River plumes form when freshwater from rivers enters the salty ocean, creating buoyant water masses that strongly influence coastal circulation. By transporting freshwater, heat, nutrients, sediments and pollutants, they impact the ocean dynamics and ecosystems on local (10-100 km) and even (beyond) regional scales (>1000 km), depending on the size and dynamics of the plume. This thesis aims to improve our understanding of the Rhine River plume and its interactions with sea-level variations. The Rhine plume, located along the Dutch coast in the Southern North Sea, is highly dynamic system, influenced by tides and winds.
Chapter 2 investigates the variability of the wind-driven response of the Rhine River plume using numerical model simulations of a spring-neap cycle forced by idealized wind conditions. The difference in wind-driven response between spring and neap tide shows how the competition between straining and mixing, both induced by tides and winds, determines the structure and evolution of the Rhine River plume.
Chapter 3 examines the plume’s effect on sea-level variability along the Dutch coast by comparing barotropic and baroclinic model simulations. The Rhine plume induces a positive steric height anomaly, elevating the mean sea level along the coast and modulating the tidal signal near the river mouth. This highlights the need to include river plumes in sea-level studies.
In Chapters 4 and 5, an innovative method is developed for estimating sound speed profiles from multibeam echosounder measurements. The inversion method is based on minimizing the discrepancies between overlapping swaths and exploits empirical orthogonal functions to describe sound speed profiles using a limited number of unknowns. Since sound speed is influenced by depth, temperature, and salinity, this proof-of-concept provides a way to offer valuable insights into the vertical structure of the water column using routinely collected data.
Overall, this thesis advances our understanding of the Rhine River plume and its contribution to sea-level variability. In addition, the development of a proof-of-concept for retrieving sound speed profiles from multibeam echosounder measurements offers a promising approach to provide valuable information on stratification in river plumes. Together, these contributions support improved modelling and understanding of coastal oceans, particularly river plumes, which will become more and more important, especially in the face of climate change and its impact on coastal regions. ... Read more

The Atlantic Meridional Overturning Circulation (AMOC) is an important element in the Earth’s Climate System, and it leads to variations in the climate systems, such as heat transformation, carbon dioxide distribution, freshwater distribution, and extreme weather conditions.
The dense water formation and export in the North Atlantic contribute to AMOC. Motivated by the aim to understand the strength and processes of AMOC in the Subpolar (SPNA), the water origins and transport routes are explored in this study, based on the inspiration of the previous studies of water pathways in the Labrador Sea.
The objective of this study is to investigate the origin of water in the Irminger Sea and the transport routes before arriving at Cape Farewell, the tip of Greenland where water continues to flow from the Irminger Sea to the Labrador Sea. Based on simulated field data in SPNA obtained from Modular Ocean Model (MOM), the Lagrangian method is applied in the Ocean Model Connectivity Modeling System (CMS), to track particles in the Irminger Sea. The properties of particles, such as location and temperature along their trajectory, are calculated at advection timesteps.
Origins of water reaching Cape Farewell through the Irminger Sea are discovered, including the source from the Denmark Strait, the Iceland-Scotland Ridge, and the South Iceland Basin. Water from these origins follows different routes before arriving at Cape Farewell and qualitative and quantitative analyses of these routes based on particle trajectories and particle numbers, provide insights into how water crosses the Irminger Sea and the importance of the routes.
In summary, water following the route from the Denmark Strait contributes 25 percent of the water through the Irminger Sea arriving at Cape Farewell. Water follows a direct and straight route along the East Greenland Boundary, as the cold source in the surface layer (50-150m). Water staying in the Irminger Basin contributes 50 percent. It travels in curved and blended routes in a deep layer (1200-1500m), as the warm source. Water from Iceland-Scotland Ridge follows a long-distance trajectory, which crosses the Iceland Basin, Reykjanes Ridges, and Irminger Basin. Water in this route contributes 20 percent. They travel in a surface layer as the warm source, while the temperature of water in this route decreases when water arrives at Cape Farewell. Water from the South Iceland Basin is in the surface layer (0-150m) along the routes, with the highest temperatures. The contribution is not accurate as the long-distance route requires a longer tracking period.
The research findings provide valuable insights into the dynamics of water masses in the Irminger Sea. It relates to the currents, and oceanic activities in the Irminger Sea, such as convention and eddies, which contributes to a better understanding of AMOC in SPNA. ... Read more

