A Christmas Coral: Past and Present Variability of Tropical Hydroclimate

Abstract

Changes in global temperature and precipitation are quickly becoming part of the everyday vernacular. Future climate trends, variability and extremes are likely to increasingly influence global supply chains, agricultural productivity, water security, health and well-being. This includes densely populated nations in the south-east Indian Ocean region, with shifts in the location, width and intensity of the Tropical Convergence Zone (TCZ) becoming particularly important as the region is heavily reliant the regular arrival of the monsoonal systems.

Since the 1970's, when reliable satellite monitoring became available, observational records of global atmospheric circulation have detected a global poleward shift in the TCZ, and an associated expansion of the Hadley circulation. The rate of expansion has been determined to be zonally dependant. Knowledge of observational shifts, and variability prior to the instrumental period are particularly poor across the southeast tropical Indian Ocean with many results only based on the last 40-50 years and focused on global zonal means.

Understanding the causes of hydroclimate variability across the southeast tropical Indian Ocean on these time-scales, is challenging due to the effect of natural variability. Climate oscillations, such as the Pacific Decadal Oscillation (PDO), have been shown to influence the position of the TCZ, and as such may have a strong influence on the observed rates and degrees of expansion within the instrumental period. Without an extended high-resolution record, it is difficult to accurately determine whether natural variability is an attributing factor in the expansion of the tropics, or whether external forcings, including increasing greenhouse gases and changing aerosols concentrations, have a larger impact. This knowledge is particularly important as climate models predict that future variability will result in a northward shift in the TCZ across the Indian Ocean. An extended high-resolution record of climate belt variability needs to be established in order to better contextualise the recent observed expansions, and constrain model projections of future variability and change.

This thesis presents an identification of variability in the TCZ across the Indian Ocean, presenting a review of present day, palaeo and future variability in the features. I will then present new palaeo coral-reconstruction for Christmas Island in the Indian Ocean, covering the last 100 years at high resolution (monthly-fortnightly). This reconstruction is developed from the composite of four newly developed high resolution coral records, along with one previously published record. Christmas Island provides an ideal location to extend instrumental records and improve palaeo-reconstructions of sea surface temperature and rainfall along a longitudinal section of the Indian Ocean. Christmas Island is a small island located on the edge of the TCZ, and both observational and palaeo records at the island should reflect whether shifts in the tropical edge are consistent prior to the instrumental period. This location provides an ideal location to fill a gap in analysis across the south-east tropical Indian Ocean. The final chapter will draw on this new reconstruction from Christmas Island, along with pre-existing (published and unpublished) records from the south-east tropical Indian Ocean. The thesis will present a hydroclimate reconstruction of the region over the last 400 years. Show more