Launch of the SWOT satellite
The "Surface Water Ocean Topography" space mission is the result of a long collaboration between NASA and CNES. Many LOPS researchers and engineers have contributed to define the performance of the revolutionary KaRIN instrument that will allow to measure the sea level with much more details. The launch went off without a hitch at 12:46 on December 16, 2022
You can follow more of SWOT deployment in space on NASA's SWOT blog
KaRIN, a new kind of instrument?
The ocean has been observed by satellite altimeter teams since the early 1990s with the ERS-1 and TOPEX-POSEIDON space missions. These altimeters measure sea level, wave height and wind speed, and have enabled a revolution in the observation of ocean dynamics. Sea level variations over distances of 10 km and more are associated with the presence of ocean currents (western boundary currents such as the Gulf Stream, eddies ...) but also the signature of seamounts (because of their effect on the gravity field) and of course the tides, which we know much better thanks to these satellites. This measurement of ocean dynamics allows us to "see" the ocean circulation at spatial scales larger than 200 km and its evolution in time at the monthly scale. Can we get more details?
Indeed, 200 km features make up the oceanic "mesoscale", giving us the position of highs and lows in the ocean, much like high and low pressure system it the atmosphere. Following that analogy, the "ocean weather", the upward movements that regulate the supply of nutrients in the surface layers, is driven by fronts that are present on much finer scales and evolve much faster. To get closer to this level of detail it is essential to have sharper images of the surface.
Other techniques can provide such images, in particular synthetic aperture radars (SAR) such as the SARs of the Sentinel 1 missions, which is used at LOPS to study winds, waves and currents.
By combining two SARs on either side of the satellite's ground track, it is possible to obtain a map of the surface topography. This is the principle of the KaRIN instrument on board SWOT, which using the Across-Track-Interferometry SAR (XTI-SAR) technique. By the way, if the two antennas are aligned with the ground track, we have access to the surface velocity: this is called ATI-SAR, which is the principle of the HARMONY mission.
XTI-SAR measurements have already been made to map land surfaces, such as the SRTM campaign on the American space shuttle, or the German space mission Tadem-X. But with SWOT it is the first time that a space mission tackles the measurement of marine topography.
SAR-XTI over the ocean: a first
Measuring ocean topography is much more complicated than land topography: the sea moves, with waves several meters high and vertical speeds of the order of a meter per second, and these effers must be filtered to access the signature of the currents. In addition, LOPS researchers have been working on other phenomena that affect the local sea level: in particular internal waves, but also infragravitational waves. The preparation of the SWOT mission has thus required a lot of theoretical work and very detailed numerical simulations. However, we have never had sea level data that looks like what KaRIN measures, so we may have some surprises. The current work aims to prepare to verify the quality of SWOT data (C-SWOT campaign in collaboration with SHOM) and analyze complementary data. Indeed, before their combination to make a sea level map ("SSH"), the SAR images should show the "roughness" of the surface (the power received by KaRIN) which depends on the wind and current variations that modulate the wave field, as illustrated below.
What are the implications for oceanographic research and applications?
Observing the details of the ocean circulation is important to understand how the "ocean machine" works and to compare theories and numerical models that represent the evolution of currents and eddies with the transfer of energy to the large structures of the circulation and also to the smaller eddies and filaments, until dissipation. The coastal zone and high latitudes are regions where eddies are particularly small and very difficult to observe with current altimeters.
LOPS will also continue to work on the influence of currents on waves: indeed, current variations are responsible for strong local amplifications of the current. We can observe the wave height varations with current altimeters, but the simulation of waves with current currents underestimates the real effect because the current gradients are too weak in the "blurred" vision given by current altimeters. Currents computed from SWOT data should give a clear improvement, with an effective resolution of about 50 km, while waiting for future space missions dedicated to the measurement of currents as for example HARMONY or ODYSEA.
The image below, taken from Gwendal Maréchal's PhD thesis, shows the impact of the spatial resolution of simulated currents on wave height variations in the Agulhas Current region. Starting from a high resolution model in the top left, the current fields is progressively smoothed to the level of today's best altimeter-derived currents (GLOBCURRENT). The images are maps of gradient in wave height averaged over 3 years, which can be compared to the measured gradients (bottom right panel).