The Atlantic Meridonial Overturning Circulation (AMOC) in the North Atlantic Ocean (NAO) plays a major role in earth’s climate and climate change. A key element of the AMOC is deep convection, which is still not fully understood. One of the unknowns is where water is exchanged between the boundary current and the regions where deep convection can occur. This is important for models to know where deep waters are formed and where they are transported to. This study focuses on the Irminger Sea (IRS), a sub-sea of the NAO. The interior of this sub-sea is a known area where deep convection can occur. Using data from the Argo Float Program, a analysis was conducted to investigate exchanges of water between the boundary current of the IRS and the area where deep convection can occur. The entries and departure locations of the Argo floats are collected and statistically compared. Furthermore, seasonality difference between winter and summer months are compared using the Mann-Whitney U-Test. Lastly, the internal pathways water takes within the interior area are analysed, by tracking where a float enters the interior area and where it afterwards leaves the area. The results show water takes many different pathways in and out of the interior area and the pathways taken within the area show the expected cyclonic pattern. There were no clear differences between summer and winter months, except in the northern part of the interior area, where in winter a clear south-western current is present, but not in summer. Future studies on the exchange between the boundary current and the interior area can use these results as an indication that the exchange happens all around the area, but the water does follow a cyclonic pattern. ... Read more

The Earth’s climate is changing, due to global warming, impacting the ocean circulation around the world. As the ocean circulation distributes large amounts of energy around the world, this can alter climate drastically if changed. The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental ocean component to comprehend climate change and further investigation enhances our capacity to predict it. The AMOC plays a pivotal role in regulating the ocean heat transport within the North Atlantic Ocean, influencing the climates of North America and Europe. This study centers its attention on the Sub-Polar Gyre (SPG), a critical region where the AMOC activity peaks. Within this region, this study aims to get a better understanding of the overturning dynamics of the SPG, on a seasonal and annual time scale. To achieve this, the reanalysis model GLORYS12 is used, which offers a detailed simulation of ocean dynamics spanning the period from 1993 to 2020. With its high-resolution, eddy-resolving capabilities, GLORYS12 is particularly well-suited for capturing the nuanced small-scale overturning processes associated with the AMOC. From these model data, the overturning is calculated from alongstream changes in boundary current transport divided in density classes. The analysis is performed for the entire SPG by dividing it into its major basins: the Iceland Basin, Irminger Sea, and Labrador Sea. Subsequently, the boundary currents of the SPG are further subdivided into seventeen individual segments, providing insights into how overturning dynamics vary along the SPG. The results reveal that the mean overturning strength in the SPG for 1993-2020 is 23.8 Sverdrups (106 m3/s (Sv)). The distribution in overturning strength between the basins is 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively. Furthermore,
the results shows overturning occurs at increasingly higher densities, the further west you go. Each basin displays a pronounced seasonal pattern, with maximum overturning occurring in March and the minimum in September. On an inter-annual time scale, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a decreasing trend of -0.04 and -0.02 Sv/year respectively, whereas the Labrador Sea has an increasing trend of 0.02 Sv/year over 1993-2020. A further division in shorter segments yields large spatial differences in overturning, both in overall strength and the distribution over density classes. However, these outcomes are less robust as flows are highly variable and numerical errors associated with the overturning calculations become more prominent. This also raises questions about the reliability of the assessment
of overturning along segments from observations to determine the local overturning dynamics. In conclusion, this study leverages GLORYS12 for a detailed basin and segmented analyses to offer a comprehensive understanding of the AMOC within the SPG. The findings provide valuable insights into the AMOC’s long-term behavior, seasonal variations, annual trends, and high spatial variability. Using this increased understanding, future research can improve on why the AMOC behaves in the observed way, by analyzing the overturning dynamics sensitivity to oceanic and atmospheric conditions ... Read more

As the environment is changing temperatures are changing, becoming more extreme. This is expected to affect the oceans and its transport, specifically the Atlantic Meridional Overturning Circulation (AMOC). The Labrador Sea is a part of the AMOC, where overturning in depth and density space occurs, due to deep convection. Deep convection is the process of seawater losing its heat to the atmosphere, due to atmospheric cooling during the winter. This causes the seawater to become colder and denser, and it therefore sinks towards the bottom of the basin. Deep convection is previously studied extensively as it is a unique and important process of the global ocean circulation system. The key process that causes the AMOC water to overturn, is due to buoyant eddies shedding from the boundary current into the interior. The buoyant eddies exchange their buoyant boundary current water with the dense interior water, causing the boundary current (and in extension the AMOC water) to cool down.

