Exploring AMOC Sensitivity

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July 3, 2019 by Helen Hill

Surface heat flux cooling perturbations used in Kostov et al.’s experiments exploring the sensitivity of the Atlantic Meridional Overturning – image courtesy, the researchers.

Reporting by Helen Hill for MITgcm


A team from the UK has been using an ocean-only configuration of the MITgcm and its adjoint to explore the sensitivity of the Atlantic Meridional Overturning Circulation to surface heat and freshwater fluxes over the Subpolar Gyre.

The Atlantic meridional overturning circulation (AMOC) transports approximately 0.5 PW of heat northward across the Equator and impacts regional and global climate, with the circulation implicated as one of the possible factors that keep Western European winters mild compared to other parts of the Northern Hemisphere. The AMOC also plays an important role in the surface uptake and vertical distribution of heat and carbon within the ocean which in turn affects the evolution of surface climate under greenhouse gas and aerosol forcing.

Yavor has been using MITgcm since XXX. In his spare time he enjoys XXX, XXX, XXX.

Yavor first started using the MITgcm in 2012, but began using it a lot more intensely during his postdoc. When not tickling the AMOC, he says he enjoys spending time as the emcee at the annual MIT PAOC retreat

Because of the AMOC’s outstanding importance for large-scale ocean and climate variability, a number of theoretical and modeling studies have explored the sensitivity of the circulation to surface buoyancy forcing at different latitudes, as well as the meridional transport connectivity in the Atlantic Ocean, ie, the extent to which AMOC anomalies are causally related across different latitudes. Here we report on a modeling study by Yavor Kostov, Helen Johnson, and David Marshall from the University of Oxford in the UK who have been using MITgcm and its adjoint to probe the mechanisms behind the large seasonal differences in the AMOC sensitivity to surface heat they observe. Their findings are online in Climate Dynamics.

Using a 1 degree, ocean-only version of MITgcm, the researchers initialized the ocean state with realistic conditions based on the ECCO version 4 state estimate and drove the model with optimized time-evolving historical atmospheric forcing also available from ECCO. To generate the adjoint and compute the AMOC sensitivities to surface buoyancy fluxes linearized about the time-evolving model background state, the team used the proprietary algorithmic differentiation software TAF. Full details of forward model and adjoint are provided in the paper.

Kostov and his co-authors found that surface heat loss out of the Subpolar Gyre in the winter strengthens the AMOC at a lead time of approximately six months, but that the same surface heat flux anomaly in the summer leads to a delayed AMOC weakening that emerges at a lag of eight months. Probing further, the team found that in response to a summer surface cooling perturbation, the AMOC progressively weakened up to a lag of approximately 80 months, with the negative overturning anomaly then persisting for years.

“Compared with the sensitivity to surface heat fluxes, seasonality in the AMOC sensitivity to surface freshwater fluxes in our experiments was less pronounced,’ says Kostov, “there was also no sign reversal between the response to summer and winter perturbations.”

In particular, the team’s experiments highlighted the role of evaporation. “Heat flux anomalies over the Subpolar Gyre trigger changes in the rate of evaporation and hence affect the salinity of the mixed layer. Surface cooling gives rise to freshening in the following months, whereas warming leads to salinification. Persistent buoyancy changes due to salinity responses counteract the impact of heat fluxes to a varying extent depending on the seasonal mixed layer depth,” says Kostov. “On the other hand, air-sea feedback mechanisms exert a positive feedback on the AMOC response to surface freshwater flux perturbations both in the summer and in the winter months.”

To find out more about this work contact Yavor


Sensitivity of the AMOC to evaporation minus precipitation [Sv per (kg m−2 s−1 sustained over 1 hour)] at various lead times in months. Red (blue) shading indicates that the AMOC strengthens in response to a hypothetical positive (negative) flux anomaly at the specified lead time. The sensitivities were smoothed using a diffusive Gaussian operator with a spatial scale of 3 grid points after Weaver and Courtier (2001) – animation credit: Y. Kostov

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Do you have news about research using MITgcm? We are looking for contributions to these pages. If you have an interesting MITgcm project (ocean, atmosphere, sea-ice, physics, biology or otherwise) that you want to tell people about, get in touch. To make a post, contact Helen