Winds and Surface Currents from the Proposed ODYSEA Doppler Scatterometer Satellite: Momentum Exchanges across the Air-Sea Interface

Interactions between winds and surface currents play a critical role in shaping vertical motions within the marine atmospheric boundary layer. Unfortunately, until now we have not been able to collect global measurements of surface currents. Even more so, we lack co-located winds and surface currents on a global scale. The ODYSEA (Ocean Dynamics and Surface Exchange with the Atmosphere) satellite concept aims to fill these gaps. ODYSEA has been tapped as one of four candidate satellite observing systems to develop a Concept Study Report for NASA’s Earth System Explorers program. In mid-2025 NASA expects to select two of the four satellite concepts, with launch dates planned for 2030 and 2032. If selected, ODYSEA will put a Doppler scatterometer into orbit, providing the first-ever simultaneous global measurements of winds and surface currents, providing approximately daily coverage of the ice-free global ocean with 5-km data postings. The mission concept builds on the success of the airborne Dopper scatterometer flown as part of the Sub-Mesoscale Ocean Dynamics Experiment (S-MODE). A Ka-band radar gives ODYSEA a small measurement footprint size, allowing high-resolution measurements as close as ~3 km from the coastline. ODYSEA's ~1500–1700-km wide swath will enable roughly daily revisits, with twice daily observations at midlatitudes. ODYSEA will offer transformative opportunities for new scientific discovery, addressing wind–current coupling and surface current variability under changing environmental conditions, while also supporting operational applications in the areas of weather forecasting, ocean forecasting, search and rescue, and maritime activities. The satellite proposal is a partnership between a multi-institutional science team, the Jet Propulsion Laboratory (JPL), the French Centre National d’Études Spatiales (CNES), and the US Space Force. The program also offers opportunities for science enhancements and operational enhancements to meet a broad menu of community needs.

A key application of ODYSEA is for ocean prediction. Present ocean forecast systems are limited by the spatio-temporal resolution of observations. ODYSEA observations will provide near-daily full global coverage, representing an enormous advancement. In planning for ODYSEA, we examine applications to the ocean prediction problem. For test cases, observation errors are provided by the ODYSEA simulator, which indicates a strong dependence on wind speed with higher errors in low wind conditions. While ocean surface current errors are relatively large at the planned 5 km posting, the errors are uncorrelated. This leads to reasonable accuracy levels for application to ocean prediction at larger scales. Because ODYSEA will retrieve the very surface current velocity, we must consider how representative the observations are, given ocean numerical model resolution and the velocity structure in the upper 1 m. In previous and ongoing research, we have demonstrated velocity assimilation in ocean prediction systems by taking advantage of surface drifter measurements. A key assumption results from geostrophic balance, which connects the observed surface velocity and underlying density structure. This assumption allows the assimilation to correct the mesoscale ocean at time scales greater than the inertial period.