Natural variability at subinertial frequencies (time scale of several days) plays an important role in the interaction between Greenland’s fjords, the continental shelf, and shelf-break exchange with the deep basins. In this study we identified the nature and driving mechanisms of this variability in four fjords in Southeast Greenland, in three high-resolution numerical simulations. We find two dominant frequency ranges in along-fjord velocity, volume transport of Atlantic Water, and along-fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross-fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby-type waves on the continental shelf and as internal Kelvin-type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.@en

We identified the nature and driving mechanisms of subinertial variability (variability at a time scale of several days) in four fjords in Southeast Greenland, in three high-resolution numerical simulations. We find two dominant frequency ranges in along-fjord velocity, volume transport of Atlantic Water, and along-fjord heat transport: one around 2–4 days and one around 10 days. The higher frequency is most prominent in the two smaller fjords (Sermilik Fjord and Kangerdlugssuaq Fjord), while the lower frequency peak dominates in the larger fjords (Scoresby Sund and King Oscar Fjord). The cross-fjord structure of variability patterns is determined by the fjord's dynamic width, while the vertical structure is determined by the stratification in the fjord. The dominant frequency range is a function of stratification and fjord length, through the travel time of resonant internal Kelvin waves. We find that the subinertial variability is the imprint of Coastal Trapped Waves, which manifest as Rossby-type waves on the continental shelf and as internal Kelvin-type waves inside the fjords. Between 50% and 80% of the variability in the fjord is directly forced by Coastal Trapped Waves propagating in from the shelf, with an additional role played by alongshore wind forcing on the shelf.@en

Fjords are the primary conduit for oceanic heat transport to the Greenland Ice Sheet. This heat transport is highly time-variable, with typical frequencies in the subinertial range (with periods of several days). We studied four fjords along the southeast coast of Greenland to determine what drives oceanic heat into these fjords. The four fjords cover a range of widths and lengths, and three different high-resolution model simulations provided a range in stratifications and surface forcings. We find that the heat transport is associated with resonant modes of internal subinertial waves. The variability in transport of deep warm (Atlantic) water is highly correlated to the principal component that is associated with the gravest vertical normal mode (EOF 1 in narrow fjords, EOF 3 in a wide fjord). The dominant frequency in the heat transport time series is thus determined by the shape of the fjord and the stratification, the latter through the wave speed of the gravest normal mode. The oceanic heat supply to the Greenland Ice Sheet is thus governed by variables that are relatively easy to measure observationally.@en

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.@en

Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global 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 the OSNAP array and other mooring records 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; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used 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 on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. 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.@en

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.@en

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.@en

Mesoscale features present at the Denmark Strait sill regularly enhance the volume transport of the Denmark Strait overflow (DSO). They are important for the Atlantic Meridional Overturning Circulation and ultimately, for the global climate system. 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 DSO 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, DSO cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase.@en

About half of the dense overflow across the Greenland-Iceland-Scotland ridge is supplied by the Denmark Strait Overflow (DSO), making Denmark Strait a critical gateway between the Arctic and the subpolar North Atlantic. Boluses and pulses are mesoscale features that cross the Denmark Strait during high-overflow transport events. They occur with the same frequency as the DSO cyclones observed downstream. However, the connection between boluses, pulses, and DSO cyclones is not well established. To investigate this connection, we designed an automated vortex detection algorithm and we applied it to a realistic, high-resolution numerical model solution. Our goal is threefold: (i) To better understand the dynamics associated with stretching of the water column in the DSO; (ii) To determine how the hydrographic properties of DSO mesoscale features evolve in space and time; (iii) To advance the understanding of the high-frequency variability characteristic of the Denmark Strait Overflow. The model shows that both boluses and pulses produce DSO cyclones, but the underlying mechanisms are different. DSO cyclones associated with boluses form in Denmark Strait and, because of potential vorticity conservation and stretching of the water column, they quickly grow as they move south. In contrast, DSO cyclones associated with pulses form downstream of the sill as slow-moving anticyclones near the East Greenland shelfbreak collapse. Regardless of their formation mechanism, DSO cyclones weaken when they encounter a rapid steepening of the bathymetric slope, and the rate of decay is slower than the earlier growth rate. The mean stratification of the cyclones decreases during their life cycle, while their initial increase in relative vorticity due to stretching is followed by a decrease. As a result, potential vorticity is only conserved during the growth phase. Last, composites of the deep cyclones show that they are preceded by downwelling and followed by upwelling during their life cycle.@en

