Overturning in the subpolar North Atlantic: a review

Abstract

The Overturning in the Subpolar North Atlantic Program (OSNAP) was initiated in the spring of 2010 through a collaborative effort involving the USA, the UK, Germany, the Netherlands and Canada. A key feature of OSNAP is a trans-basin observing system deployed in the summer of 2014 for the continuous measure of volume, heat and freshwater fluxes in the subpolar North Atlantic. This review focuses on advancements made possible by the collective OSNAP observations. Chief among those advancements is the quantification of the dominant role of the eastern subpolar North Atlantic in the production of dense waters that reside in the lower limb of the overturning: the Irminger and Iceland basins contributed approximately three times as much dense water compared with the Labrador Sea over the observational period. Other advancements include elucidation of the relationship between convective activity in the basin interior and boundary current anomalies; the spread of overflow waters in the subpolar region; the seasonality of the meridional volume, heat and freshwater fluxes; and the challenges involved in designing a simpler, less costly observing system. Collectively, OSNAP measurements are laying a framework on which to assess the overturning circulation's vulnerability to continued warming and freshening as climate change continues apace. This article is part of a discussion meeting issue ‘Atlantic overturning: new observations and challenges’.

Buoyancy forcing and the subpolar Atlantic meridional overturning circulation

“…The majority of NADW formed in the subpolar gyre is not exported rapidly, but rather is recirculated within the energetic subpolar gyre circulation before being exported to the subtropics. Recirculation in the subpolar gyre is more prevalent for UNADW than for OWs, yet both UNADW and OWs reach the subtropics through both the Deep Western Boundary Current and complex interior pathways [ 102 , 112 , 114 – 181 ]. Recirculations within the subpolar gyre, interior pathways and mixing with older waters (1) lengthens the pathways to the subtropics and (2) results in a multitude of possible pathways.…”

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 ].…”

“…It is known that changes to freshwater in the subpolar North Atlantic are happening [ 52 , 53 ] and model experiments have shown that increased freshwater input could disrupt the AMOC [ 55 ]. Therefore, an unknown aspect of the Atlantic overturning is how glacial melt from Greenland [ 47 ] and/or freshwater export from the Artic [ 56 ] might impact the AMOC and on what time scale. Furthermore, it is unknown on what time scales, and by which processes, changes in deep water formation at high latitudes might propagate south.…”

“…Questions remain too about current understanding of the processes that impact the AMOC. For example, the degree to which surface buoyancy forcing versus wind forcing, either remotely or locally, affects the AMOC and on what time scales [ 47 , 48 , 59 ]. Furthermore, there is a clear distinction between deep convective mixing and sinking and the two may not necessarily occur in the same location.…”