Woodborne Stephan, Hall Grant, Robertson Iain, Patrut Adrian, Rouault Mathieu, Loader Neil J, Hofmeyr Michele
iThemba LABS, Private Bag 11, Wits 2050, South Africa; Mammal Research Institute, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa.
Mammal Research Institute, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa.
PLoS One. 2015 May 13;10(5):e0124202. doi: 10.1371/journal.pone.0124202. eCollection 2015.
A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab (Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.
基于对四棵猴面包树（猴面包树属）进行碳同位素分析得出的南非东北部代理降雨记录，显示了过去1000年里百年和十年尺度的变化。该记录与来自津巴布韦的200年树轮记录高度吻合，并且表明南非夏季和冬季降雨区之间存在降雨偶极子现象。最湿润时期约在中世纪暖期的公元1075年，而最干旱时期约在小冰期的公元1635年、公元1695年和公元1805年。十年尺度的变化表明，降雨强迫机制是近端和远端因素之间的复杂相互作用。降雨较多的时期与厄加勒斯海流核心区域较低的海表温度以及印度洋负偶极矩指数显著相关。降雨与厄尔尼诺/南方涛动指数之间的相关性并非一成不变。在记录的大部分时间里，更湿润的条件主要与厄尔尼诺现象相关，但自公元1970年左右以来，这种关系发生了反转，目前湿润条件与拉尼娜现象相关。近端和远端海洋影响都不足以解释中世纪暖期和小冰期之间的降雨模式转变，证据表明这是副热带西风带向北移动而非热带辐合带向南移动的结果。
