Abstract
The Iceland Scotland Overflow Water (ISOW) plume supplies approximately a third of the production of North Atlantic Deep Water and is a key component of the meridional overturning circulation (MOC). The Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array in the Iceland Basin has provided high-resolution observations of ISOW from 2014 to 2020. The ISOW plume forms a deep western boundary current along the eastern flank of Reykjanes Ridge, and its total transport varies by greater than a factor of two on intra-seasonal timescales. EOF analysis of moored current meter records reveal two dominant modes of velocity variance. The first mode explains roughly 20% of the variance and shows a bottom intensified structure concentrated in the rift valley that runs parallel to the ridge axis. The transport anomaly reconstructed from the first mode explains nearly 80% of the total ISOW plume transport variance. The second mode accounts for 15% of velocity variance, but only 5% of the transport variance. The geostrophically estimated transport (2.9 Sv) recovers only 70% of the total ISOW transport along the ridge flank estimated from the direct current meter observations (4.2 Sv), implying a significant ageostrophic component of ISOW mean transport and variability. Ageostrophic flow is strongly linked to the leading mode of velocity variability within the rift valley. The ISOW transport variability along the upper and middle part of the ridge is further shown to correlate with changes in the strength of deep MOC limb across the basin-wide OSNAP array.
The Iceland Scotland Overflow (ISOW) as a major component of deep ocean circulation in the North Atlantic. Newly available mooring observations from the Overturning in the Subpolar North Atlantic Program show that this current has significant intra-seasonal variability. This study shows that the flow is strongly affected by the topography of the Reykjanes Ridge which splits the current into multiple branches on the ridge slope and in the Iceland Basin interior. Most of the ISOW transport occurs along the ridge while the basin interior flow dominates the net variability. The results show that a major portion of the flow variability along the ridge is driven by flow funneled through a rift valley along the ridge slope. Geostrophic estimates of transport (the classic method of estimating large scale transport in the ocean) in this region fail to recover the flow within the rift valley.
Iceland Scotland Overflow transport variability is governed by a combination of barotropic and topographic effects Transport variability along the Reykjanes Ridge slope is dominated by deep ageostrophic flow within an axial rift valley Ageostrophic transport accounts for similar to 30% of Iceland Scotland Overflow Water (ISOW) transport along the Reykjanes Ridge flank and is a significant component of net ISOW export