Understanding the variations of water cycle on orbital timescale and their response to astronomical parameters, greenhouse gases (GHG) and ice sheets is one of the focuses in paleoclimate study. Sea ice and precipitation, two important components of the water cycle, are paid much attention. A better understanding of their variations on orbital timescale, especially their response to external forcing and related processes and feedbacks, could provide insight on their long-term variations in the future. The latest research results show that, on orbital scale, the Arctic sea ice is more sensitive to insolation, while the Southern Ocean sea ice is more sensitive to GHG. Under the combined influence of insolation and GHG, the last nine interglacials all have much less summer Arctic sea ice, as compared to both the present-day and the future, due to the much higher Northern Hemisphere (NH) summer insolation. As compared to the future, the last nine interglacials all have much more annual and seasonal Southern Ocean sea ice due to their much lower CO2. In terms of the astronomical parameters, the Arctic sea ice is more influenced by precession, whereas obliquity plays a more important role in the Southern Ocean sea ice. The different responses of Arctic and Southern Ocean sea ice to astronomical parameters and CO2 are mainly due to their different geographical locations. The Arctic ocean is relatively closed and it is located in the northern highest latitudes, which make it only receives very little insolation during winter and it is mainly influenced by precession-dominated summer insolation. The summer insolation not only influences the summer Arctic sea ice, but also has an effect on the winter one through the summer remnant effect. In addition, the Arctic sea ice is also affected by the vegetation in the northern mid and high latitudes, which is mainly dominated by precession. As compared to the Arctic, the latitudes of Southern Ocean are lower and it is more open, which make it is more sensitive to the annual insolation and CO2. The relative effect of precession, obliquity and CO2 on precipitation largely depends on different regions and time periods. The tropical precipitation changes also show obvious half-precession cycles in response to the maximum equatorial insolation. In addition, astronomically-induced slow variation of insolation can trigger abrupt changes of the Atlantic meridional circulation (AMOC) through sea ice-ocean interactions in the Labrador and Nordic Seas, consequently leading to abrupt oscillations in large-scale temperature and precipitation. In East Asia, the summer precipitation in the northern part is more influenced by insolation, with a dominant precession signal. Precession-dominated insolation can influence the atmospheric circulation through its effect on the land-sea thermal contrast, which finally affects the summer monsoon precipitation. However, the summer precipitation in the southern region is more influenced by ice sheets, by their control on the meridional movement of the Inter-tropical Convergence Zone (ITCZ). In addition, the Eurasian ice sheet can further influence the precipitation in the southern part through a south-eastwards perturbation planetary wave. Moreover, the effects of insolation and ice sheets on the summer monsoon precipitation show strong regional and nonlinear characteristics, which depend on the extent, height and location of the ice sheets and the intensity of insolation. |
