Search

The Last Glacial Maximum (LGM, from ∼26 to 20 ka BP) was the most recent period with large ice sheets in Eurasia and North America. At that time, global temperatures were 5–7 ∘C lower than today, and sea level ∼125 m lower. LGM simulations are useful to understand earth system dynamics, including climate–ice sheet interactions, and to evaluate and improve the models representing those dynamics. Here, we present two simulations of the Northern Hemisphere ice sheet climate and surface mass balance (SMB) with the Community Earth System Model v2.1 (CESM2.1) using the Community Atmosphere Model v5 (CAM5) with prescribed ice sheets for two time periods that bracket the LGM period: 26 and 21 ka BP. CESM2.1 includes an explicit simulation of snow/firn compaction, albedo, refreezing, and direct coupling of the ice sheet surface energy fluxes with the atmosphere. The simulated mean snow accumulation is lowest for the Greenland and Barents–Kara Sea ice sheets (GrIS, BKIS) and highest for British and Irish (BIIS) and Icelandic (IcIS) ice sheets. Melt rates are negligible for the dry BKIS and GrIS, and relatively large for the BIIS, North American ice sheet complex (NAISC; i.e. Laurentide, Cordilleran, and Innuitian), Scandinavian ice sheet (SIS), and IcIS, and are reduced by almost a third in the colder (lower temperature) 26 ka BP climate compared with 21 ka BP. The SMB is positive for the GrIS, BKIS, SIS, and IcIS during the LGM (26 and 21 ka BP) and negative for the NAISC and BIIS. Relatively wide ablation areas are simulated along the southern (terrestrial), Pacific and Atlantic margins of the NAISC, across the majority of the BIIS, and along the terrestrial southern margin of the SIS. The integrated SMB substantially increases for the NAISC and BIIS in the 26 ka BP climate, but it does not reverse the negative sign. Summer incoming surface solar radiation is largest over the high interior of the NAISC and GrIS, and minimum over the BIIS and southern margin of NAISC. Summer, Physical and Space Geodesy

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53°N in the boundary current and traced backward in time in a high-resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53°N originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary-interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin and part follows a longer route toward Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary-interior exchanges are important for the pathways and the properties of water masses arriving at 53°N. It reveals a complex three-dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53°N and thus for our understanding of the AMOC., Physical and Space Geodesy, Environmental Fluid Mechanics

The Atlantic Meridional Overturning Circulation (AMOC) quantifies the strength of the northward surface transports and southward deeper transports in the Atlantic Ocean. In particular, as warm surface waters flow northward in the Atlantic they gradually cool on they journey releasing heat to the atmosphere. Then, in the subpolar and polar regions these surface waters become dense enough to sink forming cold deep waters, which return southward through the Atlantic Ocean. This oceanic circulation pattern is of high importance for the Earth’s climate since it is the main distributor of heat, biogeochemical tracers and water masses globally. For decades, both the ocean and climate communities have focused on understanding the dynamics that shape the AMOC strength and most importantly its variability. Although, a lot of progress has been made towards this goal, it is still unclear how the AMOC will respond in a changing climate. Observational and numerical modelling studies are focusing on regions within the Atlantic Ocean that are known to play an important role in shaping theAMOC dynamics. Marginal seas are considered to be such key regions. There the deep waters are formed and then are transported to the global ocean. Although deep water formation regions are located in the interior of the marginal seas it is known that they are not associated with net vertical motions. Instead, several studies suggest that the net downwelling occurs along the perimeter of the marginal seas and that the ocean eddies play an important role in regulating this process. The Labrador Sea, which is located between the west coast of Greenland and the Labrador Peninsula, is known to be one of these key regions in the subpolar North Atlantic. It is characterized by a rich activity of different scale vortices (eddies) that act as a bridge between the boundary current system that encircles the basin and its interior. Therefore, a daunting challenge in physical oceanography is to understand how t, Environmental Fluid Mechanics

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53°N in the boundary current and traced backward in time in a high-resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53°N originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary-interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin and part follows a longer route toward Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary-interior exchanges are important for the pathways and the properties of water masses arriving at 53°N. It reveals a complex three-dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53°N and thus for our understanding of the AMOC., Physical and Space Geodesy, Environmental Fluid Mechanics

