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

近几十年来，东非的降雨量显著减少，最终导致非洲之角2010-2011年的极端干旱。仪器和替代数据表明，热带印度洋对东非的长期干旱趋势具有主要影响。对东非降雨特别重要的是位于印度洋中南部的所谓塞舌尔-查戈斯温跃层脊（SCTR）的变暖。SCTR具有浅层温跃层和高平均海表温度（bb0 - 28°C）的特征，允许在不同时间尺度上强烈的海洋-大气相互作用。本项目研究(1)SCTR地区的季节内和年际海洋-大气相互作用，(2)它们对全新世轨道参数变化的响应，(3)它们对东非降雨变率的影响。由于仪器数据的时间非常有限，我将使用现代和化石珊瑚的地球化学代理数据来解决这些问题。我选择以下方法：(I)每星期或每两星期取样一次珊瑚。这对于捕捉季节内事件是必要的。（II）在SCTR区域，捕获表层海洋参数（Sr/Ca和稳定氧同位素：海表温度和盐度）的代用指标与记录表层水营养成分（稳定氮同位素）的代用指标相耦合。后者与上升流和温跃层变率有关。（III）使用东非珊瑚利用Ba/Ca和骨骼发光来重建河流径流。该项目侧重于轨道时间尺度。在全新世期间，轨道参数的变化导致了季节性的变化，这种变化与今天观测到的季节周期的年际异常相当。例如，在SCTR区域，正印度洋偶极子事件导致9 - 11月变暖。模式数据表明，这与温跃层加深和富营养水上涌减少有关。在东非，降雨量高于平均水平。在全新世中期（6kyr BP）， SCTR地区的日照高峰出现在9 - 11月（6kyr BP）。这如何影响SCTR地区的上升流，以及东非的降雨响应是什么？在轨道时间尺度上观测到的过程可使我们更好地了解海洋-大气相互作用与印度洋扇区外部强迫之间的动力关系。这应该为印度洋对全球变暖的反应提供重要的约束。

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.