A sub-fossil coral Sr/Ca record documents meridional variability of the Intertropical Convergence Zone in the eastern Indian Ocean

Abstract. Sea surface temperature (SST) variability in the south-eastern tropical Indian Ocean is crucial for rainfall variability in Indian Ocean rim countries. A large body of literature has focused on zonal variability associated with the Indian Ocean Dipole (IOD), but it is unclear whether meridional shifts in the position of the Intertropical Convergence Zone (ITCZ), which at present co-vary with the IOD, may also occur independently. We have developed a new, monthly resolved Sr/Ca record from a sub-fossil coral cored at Enggano Island (Indonesia, 5° S, 102° E). Core sections containing diagenetic phases are omitted from the SST reconstruction. U/Th dating shows that the Sr/Ca-based SST record extends from 1917–1868 and from 1861–1823 with a relative age uncertainty of ±2.4 years (2σ). At Enggano Island, coastal upwelling and cooling in austral spring is coupled to the position of the ITCZ, and impacts SST seasonality. The sub-fossil coral indicates an increase in SST seasonality due to enhanced austral spring cooling between 1917 and 1855, which we attribute to stronger SE winds and a northward shift in the position of the ITCZ in austral spring. A nearby sediment core indicates SST cooling and a shallowing of the thermocline prior to ~1930. These results are consistent with an increase in the North-South SST gradient in the eastern Indian Ocean calculated from historical temperature data, that is not seen in the zonal SST gradient. We conclude that the relationship between meridional and zonal variability in the eastern Indian Ocean is non-stationary and influenced by long-term temperature trends.