Using satellites to investigate ocean surface currents along the Northern Norwegian coast

Researchers from NERSC, NORCE, the Norwegian Meteorological Institute (MET Norway), and the University of Bergen recently published an article in which they evaluate how well ocean surface currents in Northern Norway’s coastal areas can be studied with satellites, compared to traditional methods. Ocean surface currents have implications for marine and environmental safety.

 

Why are ocean surface currents important to anyone?

Ocean surface currents are part of the global ocean circulation, which influences weather patterns and climate worldwide. Moreover, ocean surface currents largely determine material drift on the ocean surface. This can for example be oil spill, marine debris such as plastic, or damaged boats that need to be rescued. Understanding the drift speed and direction is crucial for keeping the marine environment and us safe. Even the fishing industry is dependent on understanding ocean surface currents; fish egg and larvae are transported by these currents, too. And knowing where fish will hatch means knowing where fish can be caught later on.

 

How do we observe ocean surface currents nowadays? 

Scientists have been providing clever ways to surveil ocean surface currents for decades. Instruments placed onboard ships and fixed moorings can give us answers on how these currents behave nearby. Autonomous buoys that drift with the currents also provide valuable information, allowing us to track their trajectory and pathway over time. These measurements are, however, offering limited spatial and temporal coverage. To complement these methods, therefore, satellite observations at global and regional levels are of critical importance. Radar altimetry enables us to get an overview of ocean surface geostrophic current behavior every 10 days. But methods based on radar altimetry cannot be properly used in coastal areas. High-Frequency (HF) radars located onshore, designed to fill the gap in coastal ocean surface current observations, are proven to be useful for operational applications. However, they only cover the coastal zone up to 200 km offshore. Therefore, other sources of observations are required to complement ocean surface current information acquired from moorings, drifters, and HF-radars.  

 

How can we use satellites for coastal areas?Maps of ocean surface radial velocity (RVL) in m/s along the northern Norwegian coast derived from satellite imagery on two diff: Positive and negative values indicate the detected current direction with respect to SAR antenna orientation. As the RVL retrievals are referenced to the orientation of the satellite overpass, the ascending (red pattern in left panel) and descending (blue pattern in right panel) passes are both displaying the Norwegian Coastal Current with a mean northeast flow following the coastal bathymetry towards the Barents Sea.Maps of ocean surface radial velocity (RVL) in m/s along the northern Norwegian coast derived from satellite imagery on two diff: Positive and negative values indicate the detected current direction with respect to SAR antenna orientation. As the RVL retrievals are referenced to the orientation of the satellite overpass, the ascending (red pattern in left panel) and descending (blue pattern in right panel) passes are both displaying the Norwegian Coastal Current with a mean northeast flow following the coastal bathymetry towards the Barents Sea.

To improve the information about ocean surface currents in coastal areas, researchers from NERSC, NORCE, MET Norway,  and the Geophysical Institute at the University of Bergen set out to use satellites, without using radar altimetry. The researchers instead used Doppler shift information derived from Synthetic Aperture Radar (SAR) satellite imagery from the European Space Agency Sentinel-1 mission. They studied the Norwegian coastal current flowing north- and eastwards along the northern coast of Norway over a 2-month period in late 2017 and compared the ocean surface currents with data from shore-based HF radars and surface drifting buoys. The comparison shows that distinct patterns of the Norwegian coastal current can be distinguished in SAR data, and they do match the results of traditional observations. This means that satellite-based SAR data can be used to get reliable estimates of ocean surface currents in coastal areas!

 

What does this mean for the future?Artem Moiseev, PhD candidate at NERSC, is the first author on this publication, which is his first. ©NERSCArtem Moiseev, PhD candidate at NERSC, is the first author on this publication, which is his first. ©NERSC

This study, even though it only investigated a short period in time, shows promising results to use SAR to investigate how ocean surface currents behave. Some limitations currently exist, though, and further studies are necessary to make this method mainstream.

The traditional methods for coastal observations from fixed moorings, HF radars and drifting buoys, are being included in ocean models to improve ocean surface current “forecasts”. The results of this study show potential for SAR data to be included into this ocean model system to make ocean surface current “forecasts” more precise and reliable. Marine environmental services for offshore operations, pollutant tracking, search and rescue, and fishery will improve thanks to more precise forecasting of ocean surface currents in coastal areas.

Moiseev, the first author, sums up the importance of this work: “The Doppler observations from Sentinel-1 provide an exciting opportunity to acquire high-resolution ocean surface current snapshots. Despite the challenging processing, these data show promising performance for studying ocean currents, especially in the coastal zone, where the majority of human activities are located.”

 

Reference:

Moiseev, A., Johnsen, H., Hansen, M. W., & Johannessen, J. A. (2020). Evaluation of radial ocean surface currents derived from Sentinel-1 IW Doppler shift using coastal radar and Lagrangian surface drifter observations. Journal of Geophysical Research: Oceans, 125, e2019JC015743. https://doi.org/10.1029/2019JC015743