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Forecasting and early warning systems are important investments to protect lives, properties and livelihood. While early warning systems are frequently used to predict the magnitude, location and timing of potentially damaging events, these systems rarely provide impact estimates, such as the expected amount and distribution of physical damage, human consequences, disruption of services or financial loss. Complementing early warning systems with impact forecasts has a two‐fold advantage: it would provide decision makers with richer information to take informed decisions about emergency measures, and focus the attention of different disciplines on a common target. This would allow capitalizing on synergies between different disciplines and boosting the development of multi‐hazard early warning systems. This review discusses the state‐of‐the‐art in impact forecasting for a wide range of natural hazards. We outline the added value of impact‐based warnings compared to hazard forecasting for the emergency phase, indicate challenges and pitfalls, and synthesize the review results across hazard types most relevant for Europe. Plain language summary Forecasting and early warning systems are important investments to protect lives, properties and livelihood. While such systems are frequently used to predict the magnitude, location and timing of potentially damaging events, they rarely provide impact estimates, such as the expected physical damage, human consequences, disruption of services or financial loss. Extending hazard forecast systems to include impact estimates promises many benefits for the emergency phase, for instance, for organising evacuations. We review and compare the state‐of‐the‐art of impact forcasting across a wide range of natural hazards, and outline opportunities and key challenges for research and development of impact forecasting.

The Atlantic Ocean has been the most studied and best understood of the World Oceans, for the reason of its importance for the European and American societies. There is a growing evidence that the Atlantic circulation plays a crucial role in the Earth's climate. In this article we summarize our current knowledge of the large scale currents in the Atlantic as well as the variability of the circulation on multiple space and time scales. We also outline outstanding challenges for future oceanographic investigations of the Atlantic current systems.

The Lofoten Basin in the eastern Nordic Seas plays a central role in modifying the warm Atlantic Water inflow toward the Arctic Ocean. Here, the Atlantic Water experiences increased residence times, cooling, and substantial transformation. In this study, we investigate the Atlantic Water inflow pathways to the Lofoten Basin and their vertical and seasonal variations using 2-D and 3-D Lagrangian simulations forced by a high-resolution ocean model. Atlantic Water enters the basin from all directions, but we find two main inflow pathways at all vertical levels, one close to the Lofoten Escarpment in the southeast, associated with the Slope Current, and another close to the Helgeland Ridge in the southwest, associated with the Front Current. The surface inflow exhibits a stronger seasonal forcing than the inflow at depth as well as a stronger heat loss that is dominated by water masses entering the basin from the south. At deeper levels, the warm inflow from the east cools, while the relatively colder inflow from the west warms. The 2-D and 3-D synthetic trajectories show similar pathways. However, they are affected differently by the seasonal signal, giving different heat exchange patterns. Our results have implications for how results from Lagrangian observations in the region should be interpreted.

The air–sea interface is a key gateway in the Earth system. It is where the atmosphere sets the ocean in motion, climate/weather-relevant air–sea processes occur, and pollutants (i.e., plastic, anthropogenic carbon dioxide, radioactive/chemical waste) enter the sea. Hence, accurate estimates and forecasts of physical and biogeochemical processes at this interface are critical for sustainable blue economy planning, growth, and disaster mitigation. Such estimates and forecasts rely on accurate and integrated in situ and satellite surface observations. High-impact uses of ocean surface observations of essential ocean/climate variables (EOVs/ECVs) include (1) assimilation into/validation of weather, ocean, and climate forecast models to improve their skill, impact, and value; (2) ocean physics studies (i.e., heat, momentum, freshwater, and biogeochemical air–sea fluxes) to further our understanding and parameterization of air–sea processes; and (3) calibration and validation of satellite ocean products (i.e., currents, temperature, salinity, sea level, ocean color, wind, and waves). We review strengths and limitations, impacts, and sustainability of in situ ocean surface observations of several ECVs and EOVs. We draw a 10-year vision of the global ocean surface observing network for improved synergy and integration with other observing systems (e.g., satellites), for modeling/forecast efforts, and for a better ocean observing governance. The context is both the applications listed above and the guidelines of frameworks such as the Global Ocean Observing System (GOOS) and Global Climate Observing System (GCOS) (both co-sponsored by the Intergovernmental Oceanographic Commission of UNESCO, IOC–UNESCO; the World Meteorological Organization, WMO; the United Nations Environment Programme, UNEP; and the International Science Council, ISC). Networks of multiparametric platforms, such as the global drifter array, offer opportunities for new and improved in situ observations. Ad

