Abstract
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.
Highlights
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
AMOC transport at 26◦ N (Tamoc) is estimated by combining four directly observed components Eq (1): Gulf Stream transport within the Florida Straits (Tflo), which is measured by submarine cables and calibrated by regular hydrographic sections (Baringer and Larsen, 2001; Meinen et al, 2010); Ekman transport (Tek), which here is calculated from ERA-Interim reanalysis wind fields; Western Boundary Wedge transport (Twbw), which is obtained from direct current measurements over the continental slope between the Bahamas and the WB2 mooring at 76.75◦ W; and the internal transport (Tint), the basin-wide geostrophic transport calculated from dynamic height profiles described below, relative to a reference depth of no motion at 4820 dbar
Drographic data could be used to improve the resolution of the upper mid-ocean (UMO) and AMOC transport estimates compared to the sparse transatlantic sections
Summary
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 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 re-
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