Compensation between meridional flow components of the Atlantic MOC at 26° N

E Frajka-Williams,C S Meinen,D Rayner,M O Baringer,B I Moat,G D Mccarthy,D A Cuthbertson,W E Johns,D A Smeed,H L Bryden,A J Lawrence,A Duchez

doi:10.5194/os-12-481-2016

E Frajka-Williams, C S Meinen + Show 10 more

Open Access

https://doi.org/10.5194/os-12-481-2016

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Abstract

Abstract. From ten years of observations of the Atlantic meridional overturning circulation (MOC) at 26° N (2004–2014), we revisit the question of flow compensation between components of the circulation. Contrasting with early results from the observations, transport variations of the Florida Current (FC) and upper mid-ocean (UMO) transports (top 1000 m east of the Bahamas) are now found to compensate on sub-annual timescales. The observed compensation between the FC and UMO transports is associated with horizontal circulation and means that this part of the correlated variability does not project onto the MOC. A deep baroclinic response to wind-forcing (Ekman transport) is also found in the lower North Atlantic Deep Water (LNADW; 3000–5000 m) transport. In contrast, co-variability between Ekman and the LNADW transports does contribute to overturning. On longer timescales, the southward UMO transport has continued to strengthen, resulting in a continued decline of the MOC. Most of this interannual variability of the MOC can be traced to changes in isopycnal displacements on the western boundary, within the top 1000 m and below 2000 m. Substantial trends are observed in isopycnal displacements in the deep ocean, underscoring the importance of deep boundary measurements to capture the variability of the Atlantic MOC.

Highlights

The Atlantic meridional overturning circulation (MOC) is a key part of the global ocean circulation, redistributing heat and properties around the globe
The MOC is defined as the northward transport above the depth of maximum overturning across 26◦ N, and is constructed as the sum of three components: the surface meridional Ekman transport estimated from reanalysis winds, the Gulf Stream transport through the Florida Straits – the Florida Current (FC) – measured by a submarine cable (e.g. Meinen et al, 2010), and the upper mid-ocean (UMO) transport, measured by a transbasin array of current metre and dynamic height moorings between the Bahamas and Africa
Comparing the time series of western boundary isopycnal displacements at 820 m with the UMO transport (Fig. 9a), we find significant correlation where a 10 m downward displacement of the thermocline corresponds to a 1 Sv increase in the UMO transport (Fig. 9b)

Summary

Introduction

The Atlantic meridional overturning circulation (MOC) is a key part of the global ocean circulation, redistributing heat and properties around the globe. Once the MOC records had stretched to 7 years in length, striking interannual variability and more recently a declining trend in the MOC were revealed (McCarthy et al, 2012; Smeed et al, 2014).

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