中全新世以来东亚夏季气温对轨道强迫时空响应不一致的模拟研究

基于一个海-气耦合模式FOAM（the Fast Ocean Atmosphere Model）在轨道强迫下对过去6 ka气候变化的瞬变模拟结果,本文分析了中全新世以来东亚地区夏季气温对日射变化的响应特征.研究发现,东亚地区夏季气温对日射响应具有时空不一致性：相对于现代,6 kaB.P.时北半球夏季日射偏强,东亚地区地面气温却未普遍偏高,而是约以35°N为界,北方显著偏暖,南方气温变化不明显甚至有微弱冷却.自6 ka B.P.至今,东亚40°N以北的中、高纬陆地夏季气温大致呈线性降低趋势,以南的低纬陆地夏季气温则呈量级较小的“U”型变化,即气温在约3 kaB.P.附近达最低值,前3 ka为降温趋势,后3 ka为升温趋势.这与一些地质记录反映的气温变化相一致.中全新世以来东亚夏季气温演变的时空不一致性,可能源自因海陆热力惯性不同所引起的气温对日射响应的差异.热容量较小的东亚高纬大陆夏季气温主要响应7月份日射;而热容量较大的海洋对日射的响应通常会滞后约2个月,其夏季气温主要响应5月份日射.受海洋影响,南方陆地夏季气温对日射响应呈现出与海洋相似的特点.在岁差周期上,5、6、7月份日射间的相位差相对于较长轨道时间尺度较不明显,但在相对较短的近6千年时期内,它们相继出现波谷而呈显著趋势差异,从而导致了中全新世以来东亚夏季气温变化的时空差异.正如有学者所指出的,夏季气温变化对应的可能并非同季节日射强迫,考察轨道强迫的气候响应时,如何选择日射标尺至关重要,否则可能混淆“因果”.

Simulation of spatial-temporal asynchronism of East Asian summer’s surface-air temperature response to orbital forcing since the Mid-Holocene

By using a fast ocean-atmosphere coupled model (FOAM) and its transient simulation of the past 6000 years driven by orbital forcing, we have studied the character of the East Asian summer surface-air temperature's response to orbital forcing. It is found that spatial-temporal response of East Asian summer surface-air temperature to solar radiation is significantly inconsistent since the Mid-Holocene. Compared with the present, the Mid-Holocene had enhanced summer isolation in the Northern Hemisphere. The surface-air temperature in East Asia, however, had not increased at large. Taking the 35°N as the boundary line, the north was significantly warmer in the Mid-Holocene than in the present while the south had no obvious variation-it had even slight cooling in some areas. Since the Mid-Holocene, the surface-air temperature variation in East Asia has exhibited a linear-reducing trend in the high and middle latitude regions north of the 40°N and a small-scale "U" shape trend in low latitude regions south of the 40°N, where showed a cooling trend before 3kaBP but warming thereafter. This finding is in agreement with the geologic records of the surface-air temperature variation. The asynchronous characteristic of East Asian summer surface-air temperature may result from its different isolation responses due to different thermal inertia between ocean and land. The surface-air temperature over high-latitude land responds to the July's isolation due to its small thermal capacity, while the surface-air temperature over the ocean responds to the May's isolation, lagging behind two months or so due to its large thermal capacity. Affected by ocean climate, the summer surface-air temperature over the south land of East Asia shows a similar feature to that over the ocean. In the precession cycle, the variation trends between May's, June's and July's isolations do not seem to have obvious differences on long time scales, instead they display a notable distinction on a relatively short time scale such as the past 6000 years when the wave valleys appeared one after another, which leads to spatial-temporal asynchronism of the East Asian summer surface-air temperature response to orbital forcing since the Mid-Holocene. The results show that the summer air temperature may not agree with the same season isolation forcing, so it is very important to choose the ruler of isolation when the climate response to orbital forcing is discussed.