The Southeast Greenland shelf region harbors various exchange flows that play an important role in the climate system, such as the Denmark Strait Overflow and fjord-shelf exchange. Most of the literature on these flows focuses on slow processes such as estuarine circulation and hydraulically-controlled flow over a sill. Nevertheless, there is overwhelming evidence that the volume transport variability of these processes is dominated by subinertial oscillations (timescale of several days). In this study we use a high-resolution ocean model to explore and characterize subinertial variability along the Southeast Greenland coast. We study time series and spatial patterns of sea level anomalies along the shelf break, and time series of dense-water volume transport across several sections along the coast. Using a wavelet analysis of the time series and an evaluation of the time evolution of spatial anomaly patterns, we identify strong evidence for coastal-trapped wave modes at around 4 and 10 day frequencies, propagating along the coast. We discuss the importance of these results in the light of sparse observational data and their interpretation.@en

OceanSpy is an open-source and user-friendly Python package that enables scientists and inter- ested amateurs to analyze and visualize oceanographic data sets. OceanSpy builds on software packages developed by the Pangeo community, in particular Xarray (Hoyer & Hamman, 2017), Dask (Dask Development Team, 2016), and Xgcm (“Xgcm,” n.d.). The integration of Dask facilitates scalability, which is important for the petabyte-scale simulations that are becoming available. OceanSpy can be used as a standalone package for analysis of local circulation model output, or it can be run on a remote data-analysis cluster, such as the Johns Hopkins University SciServer system (Medvedev, Lemson, & Rippin, 2016), which hosts several sim- ulations and is publicly available. OceanSpy enables extraction, processing, and visualization of model data to (i) compare with oceanographic observations, and (ii) portray the kinematic and dynamic space-time properties of the circulation.@en

Denmark Strait is dynamically relevant to the global climate system because the dense water that overflows through the strait is a major contributor to the Deep Western Boundary Current. Observations show that the Denmark Strait Overflow (DSO) pulses with high frequencies (2-5 days). The hydrography and circulation of our high-resolution numerical simulations covering the east Greenland shelf and the Iceland and Irminger Seas is consistent with available observations in Denmark Strait. The model shows that boluses and pulses, the two dominant mesoscale features observed in the strait, play a major role in controlling the variability of the DSO thickness and transport into the Irminger Sea. Indeed, the mean southward volume flux of the DSO in the model is about 30% greater in the presence of boluses and pulses. In this study we take advantage of our ~2 km resolution model to interpret the sparse observations available for this area. We describe the evolution of boluses and pulses downstream of Denmark Strait by investigating several vertical sections across the east Greenland shelf. Moreover, we use a Lagrangian particle code to track boluses and pulses and to study their hydrographic properties along the descending pathways. Combining the Eulerian and Lagrangian approaches we isolate boluses and pulses from the background state and we investigate the circulation in the Irminger basin associated with the mesoscale features.@en

Initial results are presented from a yearlong, high-resolution (~2 km) numerical simulation covering the east Greenland shelf and the Iceland and Irminger Seas. The model hydrography and circulation in the vicinity of Denmark Strait show good agreement with available observational datasets. This study focuses on the variability of the Denmark Strait overflow (DSO) by detecting and characterizing boluses and pulses, which are the two dominant mesoscale features in the strait. The authors estimate that the yearly mean southward volume flux of the DSO is about 30% greater in the presence of boluses and pulses. On average, boluses (pulses) are 57.1 (27.5) h long, occur every 3.2 (5.5) days, and are more frequent during the summer (winter). Boluses (pulses) increase (decrease) the overflow cross-sectional area, and temperatures around the overflow interface are colder (warmer) by about 2.6°C (1.8°C). The lateral extent of the boluses is much greater than that of the pulses. In both cases the along-strait equatorward flow of dense water is enhanced but more so for pulses. The sea surface height (SSH) rises by 4–10 cm during boluses and by up to 5 cm during pulses. The SSH anomaly contours form a bowl (dome) during boluses (pulses), and the two features cross the strait with a slightly different orientation. The cross streamflow changes direction; boluses (pulses) are associated with veering (backing) of the horizontal current. The model indicates that boluses and pulses play a major role in controlling the variability of the DSO transport into the Irminger Sea.@en