Arctic sea ice response to wind stress variations

[1] Timescale of sea ice response to wind stress variations and the mechanisms controlling the timescale are investigated by a coupled sea ice - ocean model. Wind stress variations account for a significant part of the changes of sea ice volume in the Arctic Ocean over the last several decades. The changes of sea ice volume associated with wind stress are mainly induced by changes of sea ice outflow from the Arctic Ocean. While outflow immediately responds to wind stress variations, responses of sea ice volume lag behind the changes of the outflow by several years. For example, when the model is driven by interannually varying wind stress with the other atmospheric forcing components given by climatology, sea ice outflow abruptly decreases in 1996 and remains almost constant for the following several years. However, the responding increase of sea ice volume is gradual and continues for several years. In order to clarify the timescale of sea ice response to an abrupt change of wind stress and the mechanisms controlling the timescale, the model is forced by two typical wind stress fields, which cause small and large outflow. Equilibrium annual mean ice thickness averaged over the Arctic Ocean is different by about 50 cm between these two wind-forcing fields. Starting from the equilibrium state obtained under one of these two fields, wind-forcing is switched to the other. Adjustment time of ice thickness is then defined as the year when annual mean ice thickness is adjusted to new equilibrium by 90% of the difference between the two equilibria. The timescale of the sea ice response and the mechanisms controlling the timescale are found to be different depending on whether the outflow increases or decreases. When the outflow increases, the timescale depends on the advection time of the sea ice flowing toward the exits. When the outflow decreases, the timescale depends on the thermodynamic growth rate of sea ice. The change in the late 1990s corresponds to the latter case.