COUPLED MODELING OF ATMOSPHERE AND OCEAN IN THE HOLOCENE

The evolution of Holocene global atmosphere-ocean system is simulated in two coupled models (the FOAM and the NCAR CSM) under the insolation forcing at 3ka, 6ka, 8ka and 11ka. The focus is the changes of the global monsoon and the upper ocean.

In the atmosphere, the evolution of six major summer monsoons are investigated: the Asian monsoon, the North African monsoon, the North American monsoon, the Australian monsoon, the South American monsoon and the South African monsoon. Special attention has been paid to the relative roles of the direct insolation forcing and oceanic feedback. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of atmospheric and oceanic circulation and ocean-atmosphere interaction. In the northern hemisphere, the coupled models show a significant enhancement of all the monsoons in the early Holocene and a gradual weakening towards the present, with the North Africa monsoon showing the largest relative changes. The monsoons are enhanced in the Holocene by a positive oceanic feedback in North Africa and North America, but are suppressed by a negative overall feedback in Asia. In the southern hemisphere, monsoons are reduced most significantly in South America, and modestly in South Africa, mainly due to direct insolation forcing. In contrast, the Australia monsoon is enhanced by an overwhelming positive oceanic feedback.

In the ocean, we focused on SST and upper ocean thermocline structure. The annual mean SST changes in the early to mid-Holocene are found to be forced mainly by the annual mean insolation forcing with an overall symmetric response of colder equator (< 0.5oC)/warmer high latitudes (< 0.4oC in the Southern Ocean and > 1oC in the Arctic). In contrast, the temperature response in the thermocline is dominated by an anti-symmetric pattern with a cooling (warming) in the Northern (Southern) Hemisphere mid-latitudes. The thermocline response is determined predominantly by surface water subduction, and, ultimately, the insolation forcing in local late winter.

The simulated evolution of monsoons during the Holocene shows a general agreement with paleoclimate observations. The simulated SST change seems to be also consistent with a synthesis of mid-Holocene paleo-SST records.