Generation of sub-surface anticyclones at Arctic surface fronts due to a surface stress

• Published in: Journal of Physical Oceanography
• Main Authors: Brannigan, Liam, Johnson, Helen, Lique, Camille, Nycander, Jonas, Nilsson, Johan
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Meteorological Society 2017
• Subjects: Arctic; Arctic Ocean
• Online Access: https://archimer.ifremer.fr/doc/00396/50763/51476.pdf; https://doi.org/10.1175/JPO-D-17-0022.1; https://archimer.ifremer.fr/doc/00396/50763/

Isolated anticyclones are frequently observed below the mixed layer in the Arctic Ocean. Some of these sub-surface anticyclones are thought to originate at surface fronts. However, previous idealized simulations with no surface stress show that only cyclone-anticyclone dipoles can propagate away from baroclinically unstable surface fronts. Numerical simulations of fronts subject to a surface stress presented here show that a surface stress in the same direction as the geostrophic flow inhibits dipole propagation away from the front. On the other hand, a surface stress in the opposite direction to the geostrophic flow helps dipoles to propagate away from the front. Regardless of the surface stress at the point of dipole formation, these dipoles can be broken up on a timescale of days when a surface stress is applied in the right direction. The dipole breakup leads to the deeper anticyclonic component becoming an isolated sub-surface eddy. The breakup of the dipole occurs because the cyclonic component of the dipole in the mixed layer is subject to an additional advection due to the Ekman flow. When the Ekman transport has a component oriented from the anticyclonic part of the dipole towards the cyclonic part then the cyclone is advected away from the anticyclone and the dipole is broken up. When the Ekman transport is in other directions relative to the dipole axis it also leads to deviations in the trajectory of the dipole. A scaling is presented for the rate at which the surface cyclone is advected that holds across a range of mixed layer depths and surface stress magnitudes in these simulations. The results may be relevant to other regions of the ocean with similar near-surface stratification profiles.