A coupled model study on the Atlantic Meridional Overturning Circulation under extreme atmospheric CO2 conditions

Rita Lecci Rita Lecci,Simona Masina Simona Masina,Marcelo Barreiro Marcelo Barreiro,Annalisa Cherchi

doi:10.4401/ag-6658

Abstract

This study investigates the climate sensitivity to a strong CO2 atmospheric forcing focusing on the North Atlantic Ocean (NA). The analysis is based on a set of 600 years long experiments performed with a state-of-the-art coupled general circulation model (CGCM) with the 1990 reference value of atmospheric CO2 multiplied by 4, 8 and 16. Extreme increases in atmospheric CO2 concentration have been applied to force the climate system towards stable states with different thermo-dynamical properties and analyze how the different resulting oceanic stratification and diffusion affect the Atlantic Meridional Overturning Circulation (AMOC). The AMOC weakens in response to the induced warming with distinctive features in the extreme case: a southward shift of convective sites and the formation of a density front at mid-latitudes. The analysis of the density fluxes reveals that NA loses density at high latitudes and gains it southward of 40°N mainly due to the haline contribution. Our results indicate that the most important processes that control the AMOC are active in the high latitudes and are related to the stability of the water column. The increased ocean stratification stabilizes the ocean interior leading to a decreased vertical diffusivity, a reduction in the formation of deep water and a weaker circulation. In particular, the deep convection collapses mainly in the Labrador Sea as a consequence of the water column stratification under high latitudes freshening.

Highlights

The Atlantic Meridional Overturning Circulation (AMOC) has a major role in the Earth’s climate system being responsible for one third of the northward global ocean heat transport [Ganachaud and Wunsch 2000]
Several studies suggest that increased greenhouse gas (GHG) concentration may cause a reduction of heat loss and an enhanced freshwater input at high latitudes making the surface waters less dense in the northern sinking regions [Mikolajewicz and Voss 2000, Thorpe et al 2001]
Increased atmospheric CO2 experiments are performed with a state-of-the-art coupled model with variable oceanic vertical diffusivity

Summary

Introduction

The Atlantic Meridional Overturning Circulation (AMOC) has a major role in the Earth’s climate system being responsible for one third of the northward global ocean heat transport [Ganachaud and Wunsch 2000]. Previous studies have shown that vertical diffusivity and stratification are potentially important factors controlling the AMOC behavior, and they may change under global warming conditions or under increased radiative and water fluxes forcing [Manabe and Stouffer 1994, Wood et al 1999, Thorpe et al 2001, Schmittner et al 2005, Weaver et al 2007, Marzeion et al 2010]. The model setup with atmosphere, ocean and sea-ice active components allows to compare the influence of high-latitude heat and freshwater fluxes on density changes. Even if the response of the ocean to the radiative forcing has been shown to depend on the mean state [Gregory et al 2005], the analysis of the changes in the perturbed experiments still can provide information about the mechanisms that control the AMOC.

Impact of increased atmospheric CO2 forcing on the AMOC

Induced Atlantic Ocean mean state and AMOC changes

AMOC driving processes under extreme atmospheric CO2 conditions

Findings

Discussion and conclusions