Sensitivity of sea -ice cover and ocean properties to wind -stress and radiative forcings from 1500 to 2000

In this thesis we investigate the sensitivity of the Arctic and Antarctic sea-ice cover and global ocean properties to wind-stress and radiative forcings from 1500 to 2000. In a first step, the conversion of the granular sea-ice model (GRAN) (Tremblay and Mysak, 1997) from Cartesian to spherical coordinates is presented. The GRAN is coupled to the Earth System Climate Model of the University of Victoria as an application. The sea-ice thickness and drift in the Arctic Ocean agree well with observations. The sea-ice volume and area fluxes through Fram Strait show good correlations with observations, although the simulated area flux has a smaller mean value as compared to the observations. To further validate the model, the results of thermodynamic component are compared with the Surface Heat Budget of the Arctic Ocean (SHEBA) datasets which were collected between autumn 1997 and autumn 1998. The simulated growth rate is larger and the melt rate is smaller than observed. The larger growth rate is caused by thinner ice at the beginning of the SHEBA period and the absence of internal heat storage, while the lower summer melt is due to smaller-than-observed surface melt. The simulated surface and bottom melt contribute equal parts to the overall melt.;In a second step, the model is used to investigate which forcings had a dominant effect on the sea-ice cover in both polar regions during the Little Ice Age (LIA), defined as the period between 1500 and 1850, and the industrial period (1850--2000). Three different reconstructed wind-stress fields which take into account the North Atlantic Oscillation, one general circulation model wind-stress field, and three different radiative forcings are used (i.e., volcanic activity, insolation changes, greenhouse gas changes). The annual surface air temperature anomalies for the Northern Hemisphere, which are used as model validation, show good agreement with reconstructed temperature anomalies, i.e., cooling during the LIA and warming afterwards. The simulated sea-ice area and volume in the Northern Hemisphere were larger during the LIA as compared to the present. The comparison between wind-driven and radiatively-driven changes shows that both forcings result in equal magnitude changes in the case of ice volume; for ice area, the wind-driven part is twice as large as the radiatively-driven part. The simulations suggest that the main radiative forcing before 1850 was volcanic forcing, whereas after 1850 the greenhouse gas changes dominated the forcing field. Additionally, an increased ice volume export from the Arctic to the North Atlantic has no significant effect on the maximum strength of the Atlantic meridional overturning circulation. In the Southern Hemisphere, no long-term trends are visible in the simulated sea-ice area and volume. The wind-driven changes are about four times larger than the radiatively-driven changes.;The above mentioned work is extended to several ocean properties. The ocean heat content changes in the upper 300 m from the tropical to mid-latitudes are mainly driven by the changes in radiative forcing. In the high-latitudes the changes in heat content are wind-driven. In the full ocean (0--3000 m) the wind-stress forcing reduces the radiatively-driven downward trend prior to 1700. After 1700 no wind-driven effect is visible in the simulations. The ocean temperature changes from the LIA to the industrial era show a radiative cooling in the upper 600 m and a dynamical downward transport of cool water to lower depths during the LIA. Changes in salinity are mainly located in the northern high-latitudes. In the surface layers the water was saltier in the Arctic Ocean and fresher in the North Atlantic during the LIA due to increased sea-ice formation and subsequent transport to the south and melting during this period. In the subsurface layers of the Arctic Ocean there was a stronger inflow of saline water during the LIA. The simulated density changes are a composite picture of the temperature and salinity changes. Further, the wind stress is the main forcing for circulation changes between the LIA and the industrial era. The added buoyancy-driven circulation changes due to changes in the radiative forcing are small. During the LIA the maximum strength of the meridional overturning circulation in the North Atlantic was reduced as compared to the industrial era. In the Southern Hemisphere, on the other hand, the ventilation rate was increased during the LIA.