Reconstructing Ocean Surface Current Combining Altimetry and Future Spaceborne Doppler Data

Abstract

The ocean surface current, a key variable for many scientific and operational applications, is only partially and indirectly observed from space. Altimetry provides the geostrophic component of the current (Fu et al., 1988), which is a dominant contribution to surface transport in most of the oceans, effectively resolving wavelengths larger than about 200 km wavelength (Ballarotta et al., 2019). The ageostrophic component, not synoptically observed yet, is locally sampled from drifting buoys (Elipot et al., 2016) or high-frequency radars near the coasts (Kim et al., 2008). If model estimates for ageostrophic current are available, in particular for the low-frequency part (Rio et al., 2014) the uncertainties are still high. Filling this gap with satellite measurements of the total surface current is the topic of active research, with several emerging concepts of spaceborne Doppler radar for either 1 km resolution local studies such as SEASTAR (Gommengin

Simultaneous estimation of Ocean mesoscale and coherent internal tide Sea Surface Height signatures from the global Altimetry record

“…Even with suited spatial localization, the number of observations would exceed the limit of matrix invisibility (and even matrix storage). To overcome this issue, one can define covariances through the construction of a reduced basis as presented in Ubelmann et al (2021) and invert the system with conjugate gradients bypassing the explicit massive inversion.…”

“…This paragraph is a summary of the method presented in details in Ubelmann et al (2021). For all component k, we consider Γ k the reduced basis of elements in the time-space domain to approximate x k as follows: where the η k are the state vectors in parameter space…”

“…The basis for mesoscales is the same as presented in Ubelmann et al (2021), where the analytical formula of the ensemble of basis elements are given. Here, we only provide an illustration for comparison with the Internal-Tide basis described in details in the next section.…”

“…The second step is the resolution of Eq. 13, as explained in details in Ubelmann et al (2021), to converge toward the solution after typically 100 iterations with a conjugate-gradient method. Then, the third step is the projection of the solution in the grid space, formally Γη but in practice this is computed sequentially by adding each column of Γ (i.e.…”

Large River Plumes Detection by Satellite Altimetry: Case Study of the Ob–Yenisei Plume

“…Topex/Poseidon, launched in 1992, was the first satellite designed to make accurate measurements necessary for observing the time-averaged surface circulation, tides, and the variability of currents [21]. The most recent gridded altimetry products have a spatial resolution of 0.1 • [22] and are steadily improving the accuracy of measurements [23].…”