Land-Ocean Asymmetry of Tropical Precipitation Changes in the Mid-Holocene

A series of model experiments were conducted using an intermediate ocean atmosphere land model for a better understanding of a distinct land sea asymmetry in tropical precipitation differences between the mid-Holocene and preindustrial climates. In austral (boreal) summer, most reduced (enhanced) precipitation occurs over continental convective regions, while most enhanced (reduced) precipitation occurs over oceanic convection zones. This land sea asymmetry of tropical precipitation is particularly clear in austral summer. During the mid-Holocene, the solar forcing presents both spatial and seasonal asymmetric patterns. While the boreal summer insolation is stronger at high latitudes of the Northern Hemisphere in the mid-Holocene than at present, the austral summer insolation is weaker with a more spatially symmetric distribution about the equator. Because of the slow response time of the ocean to forcing, the direct insolation forcing of the current season is cancelled by the ocean memory of an earlier insolation forcing, which in the case of the mid-Holocene is opposite to the current season insolation forcing. As a result, tropical sea surface temperature variation, as well as the tropical atmospheric temperature and moisture changes, is small, which gives rise to a different precipitation response from under the condition of stronger atmospheric temperature and moisture changes, such as in the case of postindustrial global warming induced by an increased concentration of atmospheric greenhouse gases. Thus, the cancellation between the direct and memory effects of the seasonally asymmetric insolation forcing leaves the net energy into the atmosphere to be responsible for the land sea asymmetry of tropical precipitation changes.