AMOC sensitivity to surface buoyancy fluxes: the role of air-sea feedback mechanisms

Yavor Kostov,Helen L Johnson,David P Marshall

doi:10.1007/s00382-019-04802-4

Yavor Kostov, Helen L Johnson + Show 1 more

Open Access

https://doi.org/10.1007/s00382-019-04802-4

Copy DOI

Journal: Climate dynamics	Publication Date: May 20, 2019
Citations: 21	License type: open-access

Affiliation: University of Oxford

Abstract

We interrogate the sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to surface heat and freshwater fluxes over the Subpolar Gyre in an ocean general circulation model and its adjoint. Surface heat loss out of the Subpolar Gyre in the winter strengthens the AMOC at a lead time of approximately 6 months. However, the same surface heat flux anomaly in the summer leads to a delayed AMOC weakening that emerges at a lag of 8 months. Under a summer surface cooling perturbation, the AMOC progressively weakens up to a lag of approximately 80 months, and then the negative overturning anomaly persists for years. Compared with the sensitivity to surface heat fluxes, seasonality in the AMOC sensitivity to surface freshwater fluxes is less pronounced, and there is no sign reversal between the response to summer and winter perturbations. We explain the mechanisms behind the large seasonal differences in the AMOC sensitivity to surface heat fluxes and highlight 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. 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.

Highlights

The Atlantic meridional overturning circulation (AMOC) transports approximately 0.5 PW of heat northward across the Equator (Buckley and Marshall 2016) and impacts regional and global climate (Stolpe et al 2018)
We show that the representation of surface boundary conditions strongly affects the RAPID-AMOC sensitivity to thermohaline forcing over the Subpolar Gyre
The seasonal sign reversal in heat flux sensitivity is not mirrored in the corresponding sensitivity to regional surface freshwater fluxes (Fig. 2b, blue, and Fig. 2c for heat and freshwater fluxes scaled in the same units)

Summary

Introduction

The Atlantic meridional overturning circulation (AMOC) transports approximately 0.5 PW of heat northward across the Equator (Buckley and Marshall 2016) and impacts regional and global climate (Stolpe et al 2018). 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 (Zou et al 2019) in the Atlantic Ocean, i.e., the extent to which AMOC anomalies are causally related across different latitudes. In our model-based study, we do not restore sea surface temperature or salinity, but instead we employ either parameterized or fully prescribed heat and freshwater fluxes as our boundary conditions. Using boundary conditions based on ECCO allows us to explore the impact of air-sea feedback mechanisms on the AMOC in a more realistic framework compared to model configurations that rely on prescribed surface restoring conditions.

Data and methods

Results

Sensitivity of the AMOC to surface heat fluxes over the subpolar gyre

Impact of air‐sea feedback on the response to surface freshwater fluxes

Discussion and conclusions