Denmark Strait Ocean Circulation Variability

Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water overflows through Denmark Strait, the channel located between Greenland and Iceland. This thesis investigates the physical processes controlling the variability of the circulation in the vicinity of Denmark Strait. As direct measurements are not sufficient to unravel most of these processes, we develop a realistic general circulation model covering the East Greenland shelf and adjacent deep ocean. The model hydrography and circulation agree well with available observations. We find that the yearly mean southward volume flux of dense water is about 30% greater in the presence of mesoscale features known as boluses and pulses. We establish the causal relationship between these features and overflow cyclones observed further south. Most of the cyclones form at the Denmark Strait sill during overflow surges and grow as they move equatorward. A fraction of the cyclones form south of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Finally, the model reveals that the eddy activity north of Denmark Strait regulates the bifurcation of the southward current along the eastern coast of Greenland and the offshore transport of fresh water at the surface

Coastal trapped waves and other subinertial variability along the Southeast Greenland coast in a realistic numerical simulation

Ocean currents along the Southeast Greenland Coast play an important role in the climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from moorings shows that the circulation in this region displays substantial subinertial variability (typically with periods of several days). For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport. It has been suggested that the subinertial variability found in observations is associated with Coastal Trapped Waves, whose properties depend on bathymetry, stratification, and the mean flow. Here, we use the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals are coherent over hundreds of kilometers along the shelf. We find Coastal Trapped Waves on the shelf and along the shelf break in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—that are consistent with a combination of Mode I waves and higher modes. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep

Localized general vertical coordinates for quasi‐Eulerian ocean models: The Nordic overflows test‐case

A generalized methodology to deploy different types of vertical coordinate system in arbitrarily defined time-invariant local areas of quasi-Eulerian numerical ocean models is presented. After detailing its characteristics, we show how the general localization method can be used to improve the representation of the Nordic Seas overflows in the UK Met Office NEMO-based eddy-permitting global ocean configuration. Three z*-levels with partial steps configurations localizing different types of hybrid geopotential/terrain-following vertical coordinates in the proximity of the Greenland-Scotland ridge are implemented and compared against a control configuration. Experiments include a series of idealized and realistic numerical simulations where the skill of the models in computing pressure forces, reducing spurious diapycnal mixing and reproducing observed properties of the Nordic Seas overflows are assessed. Numerical results prove that the localization approach proposed here can be successfully used to embed terrain-following levels in a global geopotential levels-based configuration, provided that the localized vertical coordinate chosen is flexible enough to allow a smooth transition between the two. In addition, our experiments show that deploying localized terrain-following levels via the multi-envelope method allows the crucial reduction of spurious cross-isopycnal mixing when modeling bottom intensified buoyancy driven currents, significantly improving the realism of the Nordic Seas overflows simulations in comparison to the other configurations. Important hydrographic biases are found to similarly affect all the realistic experiments and a discussion on how their interaction with the type of localized vertical coordinate affects the realism of the simulated overflows is provided

Is computational oceanography coming of age?

Computational oceanography is the study of ocean phenomena by numerical simulation, especially dynamical and physical phenomena. Progress in information technology has driven exponential growth in the number of global ocean observations and the fidelity of numerical simulations of the ocean in the past few decades. The growth has been exponentially faster for ocean simulations, however. We argue that this faster growth is shifting the importance of field measurements and numerical simulations for oceanographic research. It is leading to the maturation of computational oceanography as a branch of marine science on par with observational oceanography. One implication is that ultraresolved ocean simulations are only loosely constrained by observations. Another implication is that barriers to analyzing the output of such simulations should be removed. Although some specific limits and challenges exist, many opportunities are identified for the future of computational oceanography. Most important is the prospect of hybrid computational and observational approaches to advance understanding of the ocean

Denmark Strait Ocean Circulation Variability

Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water overflows through Denmark Strait, the channel located between Greenland and Iceland. This thesis investigates the physical processes controlling the variability of the circulation in the vicinity of Denmark Strait. As direct measurements are not sufficient to unravel most of these processes, we develop a realistic general circulation model covering the East Greenland shelf and adjacent deep ocean. The model hydrography and circulation agree well with available observations. We find that the yearly mean southward volume flux of dense water is about 30% greater in the presence of mesoscale features known as boluses and pulses. We establish the causal relationship between these features and overflow cyclones observed further south. Most of the cyclones form at the Denmark Strait sill during overflow surges and grow as they move equatorward. A fraction of the cyclones form south of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Finally, the model reveals that the eddy activity north of Denmark Strait regulates the bifurcation of the southward current along the eastern coast of Greenland and the offshore transport of fresh water at the surface

Evolution of Denmark Strait overflow cyclones and their relationship to overflow surges

