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Future sea level rise and changes in extreme weather will increase the frequency of flooding and intensify the risks for the millions of people living in low‐lying coastal areas. Concerns about coastal adaptation have been broadened due to societal awareness of the threat from rising seas, leading to a large set of potential adaptation users with diverse needs for adequate sea level projections in coastal areas beyond the current state of the art regional projections. In this paper, we provide an overview of the potential steps for improvement of regional sea level projections along the global coastline, with specific focus on the contribution from ocean dynamics to seasonal‐decadal variability of coastal sea level, and its implications for changes in frequency and magnitude of extreme sea levels. We discuss the key gaps in our knowledge and predictive capability of these dynamics as they relate to sea level variability on seasonal to decadal timescales, and conclude by suggesting ways in which these knowledge gaps could be addressed. Plain Language Summary: In the next few decades sea level rise and changes in extreme weather will increase the frequency of flooding and intensify the risks of inundation for the millions of people living in low‐lying coastal areas. The sustainable future of coastal communities in small settlements and populous megacities alike will depend on the efficacy of new coastal defense infrastructure and adequate decisions about adaptation options. The planning for these management solutions is fundamentally based on local sea level projections. Providing sea level projections at the coast is a complex task, because sea level in coastal areas is impacted by numerous local drivers (e.g., wind, bathymetry, and limited connection with the open ocean), changing water motion in time and space. We highlight several potential research avenues to improve simulations of sea level changes in coastal areas that are urgently required for improved localized sea level projections for effective coastal management in a changing climate. Key Points: There exist diverse sea level projection requirements in coastal areas beyond current state of the art regional projectionsOcean dynamics across scales is crucial in linking open ocean to shelf sea to coastal sea level variability and changeSteps for improvement of sea level projections along the global coastline are proposed, focusing on seasonal‐decadal variability [ABSTRACT FROM AUTHOR]

After the Fukushima Daiichi Nuclear Power Station (F1NPS) accident, 137Cs activity concentrations have not yet decreased to pre-accident levels because of direct release from the site and fluvial discharges of 137Cs deposited on land. It is necessary to consider dispersion processes in the coastal area to understand the impact of multiple river discharges and direct release. To achieve this goal, we carried out oceanic dispersion simulations that considered direct release and fluvial discharges and compared the results with the annual averages of observed data. We assumed that particulate 137Cs discharged from rivers to the ocean quickly resuspended and re-leached after coagulation and precipitation, and that all of the 137Cs was dispersed. The reproducibility of results was improved by considering fluvial discharges of particulate 137Cs at all sites between 2013 and 2016, except near the F1NPS. In other words, particulate 137Cs discharged from rivers was found to influence the results of ocean surface activity concentrations within a relatively short period of time. The impact of direct release was dominant for the observed 137Cs activity concentrations adjacent to the F1NPS, which was used to estimate direct releases. [ABSTRACT FROM AUTHOR]

Brett, J.; Samuell, M.; Augstein, B.; Parra, S.; Hancock, E.; Saunders, C.; Wunsch, S.; Winstead, N., and Boothby, J., 2024. Nature-based coastal protection: Measuring and modeling flow reduction by oysters. Journal of Coastal Research, 40(4), 647–660. Charlotte (North Carolina), ISSN 0749-0208. Natural coastal infrastructure protects coast lines from climate hazard impacts, such as erosion and flooding. Laboratory and modeling studies presented here examine the utility of oyster reefs as a form of natural coastal protection. Oyster reefs are known to provide both water quality improvements and erosion protection. Experiments were conducted to determine the effects of oysters on flow parameters in a recirculating flow channel. A cage of eastern oysters, Crassostrea virginica, was placed in the flow channel, blocking a portion of the flow, to imitate cultured oysters in the ocean. Particle image velocimetry was used to measure the flow behind the oyster cage to examine the impact of the oysters compared with a control test with an empty cage. Average velocities were reduced by 15%, with velocities directly behind the cage reduced 70% in fresh water and reversed in salt water. A setup similar to the laboratory experiments was simulated in a Coupled Ocean–Atmospheric–Wave–Sediment Transport (COAWST) model. In the COAWST model, the oyster reef was simulated as stiff vegetation with a drag coefficient of 8, reproducing the 70% reduction measured in the lab experiments. Finally, a regional model is presented, which demonstrates the impact of an oyster reef on waves and currents at a larger scale with a drag coefficient of 1.5, the highest numerically stable value found. Within the oyster vegetation patch, depth-averaged currents were reduced 14% and bottom currents were reduced 97%. Significant wave heights were reduced by 62% above the reef and 23% shoreward of the reef. The results here suggest that regional modeling such as COAWST can be an effective tool for assessing the impact of potential coastal resilience strategies to mitigate climate hazard impacts in a climate-changed world. [ABSTRACT FROM AUTHOR]

