Increased Oceanic Heat Transport in the Main Atlantic Inflow Branch to the Nordic Seas 1993-2013

The northernmost upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is the northward flow of warm and saline water from the Atlantic Ocean, across the Greenland-Scotland Ridge, into the Nordic Seas. This flow, termed the Atlantic inflow, is split into three separate branches where the flow between Iceland and Faroes (IF-inflow), focusing in the Faroe Current north of the Faroes, is the most intensive of these branches. Since the late 1980s, the hydrographic properties of the IF-inflow have been monitored on regular CTD cruises along a section crossing the Faroe Current north of the Faroes and since the mid-1990s, several ADCPs have been moored along the section. From these in situ observations, time series of volume and heat transport have previously been reported, but the high variability of the heat transport has made identification of trends difficult. In a new analysis, satellite altimetry data have been combined with the in situ data resulting in a new longer time series covering the whole altimeter period from 1993 to 2013. The new time series show no indication of reduced volume transport and show a clear positive trend in heat transport. The overall increase over the 2 decades of altimetry observation was 9±8% for volume transport and 18±9% for heat transport (95% confidence intervals). During the same period, the salt transport relative to the salinity of the deep Faroe Bank Channel overflow (34.93) more than doubled, potentially strengthening the feedback on AMOC. The increased heat and salt transports are partly caused by the increased volume transport and partly by increased temperatures and salinities of the Atlantic inflow, which have been claimed mainly to be caused by the weakened subpolar gyre.