Previous studies have shown that the properties of the boundary current water are strongly dependent on the eddy exchange, and therefore on the surface heat loss. However, it is not known how consecutive strong winters impact the dynamics of the Labrador Sea on various timescales, which will therefore be the focus of this thesis. Data for this research will be obtained by using an idealised model configuration of the Labrador Sea, where the hydrostatic primitive equations of motion are solved by the MIT general circulation model (MITgcm). Different types of scenarios are defined to analyse different effects on the dynamics. These scenarios are analysed by looking into how the mean basin temperature changes, how the eddy kinetic energy (EKE) and mixed layer depth (MLD) develop, and how the properties through a transect of the basin change. The effects of these interactions are then studied by looking at how the transport of water throughout the boundary current, per density class and per vertical layer change.

The thesis mainly shows that the mixed layer depth in the interior increases during a strong winter. As a result, the eddy kinetic energy increases significantly in the boundary current, as the horizontal density gradient increases, thus causing an increase in boundary current velocity in the downstream direction. Additionally, more and denser interior water accumulates, depending on how many consecutive strong winters occur. This deep convected water in the interior partly remains near the bottom of the basin. In the next winter, it is mixed again due to deep convection, consequently a positive feedback loop occurs. Meaning, that the number of consecutive winters positively impacts the interactions in the basin, as the horizontal density gradient increases, and thus the velocity and eddy kinetic energy increase as well, in respect to the previous winter. The effect of the strong winters persists in the years afterwards, as the interior remains relatively cold. Additionally, a part of the accumulated convected interior water resides too deep in the basin to be exchanged by the eddy exchange and therefore flows near the bottom out of the basin, due to a pressure difference. The flow near the bottom is a negative feedback loop, as the volume of dense convected water decreases and can therefore not be further cooled during consecutive strong winters. Finally, the properties and the transport of the boundary current water are directly related to the interior water and eddy exchange. As the MLD in the interior and eddies in the BC are still relatively large in the years after the additional surface heat loss, the export of boundary current water therefore also remains affected. In conclusion, the effect of wintertime surface heat loss on the Labrador Sea Water in the short term has the most influence on the MLD and EKE, however the influence of the MLD and EKE remains and therefore in the long term affects the export through the BC. These conclusions can help to better interpret the limited available measurements of the Labrador Sea Water. ... Read more

The production of water masses formed by convection in the Labrador Sea (i.e. Labrador Sea Water, LSW) and its variability contributes to the variability of the Atlantic Meridional Overturning Circulation (AMOC). Several studies put the role of the Labrador Sea under renewed debate, and suggest a rather complex interplay between the production of the LSW, the boundary current and the eddy field. To this end, an increased effort is put in understanding the variability of the LSW, its export routes and associated export timescales. In this study, the effects of variations in boundary current strength on the export pathways of convected water masses are investigated. The same idealized eddy-resolving numerical model is used as Georgiou et al. (2019) which has proven to be capable of capturing the key dynamics of the Labrador Sea, like the annual cycle of convection, the process and timescales of restratification, and properties of the mesoscale eddy field. Model simulations are set-up with different scenarios of the density structure of the boundary current at inflow location (i.e. southern tip of Greenland). The variations result in respectively a 5% strengthening and 5% weakening of the boundary current, which corresponds to interannual variability of observed surface velocities. The model output demonstrates that boundary current variations start a chain of reactions, significantly changing the dynamics of the Labrador Sea. This has implications for deep convection processes in the interior of the basin and thus the export product. With a passive tracer analysis it is shown that convected water masses formed in the convection area are laterally steered along isopycnals by an eddy-induced shear flow from the interior towards the boundary current at the West-Greenland coast in deeper layers. A strengthening (weakening) of the boundary current yields a lighter (denser) water mass to be exported at shallower (deeper) layers out of the interior. The most intense entrainment into the boundary current occurs where both the density and depth of the convected water masses match the local water mass properties of the boundary current, and where eddies detach from the boundary current. The associated export timescales can be linked to the location where eddies detach, and to the strength of the eddy-induced shear flow. This study further highlights the implications for linking variability in the LSW production and export to AMOC variability as the total export of convected waters in the Labrador Sea is a mixture of multiyear convected waters. Based on density alone, measurements of water masses at the exit do not directly reveal the past-year dynamical state of the Labrador Sea. This emphasizes that a proper representation of mesoscale eddies in models is necessary for representing the export timescales and water mass properties of the LSW, and their response to changing forcing. ... Read more