Boundary conditions are important constraints when solving partial differential equations to obtain solutions for the interior. Global ocean prediction models have supplied such boundary conditions for regional ocean models. However, the biases present in these global ocean models can significantly affect the accuracy of predictions within nested regional ocean models. In the present study, open boundary bias correction and data assimilation were applied using the observed temperature and salinity data. The bias correction was conducted at the upstream open boundary, the Korea Strait, while the data assimilation was performed in the interior, including Ulleung Basin in the southwestern East Sea, to reduce the prediction errors in the nested regional model. The effects of each correction method were quantified through four numerical experiments. In the Korea Strait and Ulleung Basin, root mean square errors (RMSEs) in temperature decreased by 44% in February using data assimilation compared to free run, while RMSEs in salinity decreased by 31% in August using bias correction. Bias correction of open boundary data enhanced the simulation performance in salinity near the open boundary; however, it did not lead to improvements in temperature distribution within the interior. Conversely, data assimilation improved temperature distribution within the interior; however, salinity bias persisted near the open boundary. Notably, the combined use of the two correction methods resulted in a lower RMSE in temperature in the Ulleung Basin during winter, and a lower RMSE in salinity in the Korea Strait and Ulleung Basin during the summer. Hence, the simultaneous use of open boundary bias correction and data assimilation is expected to improve the interior solution of salinity and temperature in various other regional ocean models. [ABSTRACT FROM AUTHOR]

Abstract Future sea level rise and changes in extreme weather will increase the frequency of flooding and intensify the risks for the millions of people living in low‐lying coastal areas. Concerns about coastal adaptation have been broadened due to societal awareness of the threat from rising seas, leading to a large set of potential adaptation users with diverse needs for adequate sea level projections in coastal areas beyond the current state of the art regional projections. In this paper, we provide an overview of the potential steps for improvement of regional sea level projections along the global coastline, with specific focus on the contribution from ocean dynamics to seasonal‐decadal variability of coastal sea level, and its implications for changes in frequency and magnitude of extreme sea levels. We discuss the key gaps in our knowledge and predictive capability of these dynamics as they relate to sea level variability on seasonal to decadal timescales, and conclude by suggesting ways in which these knowledge gaps could be addressed.

In the coming decades, warming and deoxygenation of marine waters are anticipated to result in shifts in the distribution and abundance of fishes, with consequences for the diversity and composition of fish communities. Here, we combine fisheries-independent trawl survey data spanning the west coast of the USA and Canada with high-resolution regional ocean models to make projections of how 34 groundfish species will be impacted by changes in temperature and oxygen in British Columbia (BC) and Washington. In this region, species that are projected to decrease in occurrence are roughly balanced by those that are projected to increase, resulting in considerable compositional turnover. Many, but not all, species are projected to shift to deeper depths as conditions warm, but low oxygen will limit how deep they can go. Thus, biodiversity will likely decrease in the shallowest waters (less than 100 m), where warming will be greatest, increase at mid-depths (100–600 m) as shallow species shift deeper, and decrease at depths where oxygen is limited (greater than 600 m). These results highlight the critical importance of accounting for the joint role of temperature, oxygen and depth when projecting the impacts of climate change on marine biodiversity. This article is part of the theme issue 'Detecting and attributing the causes of biodiversity change: needs, gaps and solutions'. [ABSTRACT FROM AUTHOR]

Outputs of the regional ocean circulation model are analyzed to demonstrate the measurable impact of the El Niño remote oceanic forcing mechanism along the US West Coast during the major heat wave period of 2014–2016. The 2‐km horizontal resolution model, based on the Regional Ocean Modeling System (ROMS), was run for the period of 2009–2018. Though the model does not assimilate observations, it performs well by comparison with time series data explaining observed variability on temporal scales from several days to seasonal and interannual. The El Niño‐related oceanic anomalies provided by a global state estimate are introduced in the regional model at the southern boundary at 24N. These propagate alongshore with coastally trapped waves (CTWs) and influence the variability off Oregon (41°–46°N). In particular, CTWs are evident in the subsurface along‐slope current, vs, and in the depth of the 26.5 kg m−3 isopycnal surface over the slope, z26.5. In summer 2014 and 2015, vs anomalies are positive (northward) and z26.5 anomalies are negative (deeper) along the US West Coast. In addition to the CTW patterns, z26.5 anomalies also exhibit slow‐moving features associated with undercurrent widening, separation, and subsurface eddy variability. Over the Oregon shelf, El Niño conditions contributed to the sharp weakening of the southward alongshore current throughout the water column in July 2014 and 2015, despite the near‐average southward, upwelling‐favorable winds. Plain Language Summary: El Niño is a large‐scale anomalous pattern that originates in the atmosphere and the ocean in the equatorial Pacific ocean. It occurs once every several years at irregular time intervals and impacts the weather and ocean variability at great distances, in particular reaching the Pacific coast of the United States. In this study, a high‐resolution, three‐dimensional computer ocean circulation model is utilized to track the El Niño‐related signal propagating as very long oceanic waves along the continental slope from Mexico to the United States and Canada in 2014 and 2015. This propagation is best seen in the subsurface alongshore currents and in the depression of the surfaces of constant density over the continental slope. The model compares well to satellite and in‐situ observations. Carefully designed numerical models can be utilized for multiyear simulations to aid scientific discoveries. Key Points: El Niño anomalies propagating with coastally trapped waves (CTWs) impact the coastal ocean variability off Oregon 2014–2016In summers 2014 and 2015, the subsurface along‐slope velocity anomalies are poleward, propagating as CTWOn the shelf, alongshore southward currents are sharply reduced in July 2014 and 2015 despite the upwelling winds are strong [ABSTRACT FROM AUTHOR]

