The impact of mesoscale variability on northward volume transport in the Irminger Sea

The Irminger Current (IC) is known to be an important contributor to the northward volume transport associated with the Atlantic Meridional Overturning Circulation (AMOC). The IC has a two-core structure with surface intensified velocities and transports warm and saline waters originating from the North Atlantic Current further north. The strength of the subpolar AMOC is continuously measured by the Overturning in the Subpolar North Atlantic Program (OSNAP) since 2014. Recent results highlight that most of the overturning in density space occurs in the array east of Greenland, in the Irminger and Iceland Basins. In previous work we looked into the transport variability of the IC on decadal to interannual time scales and could identify long-term trend related to basin-wide density changes which have the potential to impact AMOC variability. However, the impact of mesoscale variability on northward transport variability in the Irminger Sea has not been studied yet.

In this study, we explore the mesoscale variability in the IC and its impact on northward transport variability.

Previous studies showed that the western flank of the Reykjanes Ridge, where the IC is located, is a region of enhanced eddy kinetic energy. We used high resolution mooring data from 2014 – 2020 from the IC mooring array to investigate its transport variability. The mean volume transport obtained for the IC is 10.4 Sv but it strongly varies on time scales from days to months (std. dev. of 4.3 Sv). The mooring data reveals a seasonal cycle in the eddy kinetic energy with the strongest activity in winter. However, this does not coincide with a seasonal cycle in volume transport. We found the strongest EKE in the western core of the IC. In 2019, an exceptional 6-month intensification of the IC led to exceptionally strong volume transport of the IC of 19.9 Sv in August. Using sea level anomaly maps from satellite altimetry, the intensification was attributed to the presence of a mesoscale eddy in the vicinity of the moorings.  At this time, altimetry shows an anticyclone lingering next to a cyclone in the mooring array, which intensified northward velocities within the IC. We thus conclude that mesoscale variability can directly impact both the transport and the variability of the IC.

Considering the potential importance of mesoscale variability along the Reykjanes Ridge, further research will focus on estimating the mean properties of the eddies, their formation region and their faith using a high-resolution model.