The Labrador Sea is one of the deep convection sites in the world's oceans and the water masses formed here are an important component of the Atlantic Meridional Overturning Circulation (AMOC). To study this linkage, one study in particular used an idealized model of the Labrador Sea where the density variations consists only of temperature variations. In this study, it is questioned whether the assumption of neglecting salinity is appropriate, by analysing the pathways of water mass and water mass transformation in the Labrador Sea.
This is investigated by using that same idealized model (here called the reference run) and comparing this to a model where salinity variations are added whilst keeping density variations the same (Sconstant) to produce a similar circulation pattern. Furthermore, a model configuration is created which investigates if a seasonal cycle in salinity impacts the circulation pattern of the Labrador Sea (Sseasonal). The pathways of water masses in these model configurations are analyzed by Lagrangian particle tracking from A to B.
It was found that with the same initial density variations the maximum surface eddy kinetic energy (EKE) increases by 41 % when salinity is incorporated in the model. An increase in EKE is often associated with more water mass leaving the boundary current (BC) due to an increase in instabilities. Surprisingly, the opposite was found: 7.02 and 8.22 Sv are transported through the BC for the reference run and Sconstant, respectively. Furthermore it was found that most of the water mass leaves and re-enters the BC near the maximum EKE for each model configuration. An increase was found in maximum overturning in density space from an Eulerian perspective: from 3.9 to 4.8 Sv for the reference run and Sconstant, respectively, where about 10 % so called density compensation occurred for Sconstant. No significant annual changes are found when adding a seasonal cycle to the model. For all model configurations a large discrepancy exists between Eulerian and Lagrangian calculations in downwelling. This discrepancy is due to Lagrangian particles that reside in the models at the end of their simulation duration Thus the overturning in the Labrador Sea is significantly influenced by particles that have a long residence time (longer than 4 years in these model simulations). Between 22 and 25 % of the Lagrangian volume transport does not reach the outflow of each model simulation.
There are also properties that salinity did not influence: no significant changes were found between the model configurations for the overturning in depth space, the annual MLD and barotropic streamfunction. In conclusion adding salinity to the idealized model showed only minor changes in the pathways of water mass and water mass transformation: the order of magnitude of all analyzed properties stays the same. Density compensation however is neglected when no salinity variations are added in the model. This means that for a highly idealized model of the Labrador Sea, salinity variations can be neglected, when density variations due to salinity variations are represented by temperature variations. ... Read more

The Atlantic Meridional Overturning Circulation (AMOC) is a key component in the Earth System. Given its important role in the climate system, variability in the AMOC strength is expected to have great impact on the global climate. The current observational timeseries are not long enough to make climate projections for the end of the century or even longer. Therefore, coupled climate models play an important role in the making of end of century climate projections on the AMOC strength. A known issue with the current generation of global coupled climate models is that the grid resolution is generally too coarse to resolve smaller scale processes such as mesoscale eddies. Observations and modelling studies suggest that mesoscale eddies play an important role in the exchange of water between convection regions and downwelling regions. When such processes are absent or parametrized incorrectly, it can have an influence on the climate projections based on these model simulations. This study analysed the AMOC characteristics in two different simulations of the Community Earth System Model; a reference simulation (referred to as piControl) and a simulation in which the atmospheric CO2 concentrations have been increased to four times the initial concentration (referred to as 1pctCO2). First, the AMOC characteristics in the piControl simulation are analysed using both a Eulerian and a Lagrangian approach. The Eulerian analysis shows that deep mixed layers, an indicator for convection, are present in the subpolar North Atlantic. Compared to observations and higher-resolution ocean-only models are these located closer to the West-Greenland coast. Strong vertical velocities are found over the continental slopes, especially over the steep continental slopes around Greenland. Second, the Lagrangian analysis showed the consequences of the coarse grid in the model. Only a single pathway around the subpolar gyre was observed. This implies that particles will experience convection while crossing the interior of the Labrador Sea, but only will experience downwelling when their individual pathway comes close enough to the continental slopes around Greenland. Furthermore, there is only limited exchange of particles with the regions north and south of the subpolar gyre. In the export of deep waters to the subtropical gyre, does it seem that the particles are being blocked by the North Atlantic Current. Third, the changes in the AMOC characteristics in the 1pctCO2 simulation are compared to the piControl simulation. In the 1pctCO2 simulation have deep mixed layers disappeared from the subpolar North Atlantic and convection in this region has shut down. A new fresh(er) surface layer in the Labrador Sea has intensified the stratification and prohibits the formation of deepmixed layers. Instead of the deep mixed layers in the subpolar North Atlantic have new deep mixed layers emerged between 30N and 40N. In general are velocities reduced in magnitude in the 1pctCO2 simulation, but the stronger vertical velocities can still be found over the continental slopes. In conclusion, the results show that the CESM model can reproduce the two components of the AMOC reasonably well, but the connection in the formof mesoscale eddies is missing. This illustrates that a overturning streamfunction simplifies the complexity of the overturning process. The overturning streamfunction is a measure for the overturning strength, but it does not take into account how and if the convection process and the exchange between convection and downwelling are represented. ... Read more

