Tracking the Gulf Stream with satellite data
2020 and 2021
Using satellite data from multiple satellite instruments to track the Gulf Stream.
05 May 2023
07 June 2021
The Gulf Stream is one of the most famous ocean currents. First mapped by Benjamin Franklin in 1769, modern maps showing the full extent and complexity of this major current have been made by satellites since the 1990s. Radar altimetry satellites, such as Jason-3, Sentinel-3A and -B, and soon Sentinel-6 Michael Freilich, observe the sea surface height over all the oceans. Over the Gulf Stream, they observe a sea surface 'cliff', meandering with small bumps and troughs detaching and, sometimes, attaching to it.
Knowing the ocean topography in two dimensions enables analysis of the surface geostrophic currents, i.e., the ocean circulation generated by the balance between the horizontal pressure gradient forces exerted by water masses and the effect of acceleration due to the Earth’s rotation (Figure 1). Outside the equatorial band, the geostrophic currents are computed from the gradient of the surface slopes, which are observed by altimetry satellites. The satellite data are provided by EUMETSAT to the Copernicus Marine Environment Monitoring Service (Global ocean gridded L4 Sea Surface Heights and derived variables) and the Copernicus Climate Change Service (Sea level daily gridded data from satellite observations for the global ocean from 1993 to present), which provide the sea surface heights and geostrophic currents to users (Figure 2).
The impact of the Gulf Stream can be observed beyond its signature in sea surface height. The transport of water around the global oceans by its currents, is intimately connected to the transport of heat. As such, observing such currents from the yearly to decadal timescales gives information about impacts on seasonal weather (see this companion case study on how weather satellites observe the Gulf Stream — Gulf Stream visible off Virginia coast), and over decades a shift has been observed which may be linked to climate change (Kelly and Dong, 2004, The relationship of western boundary current heat transport and storage to mid-latitude ocean-atmosphere interaction, in Earth’s Climate: The Ocean-Atmosphere Interaction).
The physical environment of currents like the Gulf Stream also strongly influence life in the oceans. The interplay between strong western boundary currents and shelf-edge bathymetry often results in the production of eddies and meanders. These drive exchanges between the shallow shelf waters and those of the open ocean, influencing the provision of nutrients and light for the most basic levels of life in the ocean — the phytoplankton.
Figure 3 and Figure 4 show respective sea surface temperature (SST) and blue/green false colour images over a section of the Gulf Stream in May 2021, as retrieved from measurements made by the SLSTR and OLCI instruments on Sentinel-3A. The false colour indicates areas of higher (green) and lower (blue) concentrations of phytoplankton. The similar patterns seen in both the SST and the false colour image show how influential the physical dynamics of the current are on the growth of phytoplankton. Where the Gulf Stream ejects eddies of warm, nutrient poor waters to the north, on to the shelf, we see an associated patch of blue, low phytoplankton waters. Where the Gulf Stream entrains shelf waters, we see the tell-tale imprint in the green, high phytoplankton patches that have been dragged to the south.
The plant-like phytoplankton are the basis of nearly all marine life, providing food for many dependent animals throughout the marine chain, and ultimately, for us. They also play a critical role in the carbon and oxygen cycles of the Earth, influencing both atmospheric composition and the chemical composition of the oceans.
Constellations of satellite missions such as Jason-3, Sentinel-3 and Sentinel-6 mean that we can observe the complex interactions between the physics and biology of our oceans, in places like the Gulf Stream and beyond.
Current heat - January 2005 (AVISO)
Surface Currents in the Atlantic Ocean - The Gulf Stream (Rosenstiel School of Marine and Atmospheric Science)