A series of accidents at the Fukushima Daiichi Nuclear Power Station (F1NPS), following the 11 March 2011 earthquake and tsunami, resulted in the release of radioactive substances into the ocean. In particular, the release of large amounts of radioactive caesium has damaged the fishing industry, leading to voluntary restrictions on fishing and shipping. Oceanic dispersion simulations based on estimates of the pathways and fluxes of radioactive materials provide useful information for assessing the environmental impacts and formulating measures to mitigate the effects of the accident. For the direct release rate from the F1NPS site, an estimation method was developed using the results from nearby monitoring, and the seawater exchange rate was estimated in target volume using a numerical simulation. However, the influence of volume on the seawater exchange rate was not considered. Appropriate volumes must be considered when estimating the effects of future accidents. In addition, the directional coastal transport was underestimated in the simulations of the F1NPS accident because of the low resolution. To estimate the pathways and fluxes of radioactive material to the ocean and understand the distribution of the concentration of radioactive material based on ocean dispersion simulations, a study was conducted using a higher-resolution model. The horizontal resolution of the conventional ocean dispersion model Regional Ocean Modelling System (ROMS), was increased from 1 km to 200 m. The optimal settings of the seawater exchange rate were investigated, and the radioactive caesium transport process in the coastal direction was more accurately reflected. We found that the conventional volume for determining the seawater exchange rate, including the locations of release sources and observation points, is optimal. The validity of this finding was confirmed using experimental equations from previous oceanic tracer release experiments. To estimate future release rates, it an appropriate volume must be defined, for example, depending on the distance between the locations of the release sources and the observation point. In addition, improvements in the accuracy of the simulation of the coastal transport process were observed owing to the higher resolution, which increased reproducibility. However, with a horizontal resolution of 200 m, problems with repeatability near the harbours arose. A higher resolution, achieved using nesting or other methods, would be desirable to deal with releases smaller than those in the F1NPS accident. • A higher resolution model was developed for the assessment of the Fukushima Daiichi Nuclear Power Station accident. • A method for estimating direct leakage rates using the higher resolution model was proposed for future severe accident. • The higher resolution allowed improved transport processes of 137Cs along the coast. [ABSTRACT FROM AUTHOR]

A long simulation over the period 1901–2010 with an eddy-permitting ocean circulation model is used to study the variability of the upper layer circulation in the North Ionian Gyre (NIG) in the Eastern Mediterranean Sea (EMed). The model is driven by the atmospheric forcing from the twentieth century reanalysis data set ERA-20C, ensuring a consistent performance of the model over the entire simulation period. The main modes of variability known in the EMed, in particular the decadal reversals of the NIG upper layer circulation observed since the late 1980s are well reproduced. We find that the simulated NIG upper layer circulation prior to the observational period is characterized by long-lasting cyclonic phases with weak variability during years 1910–1940 and 1960–1985, while in the in-between period (1940–1960) quasi-decadal NIG circulation reversals occur with similar characteristics to those observed in the recent decades. Our simulation indicates that the NIG upper layer circulation is rather prone to the cyclonic mode with occasional kicks to the anticyclonic mode. The coherent variability of the NIG upper layer circulation mode and of the Adriatic Deep Water (AdDW) outflow implies that atmospheric forcing triggering strong AdDW formation is required to kick the NIG into an anticyclonic circulation 1–2 years later. A sensitivity experiment mimicking a cold winter event over the Adriatic Sea supports this hypothesis. Our simulation shows that it is the multi-decadal variability of the salinity in the Adriatic Sea that leads to periods where low salinity prevents strong AdDW formation events. This explains the absence of quasi-decadal NIG reversals during 1910–1940 and 1960–1985. [ABSTRACT FROM AUTHOR]

Satellite altimeter data assimilation into a regional ocean model southwest of Japan is studied based on an approximate Kalman filter. System error or process noise has been conventionally attributed to surface meteorological conditions. However, the effects of surface meteorological change by the data assimilation decay rapidly to depths and may not propagate into deep layers. Here, we attempted to employ lateral boundary update by the data assimilation in comparison with surface forcing control to improve the representation of the Kuroshio Current. The magnitude of process noise from the surface and lateral boundary forcings was separately determined by minimizing the root-mean-square difference of temperature. The results showed that the effect of lateral forcing propagated into deep layers, and it performed better than the experiments controlling surface conditions around the Kuroshio Current where the water depth is deep. However, the data assimilation of surface meteorological condition was found to be still effective in shallow area. The cross-spectral analysis indicated that satellite altimeter data assimilation can improve the representation of long-term Kuroshio Current variability. [ABSTRACT FROM AUTHOR]

Changes in a marine environment have a broad socioeconomic implication on fisheries and their relevant industries so that there has been a growing demand for the medium-range (months to years) prediction of the marine environment Using a medium-range ocean prediction model (Ocean Mid-range prediction System, OMIDAS) for the northwest Pacific, this study attempted to assess seasonal difference in the mid-range predictability of the sea surface temperature (SST), focusing on the Korea seas characterized as a complex marine system. A three-month re-forecast experiment was conducted for each of the four seasons in 2016 starting from January, forced with Climate Forecast System version 2 (CFSv2) forecast data. The assessment using relative root-mean-square-error was taken for the last month SST of each experiment. Compared to the CFSv2, the OMIDAS revealed a better prediction skill for the Korea seas SST, particularly in the Yellow sea mainly due to a more realistic representation of the topography and current systems. Seasonally, the OMIDAS showed better predictability in the warm seasons (spring and summer) than in the cold seasons (fall and winter), suggesting seasonal dependency in predictability of the Korea seas. In addition, the mid-range predictability for the Korea seas significantly varies depending on regions: the predictability was higher in the East Sea than in the Yellow Sea. The improvement in the seasonal predictability for the Korea seas by OMIDAS highlights the importance of a regional ocean modeling system for a medium-range marine prediction. [ABSTRACT FROM AUTHOR]

