28-year volume transport decrease in the Irminger Sea: Results from mooring and reanalysis data

As an extension of the North Atlantic Current, the Irminger Current is an important component of the overturning in the subpolar North Atlantic. It contains warm, saline Subpolar Mode Water and cold, dense North East Atlantic Deep Water flowing northward along the western flank of the Reykjanes Ridge. As part of OSNAP (Overturning in the Subpolar North Atlantic Project) the Irminger Current has been monitored since 2014 with a mooring array consisting of five moorings, all equipped with current meters, ADCPs and CTDs.

Preliminary results from the recent 6-year mooring time series until summer 2020 give new insights into the interannual transport variability of the Irminger Current. The mean volume transport is 11.3 ± 8.8 Sv with a clear maximum of the yearly mean transport in 2019 (15.7 Sv). The Irminger Current experienced a decrease in salt transport by 50% from 2016 – 2018 compared to 2014 – 2016. This signal originates from a freshwater anomaly in the eastern subpolar North Atlantic.

For an investigation of the longer-term variability we used monthly mean reanalysis data (CMEMS) from 1993 - summer 2019 and the analysis and forecast up to summer 2020 along the Irminger Current mooring array across the Irminger Sea. The reanalysis data compares well with the mooring results both in mean transport and structural representation of the Irminger Current. Volume transport in the eastern Irminger Sea and sea surface height gradient are significantly correlated by r = 0.82 on interannual time scales. The 28-year time series shows a significant negative trend in volume transport over the eastern Irminger Sea, concomitant with a significant negative trend in the sea surface height and density gradient. Hydrographic changes over the top of the Mid Atlantic Ridge are dominating the trend in density gradient as changes in the central Irminger Sea are smaller and mostly density compensating.