The Atlantic Meridional Overturning Circulation (AMOC) quantifies the strength of the northward surface transports and southward deeper transports in the Atlantic Ocean. In particular, as warm surface waters flow northward in the Atlantic they gradually cool on they journey releasing heat to the atmosphere. Then, in the subpolar and polar regions these surface waters become dense enough to sink forming cold deep waters, which return southward through the Atlantic Ocean. This oceanic circulation pattern is of high importance for the Earth’s climate since it is the main distributor of heat, biogeochemical tracers and water masses globally. For decades, both the ocean and climate communities have focused on understanding the dynamics that shape the AMOC strength and most importantly its variability. Although, a lot of progress has been made towards this goal, it is still unclear how the AMOC will respond in a changing climate. Observational and numerical modelling studies are focusing on regions within the Atlantic Ocean that are known to play an important role in shaping theAMOC dynamics. Marginal seas are considered to be such key regions. There the deep waters are formed and then are transported to the global ocean. Although deep water formation regions are located in the interior of the marginal seas it is known that they are not associated with net vertical motions. Instead, several studies suggest that the net downwelling occurs along the perimeter of the marginal seas and that the ocean eddies play an important role in regulating this process. The Labrador Sea, which is located between the west coast of Greenland and the Labrador Peninsula, is known to be one of these key regions in the subpolar North Atlantic. It is characterized by a rich activity of different scale vortices (eddies) that act as a bridge between the boundary current system that encircles the basin and its interior. Therefore, a daunting challenge in physical oceanography is to understand how t, Environmental Fluid Mechanics

The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas., Environmental Fluid Mechanics

The water masses exiting the Labrador Sea, and in particular the dense water mass formed by convection (i.e. Labrador Sea Water, LSW), are important components of the Atlantic Meridional Overturning Circulation (AMOC). Several studies have questioned the connection of the LSW production to the AMOC variability. This is partly due to the limited understanding of how this locally formed water mass leaves the interior of the Labrador Sea. In this study, the pathways and the timescales of the water masses exiting the Labrador Sea via the boundary current are investigated by Lagrangian particle tracking. This method is applied to the output of a strongly-eddying idealized model that is capable of representing the essential physical processes involved in the cycle of convection and restratification in the Labrador Sea. The Lagrangian trajectories reveal that prior to exiting the domain the water masses follow either a fast route within the boundary current or a slower route that involves boundary current-interior exchanges. The densest water masses exiting the Labrador Sea stem from this slow route, where particles experience strong water mass transformation while in the interior. In contrast, the particles that follow the fast route experience water mass transformation in the boundary current at the western side of the domain only, yielding a lighter product. Although both routes carry roughly the same transport, we show that 60% of the overturning in density space is associated with the volume transport carried by particles that follow the slow route. This study further highlights that the export of dense water masses, which is governed by the eddy activity in the basin, yields export timescales that are usually longer than a year. This underlines the necessity of resolving the mesoscale features required to capture the interior–boundary current exchange in order to correctly represent the export of the LSW., Environmental Fluid Mechanics

The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas., Environmental Fluid Mechanics

The water masses exiting the Labrador Sea, and in particular the dense water mass formed by convection (i.e. Labrador Sea Water, LSW), are important components of the Atlantic Meridional Overturning Circulation (AMOC). Several studies have questioned the connection of the LSW production to the AMOC variability. This is partly due to the limited understanding of how this locally formed water mass leaves the interior of the Labrador Sea. In this study, the pathways and the timescales of the water masses exiting the Labrador Sea via the boundary current are investigated by Lagrangian particle tracking. This method is applied to the output of a strongly-eddying idealized model that is capable of representing the essential physical processes involved in the cycle of convection and restratification in the Labrador Sea. The Lagrangian trajectories reveal that prior to exiting the domain the water masses follow either a fast route within the boundary current or a slower route that involves boundary current-interior exchanges. The densest water masses exiting the Labrador Sea stem from this slow route, where particles experience strong water mass transformation while in the interior. In contrast, the particles that follow the fast route experience water mass transformation in the boundary current at the western side of the domain only, yielding a lighter product. Although both routes carry roughly the same transport, we show that 60% of the overturning in density space is associated with the volume transport carried by particles that follow the slow route. This study further highlights that the export of dense water masses, which is governed by the eddy activity in the basin, yields export timescales that are usually longer than a year. This underlines the necessity of resolving the mesoscale features required to capture the interior–boundary current exchange in order to correctly represent the export of the LSW., Environmental Fluid Mechanics

