Abstract

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26∘ N has now been continuously measured by the RAPID array over the period April 2004–September 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic multidecadal variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy forcing by the atmosphere drove intense water-mass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45∘ N, observations suggest that the AMOC may already be increasing. Examining 26∘ N, we find that the AMOC is no longer weakening, though the recent transport is not above the long-term mean. Extending the record backwards in time at 26∘ N with ocean reanalysis from GloSea5, the transport fluctuations at 26∘ N are consistent with a 0- to 2-year lag from those at 45∘ N, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering nor how the AMOC at 26∘ N responds to intense buoyancy forcing.

Highlights

  • The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern spanning the Atlantic from south to north, transporting warm waters northward and colder waters southward

  • For periods shorter than about 60 days, Ekman transport dominates the variability of the AMOC; at other sub-annual periods, the variability is similar among all three components

  • From the nearly 15-year long record of the AMOC variability at 26◦ N, we can characterise the transport as highly variable on all timescales, with high-frequency variability dominated by rapid fluctuations in the zonal winds across 26◦ N, seasonal cycles contributed to by the upper mid-ocean (UMO) transport between the Bahamas and Canary Islands, and low-frequency variability dominated by the UMO transport and mirrored in the lower North Atlantic Deep Water (LNADW) layer (3000–5000 m)

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Summary

Introduction

The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern spanning the Atlantic from south to north, transporting warm waters northward and colder waters southward. It drives a large net northward transport of heat, with one petawatt (1 PW = 1015 W) released to the atmosphere between 26 and 70◦ N, impacting. The deeper limb of the AMOC is isolated from the atmosphere and can store energy and matter for centuries. Barber et al, 1999; Ganopolski and Rahmstorf, 2001), and the current generation of coupled climate models predicts a slowing of the AMOC over the present century in response to increasing greenhouse gases (IPCC, 2013) Changes to the AMOC during the paleoclimate period are thought to explain the abrupt shifts in climate found in paleoclimate records (e.g. Barber et al, 1999; Ganopolski and Rahmstorf, 2001), and the current generation of coupled climate models predicts a slowing of the AMOC over the present century in response to increasing greenhouse gases (IPCC, 2013)

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