Atmospheric wind biases: A challenge for simulating the Arctic Ocean in
coupled models?
Abstract
Many state-of-the-art climate models do not simulate the Atlantic Water
(AW) layer in the Arctic Ocean realistically enough to address the
question of future Arctic Atlantification and its associated feedback.
Biases concerning the AW layer are commonly related to insufficient
resolution and exaggerated mixing in the ocean component as well as
unrealistic Atlantic-Arctic Ocean exchange. Based on sensitivity
experiments with FESOM1.4, the ocean-sea ice component of the global
climate model AWI-CM1, we show that even if all impediments for
simulating AW realistically are addressed in the ocean model, new biases
in the AW layer develop after coupling to an atmosphere model. By
replacing the wind forcing over the Arctic with winds from a coupled
simulation we show that a common bias in the atmospheric sea level
pressure (SLP) gradient and its associated wind bias lead to differences
in surface stress and Ekman transport. Fresh surface water gets
redistributed leading to changes in steric height distribution. Those
changes lead to a strengthening of the anticyclonic surface circulation
in the Canadian Basin, so that the deep counterflow carrying warm AW
gets reversed and a warm bias in the Canadian Basin develops. An
underestimation of sea ice concentration can significantly amplify the
induced ocean biases. The SLP and anticyclonic wind bias in the Nordic
Seas weaken the cyclonic circulation leading to reduced AW transport
into the Arctic Ocean through Fram Strait but increased AW transport
through the Barents Sea Opening. These effects together lead to a cold
bias in the Eurasian Basin.