A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

Emma L. Worthington¹, Ben I. Moat², David A. Smeed², Jennifer V. Mecking², Robert Marsh¹, and Gerard D. McCarthy³

• ¹University of Southampton, European Way, Southampton, SO14 3ZH, UK
• ²National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK
• ³ICARUS, Department of Geography, Maynooth University, Maynooth, Co. Kildare, Ireland

Received: 16 Jul 2020 – 

Discussion started: 14 Aug 2020 – 

Revised: 09 Dec 2020 – 

Accepted: 21 Dec 2020 – 

Published: 15 Feb 2021

Abstract

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26^∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline.How to cite. Worthington, E. L., Moat, B. I., Smeed, D. A., Mecking, J. V., Marsh, R., and McCarthy, G. D.: A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline, Ocean Sci., 17, 285–299, https://doi.org/10.5194/os-17-285-2021, 2021.1 Introduction

In the Northern Hemisphere, the Atlantic meridional overturning circulation (AMOC) carries as much as 90 % of all the heat transported poleward by the subtropical Atlantic Ocean (Johns et al., 2011), with the associated release of heat to the overlying air helping to maintain north-western Europe’s relatively mild climate for its latitude. The AMOC also transports freshwater towards the Equator, and the associated deep water formation moves carbon and heat into the deep ocean (Kostov et al., 2014; Winton et al., 2013; McDonagh et al., 2015). A significant change in AMOC circulation is thus likely to have an impact on the climate of north-western Europe and further afield, with possible influences on global hydrological and carbon cycles. Although the Intergovernmental Panel on Climate Change (IPCC) says that it is unlikely that the AMOC will stop this century, they state with medium confidence that a slowdown by 2050 due to anthropogenic climate change is very likely (Stocker et al., 2013).

The importance of the AMOC means that since 2004 it has been observed by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) mooring array at 26^∘ N. The resulting observations have highlighted the great variability in AMOC transport on a range of timescales (Kanzow et al., 2010; Cunningham et al., 2007), including a decline in AMOC strength between 2004 and 2012 (Smeed et al., 2014). This reduced state persisted in 2017 (Smeed et al., 2018). The decrease is more likely to be internal variability rather than a long-term decline in response to anthropogenic forcing (Roberts et al., 2014), which the time series is currently too short to detect. Although the AMOC has been well-observed at 26^∘ N since 2004, prior to this, estimates of AMOC strength were restricted to instances of transatlantic hydrographic sections along 24.5^∘ N in 1957, 1981, 1992, 1998, and 2004, which provided only snapshots of the overturning circulation strength (Bryden et al., 2005). There are extensive additional hydrographic data around 26^∘ N, particularly at the western boundary, but these are insufficient to reconstruct the AMOC conventionally (Longworth et al., 2011). Due to the limited availability of hydrographic data, proxies have been used to reconstruct the AMOC time series earlier than 2004.

In one proxy reconstruction, Frajka-Williams (2015) used sea-surface height from satellite altimetry to estimate trans-basin baroclinic transport at 26^∘ N between 1993 to 2014. In another, Longworth et al. (2011) used temperature anomaly at the western boundary as a proxy for geostrophic transport within the upper 800 m, or thermocline layer, finding the temperature anomaly at 400 dbar explained 53 % of the variance in thermocline transport. However, both Longworth et al. (2011) and Frajka-Williams (2015) used single-layer models that do not account for the variable depth structure of the AMOC in the subtropics.

At 26^∘ N, the dynamics of the AMOC involve multiple water masses flowing in opposite directions in different layers, driven by the changing density structure with depth (Fig. 1a). Within the permanent thermocline layer, which reaches as deep as 800 m on the western boundary and 600 m on the eastern, isopycnals rise towards the eastern boundary, indicative of southward flow (Hernández-Guerra et al., 2014). Below the thermocline, isopycnals deepen towards the east, and the resulting transport profile (Fig. 1c) shows a small northward transport centred around 1000 m sandwiched between southward transports above and below. Although referred to by RAPID as Antarctic Intermediate Water (AAIW), both AAIW and Mediterranean Water are observed between 700–1600 m on the eastern boundary, with the relative contribution of each varying seasonally (Fraile-Nuez et al., 2010; Machín and Pelegrí, 2009; Hernández-Guerra et al., 2003). The transport profile also shows North Atlantic Deep Water (NADW), which has two distinct layers: Upper (UNADW) above 3000 m, primarily formed in the Labrador Sea (Talley and McCartney, 1982), and Lower (LNADW) below 3000 m, which has its origins in the overflows from the Nordic Seas (Pickart et al., 2003). Changes observed in one NADW layer are not necessarily observed in another. Smeed et al. (2014) found that the reduction in AMOC strength between 2004 and 2012 was seen in LNADW but not UNADW, while Bryden et al. (2005) found that LNADW transport estimated from transatlantic hydrographic sections at 25^∘ N decreased from −15 Sv in 1957 to less than −7 Sv in 1998 and 2004 but the UNADW transport remained between −9 and −12 Sv. Below the NADW layers, there is a small northward transport below 5000 m, Antarctic Bottom Water (AABW), that flows along the western side of the Mid-Atlantic Ridge. The partition between the upper southward and deep southward transports defines the strength of the overturning circulation: a weak AMOC is associated with a greater recirculation within the upper layers of the thermocline and weaker deep return flow; a stronger AMOC is associated with weaker thermocline recirculation and stronger deep NADW transport. For an empirical model to more fully represent AMOC dynamics, in particular lower-frequency changes, we suggest that it must represent these deeper layers. A layered-model interpretation of the density structure and the associated water mass transports is shown in Fig. 1b.

Here, we revisit the approach of Longworth et al. (2011) by using linear regression models to represent the AMOC and develop the method further to include additional layers representative of the deep circulation. Section 2 describes how we trained and validated our statistical model using the RAPID dataset and how we selected historical hydrographic data to apply to the model. Section 3 describes how these hydrographic data were used to create an extended time series of AMOC strength from 1982 to 2016. In Sects. 4 and 5, we discuss the implications of creating the longest observational time series of AMOC strength that incorporates variability in the deep NADW layers and acknowledge the limitations of using an empirical model.

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