The oceanic transport of heat and salt from the equator northward is one of the main reasons for the mild climate of Europe. This transport occurs in the upper layer of the ocean. In the north, strong cooling occurs due to the large difference in temperature between the ocean surface and the atmosphere. The cooled watermass has a higher density and therefore sinks and returns toward the south at depth. This so-called AtlanticMeridional Overturning Circulation is driven in part by the wind and in part by the difference in temperature and salinity between the equator and the poles. Polar climate change will result in warmer and fresher oceans whichwill likelyweaken this global overturning circulation. Especially processes that concern the transformation from the light (warm) watermasses to dense (cold) watermasses are sensitive to changes in buoyancy forcing. This thesis focuses on an area where a large part of this transformation from light to dense watermasses takes place; the Nordic Seas. The Nordic Seas are located between Greenland and Norway and consist of several sub-basins, like the Lofoten Basin, the Greenland Basin and the Norwegian Basin. The main aim of this thesis is to better understand the dynamical processes involved in the watermass transformation in the Nordic Seas. ... Read more

The Weddell Polynya, a large hole in the Antarctic sea ice, reappeared in 2017. The polynya forms due to deep convection, which is caused by static instability of the water column. Observations and model studies show periodic heat accumulation in the subsurface layer prior to a polynya. This heat accumulation could be caused by internal ocean dynamics: the Southern Ocean Mode. Periodic subsurface heat and salt accumulation could be the major driver in causing periodic deep convection, which is in contrast with earlier studies. These studies focus on surface processes, and see the polynya as an irregular event. In this study a simple convective model is used to look into this contrast. Model simulations excluding and including periodic subsurface heat and salt fluxes have been performed. Multiple polynya events were only simulated in the model set up including subsurface fluxes. The dominant frequency for polynya events in these simulations equals the frequency of the subsurface heat and salt accumulation. This frequency is still visible in runs with white noise added to the freshwater flux, showing the importance and dominance of the subsurface forcing. In combination with earlier studies, this study suggests that periodic subsurface processes are most dominant and govern the initial formation and periodicity of the Weddell Polynya. ... Read more

Thermohaline staircases are characterised by stepped vertical temperature, salinity and density profiles, which are formed and maintained by the double diffusion of heat and salt. Because double diffusion is the primary mixing agent in regions with staircases, it is the topic of extensive studies. Previous studies, however, are mainly theoretical and modelling orientated and observational evidence is needed to verify the results. In our study we use an extensive dataset with 460 vertical profiles of temperature and salinity. We found that staircases in the Caribbean Sea are related through temperature and salinity, indicating that
staircases in the Caribbean Sea are constant in time and space. Individual steps, however, differ and were characterised in four types: well-developed steps, transitional layers, inversions and absence of steps. A case study of a strong anticyclonic eddy gave the indication that steps are influenced by short term processes. The eddy induces lateral gradients and hereby positions the water masses in the interior and exterior of the eddy such that thermohaline
intrusions are initiated. The apparent preconditioning by the eddy, leading to thermohaline intrusions, allows us to speculate that the eddy is a catalyst in double diffusive diapycnal buoyancy transport. ... Read more

