Towards two decades of ocean mass and heat transports at 26.5N: Extension of the RAPID-MOCHA Array

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Climate models suggest that variability of the Atlantic meridional overturning circulation (AMOC) is linked to sea surface temperature changes on interannual to multi-decadal time scales that can have far reaching societal impacts. Additionally, climate models continue to project a substantial weakening of the AMOC over the next century that could feed back on the climate system through reduced northward heat transport as well as reduced CO2 uptake in the North Atlantic. The specific role of the AMOC in the climate system has been difficult to determine due to a lack of long term measurements of its variability. At the end of its current phase, the RAPID-MOCHA observing system along 26.5°N will have provided continuous estimates of the AMOC for just under 19 years, the longest full-depth, basinwide measurements of oceanic transport currently available. These observations have provided a critical benchmark for the evaluation of forced, data-assimilative, and coupled climate models, and have formed a fundamental building block around which international programs are being developed to monitor the AMOC throughout the Atlantic basin. This project will continue the U. S. contribution to RAPID-MOCHA for an additional 42 months, from January 2023 to June 2026, extending the time series of oceanic mass, heat, and freshwater transports to 22.3 years. The results from this program will continue to provide oceanic observations crucial for the assessment and improvement of forced, data-assimilative, and coupled climate models. The proposed program continues a successful collaborative effort between the P.I.'s and investigators from the U.K. National Oceanography Centre, who are submitting a companion proposal to maintain their part of the observing system for the same time period through U.K. funding sources. The program also collaborates with NOAA AOML in their Western Boundary Time Series program, and provides training for graduate students and postdoctoral researchersThe U.S. contribution to the in-situ observation system includes: (1) Florida Current monitoring using a submarine cable, calibrated by regular cruises with hydrographic and velocity measurements, and (2) continuous moored time series measurements within the Deep Western Boundary Current and Antilles Current region east of the Bahamas, including hydrographic and velocity sections at approximately 11-month intervals. The maintenance of the repeat hydrographic sections will be conducted in collaboration with NOAA which also supports independently the Florida Current monitoring. U.K. collaborators will provide support for the eastern basin moorings that complete the 26.5°N trans-basin time series observing system. Limited new instrumentations on moorings will also be included to derive ocean bottom pressure gradient on the boundary east of the Bahamas. The RAPID-MOCHA observations will allow the team to document and understand the causes and impacts of the AMOC interannual to decadal variability, to investigate the control of western boundary pressure on transports, and to link changes observed at 26.5°N to other AMOC indices and results from on-going AMOC measurement programs around the Atlantic basin. By quantifying the interannual to decadal variability of the meridional circulations of mass, heat, and freshwater in the North Atlantic Ocean at 26.5°N, this project will further contribute to the understanding of the processes leading to changes of sea surface temperature, air-sea fluxes, and ocean heat and freshwater storage that are integral components of the North Atlantic regional climate and global climate. The time series so far have shown a longer-term decline with respect to the early years of the program, the origin of which---natural variability or response to anthropogenic climate change---is yet to be determined from longer observations. The AMOC constitutes a relative rapid conduit between the atmosphere and the deep ocean that is crucial to continuously observe on climate-relevant temporal scales to understand the ramifications of anthropogenically-forced climate change.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.