Rapid eddy-mediated export of waters formed by boundary convection in the Irminger Sea

The deep limb of the Atlantic's overturning circulation is a critical component of climate stability and variability on long time scales. The standard view of the overturning circulation's deep limb emphasizes the role of deep convection, yet it is well known that in order for waters to enter the overturning circulation's deep limb, they must not only be transformed, but also exported equatorward. Here, we show the first observations of both the formation and export of waters formed by convection. Our focus is on two years (2014-2016) of OSNAP (Overturning in the Subpolar North Atlantic Program) and OOI (Ocean Observatories Initiative) mooring observations which extend from the southeastern coast of Greenland to the central Irminger Sea and coincide with a period of intense deep convection in the Irminger Sea (Fig d,e).

We identify two water masses with distinct T-S properties that are formed via convection in the Irminger Sea: upper and deep Irminger Sea Intermediate Water (ISIW). The prefixes upper and deep were chosen as these water masses are analogous to upper and deep Labrador Sea Water. Deep ISIW is formed by deep convection in the basin interior (Fig c), whereas upper ISIW is ventilated Atlantic-origin water formed at the edge of the boundary current (i.e. boundary convection, Fig b). Upper ISIW is subducted into the boundary current core and exported within three months of formation, whereas deep ISIW is subducted into the boundary current less effectively (Fig a). Upper ISIW is exported in the boundary current at twice the rate of deep ISIW. Our observations are consistent with an eddy flux mechanism: layer thickness anomalies propagate down-gradient into the boundary current and T-S properties are homogenized within density layers. Eddy activity increases in the winter when isopycnals are steep, particularly in the upper ISIW layer and near the boundary current core. Our results imply that waters formed by boundary convection are potentially a more significant component of the overturning circulation than waters formed by deep convection, challenging the standard paradigm.