The Rhine Region of Fresh Water Influence (ROFI) is a shallow frictional river plume in front of the Dutch coast. Each tidal cycle a new tidal plume front with fresh-water is released. Recently, internal gravity waves have been observed in this plume. Using a Froude number analysis, support for the internal wave generation mechanism by a tidal plume front is found. It is shown, that on averaged neap tides the density stratification is large. This results in a larger area where the internal waves can be released from the tidal plume fronts compared to spring tides. As the Rhine ROFI is located in shallow water, it is investigated what the effect of bathymetry variation is on internal waves. This is studied by extending the standard KdV model derivation to account for a variable bed. This resulted in a small correction on the propagation speed inside the KdV equation. Observations of internal waves have been used to validate the KdV model. By scaling analysis of the observed waves it is obtained that the relative wave height and relative depth balance for most of the observed events. For these events the wave period and velocity amplitude of the KdV model are well matched with the measurements. This showed that the KdV model may be used for a first estimate of internal waves in the Rhine ROFI. By developing a TGE fitting procedure, it was possible to obtain the pycnocline depth and the direction of wave propagation with only limited data available. Therefore, the parameters have been varied and the error between the TGE solution and the velocity potential obtained from the velocity measurements has been minimized. ... Read more

In highly dynamic and vulnerable tidal systems such as the Wadden Sea, the importance of understanding natural processes and how they are hampered by anthropogenic pressure is highly demanding. Within these processes the sediment transport is one of the most challenging movements to be monitored. With this in mind, suspended particulate matter (SPM) transport in the Marsdiep inlet, the southeastern most tidal inlet in the Dutch Wadden Sea, is monitored with high frequency acoustic backscattering measurements obtained with acoustic Doppler current profiler (ADCP) on Texels Eigen Stoomboot Onderneming (TESO) ferry. The calibration of ADCP measurements is practiced with another device - optical backscatter sensor (OBS). In order to obtain reliable suspended particulate matter concentration (SPMC) measurements, the first step is to calibrate OBS output with high precision. Based on the studies done in the past, the calibration needs to be done locally and regularly as the OBS is sensitive to the variability of SPM properties. The objective of the present study is to formulate an improved OBS calibration method with in situ water samples taken from the Royal Netherlands Institute for Sea Research (NIOZ) jetty. This was achieved by applying pumping suction method to collect the water samples while measuring optical backscattering signal with Campbell Scientific OBS3+ device. Subsampling of the water samples was tested and the results revealed that subsampling leads to undesirable outcome. Procedural control filters that were applied to the laboratory procedure showed filter mass loss that needs to be taken into the account, and the analysis of salt retention showed 1.06 mg of salt remaining on the filters after filtration procedure. Moreover, loss on ignition (LOI) technique revealed the amount of organic content of SPMC which is linearly correlated to full SPMC. The analysis of spring-neap tidal cycle showed that during neap tide there was 0:5 mg l^-1 more organic SPMC compared to the one during spring tide. Finally, the sources of uncertainties were identified and the guidance for further research was suggested. ... Read more

As a key component to the bottom limb of the Atlantic Meridional Overturning Circulation (AMOC), the Labrador Sea is one of the regions where deep ocean convection takes place. This convection is driven by atmospheric cooling during winter, which brings the surface water into the intermediate and deep layers by uniformizing water mass properties. This homogeneous layer is called Mixed Layer (ML). As a result of this convection, stratification is no longer maintained, and the Mixed Layer Depth (MLD) deepens. During this deepening, an enormous amount of potential energy is converted to kinetic energy, and meso- and sub-mesoscale instabilities develop. After wintertime, the MLD starts to shallow again. Atmospheric-induced convection ceases or decreases significantly and physical components return to stratified conditions. Baroclinic instabilities grown to mesoscale or geostrophic scale play a role in restratifying the ML through the formation of coherent ocean eddies. This chain of processes follows a seasonal cycle that strongly depends on the imbalance between horizontal and vertical buoyancy gradients. A practical way to quantify this imbalance is the use of the Ertel potential vorticity or a derived magnitude as the Richardson angle, which allow to infer the existence of instabilities and to classify them respectively.
This study analyzes the physical processes behind the MLD seasonal variability in the Labrador Sea. To this end, high-resolution model data (1/12° × 1/12°) from a global simulation has been used. An evaluation of spatial and temporal patterns of the MLD and energy conversion is provided, and the dominant types of instabilities are determined. It is hypothesized that these instabilities drive the energy conversion and the growth of coherent mesoscale eddies, which can modify the MLD and restratify the ocean. Finally, the sequential interactions among the processes are investigated to provide better understanding about seasonal MLD variability. This study shows that the density-based MLDs with a threshold of 0.03 kg m^-3 are the most credible values, and the spatial and temporal patterns of energy conversion and gravitational/symmetric instabilities are in phase with the MLD variability. The energy conversion is investigated by means of the available potential energy (APE), kinetic energy (KE) and Energy Ratio (ER) which is introduced in this study, and a large amount of gravitational and/or symmetric instabilities is found within ML, especially in the upper ocean layers. The role of baroclinic instabilities is investigated with the Eady growth rate, while the presence of coherent mesoscale eddies is inferred from the Okubo-Weiss parameter and the Eddy Kinetic Energy, whose size is limited by the internal Rossby radius. This study shows that the MLD variability is the result of changes in the conversion between the available potential energy (APE) and kinetic energy (KE) as well as of the competition between ravitational/symmetric and baroclinic instabilities. The former favoring MLD deepening, and the latter favoring MLD shallowing. ... Read more

