that collects data with flexible on-demand capabilities at much greater spatial resolution than sensors on aircraft and satellites, and resultant imagery can be readily rendered in three dimensions through Structure from Motion (SfM) photogrammetric processing. In this study, UAS data at 5 cm resolution was collected at an engineered wetland at Poplar Island, located in Chesapeake Bay, MD United States five times throughout 2019 to 2022. The wetland is dominated by two vegetation species: Spartina alterniflora and Spartina patens that were originally planted in 2005 in low and high marsh elevation zones respectively. During each survey, UAS multispectral reflectance, canopy elevation, and texture were derived and used as input into supervised random forest classification models to classify species-specific marsh vegetation. Overall accuracy ranged from 97% to 99%, with texture and canopy elevation variables being the most important across all datasets. Random forest classifications were also applied to down-sampled UAS data which resulted in a decline in classification accuracy as spatial resolution decreased (pixels became larger), indicating the benefit of using ultra-high resolution imagery to accurately and precisely distinguish between wetland vegetation. High resolution vegetation classification maps were compared to the 2005 as-built planting plans, demonstrating significant changes in vegetation and potential instances of marsh migration. The amount of vegetation change in the high marsh zone positively correlated with interannual variations in local sea level, suggesting a feedback between vegetation and tidal inundation. This study demonstrates that UAS remote sensing has great potential to assist in large-scale estimates of vegetation changes and can improve restoration monitoring success.

Sea Level Variability in the Equatorial Malacca Strait: The Influence of Climatic–Oceanographic Factors and Its Implications for Tidal Properties in the Estuarine Zone

The sea level trend in the equatorial Malacca Strait is a significant issue that needs to be reviewed since it is an area of interest. Assessing its future impact on estuarine tidal characteristics is worth studying because it relates to the potency of coastal damages. This study aimed to discuss the relationship between sea level variations and anomalies and their possible triggering factors and to estimate the future impacts on the tidal properties in the estuarine zone. Tide gauge and altimetry data in the Tanjong Pagar site were used to assess the sea level trends over 27 years of observation (from 1992 to 2019). Both altimetry and tide gauge data showed an upward trend, with 0.24 cm/year and 0.39 cm/year, respectively. Due to the near-equatorial area of interest, sea level variability is more synchronized with ENSO rather than IOD. At some points, ENSO shapes the sea level fluctuation, with an R2 of less than 10%. For specific periods, the coupling effects between MJO and La Niña may trigger higher evaporation in the maritime continent, triggering increasing sea levels. Of particular concern, among the other assessed factors, the zonal currents and winds (wind-driven currents) are strongly correlated with sea level variations, primarily during the NE monsoon and the second transitional periods, with a determination coefficient of about 18–36%. As a result of sea level rises, it is estimated that tidal constituent amplitudes will increase by about 8.9% and 18.3% in 2050 and 2100, respectively. The increase in tidal range will possibly relate to the tidal bore passage in the Kampar estuary. Therefore, more advanced hydrodynamic modeling is necessary to determine the impact of sea level rises on tidal bore generation.