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Abstract Ocean heat transport (OHT) plays a key role in climate and its variability. Here, we identify modes of low-frequency North Atlantic OHT variability by applying a low-frequency component analysis (LFCA) to output from three global climate models. The first low-frequency component (LFC), computed using this method, is an index of OHT variability that maximizes the ratio of low-frequency variance (occurring at decadal and longer timescales) to total variance. Lead-lag regressions of atmospheric and ocean variables onto the LFC timeseries illuminate the dominant mechanisms controlling low-frequency OHT variability. Anomalous northwesterly winds from eastern North America over the North Atlantic act to increase upper ocean density in the Labrador Sea region, enhancing deep convection, which later increases OHT via changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The strengthened AMOC carries warm, salty water into the subpolar gyre, reducing deep convection and weakening AMOC and OHT. This mechanism, where changes in AMOC and OHT are driven primarily by changes in Labrador Sea deep convection, holds not only in models where the climatological (i.e., time-mean) deep convection is concentrated in the Labrador Sea, but also in models where the climatological deep convection is concentrated in the Greenland-Iceland-Norwegian (GIN) Seas or the Irminger and Iceland Basins. These results suggest that despite recent observational evidence suggesting that the Labrador Sea plays a minor role in driving the climatological AMOC, the Labrador Sea may still play an important role in driving low-frequency variability in AMOC and OHT. 

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.