Atlantic Water takes various pathways through the Nordic Seas, and its transformation to denser waters forms a crucial connection to the lower limb of the Atlantic Meridional Overturning Circulation. Circulation maps often schematize two distinct pathways of Atlantic Water: one following the Norwegian Atlantic Slope Current along the continental slope of Norway and one following the Norwegian Atlantic Front Current along the Mohn and Knipovich Ridges. In this paper, the connectivity between the northward flow along these ridges is investigated. Analyzing trajectories of surface drifters and ARGO floats, we find that only 8% of the floats that travel near the mid-ocean ridges take the frontal pathway to the north. Indeed, by tracing numerical particles in a realistic numerical simulation, part of the water mass traveling along the Mohn Ridge follows the 2,500 m isobath eastward and joins the slope current, instead of flowing north along the Knipovich Ridge. Furthermore, north of 74°N, frequent exchange between the slope current and the front current is observed. Therefore, the slope current and front current are less isolated than often schematized. Additionally, the observational data set reveals substantial cross-ridge exchange; 31% of the floats that travel within 60 km from the mid-ocean ridges cross it. Results from numerical simulations indicate that the cross-ridge exchange leads to cooling and freshening of the Atlantic Water along the front. Deployments of floats near the mid-ocean ridges are needed to investigate the pathway of Atlantic Water and its exchange across the ridge in more detail., Environmental Fluid Mechanics

Environmental Fluid Mechanics

In this study, an idealized eddy-resolving model is employed to examine the interplay between the downwelling, ocean convection and mesoscale eddies in the Labrador Sea and the spreading of dense water masses. The model output demonstrates a good agreement with observations with regard to the eddy field and convection characteristics. It also displays a basin mean net downwelling of 3.0 Sv. Our analysis confirms that the downwelling occurs near the west Greenland coast and that the eddies spawned from the boundary current play a major role in controlling the dynamics of the downwelling. The magnitude of the downwelling is positively correlated to the magnitude of the applied surface heat loss. However, we argue that this connection is indirect: the heat fluxes affect the convection properties as well as the eddy field, while the latter governs the Eulerian downwelling. With a passive tracer analysis we show that dense water is transported from the interior towards the boundary, predominantly towards the Labrador coast in shallow layers and towards the Greenland coast in deeper layers. The latter transport is steered by the presence of the eddy field. The outcome that the characteristics of the downwelling in a marginal sea like the Labrador Sea depend crucially on the properties of the eddy field emphasizes that it is essential to resolve the eddies to properly represent the downwelling and overturning in the North Atlantic Ocean, and its response to changing environmental conditions., Accepted Author Manuscript, Environmental Fluid Mechanics

The pathways and watermass transformation of the North Icelandic Irminger Current (NIIC) in the Nordic Seas are investigated by tracing the NIIC watermass in two ocean circulation models: the Modular Ocean Model (MOM) and the Parallel Ocean Program (POP). The two simulations use identical atmospheric forcing and have a horizontal resolution of 0.1°. However, the models differ strongly in their representation of the sea-ice cover in the Nordic Seas and, possibly as a consequence, display a different hydrography. Results from observational studies point towards a fast overturning loop north of Iceland that connects the NIIC watermass to the Denmark Strait Overflow Water (DSOW). However, our Lagrangian analysis shows that only 0.2 Sv of the entering NIIC water exits as DSOW in the two models. In POP, the main transformation to dense water takes place along a short path north of Iceland. In MOM however, the contributing part of the NIIC to DSOW takes a long path through the Nordic Seas and reaches Denmark Strait as part of the East Greenland Current (EGC). A small contribution of the NIIC watermass to the Iceland Scotland Overflow Water (ISOW) is found in both MOM and POP (7.8%, respectively 2.1% of the NIIC watermass). In the model simulations studied, the part of the NIIC watermass that is not connected to the overflows takes many different pathways through the Nordic seas. Analysis of the depth distribution and the thermohaline changes of the particles indicates that the watermass transformation that takes place north of Iceland is crucial for diversifying the pathways of the NIIC water., Accepted Author Manuscript, Environmental Fluid Mechanics