Denmark Strait, the channel located between Greenland and Iceland, is a critical gateway between the Nordic Seas and the North Atlantic. Mesoscale features crossing the strait regularly enhance the volume transport of the Denmark Strait overflow. They interact with the dense water masses descending into the subpolar North Atlantic and therefore are important for the Atlantic Meridional Overturning Circulation. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and overflow cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, the cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase

Mesoscale features dominating the Denmark Strait overflow and our plan to improve the representation of overflows in global models

Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water overflows through Denmark Strait, the channel located between Greenland and Iceland. Mesoscale anomalies play a major role in controlling the amount of overflow water entering the global circulation. The observed mesoscale anomalies are well represented by regional general circulation models of O(1) km resolution. MITgcm numerical solutions show that the yearly mean southward volume flux of dense water is about 30% greater in the presence of these mesoscale features [1], and reveal a causal relationship between the overflow surges at the sill and cyclones controlling the overflow variability further downstream [2]. Global simulations at kilometric scale are computationally expensive. Therefore, the difficulty to resolve the mesoscale dynamics dominating overflows is a significant weakness of current global/climate models. Recent developments of the AGRIF nesting tool allow to embed in NEMO configurations multiple nests with refined grids and independent vertical coordinate systems. We are implementing two-way nests in a global eORCA12 configuration to asses their impact in key overflow regions, such as the Greenland-Scotland Ridge and Gibraltar Strait

Atlantic-Origin Overflow Water in the East Greenland Current

Dense water masses transported southward along the east coast of Greenland in the East Greenland Current (EGC) form the largest contribution to the Denmark Strait Overflow. When exiting Denmark Strait these dense water masses sink to depth and feed the deep circulation in the North Atlantic. Based on one year of mooring observations upstream of Denmark Strait and historical hydrographic profiles between Fram Strait and Denmark Strait, we find that a large part (75%) of the overflow water (⁠ ≥ 27.8 kg m−3) transported by the EGC is of Atlantic origin (potential temperature θ > 0°C). The along-stream changes in temperature of the Atlantic-origin Water are moderate north of 69°N at the northern end of Blosseville basin, but southward from this point the temperature decreases more rapidly. We hypothesize that this enhanced modification is related to the bifurcation of the EGC taking place close to 69°N into the shelfbreak EGC and the separated EGC. This is associated with enhanced eddy activity and strong water mass modification reducing the intermediate temperature and salinity maxima of the Atlantic-origin Water. During periods with a large (small) degree of modification the separated current is strong (weak). Output from a high-resolution numerical model supports our hypothesis and reveals that large eddy activity is associated with an offshore shift of the surface freshwater layer that characterizes the Greenland shelf. The intensity of the eddy activity regulates the density and the hydrographic properties of the Denmark Strait Overflow Water transported by the EGC system

Atlantic-origin overflow water in the East Greenland current

Dense water masses transported southward along the east coast of Greenland in the East Greenland Current (EGC) form the largest contribution to the Denmark Strait Overflow. When exiting Denmark Strait these dense water masses sink to depth and feed the deep circulation in the North Atlantic. Based on one year of mooring observations upstream of Denmark Strait and historical hydrographic profiles between Fram Strait and Denmark Strait, we find that a large part (75%) of the overflow water ( σ θ ≥ 27.8 kg m−3) transported by the EGC is of Atlantic origin (potential temperature θ > 0°C). The along-stream changes in temperature of the Atlantic-origin Water are moderate north of 69°N at the northern end of Blosseville basin, but southward from this point the temperature decreases more rapidly. We hypothesize that this enhanced modification is related to the bifurcation of the EGC taking place close to 69°N into the shelfbreak EGC and the separated EGC. This is associated with enhanced eddy activity and strong water mass modification reducing the intermediate temperature and salinity maxima of the Atlantic-origin Water. During periods with a large (small) degree of modification the separated current is strong (weak). Output from a high-resolution numerical model supports our hypothesis and reveals that large eddy activity is associated with an offshore shift of the surface freshwater layer that characterizes the Greenland shelf. The intensity of the eddy activity regulates the density and the hydrographic properties of the Denmark Strait Overflow Water transported by the EGC system

OceanSpy: A Python package to facilitate ocean model data analysis and visualization

Simulations of ocean currents using numerical circulation models are becoming increasingly realistic. At the same time, these models generate increasingly large volumes of model output data. These trends make analysis of the model data harder for two reasons. First, researchers must use high-performance data-analysis clusters to access these large data sets. Second, they must post-process the data to extract oceanographically-useful information. Moreover, the increasing model realism encourages researchers to compare simulations to observations of the natural ocean. To achieve this task model data must be analyzed in the way observational oceanographers analyze field measurements; and, ideally, by the observational oceanographers themselves. The OceanSpy package addresses these needs