To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.

The 79-North Glacier (79-NG), a major glacier in North-East Greenland that was subject to an increased melt in the last years. The rate of ice loss by basal melting at marine-terminating glaciers is linked to the flow of warm water masses to the glacier that enhance basal melting. However, the sources and pathways of the warm waters on the shelf, their variability and mechanisms of the heat transfer involved are variable regionally and still clearly known. We use Lagrangian particle simulations based on output from a high resolution FESOM (Finite Element Sea Ice-Ocean Model) model to backtrack the source water masses of the warm water inflow to the 79NG. The Lagrangian particles visualize pathways of these waters. They also disclose intense eddy mixing processes in the Fram Strait and the vivid changes of temperature and salinity along the water mass trajectories.

More than 25% of mean global sea level rise is caused by mass loss of Greenland Ice Sheet (GrIS). A significant part of this melt is attributed to the interaction between marine terminating glaciers of the GrIS and the surrounding warm ocean waters. Recent moored and ship-based observations suggest that Atlantic Intermediate Water (AIW) warmer than 1°C reaches the 79NG via the trough system on the East Greenland continental shelf, and that the southern route through the Norske Trough, is preferred. The origins and pathways of these warm AIW are elusive however due sparseness of observations from this remote and harsh environment. Here we use a a high resolution FESOM (Finite Element Sea Ice-Ocean Model) and a suite of Lagrangian model and analysis tools to study the water pathways to the NEGIS and 79NG. We deploy 1915 Lagrangian particles at the Norske Trough and track them backwards to the Fram Strait and further, to the Atlantic and Arctic origins of these waters. The particle trajectories reveal that the warm water in the Trough is a mixture of three source waters: about 60% of particles crossed the northern section (80.64°N) towards the Arctic sea, about 20% crossed the Fram section (78.5°N) southward to the North Atlantic, and nearly 10% entered through an alternative route, the Westwind Trough. We calculate the travel time and water property changes of AIW along the trajectories. The released particles with temperature higher than 2°C reached Fram section after 200 days and colder water (1-2°C) take half this time (100 days). The interannual variability of particle crossings the Fram section has good agreement with the observed variability of AW temperature anomalies in Fram Strait at depth of 250m. Using Lagrangian statistics, we assess the role of the eddy mixing processes at the Fram Strait and that of the mesoscale recirculations in the Trough system.

The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Because of the strong connection of these patterns with surface climate, recent years have shown an increased interest and an increasing skill in forecasting them. However, it is unclear what the intrinsic limits of short-term predictability for the NAO and AO patterns are. This study compares the variability and predictability of both patterns, using a range of data and index computation methods for the daily NAO and AO indices. Small deviations from Gaussianity are found along with characteristic decorrelation time scales of around one week. In the analysis of the Lyapunov spectrum it is found that predictability is not significantly different between the AO and NAO or between reanalysis products. Differences exist, however, between the indices based on EOF analysis, which exhibit predictability time scales around 12–16 days, and the station-based indices, exhibiting a longer predictability of 18–20 days. Both of these time scales indicate predictability beyond that currently obtained in ensemble prediction models for short-term predictability. Additional longer-term predictability for these patterns may be gained through local feedbacks and remote forcing mechanisms for particular atmospheric conditions.

