Geostrophic current velocities

Tracking the Gulf Stream with satellite data

2020 and 2021

Geostrophic current velocities
Geostrophic current velocities

Using satellite data from multiple satellite instruments to track the Gulf Stream.

Last Updated

09 June 2021

Published on

07 June 2021

By Vinca Rosmorduc (CLS), Ben Loveday (Innoflair), Hayley Evers-King (EUMETSAT) and Annamaria Luongo (SpaceTec Partners)

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 & 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).

Figure 1: Absolute geostrophic current velocities over the Gulf Stream for 2020, computed from the dynamic heights of the ocean as observed by all altimeters (Sources: CMEMS/CLS).
Absolute Dynamic Topography from Jason-3
Figure 2: The Absolute Dynamic Topography from Jason-3 (level 2 Non Time Critical data along the track #126) at four different dates of 2020 (left), and its location on a map (right). The along-track plot shows the steep drop in the surface height which correspond to the main flow of the Gulf Stream, and its moves along the year (data CNES/EUMETSAT, figure CLS).

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.

Example SLSTR Nadir view sea surface skin temperature image for the Gulf Stream, 15 May 2021
Figure 3: Example SLSTR Nadir view sea surface skin temperature image for the Gulf Stream on 15 May 2021. All points below quality level 4 are flagged (removing clouds, land and spurious pixels).
OLCI Level-2 WFR product, 15 May 2021
Figure 4: Enhanced colour RGB image for the same scene as shown in Figure 3, derived from the corresponding OLCI Level-2 WFR product. All standard flags are applied (removing clouds, land, glint and spurious pixels).

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.

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