The North Atlantic Ocean Is in a State of Reduced Overturning

D A Smeed,C S Meinen,W E Johns,S A Josey,E Frajka‐Williams,G D Mccarthy,D Rayner,C Beaulieu,M O Baringer,H L Bryden,B I Moat

doi:10.1002/2017gl076350

Abstract

AbstractThe Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004–2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream and altered patterns of heat content and sea surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air‐sea fluxes close to the western boundary reveal that the changes in ocean heat transport and sea surface temperature have altered the pattern of ocean‐atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal‐scale variability of North Atlantic climate.

Highlights

The Atlantic Meridional Overturning Circulation (AMOC) transports warm near-surface waters northward and colder deep waters southward, resulting in a large transport of heat
A number of studies with coupled climate models and forced ocean models have found that decadal-scale reductions in the AMOC are manifest by a weakening of the northward western boundary current (WBC) (e.g., Thomas et al, 2012) and a reduction of the heat transported by the North Atlantic Current into the eastern subpolar gyre (Zhang, 2008)
These are very similar to the changes that we have identified as concurrent with the observed reduction in the AMOC

Summary

Introduction

The Atlantic Meridional Overturning Circulation (AMOC) transports warm near-surface waters northward and colder deep waters southward, resulting in a large transport of heat. These results provide strong observational evidence that the AMOC is a major factor in decadal-scale variability of North Atlantic climate. The time series of the vertical profile of the overturning transport is calculated from the data From this the transports associated with the upper and lower North Atlantic Deep Waters (NADW) are derived. To formally examine whether or not the reduction of the AMOC has continued over the 5 years since the decline was first reported, we have used change-point analysis (Beaulieu et al, 2012) For this analysis, we used de-seasonalized monthly mean values of the RAPID 26°N time series. The 95% confidence intervals for these means are shown by shading

Concurrent Changes in the North Atlantic

Findings

Discussion