To model tracer spreading in the Ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective-diffusive tracer equations online. Differences in both approaches raise the question whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion which is included in the advection-diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations scale dynamics are resolved and a sufficient amount of floats is used. This study compares the spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer at a depth of 160 m and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D-horizontal distributions show no signigicant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved sub-daily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4048 m^2/s in the Cape Basin.

More than 25% of mean global sea level rise is caused by mass loss of Greenland Ice Sheet (GrIS). A significant part of this melt is attributed to the interaction between marine terminating glaciers of the GrIS and the surrounding warm ocean waters. However, the sources and pathways of the warm waters on the shelf, their variability and mechanisms of the heat transfer involved are variable regionally and yet largely unknown. In this work, we focus on the 79-North Glacier (79-NG), a major glacier in North-East Greenland that was subject to an increased melt in the last years. Recent observations show that Atlantic Intermediate Water (AIW) warmer than 1°C reaches the 79NG via the trough system on the East Greenland continental shelf. In particular, these observations indicate that AIW reaches the glacier rather through the southern Norske Trough than through the northern Westwind Trough. Here we employ Lagrangian modelling and analysis using a high resolution FESOM (Finite Element Sea Ice-Ocean Model) simulation. Particle trajectories representing warm AIW mass are calculated to determine the pathways of this water mass on the adjacent shelf in the Norske Trough, and we analyze the water property changes along the trajectories. Moreover, to identify the sources of the AIW in the vicinity of the 79-NG, we compute backward particle trajectories

Highlights: • Lagrangian ocean analysis is a powerful way to analyse the output of ocean circulation models • We present a review of the Kinematic framework, available tools, and applications of Lagrangian ocean analysis • While there are unresolved questions, the framework is robust enough to be used widely in ocean modelling Abstract: Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.

In recent years, a substantial effort has been placed on quantifying oceanic eddy trasport with Lagrangian diffusivity coefficients estimated either from surface and subsurface Lagrangian observations (drifters and floats) or from Lagrangian particle trajectories advected by velocity output from eddy-permitting/-resolving ocean models. A tantalizing aspect of these studies is a possible application of these Lagrangian diagnostics to parameterize eddy transport of tracers in global circulation models. The questions remain however regarding a large spread and high values of the Lagrangian diffusivity estimates and their practical implementation in model simulations. I will address these questions by compiling my various studies, reflections and discussions on the topics of Lagrangian diffusivity and Lagrangian modeling. First, I will take up different factors associated with the calculation of Lagrangian diffusivities that may add onto the natural spatio-temporal variability of the eddy field and bias the estimated eddy diffusivity coefficients high. In the second part, I will consider implementation of Lagrangian diffusivities to represent eddy transport processes with cautious remarks on their usage as mixing (Redi) coefficients in global circulation models. I will discuss, and show examples of, an alternative approach employing Lagrangian (stochastic) modeling to represent lateral turbulent transport (eddy mixing) and remarking on specific applications ("to diffuse or not to diffuse"), the choice of the stochastic model, and the impact of spatial variability of the diffusivities.

Highlights: • Water mass transformation in Denmark Strait Overflow is localized in space/time • High transformation co-locates with maxima in eddy velocity variance and shear • Overflow eddies modulate the transformation, eddy heat flux divergence and shear Abstract: The Denmark Strait Overflow (DSO) is a major export route for dense waters from the Nordic Seas forming the lower limb of the Atlantic Meridional Overturning Circulation, an important element of the climate system. Mixing processes along the DSO pathway influence its volume transport and properties contributing to the variability of the deep overturning circulation. They are poorly sampled by observations however which hinders development of a proper DSO representation in global circulation models. We employ a high resolution regional ocean model of the Irminger Basin to quantify impact of the mesoscale flows on DSO mixing focusing on geographical localization and local time–modulation of water property changes. The model reproduces the observed bulk warming of the DSO plume 100–200 km downstream of the Denmark Strait sill. It also reveals that mesoscale variability of the overflow (‘DSO-eddies’, of 20-30 km extent and a time scale of 2–5 day) modulates water property changes and turbulent mixing, diagnosed with the vertical shear of horizontal velocity and the eddy heat flux divergence. The space–time localization of the DSO mixing and warming and the role of coherent mesoscale structures should be explored by turbulence measurements and factored into the coarse circulation models.

