As Earth’s climate changes, so too will its oceans. Water temperatures are climbing, sea levels are rising, and ocean currents are shifting. Researchers typically use radar altimeters, which send a microwave pulse toward the ocean and measure the time it takes to rebound, to study the ocean surface, but the usefulness of altimetry data is limited to large areas and long temporal scales. In a new study, Le Goff et al. turn to maritime data to create a more precise picture of ocean currents.
Historically, data on ocean surface currents were based on ships’ logs, which tracked how intense currents affected a vessel’s course or speed. But today’s ships are equipped with much more precise geopositioning technologies. Merchant ships continually transmit their position, bearing, and speed through the Automatic Identification System (AIS), providing mountains of data that are more precise than ever before. Previous studies have shown that surface current velocities from AIS data match well with those predicted by high-frequency radar measurements.
Here the research team focused on the northern reaches of the Agulhas Current, a strong current that roughly follows the continental shelf break off the eastern coast of South Africa. The current, which has surface velocities of up to 2 meters per second, passes through a region with heavy maritime traffic. Using AIS data from vessels in transit through the region in 2016 and mathematical modeling, the team was able to reconstruct the surface current. The authors used surface current estimates collected by satellites and drifting buoys to validate the AIS-based observations. The study shows how AIS data could be a critical part of a more comprehensive current monitoring system.
According to the authors, the methods could be applied to other regions with heavy maritime traffic, such as the Mediterranean Sea. Monitoring ocean currents is critical: As Earth’s climate changes, so too will ocean surface currents, leading to changes in sea surface temperature and salinity that will ripple throughout marine ecosystems. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2021JC017228, 2021)
—Kate Wheeling, Science Writer