Sea level on the northwestern European shelf (NWES) varies substantially from year to year. Removing explained parts of interannual sea level variability from observations helps to improve estimates of long‐term sea level trends. To this end, the contributions of different drivers to interannual sea level variability need to be understood and quantified. We quantified these contributions for the entire NWES by performing sensitivity experiments with a high‐resolution configuration of the Regional Ocean Modeling System (ROMS). The lateral and atmospheric boundary conditions were derived from reanalyses. We compared our model results with satellite altimetry data and used our sensitivity experiments to show that nonlinear feedbacks cause only minor interannual sea level variability on the shelf. This indicates that our experiments can be used to separate the effects of different drivers. We find that wind dominates the variability of annual mean sea level in the southern and eastern North Sea (up to 4.7‐cm standard deviation), whereas the inverse barometer effect dominates elsewhere on the NWES (up to 1.7‐cm standard deviation). In contrast, forcing at the lateral ocean boundaries results in small and coherent variability on the shelf (0.5‐cm standard deviation). Variability driven by buoyancy fluxes ranges from 0.5‐ to 1.3‐cm standard deviation. The results of our sensitivity experiments explain the (anti)correlation between interannual sea level variability at different locations on the NWES and can be used to estimate sea level rise from observations in this region with higher accuracy.Plain Language Summary: Sea level on the continental shelf northwest of Europe is rising in the long term but is also varying strongly from year to year. This makes it difficult to determine the rate of sea level rise from observations. To improve long‐term trends computed from sea level observations, the causes of short‐term sea level variability need to be understood. Therefore, we test the influences of different components of the atmosphere and ocean on year‐to‐year sea level variability, using a numerical ocean model for northwestern Europe. We find that the varying strength and direction of winds causes large variability of sea level in the southern and eastern North Sea. In other places on the continental shelf, sea level variability is mainly influenced by the variability of atmospheric pressure. We also find that the ocean outside our model domain drives small and uniform sea level variability on the continental shelf and that there is a moderate influence of solar radiation, precipitation, and evaporation. Our results help to understand how sea level varies at different locations in northwestern Europe and to obtain better estimates of the rate of long‐term sea level rise.Key Points: We use a ROMS setup to quantify the contributions of different drivers to interannual sea level variability on the NWES for 1995–2018Atmospheric variability is the main driver, whereas ocean variability drives small and uniform sea level variability on the shelfWind drives large variability in the North Sea, north of the United Kingdom and around Norway, while the inverse barometer effect dominates elsewhere [ABSTRACT FROM AUTHOR]

Wintertime sea surface temperature (SST) anomalies that disappear in summer and recur in the following winter are known as "reemergence." The role of reemergence in recurrent SST anomalies in an area in the central North Pacific is investigated quantitatively for the first time by conducting an online mixed layer heat budget analysis with a realistic ocean model simulation. In contrast to past studies that have emphasized the importance of vertical entrainment of subsurface temperature anomalies, it is shown that several mechanisms are operating in recurrence of SST anomalies during 1977–1999. In particular, anomalous Ekman meridional advection of the mean meridional temperature gradient induced by zonal wind stress anomalies plays an important role in many years. Also, coincidence in the sign of SST anomalies in winter and surface heat flux anomalies in the following autumn leads to recurrence of SST anomalies. Plain Language Summary: Sea surface temperature anomalies formed in a winter tend to reemerge in the subsequent winter. This process known as reemergence is considered as a key oceanic mechanism of decadal climate modes that have large impacts on global climate and ecosystems. Based on quantitative analyses of a realistic ocean model simulation, we show that recurrence of sea surface temperature occurs through two mechanisms: (1) wind‐induced anomalous meridional advection in autumn favoring reappearance of anomalously warm or cool surface water and (2) coincidence in the sign of sea surface temperature anomalies in winter and surface heat flux anomalies in the following autumn. Key Points: The role of reemergence in the North Pacific is investigated for the first time with a completely closed mixed layer heat budgetRecurring sea surface temperature is not necessarily due to entrainment of anomalous subsurface temperature formed in the previous winterAnomalous Ekman meridional advection and coincidence in the sign of surface heat flux anomalies are the key for recurrence [ABSTRACT FROM AUTHOR]

Ningaloo Niño/Niña is the dominant mode of interannual variability of sea surface temperature (SST) in the southeastern Indian Ocean. According to previous studies, both local air-sea interaction and remote forcing contribute to generation and amplification of the Ningaloo Niño/Niña. The latter forcing includes the atmospheric teleconnection and oceanic wave propagation through the Indonesian archipelago, mainly associated with the El Niño/Southern Oscillation (ENSO). One of the most important characteristics of the Ningaloo Niño/Niña is their asymmetry; the Ningaloo Niño is stronger than the Ningaloo Niña. Using a regional ocean modeling system (ROMS), the impact of oceanic wave propagation on the amplitude and asymmetry of SST anomalies associated with the Ningaloo Niño/Niña is investigated. For these purposes, a sensitivity experiment in which oceanic lateral boundary conditions are replaced by the monthly climatology is conducted. It is shown that the oceanic teleconnection transmitted from the western tropical Pacific can explain about 30% of the observed amplitude asymmetry in SST anomalies. Results from composite and heat budget analyses suggest that coastal downwelling (upwelling) Kelvin waves from the western tropical Pacific associated with La Niña (El Niño) that often co-occurs with the Ningaloo Niño (Niña) are stronger (weaker) and contribute to the asymmetry. [ABSTRACT FROM AUTHOR]