Seasonal variability in pathways of warm water masses toward the Kangerdlugssuaq Fjord-Glacier system (KF/KG), southeast Greenland, is investigated by backtracking Lagrangian particles seeded at the fjord mouth in a high-resolution regional ocean model simulation in the ice-free and the ice-covered seasons. The waters at KF are a mixture of Atlantic-origin water advected from the Irminger Basin (FF for Faxaflói), the deep waters from the Denmark Strait and the waters from the Arctic Ocean, both represented by the Kögur section (KO). Below 200m depth, the warm water is a mixture of FF and KO water masses, and is warmer in winter than in summer. We find that seasonal differences in pathways double the fraction of FF particles in winter, causing the seasonal warming and salinification. Seasonal temperature variations at the upstream sections (FF and KO) have a negligible impact on temperature variations near the fjord. Successful monitoring of heat flux to the fjord therefore needs to take place close to the fjord, and cannot be inferred from upstream conditions.

The Denmark Strait Overflow (DSO) supplies about one-third of the North Atlantic Deep Water and is critical to global thermohaline circulation. Knowledge of the pathways of DSO through the Irminger Basin and its transformation there is still incomplete, however. The authors deploy over 10 000 Lagrangian particles at the Denmark Strait in a high-resolution ocean model to study these issues. First, the particle trajectories show that the mean position and potential density of dense waters cascading over the Denmark Strait sill evolve consistently with hydrographic observations. These sill particles transit the Irminger Basin to the Spill Jet section (65.25°N) in 5–7 days and to the Angmagssalik section (63.5°N) in 2–3 weeks. Second, the dense water pathways on the continental shelf are consistent with observations and particles released on the shelf in the strait constitute a significant fraction of the dense water particles recorded at the Angmagssalik section within 60 days (~25%). Some particles circulate on the shelf for several weeks before they spill off the shelf break and join the overflow from the sill. Third, there are two places where the water density following particle trajectories decreases rapidly due to intense mixing: to the southwest of the sill and southwest of the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, the overflow particles exhibit a wide range of densities.

We examine the trajectories of 168 drifters as a proxy for Atlantic water flowing through the Nordic Seas. The drifters were released at or passed through the Svinøy section, off the west coast of Norway. For comparison, we generate a set of synthetic trajectories using a stochastic model, with a range of diffusivities. With the both sets, we determine the transit times of both drifters and particles from Svinøy to Arctic gateways: the Barents Sea and Spitsbergen. The mean arrival times to these locations are roughly 200 and 500 days, respectively. This implies ample time for cooling of the surface waters, which increases densities and permits subduction before reaching the Arctic. However a range of transit times is seen; some parcels reach Fram Strait in only 4 months while others are still in the southern Nordic Seas after 2 years. The results do not support the idea that temperature or salinity anomalies can exist as coherent packets. The drifters passing Svinøy, when treated as a group, quickly spread over large distances, mixing with water in the Norwegian and Lofoten Basins. Thus an anomaly entering the Nordic Seas would quickly be obliterated. However, the velocity of the clusters center of mass is consistent with anomaly propagation speeds inferred previously from hydrographic measurements, suggesting the observed variability is advective in nature.

