NCCS-Hosted Simulations Probe the Interactions of the
Freshwater Yukon River and the Salty Arctic Ocean
The NASA Center for Climate Simulation (NCCS) Discover supercomputer powered a model NASA Goddard Space Flight Center scientists developed to simulate the physical properties and transport of water in the lower Yukon River and Northern Bering Sea — water that ultimately reaches the freshest of the world’s major oceans, the Arctic Ocean.
“The Yukon River is one of the largest rivers in the world, and a lot of the freshwater and potentially dissolved material that enters the ocean from the river is transported into the Arctic,” said J. Blake Clark, an assistant research scientist in NASA Goddard’s Ocean Ecology Laboratory and affiliated with GESTAR II through the University of Maryland, Baltimore County. “We undertook this study to understand the balance between freshwater input and external forces such as wind and tides on the structure of the freshwater Yukon River plume and the transport of water in the coastal ocean.”
Clark collaborated on the computational study with his postdoctoral mentor, Antonio Mannino, a NASA Goddard Ocean Ecology Laboratory research oceanographer. Their study appears in the journal Frontiers in Marine Science.
For this research, Clark and Mannino used the open-source Finite Volume Community Ocean Model (FVCOM). They implemented the FVCOM model for a region spanning from 200 kilometers (km) upstream of the coast at Pilot Station, Alaska, to south of the Yukon delta northward to west of Nome, Alaska, encompassing nearly all of Norton Sound and the Yukon delta (see figures A-C below).
YukonFVCOM, as the researcher team calls this implementation of the model, ran at spatial resolutions ranging from roughly 100 meters in shallow areas of the coasts and deltas to roughly 2.5 km offshore. The simulation covered 7 years (2004–2005, 2015–2019) of mostly ice-free months (April–September) to capture varying river flow and weather conditions. Model input data came from the National Oceanic and Atmospheric Administration (NOAA) North American Regional Reanalysis (weather) and the U.S. Geological Survey National Water Information System (river flow and temperature).
Clark and Mannino ran YukonFVCOM on the NCCS Discover supercomputer, using 1,120 ”Skylake” cores for approximately 89,740 core hours. Open access to model output data (including variables such as temperature, salinity, and velocity) is available on the NCCS DataPortal.
“This model has over 700,000 grid cells, and it would not be feasible to run the model on a lab-scale computer system,” Clark said. “NCCS and the Discover supercomputer allowed me to efficiently perform all aspects of the research, including model development and testing, output analysis, and big data processing.”
The YukonFVCOM simulation shows that phenomena occurring on differing timescales largely determine the position, current speed, and strength of the river plume. On short timescales (days to weeks) the dominant factor is wind direction and speed. On interseasonal and interannual timescales, the dominant factor is the timing and volume of incoming river freshwater as it flows over heavier ocean saltwater.
“Wind can steer the freshwater plume in the ocean for hundreds of miles, which will affect where sediment and carbon is transported and processed in the ocean,” Clark said. “There is a very large ‘bulge’ of freshwater [see left panel in figure below] that develops to the west before currents push water back towards the coast and then further north to the Bering Strait.”
In contrast, “the freshwater input drives these strong currents on longer timescales due to the lighter freshwater flowing over heavier saltwater in the bottom layer,” Clark said. Validation for the YukonFVCOM model results comes from observations of sea surface temperature taken by NASA MODIS and NOAA VIIRS satellite instruments.
In a follow-on NASA Carbon Cycle Science project, Clark and Mannino have linked the YukonFVCOM hydrodynamic model to a full carbon cycle model and prepared a soon-to-be published paper describing the transport and transformation of carbon in the Yukon River delta and plume. The team is running the updated model on Discover and will expand their simulations into other years and test additional scenarios. “We will produce estimates on how the river and ocean carbon cycle will look in the future under various climate change scenarios such as increased temperature and atmospheric carbon dioxide,” Clark said.
Related Link
- Clark, J.B., and A. Mannino, 2022: The Impacts of Freshwater Input and Surface Wind Velocity on the Strength and Extent of a Large High Latitude River Plume. Frontiers in Marine Science, Feb. 10, 2022, doi:10.3389/fmars.2021.793217.
Jarrett Cohen, NASA Goddard Space Flight Center
June 14, 2022