Feb. 26, 2019 — Change is in the air, ice, and water, of the Arctic Ocean. The North Pole sits steadfastly in the middle, surrounded by about five million square miles of floating sea ice of the Arctic pack. In 2018, the air warmed near the pole, making the second-warmest Arctic air temperatures on record. What’s more, sea ice shrank to its second-lowest cover on record, both according to polar-orbiting satellite data from the National Oceanic and Atmospheric Administration. Recent observed warming in the ocean has also contributed to the decline in sea ice (e.g., Polyakov et al., 2018). Wildlife such as caribou and reindeer depend on sea ice to get across ocean water. Closer to home, shrinking sea ice is linked to cold snaps from a weakened polar vortex, according to a 2018 study. An award-winning simulation shows the complex nature of the circulation happening at one of Earth’s most remote and hard-to-reach places, the Arctic Ocean.

SC18 Visualization Showcase

The Texas Advanced Computing Center (TACC) shared an award with UT Austin’s Oden Institute for Computational Engineering and Sciences (ICES) for the Best Scientific Visualization & Data Analytics Showcase, “Circulation in the Arctic Ocean and its Marginal Seas: From Low Latitudes to the Pole and Back.” The SC18 supercomputing conference gave the award in November of 2018 to the team of Greg Foss and Briana Bradshaw of TACC, and An T. Nguyen, Arash Bigdeli, Victor Ocaña and Patrick Heimbach of ICES.

The visualization was part of a public-outreach component of an NSF-funded project (PLR-1603903) aimed at understanding and quantifying the Arctic ocean-sea ice mean state and its changes in response to the Earth’s recent warming. It seeks to capture important physical processes and aspires to engage the general public to facilitate the conversation on Arctic Ocean research. The research was carried out at The Oden Institute for Computational Engineering and Sciences, the Institute for Geophysics, and the Jackson School of Geosciences, UT Austin. A version of the visualization was also shown at the exhibition “Exploring the Arctic Ocean,” which ran at the UT Visual Arts Center in Austin through the fall 2018 semester.

“This project was the sort of thing I love working on because it’s very visual and easy to relate to,” said Greg Foss, visualization specialist at TACC. Foss worked with ParaView, a free visualization package from Kitware that TACC provides and supports for its users.

The Arctic Ocean visualization also used Intel OSPRay, an open, scalable, and portable ray tracing engine. “I used OSPRay specifically in this project because it brought out fine, detailed structures I didn’t see with ParaView’s default OpenGL renderer,” Foss added.

High Performance Computing

The visualization was produced with the TACC Stampede2and Maverick supercomputers, and the simulation data was generated with the Sverdrup cluster at ICES. High performance computing was needed to capture mesoscale eddies smaller than 15 km that transport heat and freshwater across the Arctic ocean. Each frame of the visualization showed a one-day average of ocean water temperatures at depths 100-500m below the surface, and the visualization spanned eleven years (2006-2016).

The ICES team used the NASA Estimating the Circulation and Climate of the Ocean (ECCO) model-data inversion framework, which combines observations with the MIT general circulation model and its underlying fluid dynamical equations to seek a best representation of the world ocean. Observations include satellite altimetry, Argo buoys, and NASA Gravity Recovery and Climate Experiment satellites among others. Data from a simulation of the ocean circulation in the Arctic and its marginal seas from 2006-2016 was used for the visualization.

“It’s very active data,” Foss explained. “You can look anywhere in it and you’ll see all kinds of different structures happening. The animation essentially tours a selection of these interesting features.”

Northward flow of Atlantic Water

The main feature highlighted in the rendering includes the flow of warm water from the Atlantic Ocean, originated from the Gulf Stream, into the Nordic and Barents Seas, and further north into the Arctic Ocean. This current may be viewed as the upper limb of the Atlantic Meridional Overturning Circulation, a conceptualized view of the Atlantic circulation, by which warm, light waters are carried northward, overturn vertically, and return southward as cold, dense waters.

