Unraveling the coupled ocean-atmosphere system in the Indian Ocean using paired δ18O and Sr/Ca records from a coral core originating from Cocos (Keeling) Island

Abstract

This thesis researches the coupling of the ocean and atmosphere systems in the eastern Indian Ocean at the key location of Cocos (Keeling) Island, by studying the link between SST’s, atmospheric convection, precipitation patterns, and cyclones. Which known ocean-atmosphere systems influence the climate around Cocos (Keeling) island and which other climate forcing factors are active in this region? The study uses δ18O and Sr/Ca isotopes from a continuous coral core (Darwin Long) to reconstruct past SST and δ18OSW for the time period 1871 to 2009 AD. Spectral analysis is used to research periodicities of dominant cycles. The data shows that climate variability at Cocos (Keeling) island is mainly influenced by annual seasonal variability. Also the eleven year sunspot cycle is thought to be of major influence at this site. The coupled ocean-atmosphere systems ENSO and IOD seem to be of minor influence in this region, but are possibly expressed during short events. There might also be a PDO signal present in the data records, but this should be studied in further research. Furthermore, there are four very distinct events present in our δ18O and Sr/Ca records indicating SST cooling of 2-4°C, occurring around the years 1883, 1910-1915, 1946 and 1965-1970 AD. The cause of these events are different, but mainly related to minima in solar activity and possible ENSO and/or IOD influences. The 1883 event could be caused by the volcanic eruption of the Krakatoa. Finally, SST’s gradually increase from around 1975 to around 2000 AD. However, a similar shift in influences of different ocean-atmosphere systems found around the Chagos Archipelago due to crossing of a SST threshold value, is not recognized in the Cocos (Keeling) region.