Measuring interannual variability of the AMOC and meridional ocean heat transport at 26.5N: The RAPID-MOCHA Array

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Overview: Climate models suggest that variability of the Atlantic meridional overturning circulation (AMOC) is linked to sea surface temperature changes on inter-annual 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 carbon dioxide uptake in the North Atlantic (2007 Report from the Intergovernmental Panel on Climate Change). 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. However, the joint US-UK ocean observing system along 26.5°N (RAPID-MOCHA) has now provided continuous estimates of the AMOC for nearly a decade that have revealed the nature of its seasonal to inter-annual variability, the only such full-depth, basin-wide measurements currently available. The RAPID-MOCHA program has already provided a critical benchmark for evaluation of forced, data-assimilative, and coupled climate models, and is forming a fundamental building block around which international programs are being developed to monitor the AMOC throughout the Atlantic basin.Intellectual merit: The goal of this project is to maintain this measurement system for an additional 6 years, from 2014 to 2020, to quantify the inter-annual variability of the AMOC and its relationship to changes in surface forcing over the North Atlantic. The ocean meridional heat transport (MHT) at 26.5°N will also be monitored, including the time varying contributions by the overturning and gyre components of the heat transport. The last few years of the time series have shown significant inter-annual time-scale changes with respect to prior years, which are believed to be linked to changes in North Atlantic Oscillation (NAO) forcing. Ocean heat content and Sea Surface Temperature changes in the mid-latitude North Atlantic also appear to be linked to the recent AMOC changes. The results of the RAPID-MOCHA project thus far have been primarily focused on an accurate determination of the mean AMOC strength and vertical structure, its associated heat transport, and the dynamics of the seasonal cycle and shorter-term variations. The focus going forward will be to document and understand the causes of inter-annual variability and to link AMOC changes observed at 26.5°N to other AMOC indices and results from developing AMOC measurement programs around the Atlantic basin.The broader impacts of the project are to provide the basis for long-term monitoring of the AMOC, so that its relationship to observed climate fluctuations can be understood, and our ability to assess climate model predictions can be improved. This project will continue a successful international collaboration with investigators from the U.K. National Oceanography Centre as part of the U.K. RAPID-WATCH program. The U.S. contribution to this in-situ ocean observation system includes: (1) Florida Current monitoring using the operational submarine cable, calibrated by regular cruises with expendable temperature dropsondes, shipboard Acoustic Doppler Current Profiler (ADCP), and hydrographic/lowered-ADCP measurements, and (2) continuous moored time series measurements across the Deep Western Boundary Current and Antilles Current region east of the Bahamas, including hydrographic/lowered-ADCP sections at nine-month intervals in concert with scientists at the National Oceanic and Atmospheric Administration (NOAA. The Florida Current monitoring is independently supported by NOAA. The U.K. will provide support for the extended array of moorings along 26.5°N that completes the trans-basin continuous time series observing system.This project is a contribution to the US AMOC under the US CLIVAR (CLImate VARiability and predictability) program to maintain the moored component of element for a six-year period extending from March 2014 to March 2020.