Antarctic krill (Euphausia superba) is a key species in the marine ecosystem of the Southern Ocean, but is also the target of a commercial fishery, with an important fishing ground in the South Orkney Islands region. The potential for competition for krill between predators and the fishery requires risk management strategies for the fishery, underpinned by an understanding of the key physical and behavioural drivers of krill movement and retention in target areas. Here, we present the results of a regional modelling study, combining a high-resolution ocean-sea ice model and an individual-based model parameterised for krill, to elucidate the roles of oceanographic variability and krill behaviour on patterns of transport and retention on and around the South Orkney Plateau. Simulations suggest that oceanic transport from sources around the Antarctic Peninsula is restricted by the northward flowing Antarctic Slope Current. Around the South Orkney Plateau, anticyclonic flows associated with the Weddell Front and the shelf edge transport krill rapidly towards the main fishing grounds to the northwest of the plateau. Transport onto the shelf and subsequent retention are influenced by the strength and direction of regional winds; weaker on-shelf transport and shorter retention times are associated with stronger westerly and northerly winds. The incorporation of sea-ice associated behaviour, whereby krill are moved with sea ice when sea ice is present, significantly modifies the patterns from purely oceanic transport; it reduces the influence of strong regional oceanic flows and increases transport of krill to the South Orkneys region from the Antarctic Peninsula. The inclusion of diel vertical migration also modifies predicted patterns from oceanic transport, but to a lesser extent, and moderates the impact of including sea-ice associated behaviour. We highlight the importance of understanding the behaviour of krill, including age-dependent behavioural changes in response to sea ice conditions, for modelling and management of Antarctic krill populations. • Numerical modelling reveals drivers of krill transport and distribution variability. • Antarctic Slope Current restricts transport of krill from the Antarctic Peninsula. • Sea-ice associated behaviour significantly alters krill transport and retention. • Diel vertical migration impact on simulated krill transport and distribution is relatively weak. • Stronger westerly and northerly winds increase influx but reduce local retention. [ABSTRACT FROM AUTHOR]

Impacts of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are assessed through novel sensitivity experiments using a regional ocean model (Regional Ocean Model System) and detailed diagnostics of heat and momentum budget. During the pIOD, density stratification in the eastern equatorial Indian Ocean is enhanced due to anomalous surface freshening and subsurface saltening. This causes momentum inputs from the wind forcing to be more strongly trapped in the surface layer, and zonal and vertical current anomalies to be more confined to the upper layer. As a result, upward transports of cold water from below the thermocline to the surface layer are significantly suppressed, and the cooling in the eastern equatorial Indian Ocean is suppressed by as much as 1.0 °C. The above arguments are further corroborated by a set of sensitivity experiments using a linear continuously stratified ocean model, which can isolate the effect of stratification change caused by salinity anomalies associated with the pIOD in the Regional Ocean Model System simulation. Our results suggest that salinity does play an active role in the evolution of the pIOD, rather than being passively affected by large‐scale anomalous atmospheric and oceanic conditions. Plain Language Summary: Salinity modulates the upper ocean circulation and temperature by changing the density of seawater and dynamical/themodynamical processes that control them. Recent studies have demonstrated that the tropical Indian Ocean experiences significant salinity variation associated with the Indian Ocean Dipole (IOD), which is a dominant climate mode in the tropical Indian Ocean and exerts widespread impacts on the global climate. However, how these salinity anomalies affect the upper ocean properties and feedback onto the IOD are not quantitatively understood. Here, we investigate the roles played by salinity variation associated with the positive IOD (pIOD) by conducting a series of sensitivity experiments using a regional ocean model. The results from our experiments show that these pIOD‐related salinity anomalies act to shift the current anomalies upward and weaken the surface cooling by about 1.0 °C in the eastern equatorial Indian Ocean. This is because enhanced salinity stratification traps momentum imparted by wind to the upper layer and alters the structure of the equatorial circulation. The validity of this mechanism is supported by detailed heat and momentum budget analyses, as well as sensitivity experiments using a simplified ocean model. These results provide clear evidences that salinity plays an active role in the evolution of the pIOD. Key Points: Role of salinity in the evolution of the positive Indian Ocean Dipole is quantitatively assessed using a regional ocean modelSensitivity experiments reveal significant impacts of salinity anomalies on the upper ocean temperature and currents anomaliesEnhanced stratification and associated weakening of upwelling suppress the sea surface temperature cooling in the east by 1.0 °C [ABSTRACT FROM AUTHOR]

