27 Apr 2023
 | 27 Apr 2023
Status: this preprint is open for discussion.

The Iceland-Faroe warm-water flow towards the Arctic estimated from satellite altimetry and in situ observations

Bogi Hansen, Karin Margretha Húsgarð Larsen, Hjálmar Hátún, Steffen Malskær Olsen, Andrea Martina Ulrike Gierisch, Svein Østerhus, and Sólveig Rósa Ólafsdóttir

Abstract. The inflow of warm and saline Atlantic water to the Arctic Mediterranean (Nordic Seas and Arctic Ocean) between Iceland and the Faroes (IF-inflow) is the strongest Atlantic inflow branch, in terms of volume transport, and associated with a large transport of heat towards the Arctic. The IF-inflow is monitored on a section east of the Iceland-Faroe Ridge (IFR) by use of Sea Level Anomaly (SLA) data from satellite altimetry, a method that has been calibrated by in situ observations gathered over two decades. Monthly averaged surface velocity anomalies calculated from SLA data were strongly correlated with anomalies measured by moored Acoustic Doppler Current Profilers (ADCPs) with consistently higher correlations when using the reprocessed SLA data released in December 2021 rather than the earlier version. In contrast to the earlier version, the reprocessed data also had the correct conversion factor required by geostrophy. Our results show that the IF-inflow crosses the IFR in two separate branches. The Icelandic branch is a jet over the Icelandic slope with average surface speed exceeding 20 cm s−1, but it is narrow and shallow with an average volume transport less than 1 Sv (106 m3 s−1). Most of the Atlantic water crosses the IFR close to its southernmost end in the Faroese branch. Between these two branches, water from the Icelandic branch turns back onto the ridge in a retroflection with a recirculation over the northernmost bank on the IFR. Combining multi-sensor in situ observations with satellite SLA data, monthly mean volume transport of the IF-inflow has been determined from January 1993 to December 2021. The IF-inflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which is expected to weaken under continued global warming. Our results show no weakening of the IF-inflow. Annually averaged volume transport of Atlantic water through the monitoring section had a statistically significant (95 % confidence level) increasing trend of (0.12 ± 0.10) Sv per decade. Combined with increasing temperature, this caused an increase of 13 % in the heat transport, relative to 0 °C, towards the Arctic of the IF-inflow over the 29 years of monitoring. The near-bottom layer over most of the IFR is dominated by cold water of Arctic origin that may contribute to the overflow across the ridge. Our observations confirm a dynamic link between the overflow and the Atlantic water flow above. The results also provide support for a previously posed hypothesis that this link may explain the difficulties in reproducing observed transport variations of the IF-inflow in numerical ocean models, with consequences for its predictability under climate change.

Bogi Hansen et al.

Status: open (until 22 Jun 2023)

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Bogi Hansen et al.

Bogi Hansen et al.


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Short summary
Based on in situ observations combined with Sea Level Anomaly (SLA) data from satellite altimetry, volume as well as heat (relative to 0 °C) transport of the Iceland-Faroe warm-water inflow towards the Arctic (IF-inflow) increased from 1993 to 2021. The reprocessed SLA data released in December 2021 represent observed variations accurately. The IF-inflow crosses the Iceland-Faroe Ridge in two branches with retroflection in between. The associated coupling to overflow reduces predictability.