Estimation of sea surface salinity at high latitudes using the Cryorad 0.4-2GHz wideband radiometer.

The salinity of polar oceans is undergoing significant changes due to sea ice melt and increased continental runoff, which have resulted in a decrease in sea surface salinity (SSS) across most regions of the Arctic Ocean. Similarly, changes in the extent and thickness of Antarctic sea ice are altering SSS in the Southern Ocean, intensifying upper ocean stratification. These shifts profoundly impact ocean circulation, the ocean's capacity to absorb atmospheric heat and carbon, and ultimately, Earth’s climate. Notably, variations in SSS play a crucial role in the potential collapse of the Atlantic Meridional Overturning Circulation, with timing potentially earlier than anticipated by current climate models.

Accurate SSS estimates are essential for monitoring freshwater fluxes at ocean boundaries (e.g., sea ice melting and formation, river runoff, and precipitation), surface hydrography variability affecting deep water formation and overturning circulation, and exchanges with other ocean basins—all of which influence global climate. However, current climate models struggle to accurately represent high-latitude water mass properties due to simplistic depictions of processes like lateral mixing, convection, and entrainment, especially in marginal ice zones. These limitations hinder the models’ ability to predict climate change impacts effectively.

SSS is recognized as an Essential Climate Variable (ECV) by the Global Climate Observing System (GCOS) and an Essential Ocean Variable by the Global Ocean Observing System (GOOS). While current 1.4 GHz (L-band) radiometer missions have revolutionized global SSS measurements at scales of 40–150 km with revisit intervals of 3 to 8 days, their sensitivity to SSS diminishes significantly in cold waters (by a factor of ~3 between 30°C and 0°C), leading to greater uncertainties in polar regions.

The CryoRad mission, an ESA Earth Explorer 12 candidate, features a radiometer with an extended frequency range of 0.4–2 GHz, designed to improve SSS measurement accuracy in cold waters by at least a factor of two compared to L-band radiometers.

As part of the CNES study on "Salinity Estimation in Cold Seas Using Multiband 0.4–2 GHz" and the ESA CryoRad Phase 0 Science and Requirements Consolidation Study (SciReC), we conducted simulations using a simplified CryoRad instrument model. These simulations demonstrate the mission's potential to enhance SSS retrieval at high latitudes. The uncertainties in SSS retrieval were evaluated, considering various radiometric measurement factors such as sea surface temperature, wind speed, and atmospheric influences, which were modeled using radiative transfer principles validated for L-band and extrapolated to lower frequencies.

This simulator was used to perform an initial sensitivity analysis for Level 2 and Level 3 salinity estimates. Our presentation will detail the simulator's implementation, including the direct and inverse models, inversion strategies, and the performance achieved in estimating SSS within the context of an academic case study.