Abstract
The future international Surface Water and Ocean Topography (SWOT) Mission, planned for launch in 2021, will make high-resolution 2D observations of sea-surface height using SAR radar interferometric techniques. SWOT will map the global and coastal oceans up to 77.6° latitude every 21 days over a swath of 120 km (20 km nadir gap). Today’s 2D mapped altimeter data can resolve ocean scales of 150 km wavelength whereas the SWOT measurement will extend our 2D observations down to 15-30 km, depending on sea state. SWOT will offer new opportunities to observe the oceanic dynamic processes at these scales, that are important in the generation and dissipation of kinetic energy in the ocean, and act as one of the main gateways connecting the interior of the ocean to the upper layer. The active vertical exchanges linked to these scales have impacts on the local and global budgets of heat and carbon, and on nutrients for biogeochemical cycles. This review paper highlights the issues being addressed by the SWOT science community to understand SWOT’s very precise SSH / surface pressure observations, and it explores how SWOT data will be combined with other satellite and in-situ data and models to better understand the upper ocean 4D circulation (x,y,z,t) over the next decade. SWOT’s new SAR-interferometry technology aims to observe ocean SSH scales down to 15-30 km in wavelength. At these scales, SSH includes “balanced” geostrophic eddy motions and high-frequency internal tides and internal waves. This presents both a challenge in reconstructing the 4D upper ocean circulation, or in the assimilation of SSH in models, but also an opportunity to have global observations of the 2D structure of these phenomena, and to learn more about their interactions. At these small scales, the ocean dynamics evolve rapidly, and combining SWOT 2D SSH data with other satellite or in-situ data with different space-time coverage is also a challenge. SWOT’s new technology will be a forerunner for the future altimetric observing system, and so advancing on these issues today will pave the way for our future.
Highlights
Over the last 25 years, satellite altimetric sea surface height (SSH) observations have greatly advanced our understanding of the large-scale ocean circulation and its interaction with the larger mesoscale dynamics (Fu and Cazenave, 2001; Morrow et al, 2018b)
The primary oceanographic objective of the Surface Water and Ocean Topography (SWOT) mission is to characterize the ocean mesoscale and submesoscale circulation determined from ocean surface topography, from the large scale down to around 15 km wavelength (Fu et al, 2012; Fu and Ubelmann, 2014)
The spatial “transition” scale at which balanced motions dominate over unbalanced motions and the seasonal patterns of their relative strength are beginning to be better understood from modeling studies and in situ observations (Qiu et al, 2017, 2018)
Summary
Over the last 25 years, satellite altimetric sea surface height (SSH) observations have greatly advanced our understanding of the large-scale ocean circulation and its interaction with the larger mesoscale dynamics (Fu and Cazenave, 2001; Morrow et al, 2018b). These SSH observations reflect the ocean surface pressure field and give us the ability to monitor depth-integrated ocean dynamics. SWOT observations will fill the gap in our knowledge of the 15–150 km 2D SSH dynamics, which are important for setting the anisotropic structure of the ocean horizontal circulation and for understanding the ocean’s kinetic energy budget. Plans to have a global series of fine-scale experiments based on regional studies during the fast sampling phase, as part of the SWOT “Adopt-a-crossover” initiative, will be presented in a companion OceanObs2019 paper by d’Ovidio et al “Frontiers in fine scale in-situ studies: opportunities during the SWOT fast sampling phase.”
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