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Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability.This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’. 

The RAPID-MOCHA-WBTS (RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series) program has produced a continuous heat transport time series of the Atlantic Meridional Overturning Circulation (AMOC) at 26N that started in April 2004. This release of the heat transport time series covers the period from April 2004 to December 2020.The 26N AMOC time series is derived from measurements of temperature, salinity, pressure and water velocity from an array of moored instruments that extend from the east coast of the Bahamas to the continental shelf off Africa east of the Canary Islands. The AMOC heat transport calculation also uses estimates of the heat transport in the Florida Strait derived from sub-sea cable measurements calibrated by regular hydrographic cruises. The component of the AMOC associated with the wind driven Ekman layer is derived from ERA5 reanalysis. This release of the data includes a document with a brief description of the heat transport calculation of the AMOC time series and references to more detailed description in published papers. The 26N AMOC heat transport time series and the data from the moored array are curated by the Rosenstiel School of Marine, Atmospheric and Earth Science at the University of Miami. The RAPID-MOCHA-WBTS program is a joint effort between the NSF (Principal Investigators Bill Johns and Shane Elipot, Uni. Miami) in the USA, NERC in the UK (PI Ben Moat, David Smeed, and Brian King, NOC) and NOAA (PIs Denis Volkov and Ryan Smith). 

Abstract Northward ocean heat transport at 26°N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large—approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009–16) on an annual average basis associated with a 2.5-Sv (1 Sv ≡ 106 m3 s−1) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26°N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2°C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport. 

The Mediterranean Sea can be viewed as a “barometer” of the North Atlantic Ocean, because its sea level responds to oceanic-gyre-scale changes in atmospheric pressure and wind forcing, related to the North Atlantic Oscillation (NAO). The climate of the North Atlantic is influenced by the Atlantic meridional overturning circulation (AMOC) as it transports heat from the South Atlantic toward the subpolar North Atlantic. This study reports on a teleconnection between the AMOC transport measured at 26.5°N and the Mediterranean Sea level during 2004–17: a reduced/increased AMOC transport is associated with a higher/lower sea level in the Mediterranean. Processes responsible for this teleconnection are analyzed in detail using available satellite and in situ observations and an atmospheric reanalysis. First, it is shown that on monthly to interannual time scales the AMOC and sea level are both driven by similar NAO-like atmospheric circulation patterns. During a positive/negative NAO state, stronger/weaker trade winds (i) drive northward/southward anomalies of Ekman transport across 26.5°N that directly affect the AMOC and (ii) are associated with westward/eastward winds over the Strait of Gibraltar that force water to flow out of/into the Mediterranean Sea and thus change its average sea level. Second, it is demonstrated that interannual changes in the AMOC transport can lead to thermosteric sea level anomalies near the North Atlantic eastern boundary. These anomalies can (i) reach the Strait of Gibraltar and cause sea level changes in the Mediterranean Sea and (ii) represent a mechanism for negative feedback on the AMOC. 

Abstract. The strength of the Atlantic meridional overturning circulation(AMOC) at 26∘ N has now been continuously measured by the RAPIDarray over the period April 2004–September 2018. This record provides uniqueinsight into the variability of the large-scale ocean circulation,previously only measured by sporadic snapshots of basin-wide transport fromhydrographic sections. The continuous measurements have unveiled strikingvariability on timescales of days to a decade, driven largely bywind forcing, contrasting with previous expectations about a slowly varyingbuoyancy-forced large-scale ocean circulation. However, these measurementswere primarily observed during a warm state of the Atlantic multidecadalvariability (AMV) which has been steadily declining since a peak in2008–2010. In 2013–2015, a period of strong buoyancy forcing by theatmosphere drove intense water-mass transformation in the subpolar NorthAtlantic and provides a unique opportunity to investigate the response ofthe large-scale ocean circulation to buoyancy forcing. Modelling studiessuggest that the AMOC in the subtropics responds to such events with anincrease in overturning transport, after a lag of 3–9 years. At45∘ N, observations suggest that the AMOC may already beincreasing. Examining 26∘ N, we find that the AMOC is no longerweakening, though the recent transport is not above the long-term mean.Extending the record backwards in time at 26∘ N with oceanreanalysis from GloSea5, the transport fluctuations at 26∘ N areconsistent with a 0- to 2-year lag from those at 45∘ N, albeit withlower magnitude. Given the short span of time and anticipated delays in thesignal from the subpolar to subtropical gyres, it is not yet possible todetermine whether the subtropical AMOC strength is recovering nor how theAMOC at 26∘ N responds to intense buoyancy forcing.