Mechanisms of salinity anomalies associated with the positive Indian Ocean Dipole (pIOD) are investigated through a series of sensitivity experiments and an online budget analysis using a regional ocean model. Special emphasis is placed on the contribution from the rectified effects due to high‐frequency variability, which was not quantitatively discussed in previous studies. The results from sensitivity experiments show that positive sea surface salinity (SSS) anomalies in the southeastern tropical Indian Ocean are primarily caused by reduction in precipitation and partly by enhanced evaporation due to increased wind speed, while negative SSS anomalies in the central‐eastern equatorial Indian Ocean are generated by zonal advection anomalies induced by anomalous wind stress, consistent with previous studies. Completely new results are that the modulation of nonlinear salinity advection associated with mesoscale eddies also plays an important role in determining the spatial pattern of SSS anomalies, especially in the southeastern tropical Indian Ocean. On the other hand, subsurface salinity anomalies are almost entirely caused by wind stress effects mediated by ocean dynamical processes. Further decomposition of advective anomalies suggests that they are mainly explained by the pIOD‐related current anomalies governed by equatorial wave dynamics. However, a vertical shift of nonlinear freshening due to high‐frequency variability also substantially contributes to the generation of positive subsurface salinity anomalies in the eastern equatorial Indian Ocean. Our results show that large‐scale oceanic changes in response to the pIOD‐related atmospheric anomalies are the key drivers of the observed salinity anomalies, while some nonlinear effects also seem to be at work. Plain Language Summary: The positive Indian Ocean Dipole (pIOD) is characterized by anomalous cooling in the eastern tropical Indian Ocean and warming in the western tropical Indian Ocean and exerts significant impacts on the local and global climate. The signatures of the pIOD have been detected not only in the upper ocean temperature but also in salinity, which is a fundamental parameter along with temperature. Accompanied by the anomalous atmospheric circulation and precipitation pattern, surface and subsurface salinity in the tropical Indian Ocean is known to undergo significant variations associated with the pIOD. In this study, the relative importance of various factors in the generation of these salinity variation and physical processes behind them are quantitatively assessed using a regional ocean model. The results demonstrate that the anomalous large‐scale ocean circulation induced by wind anomalies is the dominant factor that causes significant salinity variation, while anomalous precipitation and evaporation also play a secondary role. More comprehensive analyses reveal that contributions from mesoscale eddies and short timescale variations, which have been overlooked in past studies, are also important for the generation of salinity anomalies. These results offer novel mechanisms explaining the observed salinity variation, in addition to well‐documented processes proposed by previous studies. Key Points: Processes contributing to the generation of salinity anomalies associated with the positive Indian Ocean Dipole are quantitatively assessedLarge‐scale wind stress anomalies that modulate salinity advection/diffusion are the primal drivers of the salinity anomaliesNonlinear advection due to rectification of high‐frequency variability also plays an important role [ABSTRACT FROM AUTHOR]

Tropical and extratropical storms commonly occur in the Northwest Atlantic Ocean, sometimes causing catastrophic losses to coastal fisheries. Still, their influence on fish movements and range shifts is poorly known. We coupled biotelemetry observations of black sea bass in the U.S. Mid-Atlantic Bight with numerical modelling of the coastal ocean to evaluate the influence of Hermine (3–8 September 2016) on cold pool thermal destratification and fish evacuation. Spring through fall, black sea bass is a sedentary species, with movements focused on structure where they support important commercial and recreational fisheries. During summer 2016, we characterized the movements of 45 acoustically tagged black sea bass at three sites deploying acoustic receivers moored in shelf waters 18–31 km east of Ocean City, Maryland, and at depths 20–32 m in the southern Mid-Atlantic Bight. On 3 September 2016, cyclonic winds of Hermine caused rapid destratification of the water column. At experimental sites, bottom temperatures rose from 13 to 23°C in 10 h. An oceanographic model and observing data showed that the effects of this destratification dominated large portions of the Mid-Atlantic Bight and had long term effects on seasonal evolution of the shelf temperature. Nearly half of remaining black sea bass on 3 September (40%) permanently evacuated the experimental sites. Those that remained showed long-term depressed activity levels. Although the cause of this incomplete evacuation is unknown, it exemplifies partial migration, which may buffer black sea bass to regional impacts of changed timing or increased incidence of tropical storms. [ABSTRACT FROM AUTHOR]

Highlights • We present regional ocean simulations driven by a new set of atmospheric fields. • Tyrrhenian large-scale circulation is well captured with 50 km and 25 km fields. • Resolved dynamical structures are better represented with fields of 25 km. • Improvements with 25 km fields are largely set by wind representation. Abstract The Tyrrhenian circulation, driven by the interplay of transport at its main straits and local climate, presents a marked seasonal variability. In this work we investigate the role of the horizontal resolution of the atmospheric fields in the long-term seasonal variability of the Tyrrhenian Sea circulation. We adopt the eddy-permitting ocean circulation model NEMO-MED12. Simulations cover the 1997–2008 period and are driven by atmospheric fields from PROMES regional climate model with a horizontal resolution of 50 km and 25 km. Given that this is the first time that PROMES variables have been used to run ocean simulations, we first evaluate the overall performance of this setup taking into consideration the entire model domain, which covers the whole Mediterranean Sea. We show that PROMES variables allow a good representation of the Mediterranean boundary fluxes and water masses. Focusing on the Tyrrhenian Sea, the comparison of the model results to altimeter data shows that the main features of the seasonal variability of the Tyrrhenian large-scale circulation are well captured regardless of the resolution of the atmospheric fields. However, the prescription of 25 km resolution atmospheric fields improves substantially the representation of the dynamical structures resolved by the model. Based on our results, we update intermediate-depth circulation patterns, very little studied so far. We show that changes observed with 25 km fields are not due to a better simulation of transport across Corsica, Sardinia and Sicily straits, but rather to the improved representation of winds. [ABSTRACT FROM AUTHOR]