Arctic Circumpolar Boundary Current

Warm Atlantic Water is shown flowing through the Norwegian-Greenland Seas across Fram Strait into the Eastern Arctic. The flow continues as a boundary current flowing counter-clockwise toward Siberia and into the Western Arctic, hugging the steep slope between the continental shelf and the deep Arctic interior. Mesoscale eddies (the ocean’s analogue of atmospheric storm systems), generated from the turbulent flow, extract heat from the main current and spread it into the Arctic Ocean interior at depths 50–700 m below the surface.

Summertime Bering Strait

Throughout the year, water from the Pacific Ocean enters the Arctic through the Bering Strait. This water is lighter (less dense) than its Atlantic counterpart, and thus resides at depths 0–250 m, which is above the Atlantic Water layer in the Western Arctic. During the summer months, Pacific-source water can have temperatures as high as 10 degrees Celsius. In order to highlight the ‘warm’ water in the range 2–4oC this segment of the visualization shows a temperature range extended up to 3.25 degrees Celsius.

Circulation in the Beaufort Sea

In the Beaufort Sea, two circulation regimes dominate: a wind-driven clockwise circulation in the upper 200 m within the Beaufort Sea interior, and a counterclockwise circulation of the Atlantic Water current along the basin rim between 200–700 m. The clockwise circulation, typically strongest in the winter months, result in convergence of the fresher Pacific Water and sea ice melt in the Beaufort Sea. As the two circulation regimes and various water masses interact, mesoscale eddies of typical length-scales 1–15 km proliferate throughout the basin interior. Through a mechanism termed “eddy-stirring,” these eddies mix temperature and salinity and act to flatten out stratification.

Fram Strait and the East Greenland Current

The Fram Strait is one of the two major gateways through which Atlantic Water enters the Arctic Ocean. Atlantic Water enters on the Svalbard side along the West Spitsbergen current at or near the surface. Once inside the Arctic, due to the presence of sea ice and fresh water (in this context, “fresh” describes water with small salt content compared to typical sea water), Atlantic Water subducts below the fresh layer and occupies the Eastern Arctic at depths below 50 m. On the western side of Fram Strait (off Northeast Greenland), cold and fresh water (of Pacific-source, river runoff, and ice melt) exits near the surface as the East Greenland Current, and modified Atlantic Water exits at depth. The large velocity gradient at the strait (northward inflow and southward outflow) generates large numbers of mesoscale eddies.

Greenland Sea

The Greenland/Iceland/Norwegian (GIN) and Labrador Seas are key regions of weak stratification (well mixed and with only small vertical density gradients) where surface water loses heat to the cold winter-time surface atmosphere, leading to convective mixing and deep water formation that supplies the deep southward return flows.

Kangerdlugssuaq Fjord and Outlet Glacier

The last feature labeled in the video shows oceanic heat transport toward the Greenland ice sheet’s marine margin in Kangerdlugssuaq Fjord. Through narrow, deep fjords such as this one, interactions take place between the ocean and the Greenland Ice Sheet. Warm subsurface Atlantic water can penetrate to the base of Greenland outlet glaciers and enhance glacial melting, contributing to the observed recent (since the late 1990s) accelerated Greenland ice mass loss.

More Than Just a Picture

The swirling details of the visualization capture key features of the Arctic and subpolar North Atlantic Ocean circulation. Capturing these features is essential for faithfully representing the time-evolving Arctic coupled atmosphere-ocean-cryosphere system and its role in climate variability and change. Such representation in models is impossible without the high performance computing resources of TACC and ICES. Advanced visualization of these complex and vast data sets leads to rapid progress in process understanding.

Greg Foss explained what he looks for in a good visualization. “Ideally, that there is enough information presented in such a way that it is faithful to the data and that the graphics are transparent to that. That’s what I hope happens, that you see the data in a new or unexpected way,” Foss said.

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Source: Jorge Salazar, TACC