Deployment of a Long-Term Self-Calibrating Ocean Bottom Pressure Recorder in the Nordic Seas for GRACE-FO, GRACE-C, and NGGM Validation

Abstract

We report the successful deployment of a Sonardyne Fetch AZA ocean bottom pressure recorder at the Oceanographic Weather Station M (66°N, 2°E) in the Nordic Seas in June 2026. The deployment and data recovery efforts are supported by NASA's GRACE-FO Science Team and the University of Bergen, with the goal of providing an independent, long-term in situ record for the validation and interpretation of satellite gravimetry observations. The instrument was deployed at approximately 2,034 m depth, about 2 km west of the long-term physical oceanographic mooring at Station M, and is recording hourly ocean bottom pressure and temperature. The instrument is equipped with batteries designed to support up to 10 years of operation and an acoustic transducer that enables data to be retrieved remotely from the surface without recovering or disturbing the instrument on the seafloor. A key capability of the Fetch AZA is its in situ self-calibration system, which enables characterization and correction of long-term pressure-sensor drift – a major limitation of conventional bottom pressure records for resolving low-frequency ocean mass variability. An initial data harvest approximately 11 hours after deployment successfully recovered the first measurements and confirmed proper instrument operation. Strong acoustic communications were also established from multiple ranging locations approximately 1 km from the triangulated instrument position, demonstrating the feasibility of future acoustic data recovery. Annual data recovery is planned to build a multi-year, drift-corrected ocean bottom pressure record for comparison with GRACE-FO and future GRACE-C and NGGM observations, and for validation and improvement of their data products.