Assessing the potential of future gravity missions to monitoring the Atlantic Meridional Overturning Circulation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a major component of the climate system, redistributing heat, freshwater and carbon through the Atlantic. Although substantial weakening is projected under continued climate change, the magnitude and timing of future change remain uncertain. Detecting a sustained anthropogenic trend is particularly difficult because direct observations are spatially limited and short relative to the AMOC’s strong natural variability. Satellite gravimetry offers a complementary observing strategy because AMOC variability produces a characteristic ocean-bottom-pressure (OBP) signal across the western boundary of the North Atlantic. However, the narrow cross-slope scale of this signal, together with measurement noise and leakage from continental hydrology, presents a demanding observational challenge. We assess the feasibility of recovering AMOC variability and long-term change using simulated gravimetry observations. Monthly output from a 1/12° North Atlantic ocean model is used to establish the relationship between AMOC transport and OBP over the upper and lower continental slope. Modelled OBP fields are transformed into spherical harmonics and progressively truncated to quantify the effects of limited spatial resolution. We then apply simulated mission errors and filtering representative of GRACE-C-like, Next Generation Gravity Mission (NGGM) and MAGIC configurations. Spherical-harmonic truncation substantially attenuates the narrow western-boundary OBP signals, but their cross-slope contrast, and hence the associated AMOC variability, remains recoverable at resolutions achievable by future missions. The most robust estimate is obtained from the difference between lower- and upper-slope OBP averaged over carefully selected latitude ranges. GRACE-C-like errors are generally too large for reliable recovery, whereas NGGM and MAGIC reproduce much of the modelled interannual-to-decadal variability; MAGIC provides a smaller additional improvement over NGGM alone. Sustained trends are also recoverable over plausible mission lifetimes, with trend uncertainties of approximately 0.8 Sv decade⁻¹ and weakening rates of around 2 Sv decade⁻¹ or greater becoming detectable in the simulations. These results demonstrate that next-generation satellite gravimetry could provide basin-scale monitoring of AMOC variability and emerging trends, complementing sustained in-situ arrays and extending observations beyond a single latitude.