Tropical Indian Ocean and East African seasonality during the Holocene: orbital forcing and ocean atmosphere interactions. (IndoOrb)

近几十年来，东非降雨量大幅减少，在2010-2011年非洲之角的极端干旱中达到顶峰。仪器和代理数据表明，热带印度洋对东非长期干旱趋势有主要影响。对东非降雨特别重要的是位于印度洋中南部的所谓的塞舌尔-查戈斯温跃层脊变暖。SCTR以浅温跃层和高平均海面温度(&gt；28°C)为特点，允许在不同时间尺度上发生强烈的海-气相互作用。该项目调查(I)南海区域的季节内和年际海洋-大气相互作用，(Ii)它们对全新世期间轨道参数变化的响应，(Iii)它们对东非降雨量变化的影响。由于仪器数据的时间非常有限，我将使用现代珊瑚和化石珊瑚的地球化学代理数据来解决这些问题。我选择了以下方法：(I)珊瑚将以每周至每两周一次的分辨率进行采样。这是捕捉季节内事件所必需的。(2)在南太平洋区域，记录表层海洋参数(锶/钙和稳定氧同位素：海表面温度和盐度)的指标与记录表层水营养物质含量(稳定氮同位素)的指标相结合。后者与上升流和温跃层变化有关。(Iii)使用来自东非的珊瑚，利用钙/钙比和骨骼发光法重建河流径流。该项目的重点是轨道时间尺度。在全新世期间，轨道参数的变化导致了季节性的变化，可与今天观察到的季节周期的年际异常相媲美。例如，在南海区域，正的印度洋偶极子事件在9-11月造成气候变暖。模型数据表明，这与温跃层加深和营养丰富的水的上升流减少有关。在东非，降雨量高于平均水平。在全新世中期(6kyr BP)，SCTR区的日照强度在9-11月达到峰值(6kyr BP)。这对南海地区的上升流有何影响？东非地区的降雨反应如何？在轨道时间尺度上观察到的过程可能有助于更好地了解印度洋区域海洋-大气相互作用和外部强迫之间的动力关系。这应该会对印度洋应对全球变暖提供重要的制约。

East African rainfall has declined significantly over recent decades, culminating in the extreme drought of 2010-2011 at the Horn of Africa. Instrumental and proxy data suggest that the tropical Indian Ocean has a primary influence on the long-term drying trend of East Africa. Of particular importance for East African rainfall is the warming of the so-called Seychelles-Chagos thermocline ridge (SCTR) located in the south-central Indian Ocean. The SCTR features a shallow thermocline and high mean sea surface temperatures (>28°C), allowing strong ocean-atmosphere interactions on various time scales. This project investigates (I) intraseasonal and interannual ocean-atmosphere interactions in the SCTR region, (II) their response to changes in orbital parameters during the Holocene and (III) their impact on rainfall variability in East Africa. As the instrumental data is very limited in time, I will use geochemical proxy data from modern and fossil corals to address these problems. I have chosen the following approach: (I) the corals will be sampled at weekly to bi-weekly resolution. This is necessary to capture intraseasonal events. (II) In the SCTR region, proxies that capture surface ocean parameters (Sr/Ca and stable oxygen isotopes: sea-surface-temperature and salinity) are coupled with proxies that record the nutrient content of surface waters (stable nitrogen isotopes). The latter is related to upwelling and thermocline variability. (III) Corals from East Africa are used to reconstruct river runoff using Ba/Ca and skeletal luminescence. The project focuses on orbital time scales. During the Holocene, changes in orbital parameters led to changes in seasonality that are comparable to interannual anomalies of the seasonal cycle observed today. For example, in the SCTR region positive Indian Ocean Dipole events cause a warming in September-November. Model data suggest that this is coupled with a deepening of the thermocline and reduced upwelling of nutrient-rich waters. In East Africa, rainfall is above average. During the mid-Holocene (6kyr BP), insolation in the SCTR region peaks in September-November (6 kyr BP). How does this affect upwelling in the SCTR region, and what is the rainfall response in East Africa? The processes observed on orbital time scales may allow a better understanding of the dynamical relationship between ocean-atmosphere interactions and external forcing in the Indian Ocean sector. This should provide important constraints on the response of the Indian Ocean to global warming.