Ocean general circulation models even with realistic behavior still incorporate large uncertainties from external forcing. This study involves the realization of ensemble experiments using a regional model configured for the Southwest Atlantic Ocean to investigate uncertainties derived from the external forcing such as the atmosphere and bathymetry. The investigation is based on perturbing atmospheric surface fluxes and bathymetry through a series of ensemble experiments. The results showed a strong influence of the South Atlantic Convergence Zone on the underlying ocean, 7 days after initialization. In this ocean region, precipitation and radiation flux perturbations notably impacted the sea surface salinity and sea surface temperature, by producing values of ensemble spread that exceeded 0.08 and 0.2 °C, respectively. Wind perturbations extended the impact on currents at surface, with the spread exceeding 0.1 m/s. The ocean responded faster to the bathymetric perturbations especially in shallow waters, where the dynamics are largely dominated by barotropic processes. Ensemble spread was the largest within the thermocline layer and in ocean frontal regions after a few months, but by this time, the impact on the modeled ocean obtained from either atmospheric or bathymetric perturbations was quite similar, with the internal dynamics dominating over time. In the vertical, the sea surface temperature exhibited high correlation with the subsurface temperature of the shallowest model levels within the mixed layer. Horizontal error correlations exhibited strong flow dependence at specific points on the Brazil and Malvinas Currents. This analysis will be the basis for future experiments using ensemble‐based data assimilation in the Southwest Atlantic Ocean. Plain Language Summary: The numerical models are powerful tools to provide knowledge about the ocean state concerning currents eddies, meanders, and other ocean dynamic and thermodynamic processes on a range of temporal and spatial scales. An accurate numerical model makes possible to get a tridimensional ocean representation with some confidence during time. Even though the ocean numerical models have been incorporating improvements, mainly due to a growing evolution of the computational resources, they are still somewhat limited and bring uncertainties on their simulations due many reasons that are related to the applied physical parameterization, atmospheric forcing, bathymetry, and some other issues. It is crucial to investigate and to know these uncertainties. This study goes further on the uncertainty investigations in order to create the basis (prior step) for an ensemble‐based data assimilation system for the Southwest Atlantic Ocean. Our results indicated that uncertainty in wind forcing plays a major role in the determination of uncertainty in the ocean state. Compared to atmospheric forcing, the uncertainty in bathymetry produced a larger impact on the ocean representation, especially in shallow waters, though this may be in part due to excited waves at the initial time. Key Points: The 10‐m wind perturbations caused the largest impact on the ocean spread at shorter time scales, considering only the atmospheric forcingThe perturbed bathymetries produced a larger impact on the ensemble model simulations, probably due to excited waves at the initial timeFlow‐dependent error correlations are identified that follow the western boundary currents pathway [ABSTRACT FROM AUTHOR]

Assessment of marine downscaling of global model simulations to the regional scale is a prerequisite for understanding ocean feedback to the atmosphere in regional climate downscaling. Major difficulties arise from the coarse grid resolution of global models, which cannot provide sufficiently accurate boundary values for the regional model. In this study, we first setup a stretched global model (MPIOM) to focus on the North Sea by shifting poles. Second, a regional model (HAMSOM) was performed with higher resolution, while the open boundary values were provided by the stretched global model. In general, the sea surface temperatures (SSTs) in the two experiments are similar. Major SST differences are found in coastal regions (root mean square difference of SST is reaching up to 2°C). The higher sea surface salinity in coastal regions in the global model indicates the general limitation of this global model and its configuration (surface layer thickness is 16 m). By comparison, the advantage of the absence of open lateral boundaries in the global model can be demonstrated, in particular for the transition region between the North Sea and Baltic Sea. On long timescales, the North Atlantic Current (NAC) inflow through the northern boundary correlates well between both model simulations (R~0.9). After downscaling with HAMSOM, the NAC inflow through the northern boundary decreases by ~10%, but the circulation in the Skagerrak is stronger in HAMSOM. The circulation patterns of both models are similar in the northern North Sea. The comparison suggests that the stretched global model system is a suitable tool for long-term free climate model simulations, and the only limitations occur in coastal regions. Regarding the regional studies focusing on the coastal zone, nested regional model can be a helpful alternative.

A 6 year, 2009-2014 simulation using a 2 km horizontal resolution ocean circulation model of the Northeast Pacific coast is analyzed with focus on seasonal and interannual variability in along-slope subsurface oceanic properties. Specifically, the fields are sampled on the isopycnal surface [ABSTRACT FROM AUTHOR]