In this study, an idealized eddy-resolving model is employed to examine the interplay between the downwelling, ocean convection and mesoscale eddies in the Labrador Sea and the spreading of dense water masses. The model output demonstrates a good agreement with observations with regard to the eddy field and convection characteristics. It also displays a basin mean net downwelling of 3.0 Sv. Our analysis confirms that the downwelling occurs near the west Greenland coast and that the eddies spawned from the boundary current play a major role in controlling the dynamics of the downwelling. The magnitude of the downwelling is positively correlated to the magnitude of the applied surface heat loss. However, we argue that this connection is indirect: the heat fluxes affect the convection properties as well as the eddy field, while the latter governs the Eulerian downwelling. With a passive tracer analysis we show that dense water is transported from the interior towards the boundary, predominantly towards the Labrador coast in shallow layers and towards the Greenland coast in deeper layers. The latter transport is steered by the presence of the eddy field. The outcome that the characteristics of the downwelling in a marginal sea like the Labrador Sea depend crucially on the properties of the eddy field emphasizes that it is essential to resolve the eddies to properly represent the downwelling and overturning in the North Atlantic Ocean, and its response to changing environmental conditions., Accepted Author Manuscript, Environmental Fluid Mechanics

Environmental Fluid Mechanics

The pathways and watermass transformation of the North Icelandic Irminger Current (NIIC) in the Nordic Seas are investigated by tracing the NIIC watermass in two ocean circulation models: the Modular Ocean Model (MOM) and the Parallel Ocean Program (POP). The two simulations use identical atmospheric forcing and have a horizontal resolution of 0.1°. However, the models differ strongly in their representation of the sea-ice cover in the Nordic Seas and, possibly as a consequence, display a different hydrography. Results from observational studies point towards a fast overturning loop north of Iceland that connects the NIIC watermass to the Denmark Strait Overflow Water (DSOW). However, our Lagrangian analysis shows that only 0.2 Sv of the entering NIIC water exits as DSOW in the two models. In POP, the main transformation to dense water takes place along a short path north of Iceland. In MOM however, the contributing part of the NIIC to DSOW takes a long path through the Nordic Seas and reaches Denmark Strait as part of the East Greenland Current (EGC). A small contribution of the NIIC watermass to the Iceland Scotland Overflow Water (ISOW) is found in both MOM and POP (7.8%, respectively 2.1% of the NIIC watermass). In the model simulations studied, the part of the NIIC watermass that is not connected to the overflows takes many different pathways through the Nordic seas. Analysis of the depth distribution and the thermohaline changes of the particles indicates that the watermass transformation that takes place north of Iceland is crucial for diversifying the pathways of the NIIC water., Accepted Author Manuscript, Environmental Fluid Mechanics

The specific differential phase (Kdp) is a polarimetric radar observable which can be advantageous for rain rate estimation as it is not affected by absolute calibration errors and resistive to hail contamination. Its estimation, starting from the differential phase (?dp) consists of many processing steps and considerations which are more pronounced for the X-band due to the presence of non-negligible differential scattering phase (?co) and attenuation. In the thesis, the estimation of differential phase (?dp) and specific differential phase (Kdp) with the X-band Doppler polarimetric radar IDRA located in Cabauw, the Netherlands, is presented along with an approach to assess the estimated results based on the self-consistency of the radar observables. The effect of attenuation and a potential correction algorithm are also discussed., IRCTR / ATMOS, Telecommunications, Electrical Engineering, Mathematics and Computer Science

The specific differential phase (Kdp) is a polarimetric radar observable which can be advantageous for rain rate estimation as it is not affected by absolute calibration errors and resistive to hail contamination. Its estimation, starting from the differential phase (?dp) consists of many processing steps and considerations which are more pronounced for the X-band due to the presence of non-negligible differential scattering phase (?co) and attenuation. In the thesis, the estimation of differential phase (?dp) and specific differential phase (Kdp) with the X-band Doppler polarimetric radar IDRA located in Cabauw, the Netherlands, is presented along with an approach to assess the estimated results based on the self-consistency of the radar observables. The effect of attenuation and a potential correction algorithm are also discussed., IRCTR / ATMOS, Telecommunications, Electrical Engineering, Mathematics and Computer Science