The Arctic Front and its variability in the Norwegian Sea

Roshin P Raj,Laurent Bertino,Sourav Chatterjee,Marcos Portabella,Antonio Turiel

doi:10.5194/os-15-1729-2019

Abstract

Abstract. The Arctic Front (AF) in the Norwegian Sea is an important biologically productive region which is well-known for its large feeding schools of pelagic fish. A suite of satellite data, a regional coupled ocean–sea ice data assimilation system (the TOPAZ reanalysis) and atmospheric reanalysis data are used to investigate the variability in the lateral and vertical structure of the AF. A method, known as “singularity analysis”, is applied on the satellite and reanalysis data for 2-D spatial analysis of the front, whereas for the vertical structure, a horizontal gradient method is used. We present new evidence of active air–sea interaction along the AF due to enhanced momentum mixing near the frontal region. The frontal structure of the AF is found to be most distinct near the Faroe Current in the south-west Norwegian Sea and along the Mohn Ridge. Coincidentally, these are the two locations along the AF where the air–sea interactions are most intense. This study investigates in particular the frontal structure and its variability along the Mohn Ridge. The seasonal variability in the strength of the AF is found to be limited to the surface. The study also provides new insights into the influence of the three dominant modes of the Norwegian Sea atmospheric circulation on the AF along the Mohn Ridge. The analyses show a weakened AF during the negative phase of the North Atlantic Oscillation (NAO−), even though the geographical location of the front does not vary. The weakening of AF during NAO− is attributed to the variability in the strength of the Norwegian Atlantic Front Current over the Mohn Ridge associated with the changes in the wind field.

Highlights

Ocean fronts are boundaries between distinct water masses with large gradients in temperature or salinity (e.g. Bakun, 1996; D’Asaro et al, 2011)
The comparatively large temperature gradient associated with the Arctic Front (AF) at the Mohn Ridge (Piechura and Walczowski, 1995) can be identified
This cold tongue is caused by mixing of the water mass carried by the North Icelandic Irminger Current into the surrounding waters, mostly in the East Icelandic Current, which flows into the south-west Norwegian Sea, where it forms a cold tongue of Arctic character (Helland-Hansen and Nansen, 1909; Blindheim and Malmberg, 2005)

Summary

Introduction

Ocean fronts are boundaries between distinct water masses with large gradients in temperature or salinity (e.g. Bakun, 1996; D’Asaro et al, 2011). The Atlantic Water is carried into the Norwegian Sea via the Norwegian Atlantic Current (e.g. Orvik et al, 2001; Raj et al, 2016), which is a two-branch current system, with an eastern branch following the shelf edge as a barotropic slope current, and a western branch following the western rim of the Norwegian Sea as a topographically guided front current (Poulain et al, 1996; Orvik and Niiler, 2002; Skagseth and Orvik, 2002; Orvik and Skagseth, 2003) These two branches are known as the Norwegian Atlantic Slope Current (NwASC; Skagseth and Orvik, 2002) and the Norwegian Atlantic Front Current (NwAFC; Mork and Skagseth, 2010) respectively.

Methods

Results

Conclusion