Towards two decades of Atlantic Ocean mass and heat transports at 26.5° N

Abstract

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’.

Should AMOC observations continue: how and why?

“…where T is northward transport in Sv associated with each density and MOC σ (y, t) is now a function of density σ (following [24,25]). We also refer the reader to the recent article by Waldman et al [26] which, for the streamfunction in depth-space, usefully reframes the decomposition of the overturning into component parts.…”

“…at RAPID, no net throughflow is allowed by the calculation, so that . This then gives us the overturning at a given latitude, The definition can, and perhaps should, be used in density-space rather than depth-space, whereupon the streamfunction is where T is northward transport in Sv associated with each density and is now a function of density (following [ 24 , 25 ]). We also refer the reader to the recent article by Waldman et al [ 26 ] which, for the streamfunction in depth-space, usefully reframes the decomposition of the overturning into component parts.…”

“…This is of course an oversimplification because it neglects the potential for water to turn or recirculate at a given latitude, but is also clearly not the view that one would derive from the AMOC transports provided by observing arrays ( figure 2 ) which highlight the extensive high-frequency variability of transport estimates. Even so, the means of these time series are all roughly the same (17 Sv, [ 25 ]) which could indicate that after averaging over sufficiently long time periods, the AMOC is conveyor like.…”

Atlantic overturning: new observations and challenges

“…This means that observations of the AMOC need to continue in some form or another, but it is currently unclear what that form should be [ 46 ]. Observations at some locations in the Atlantic are more challenging [ 47 ] than at others [ 48 ] due to the complexities of the flow and the topography. Extending the observational record back in time by the use of proxies presents its own challenges [ 49 ], especially as different paleo proxies provide information on different aspects of the AMOC [ 50 ].…”

“…As it is ocean heat transport (OHT) rather than the AMOC per se that is the climatically important quantity, the relationship between OHT and AMOC has been a special focus in studies to date [ 47 , 48 ]. However, there are indications from the RAPID measurements at 26° N that this relationship can change, and this raises the important question of how it might change under future global warming [ 48 ].…”