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

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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.