Clarifying the Relation between AMOC and Thermal Wind: Application to the Centennial Variability in a Coupled Climate Model

Robin Waldman,Joël Hirschi,Rym Msadek,Aurore Voldoire,Christophe Cassou

doi:10.1175/jpo-d-19-0284.1

Abstract

AbstractThis work aims to clarify the relation between the Atlantic meridional overturning circulation (AMOC) and the thermal wind. We derive a new and generic dynamical AMOC decomposition that expresses the thermal wind transport as a simple vertical integral function of eastern minus western boundary densities. This allows us to express density anomalies at any depth as a geostrophic transport in Sverdrups (1 Sv ≡ 106 m3 s−1) per meter and to predict that density anomalies around the depth of maximum overturning induce most AMOC transport. We then apply this formalism to identify the dynamical drivers of the centennial AMOC variability in the CNRM-CM6 climate model. The dynamical reconstruction and specifically the thermal wind component explain over 80% of the low-frequency AMOC variance at all latitudes, which is therefore almost exclusively driven by density anomalies at both zonal boundaries. This transport variability is dominated by density anomalies between depths of 500 and 1500 m, in agreement with theoretical predictions. At those depths, southward-propagating western boundary temperature anomalies induce the centennial geostrophic AMOC transport variability in the North Atlantic. They are originated along the western boundary of the subpolar gyre through the Labrador Sea deep convection and the Davis Strait overflow.

Highlights

The Atlantic meridional overturning circulation (AMOC) defines the zonally integrated Eulerian mean circulation in the meridional plane over the Atlantic Ocean
500-m depth, the behavior differs between both basins: in the Labrador Sea, we find the cold anomaly in the 500–1500-m depth range that dominates the density signal at those depths (Fig. 9c) and drives, as was shown in section 3, the AMOC transport variability in CNRM-CM6
We have proposed a general framework that relates meridional oceanic transports outside the equator to densities at lateral boundaries

Summary

Introduction

The Atlantic meridional overturning circulation (AMOC) defines the zonally integrated Eulerian mean circulation in the meridional plane over the Atlantic Ocean. At its main observation site, in the subtropical North Atlantic Ocean (26.58N), it displays two main overturning cells, an upper cell (above typically 4000-m depth) of magnitude 17.2 Sv (1 Sv [ 106 m3 s21) associated with the formation and subsequent southward transport of the North Atlantic Deep Water (NADW) and a much weaker (;22 Sv) bottom cell related to the northward transport and transformation of the Antarctic Bottom Waters (AABW) (McCarthy et al 2015). The upper cell, tightly related to the formation of deep waters in the subpolar North Atlantic (Buckley and Marshall 2016), largely dominates the oceanic meridional heat transport by advecting warm upper-limb (typically upper 1000 m) waters northward and colder lower-limb waters southward, causing a net northward heat transport at all latitudes (Ganachaud and Wunsch 2003; Trenberth and Fasullo 2017). It can only be Denotes content that is immediately available upon publication as open access

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Conclusion