Direct measurement of ocean surface velocity from space with a Synthetic Aperture Radar has shown to be a promising method to observe ocean surface currents. In this thesis report a method for Total Surface Current Vector (TSCV) retrieval using an experimental Bidirectional (BiDi) Along-Track Interferometric (ATI) acquisition mode with TanDEM-X is presented. Errors of retrieval results from simulated data and from real data are studied to assess the quality of the proposed method. The available data consists of a StripMap acquisition at the coast of Tromso and a data set acquired over Novaya Zemlya.
The measurement concept relies on the ATI phase, which provides an estimate of the first moment of the Doppler spectrum associated to total surface velocity. Observing with two beams squinted as far as 13.2 degree apart in azimuth on ground, allows the Doppler velocity to be observed in line of sight of the beams. Projection to the ocean surface gives a velocity field. This Doppler velocity field consists of a Normalized Radar Cross Section (NRCS) weighted average of velocities of sea-state dependent biases such as short wind generated waves, long swell waves and underlying currents.
Assuming the surface velocity is dominated by wind generated waves and underlying currents, the method attempts to solve for TSCV simultaneously with the surface wind vector by coupling geophysical model functions (GMF) for returned Doppler Centroid (DC) and NRCS from an ocean surface shaped by wind.
For NRCS the empirical GMF XMOD2 for X-band radar is used, based on the same regression algorithm as the widely used CMOD5 in scatterometry. For DC a GMF based on statistics of the sea surface and the Kirchhoff Approximation developed by IFREMER is used.
A cost function of the wind vector is defined as the squared difference between NRCS observations and values of the GMF in both beams. The wind speed magnitude and wind direction for which this cost function is minimal provide an estimate to the local wind vector and evaluating the GMF for DC with the estimated wind vector results in a component of surface motion caused by wind generated waves. Wind wave induced surface velocities and TSCV can then be separated.
Retrieval from simulated data shows that the wind retrieval algorithm gives an ambiguous result for the wind direction. To constrain the solution ECMWF ERA-5 reanalysis wind data is added to the cost function as an additional term with a low weight factor.
Error analyses on the propagation of data errors shows success of the method relies on calibration quality that itself depends on local conditions of the acquired data. Comparison of retrieved wind using different GMF's indicates there is a high uncertainty in the models. The average of retrieved wind vector field over the image is highly similar to the lower resolution ECMWF ERA-5 wind vector data. TSCV results appear good for data with small ATI phase errors, but are dependent on the accuracy of used GMF's. ... Read more

Sandy beaches comprise large parts of the world's shorelines and act as a natural buffer for many exposed people and assets that are concentrated in the coastal zone. Many coastal communities are vulnerable to the impact of sea-level rise (SLR) that can amplify the episodic erosion from storms and drive structural erosion. The way communities adapt to SLR hinge critically on future SLR projections. One of the major uncertainties is the potential rapid disintegration of large fractions of the Antarctic ice sheet (AIS) that can accelerate sea-level rise, albeit neglected in the latest SLR estimates of the 'Intergovernmental Panel on Climate Change (IPCC)'. Accounting for rapid AIS mass loss in coastal impact assessments is essential for risk-averse coastal managers that disfavour events with large consequences.

Although methods to predict future erosion estimates under SLR have been developed, hitherto no study has assessed the impact of different cases of AIS dynamics to erosion estimates. Here, a case-study to the island of Sint Maarten is considered to evaluate the implications for strategies to manage coastal erosion under SLR uncertainty. Regional SLR projections are made for a case consistent with the IPCC, a case with a skewed probability distribution function of the AIS dynamics and a high-end scenario of Antarctic mass loss. SLR projections are incorporated within a probabilistic erosion framework using synthetic storm time series for two beaches on the island. Future retreat distances from storms and long term coastal recession are calculated, and the different scenarios are compared and contrasted.

