Context
Remote sensing has revolutionized oceanography, starting from sea surface temperature, ocean color, sea level, winds, waves, and the recent addition of sea surface salinity. Now the oceanographic community is at the doorstep of yet another revolution with the direct measurement of surface velocities related to currents, winds and waves. After demonstrations using pairs of interferometric synthetic aperture radars (InSAR) and the Doppler centroid from single SARs, Doppler Oceanography from Space has demonstrated its feasibility, and a global monitoring mission concepts, SKIM (Sea surface KInematics Multiscale monitoring: an ESA* satellite mission - https://www.skim-ee9.org/) is at detailed design and proposal stages for ESA and NASA**. It is also possible to use today’s SAR data for measuring a single component of this velocity vector.

Meeting
A meeting held in Brest, France, on October 10 to 12, 2018, gathered 100 international participants from academia, industry and space agencies. The event was technically co-sponsored by the IEEE Oceanic Engineering Society (OES) and with the support of ESA, CNES***, Brest Metropole (Sea Tech Week), IFREMER, IUEM****, Labex Mer***** and LOPS (Laboratoire d’Océanographie Physique et Spatiale). The workshop was organized around 24 oral presentations and 15 posters. It reviewed the gaps in the observation capabilities of currents, winds and waves, recent developments in radar technology, processing and the understanding of Doppler data. The meeting was recorded live on https://www.youtube.com/channel/UCG9o60slGCjji-jOFOngYgw or Facebook https://www.facebook.com/SKIM4EE9/.

Figure 1. Reproduced from Sudre et al. (2013): correlation of V-component of current estimated from satellite data (filtered for time scales > 30 days) with in situ drifter measurements.

Physical Oceanography Background
Gaps are particularly important for tropical currents, high latitudes, extreme winds, and high-resolution currents. Today’s tropical currents are estimated from near-surface drifters or the surface drift of Argo floats, with a very poor spatial coverage for the first (> 2000 km), and a very poor temporal coverage for the latter (30 minutes every week). Estimates of surface currents are otherwise made by combining satellite altimetry and wind from models or scatterometers. At the equator, even for time scales longer than 30 days, these estimates are very poorly correlated with drifter data (Sudre et al. 2013, see figure below for V component), so that we basically know better the winds on Mars than the surface currents at the equator of our own planet. This severely limits our understanding of the heat balance in the equatorial cold tongues and the forecasting capabilities of patterns such as the African monsoon.
     At high latitude, sea ice is hiding most of the dynamics from the measurement capabilities of satellite altimeters, and where the sea ice is receding, the structures are too small to be resolved. Doppler radars can come in to measure near-ice current jets and the mesoscale of the emerging Arctic, which play a dominant role in defining the dynamics of the ice edge and transporting freshwater in the Arctic basin and around Greenland, both hugely important in global ocean circulation and its role in regulating the climate and weather.
     Another area of great science and applications opportunities is opened when waves and currents, or winds and currents are measured simultaneously. This would allow a better understanding of extreme sea states and extreme waves, and a better understanding on the ocean energy cycle, from the wind-work to the energy cascade in the ocean circulation.

Technical Background
Most of these scientific requirements are easily achievable by recent technical developments in radar technology and our understanding or Doppler properties of radar backscatter from the ocean. Exploring ocean currents, winds and waves from space can now use mature Doppler radar technology, in particular SKIM will fill two important blind spots: in the tropics and in the marginal ice zone, and expand the effective space and time resolution of the altimeter constellation by a factor 2 or more. The novel direct measurement of surface currents in the top two meters will produce the first maps of the equatorial upwellings that are critical for understanding and forecasting the heat budget at the equator with far-reaching weather and climate consequences, for example on the African monsoon. OSCV (Ocean Surface Current Velocity) maps will also allow the first monitoring of the highly dynamic currents at the ice edge. Adding this new and fundamental variable to Earth Observation capability together with high fidelity measurements of wave spectra will allow scientists to address a wide range of questions, including:

• How OSCV and waves influence upper ocean mixing and large-scale circulation?
  
• How do OSCV and waves influence the dynamics of the ice edge in the Arctic and Antarctic?
  
• What are the roles of eddies, wind-driven flows and waves in setting the surface concentration of marine litter and shaping marine ecosystems?

Figure 2. Top panels show the particular sampling of the SKIM concept, from 4 m resolution in range and 300 m in azimuth within footprints distributed across a 300 km wide swath. The bottom panels show simulated currents (with the MITgcm model) and their sampling in 3 days over the tropical Atlantic, for a particular choice of orbit.

Notes
*ESA: European Space Agency
**NASA: National Aeronautics and Space Administration
***Centre National d’Etudes Spatiales (the French Space Agency)
****IUEM: Institut Universitaire Européen de la Mer (part of the Brest University)
*****Labex Mer is a “Laboratory of Excellence” for the sea (la mer)