Across-slope bottom boundary layer (BBL) fluxes on the shelf-edge connect this region to deeper waters. Two proposed ways in which across-slope BBL fluxes can occur, in regions that have a slope current aligned to the bathymetry, are the frictional veering of bottom currents termed the 'Ekman drain' and through local wind-forced downwelling (wind-driven surface Ekman flow with an associated bottom flow). We investigate the variability, magnitude and spatial scale of BBL fluxes on the Shetland shelf, which has a prominent slope current, using a high-resolution (∼2 km) configuration of the MITgcm model. Fluxes are analysed in the BBL at the shelf break near the 200 m isobath and are found to have a seasonal variability with high/low volume transport in winter/summer respectively. By using a multivariate regression approach, we find that the locally wind-driven Ekman transport plays no explicit role in explaining daily bottom fluxes. We can better explain the variability of the across-slope BBL flux as a linear function of the speed and across-slope component of the interior flow, corresponding to an Ekman plus mean-flow flux. We estimate that the mean-flow is a greater contributor than the Ekman flux to the BBL flux. The spatial heterogeneity of the BBL fluxes can be attributed to the mean-flow, which has a much shorter decorrelation length compared to the Ekman flux. We conclude that both the speed and direction of the interior current determines the daily BBL flux. The wind does not explicitly contribute through local downwelling, but may influence the interior current and therefore implicitly the BBL fluxes on longer timescales. [ABSTRACT FROM AUTHOR]

An original methodology for integrating the water cycle from the rain water to the open ocean by numerical models was set up using an offline coupling technique. The different components of the water continuum, including watersheds, estuaries and ocean, for Western Iberia were reproduced using numerical components of the MOHID Water Modelling System (). This set of models, when combined through this novel methodology, is able to fill information gaps, and to include, in a realistic mode, the fresh water inputs in terms of volume and composition, into a regional ocean model. The designed methodology is illustrated using the Tagus River, estuary and its region of fresh water influence as case study, and its performance is evaluated by means of river flow and salinity observations. [ABSTRACT FROM AUTHOR]

This paper discusses the combined use of tools from dynamical systems theory and remote sensing techniques and shows how they are effective instruments which may greatly contribute to the decision making protocols of the emergency services for the real-time management of oil spills. This work presents the successful interplay of these techniques for a recent situation, the sinking of the Oleg Naydenov fishing ship that took place in Spain, close to the Canary Islands, in April 2015. [ABSTRACT FROM AUTHOR]

The overall goal is to downscale ocean conditions predicted by an existing global prediction system and evaluate the results using observations from the Gulf of Maine, Scotian Shelf and adjacent deep ocean. The first step is to develop a one-way nested regional model and evaluate its predictions using observations from multiple sources including satellite-borne sensors of surface temperature and sea level, CTDs, Argo floats and moored current meters. It is shown that the regional model predicts more realistic fields than the global system on the shelf because it has higher resolution and includes tides that are absent from the global system. However, in deep water the regional model misplaces deep ocean eddies and meanders associated with the Gulf Stream. This is not because the regional model’s dynamics are flawed but rather is the result of internally generated variability in deep water that leads to decoupling of the regional model from the global system. To overcome this problem, the next step is to spectrally nudge the regional model to the large scales (length scales > 90 km) of the global system. It is shown this leads to more realistic predictions off the shelf. Wavenumber spectra show that even though spectral nudging constrains the large scales, it does not suppress the variability on small scales; on the contrary, it favours the formation of eddies with length scales below the cutoff wavelength of the spectral nudging. [ABSTRACT FROM AUTHOR]

The use of multiple passive tracers in a dynamical ocean model is presented as a method to study water mass origins. The multiple passive tracer method is successfully applied to determine the origin regions of the Atlantic cold tongue (ACT), using 31 passive tracers. At the time of the ACT minimum in sea surface temperature, 38 ± 3% of the surface water in the central equatorial Atlantic is found to originate in the western side of the basin, with only 11 ± 2% originating in the east. Conversely, for the ACT surface cooling off the west coast of Africa, only 2 ± 1% is from the west of the Atlantic basin, with 71 ± 7% from the east. The region of the North Brazil undercurrent is found to be the most important origin region for the central Atlantic ACT region, for a 6-8 month lead time. Using the Met Office EN4 observational dataset of ocean temperatures, correlations are found between January temperatures in the North Brazil undercurrent region and both the ACT mean temperature and ACT spatial extent in July-August, significant at the 0.95 level. This suggests that the 6-month lead-time predictability of the ACT may be increased by improved knowledge of the North Brazil undercurrent temperature. Results from previous studies suggest that such increased predictability of the ACT could potentially help improve predictions of the West African monsoon. [ABSTRACT FROM AUTHOR]

The mean flow and meso-scale eddy variability in the region around the Grand Banks of Newfoundland (GBN) are quantified by analyzing surface drifter observations, mean dynamic topography (MDT), along-track satellite altimeter observations, and the solutions of two high-resolution ocean models. By increasing the horizontal resolution from 6.5 km (CREG12) to 2.2 km (GBN36), the modeled mean kinetic energy (MKE) decreases and the eddy kinetic energy (EKE) increases in the study area. GBN36 obtains the MKE of surface geostrophic currents and total currents similar to that derived from MDT and drifter data, whereas CREG12 overestimates these quantities by 40-53 %. CREG12 and GBN36 underestimate the EKE of surface geostrophic currents by 46 and 30 %, respectively, with respect to the EKE derived from along-track altimeter data. The models do not reproduce the strong eddy activity in the Gulf Stream and its downstream region, possibly related to a northward shift of the Gulf Stream position by 0.5° in latitude compared with observations. Both models obtain wavenumber spectra of sea level anomaly in close agreement with the spectrum derived from along-track altimeter data, with a slope of −5 at wavelengths near 100 km on logarithmic spectral density scales. [ABSTRACT FROM AUTHOR]