On the Recent Ambiguity of the North Atlantic Subpolar Gyre Index

The so-called gyre index appears to be related to core aspects of the North Atlantic subpolar gyre, meridional overturning circulation, hydrographic properties in the Atlantic inflows toward the Arctic, and in marine ecosystems in the northeast Atlantic Ocean. Recent publications, however, present a more linear version of this index with less of the key interannual-to-decadal variability. This has introduced uncertainty about the meaning and usefulness of the gyre index. We claim that these concerns are primarily caused by the fact that the recently produced gyre index is not the same as the original gyre index and discuss possible reasons.