Climatic response to Milankovitch forcing as simulated by a zonally averaged ocean-atmosphere-sea ice model

A reduced complexity ocean-atmosphere-sea ice model is developed to study the transient response of climate to Milankovitch forcing. The ocean component is a zonally averaged model of the meridional overturning circulation in different basins. The atmospheric component is a one-dimensional (latitudinal) energy-moisture balance model. A thermodynamic sea-ice model is used. The following numerical experiments are performed with this model.Second, one more time-dependent solution of the model is obtained for the last 1 Myr. For this solution, the model version without the Arctic Ocean and sea ice is extended to include an active hydrological cycle. When a changing hydrological cycle is included it is found that (i) the response of annual-mean surface air and ocean temperatures does not significantly change, and (ii) annual-mean hydrological variables (e.g., evaporation rate) mainly oscillate near the obliquity frequency.The long-term deep ocean temperature changes found in our model solutions are qualitatively consistent with similar variability observed in benthic foraminiferal delta18O records. The simulated hydrological changes are consistent with a mechanism previously postulated to explain the imprint of obliquity changes in deuterium excess records from polar ice sheet cores.First, two time-dependent solutions of the model are obtained for the time interval 5-3 Myr BP. One solution does not include sea ice and the Arctic Ocean the other solution does include these two features. A fixed hydrological cycle is used in both solutions. The main results from these experiments are the following: (i) changes in annual-mean surface air temperature mainly follow the obliquity-driven changes in annual-mean insolation, (ii) the partition of annual-mean ocean temperature variability among different Milankovitch frequencies significantly varies with latitude and depth, (iii) sea ice acts as a time-integrator of summer insolation changes so that the area and volume of sea-ice cover mainly oscillate at the obliquity frequency, and (iv) austral sea ice diminishes the temperature variability of deep water of southern origin.