Verschuren Dirk, Sinninghe Damsté Jaap S, Moernaut Jasper, Kristen Iris, Blaauw Maarten, Fagot Maureen, Haug Gerald H
Limnology Unit, Department of Biology, Ghent University, Ledeganckstraat 35, 9000 Gent, Belgium.
Nature. 2009 Dec 3;462(7273):637-41. doi: 10.1038/nature08520.
External climate forcings-such as long-term changes in solar insolation-generate different climate responses in tropical and high latitude regions. Documenting the spatial and temporal variability of past climates is therefore critical for understanding how such forcings are translated into regional climate variability. In contrast to the data-rich middle and high latitudes, high-quality climate-proxy records from equatorial regions are relatively few, especially from regions experiencing the bimodal seasonal rainfall distribution associated with twice-annual passage of the Intertropical Convergence Zone. Here we present a continuous and well-resolved climate-proxy record of hydrological variability during the past 25,000 years from equatorial East Africa. Our results, based on complementary evidence from seismic-reflection stratigraphy and organic biomarker molecules in the sediment record of Lake Challa near Mount Kilimanjaro, reveal that monsoon rainfall in this region varied at half-precessional ( approximately 11,500-year) intervals in phase with orbitally controlled insolation forcing. The southeasterly and northeasterly monsoons that advect moisture from the western Indian Ocean were strengthened in alternation when the inter-hemispheric insolation gradient was at a maximum; dry conditions prevailed when neither monsoon was intensified and modest local March or September insolation weakened the rain season that followed. On sub-millennial timescales, the temporal pattern of hydrological change on the East African Equator bears clear high-northern-latitude signatures, but on the orbital timescale it mainly responded to low-latitude insolation forcing. Predominance of low-latitude climate processes in this monsoon region can be attributed to the low-latitude position of its continental regions of surface air flow convergence, and its relative isolation from the Atlantic Ocean, where prominent meridional overturning circulation more tightly couples low-latitude climate regimes to high-latitude boundary conditions.
外部气候强迫因素，如太阳辐射的长期变化，在热带和高纬度地区会产生不同的气候响应。因此，记录过去气候的时空变化对于理解这些强迫因素如何转化为区域气候变异性至关重要。与数据丰富的中高纬度地区不同，来自赤道地区的高质量气候代用记录相对较少，特别是来自经历与热带辐合带每年两次过境相关的双峰季节性降雨分布的地区。在此，我们展示了来自赤道东非过去25000年期间水文变化的连续且分辨率良好的气候代用记录。我们的结果基于来自乞力马扎罗山附近查拉湖沉积物记录中的地震反射地层学和有机生物标志物分子的补充证据，揭示该地区的季风降雨以半岁差（约11500年）的间隔变化，与轨道控制的太阳辐射强迫同步。当半球间太阳辐射梯度最大时，从印度洋西部平流输送水汽的东南季风和东北季风交替增强；当两个季风都不增强且当地3月或9月适度的太阳辐射削弱随后的雨季时，则盛行干旱条件。在亚千年时间尺度上，东非赤道地区水文变化的时间模式具有明显的高北纬特征，但在轨道时间尺度上，它主要响应低纬度太阳辐射强迫。这个季风区域低纬度气候过程占主导地位可归因于其地表气流汇聚大陆区域的低纬度位置，以及它与大西洋相对隔离，在大西洋，显著的经向翻转环流将低纬度气候系统与高纬度边界条件更紧密地联系在一起。