For a future 1/100 year retreat distance of storm erosion, often used for zoning policies, estimates may be exceeded up to 1.11-2.22 times as frequent for inclusion of skewness, and 2.22-67 times as frequent for the high-end scenario compared to the IPCC case. These numbers further increase when additional climate model uncertainty is introduced. In terms of long-term recession, the 1% exceedance probability in 2100 for the IPCC case has a 2-4.5% exceedance probability for a skewed distribution function and a 37-88% exceedance probability under a high-end scenario of the AIS. Lower exceedance probabilities, essential for risk-averse coastal managers, are underestimated relatively more leading to potential disillusion about the safety level that is set.

In conclusion, precluding AIS uncertainty from SLR projections that feed coastal impact assessments may lead to ill-informed decisions on SLR adaptation. Risk-averse coastal managers should thus be better informed on deep uncertainty in SLR projections to prevent maladaptation of vulnerable areas.
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A freshening of the boundary current (Labrador current and West Greenland Current) supresses deep convection in the Labrador Sea. Furthermore, the fresher water contributes to the ``strength'' of Irminger Rings and increases their life time. This leads to a larger spread of high EKE into the interior of the Labrador Sea and confines the deep convection region more towards the west. ... Read more

Tropical cyclones have the ability to very quickly increase in strength. This process is called rapid intensification and as a result, tropical cyclones can transform into hurricanes. Rapid intensification is related to the availability of heat and the amount of negative feedback of the ocean on the tropical cyclone. Negative feedback results in the weakening of the tropical cyclone. Cyclones passing over a warm ocean anomaly have access to more heat and due to the relatively high temperatures, the amount of negative feedback is reduced considerably. A necessary condition for rapid intensification is therefore the presence of a warm ocean anomaly, often being warm core eddies. This paper relates the rapid intensification of tropical cyclone Matthew to the presence of warm core eddies in the track of Matthew. Results show that there is no extensive evidence found for the presence of a warm core eddy before rapid intensification took place. Although maps of the sea surface height and sea surface temperature indicate the possible existence of a warm core eddy, surface velocities do not show the characteristic rotation flow of an eddy. The enthalpy flux is considerably large just before the rapid intensification of Matthew indicating that the negative feedback by the ocean is reduced and heat is available for transport. The rapid intensification of Matthew might be linked to other physical mechanisms that have been overlooked. Possible mechanisms identified are the Amazon-Orinoco river plume and La Ni˜na. Further studies on the rapid intensification of tropical cyclone Matthew should therefore take into account these mechanisms and study their influence on rapid intensification. ... Read more

Climate models predict increased Arctic precipitation and subsequent Arctic freshening as a response to increased green house gas concentrations. Eulerian studies have shown that with increased Arctic precipitation AMOC (Atlantic Meridional Overturning Circulation) strength decreases. Decrease in AMOC strength comes with a decreased redistribution of heat from lower to higher latitudes which can have severe effects on our climate. Therefore, understanding the effects and mechanisms of Arctic precipitation change is a crucial building block for predicting and possibly preventing climate change. This study used a Lagrangian approach. The pathways of water at Fram Strait were investigated for present-day climate (control run) and two scenario runs with increased Arctic precipitation (+50% and +300% respectively). Importantly, it was found that Arctic water reaches the Labrador Sea through Denmark Strait for all three runs. Thus, the extra fresh water in the Arctic can possibly impact sinking and convection zones in the Labrador Sea. The total amount of Arctic water, passing Denmark Strait from Fram Strait, increases for the weak scenario and decreases for the strong scenario of this study. On the other side of Iceland, for the strong scenario of this study, Arctic water stops passing the Iceland-Faroe-Ridge through the Faroe Bank Channel. The amount of Arctic water going into and staying in the Nordic Seas remained almost unchanged with increased Arctic precipitation. The two routes passing from Fram Strait into the North Atlantic were analysed further with respect to depth changes and properties. On both routes particles were fresher compared to the control run when increasing Arctic precipitation. For the weak scenario particles were usually colder than the control run on both routes. For the strong scenario, particles were only colder at Fram Strait, but got warmer than the control run along the pathway. ... Read more