AMOC sensitivity to air-sea fluxes parametrization

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the climate system, yet it exhibits a large inter-model spread and its future evolution remains uncertain. Here, we investigate the sensitivity of the AMOC to air–sea turbulent flux parameterizations using the IPSL climate model with five different bulk formulations. Differences in AMOC strength across the simulations reach up to 2 Sv (≡106 m3 s−1). They are driven by the relative strength of the subtropical and subpolar gyres, which controls heat and salt transport in regions of deep water formation. We quantify the respective contributions of the parameterization and feedbacks from wind, temperature, and specific humidity to the simulated flux differences. The behaviour of the surface roughness length at high wind speeds plays a key role and triggers both local and remote feedbacks. We show that these feedbacks dominate surface fluxes and subpolar gyre differences in the North Atlantic, leading to opposite results compared to the direct effect of the wind stress parameterization. This study emphasises the role of the interplay between heat and momentum fluxes and the way they are mediated by wind speed. Several aspects emerge from this study that could guide developments of air–sea turbulent flux parameterizations and help to understand the results in a coupled model.