The Norwegian Atlantic Current (NwAC) and its eddy field are examined using data from surface drifters. The data set used spans nearly 20 years, from June 1991 to December 2009. The results are largely consistent with previous estimates, which were based on data from the first decade only. With our new data set, statistical analysis of the mean fields can be calculated with larger confidence. The two branches of the NwAC, one over the continental slope and a second further offshore, are clearly captured. The Norwegian Coastal Current is also resolved. In addition, we observe a semipermanent anticylonic eddy in the Lofoten Basin, a feature seen previously in hydrography and in models. The eddy kinetic energy (EKE) is intensified along the path of the NwAC, with the largest values occurring in the Lofoten Basin. The strongest currents, exceeding 100 cm s−1, occur west of Lofoten. Lateral diffusivities were computed in five domains and ranged from 1–5 × 107 cm2 s−1. The Lagrangian integral time and space scales are 1–2 days and 7–23 km, respectively. The data set allows studies of seasonal and interannual variations as well. The strongest seasonal signal is in the NwAC itself, as the mean flow strengthens by approximately 20% in winter. The EKE and diffusivities on the other hand do not exhibit consistent seasonality in the sampled regions. There are no consistent indications of changes in either the mean or fluctuating surface velocities between the 1990s and 2000s.

During the 4th International Polar Year 2007–2009 (IPY), it has become increasingly obvious that we need to prepare for a new era in the Arctic. IPY occurred during the time of the largest retreat of Arctic sea ice since satellite observations started in 1979. This minimum in September sea ice coverage was accompanied by other signs of a changing Arctic, including the unexpectedly rapid transpolar drift of the Tara schooner, a general thinning of Arctic sea ice and a double-dip minimum of the Arctic Oscillation at the end of 2009. Thanks to the lucky timing of the IPY, those recent phenomena are well documented as they have been scrutinized by the international research community, taking advantage of the dedicated observing systems that were deployed during IPY. However, understanding changes in the Arctic System likely requires monitoring over decades, not years. Many IPY projects have contributed to the pilot phase of a future, sustained, observing system for the Arctic. We now know that many of the technical challenges can be overcome. The Norwegian projects iAOOS-Norway, POLEWARD and MEOP were significant ocean monitoring/research contributions during the IPY. A large variety of techniques were used in these programs, ranging from oceanographic cruises to animal-borne platforms, autonomous gliders, helicopter surveys, surface drifters and current meter arrays. Our research approach was interdisciplinary from the outset, merging ocean dynamics, hydrography, biology, sea ice studies, as well as forecasting. The datasets are tremendously rich, and they will surely yield numerous findings in the years to come. Here, we present a status report at the end of the official period for IPY. Highlights of the research include: a quantification of the Meridional Overturning Circulation in the Nordic Seas (“the loop”) in thermal space, based on a set of up to 15-year-long series of current measurements; a detailed map of the surface circulation as well as characterization of

We compare two methods for estimating mean velocities and diffusivities from surface drifter observations, using data from the Nordic Seas. The first is the conventional method of grouping data into geographical bins. The second relies on a "clustering" algorithm, and groups velocity observations according to nearest-neighbor distance. Capturing the spatial variability of the mean velocity requires using bins with a length scale of ˜50km. However, because many bins have few observations, the statistical significance varies substantially between bins. Clustering yields sets with approximately the same number of observations, so the significance is more uniform. At the densely sampled Svinøy section, clusters can be used to construct the mean flow field with <=10km resolution. Clustering also excels at the estimation of eddy diffusivities, allowing resolution at the 20 km scale in the densely sampled regions. Taking bathymetry into account in the clustering process further improves mean estimates where the data is sparse. Clustering the available surface drifter data, extended by recent deployments from the POLEWARD project, reveals new features in the surface circulation. These are a large anticyclonic vortex in the center of the Lofoten Basin and two anticyclonic recirculations at the Svinøy section. Clustering also yields maps of the eddy diffusivities at unprecedented resolution. Diffusivities are suppressed at the core of the Norwegian Atlantic Current, while they are elevated in the Lofoten Basin and along the Polar Front.