Changes in the Surface Salinity Gradient and Transport of the Irminger Current: The Climate Perspective

Here we use a new analysis schema, the Freshening Length, to study the transport in the Irminger Current on the east and west sides of Greenland. The Freshening Length schema relates the transports on either side of Greenland to the corresponding surface salinity gradients by analyzing climatological data from a data assimilating global ocean model. Surprisingly, the warm and salty waters of the Current are clearly identified by a salinity maximum that varies nearly linearly with distance along the Current’s axis. Our analysis of the climatological salinity data based on the Freshening Length schema shows that only about 20 % of the transport east of Greenland navigates the southern tip of Greenland to enter the Labrador Sea in the west. The other 80 % disperses into the ambient ocean. This independent quantitative estimate based on a 37-year long record complements seasonal to annual field campaigns that studied the connection between the seas east and west of Greenland more synoptically. A temperature-salinity analysis shows that the Irminger Current east of Greenland is characterized by a compensating isopycnal exchange of temperature and salinity, while west of Greenland the horizontal convergence of less dense surface water is accompanied by downwelling/subduction.

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

<p>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.</p><p>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.</p><p>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.</p>

New Insight Into the Formation and Evolution of the East Reykjanes Ridge Current and Irminger Current

<p>The Reykjanes Ridge strongly influences the circulation of the North Atlantic Subpolar Gyre as it flows to the Irminger Sea from the Iceland Basin. The circulation is composed of two main along‐ridge currents: the southwestward East Reykjanes Ridge Current (ERRC) in the Iceland Basin and the northeastward Irminger Current (IC) in the Irminger Sea. To study their interconnection through the ridge, as well as their connections with the interior of each basin, velocity and hydrological measurements were carried out along and perpendicular to the crest of the Reykjanes Ridge in June–July 2015 as part of the Reykjanes Ridge Experiment project. This new data set changes our view of the ERRC and IC as it reveals undocumented along‐stream evolutions of their hydrological properties, structures, and transports. These evolutions are due to flows connecting the ERRC and IC branches at specific locations set by the bathymetry of the ridge and to significant connections with the interiors of the basins. Overall, the ERRC transport increases by 3.2 Sv between 63°N and 59.5°N and remains almost constantly southward. In the Irminger Sea, the increase in IC transport of 13.7 Sv between 56°N and 59.5°N, and the evolution of its properties are explained by both cross‐ridge flows and inflows from the Irminger Sea. Further north, bathymetry steers the IC northwestward into the Irminger Sea. At 63°N, the IC water masses are mostly issued from the cross-ridge flow.</p>

Seasonal and Interannual Variations of Irminger Ring Formation and Boundary–Interior Heat Exchange in FLAME

The contribution of warm-core anticyclones shed by the Irminger Current off West Greenland, known as Irminger rings, to the restratification of the upper layers of the Labrador Sea is investigated in the 1/12° Family of Linked Atlantic Models Experiment (FLAME) model. The model output, covering the 1990–2004 period, shows strong similarities to observations of the Irminger Current as well as ring observations at a mooring located offshore of the eddy formation region in 2007–09. An analysis of fluxes in the model shows that while the majority of heat exchange with the interior indeed occurs at the site of the Irminger Current instability, the contribution of the coherent Irminger rings is modest (18%). Heat is provided to the convective region mainly through noncoherent anomalies and enhanced local mixing by the rings facilitating further exchange between the boundary and interior. The time variability of the eddy kinetic energy and the boundary to interior heat flux in the model are strongly correlated to the density gradient between the dense convective region and the more buoyant boundary current. In FLAME, the density variations of the boundary current are larger than those of the convective region, thereby largely controlling changes in lateral fluxes. Synchronous long-term trends in temperature in the boundary and the interior over the 15-yr simulation suggest that the heat flux relative to the temperature of the interior is largely steady on these time scales.