Tracing the pathways of the upper limb of the North Atlantic Meridional Overturning Circulation

The warm sea surface temperatures (SSTs) of the eastern subpolar gyre (ESG) in the North Atlantic have been widely linked to the climate and climate variability of Great Britain and northwestern Europe. The source of the ESG waters, and its heat, has long been identified as surface subtropical waters that flow into the subpolar gyre as part of the upper limb of the Meridional Overturning Circulation. Recent studies, however, have cast doubt on that identification. Here we use synthetic floats launched in a high-resolution ocean general circulation model to identify the supply waters to the ESG and to determine the influence of those pathways on SSTs in that climatically important region. The synthetic floats reveal two pathways: a dominant subsurface subtropical to subpolar pathway and a less traveled surface pathway carrying recirculated waters eastward from the western subpolar gyre. The former pathway supplies anomalously warm water to the region; the latter pathway supplies anomalously cool water.