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Modelling climate and water isotope variability in southern Indonesia with GISS ModelE-R

Sophie C. Lewis1, Allegra N. LeGrande2, Maxwell C. Kelley2, Gavin A. Schmidt2, Michael K. Gagan1, Linda K. Ayliffe1

1  Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
2   NASA Goddard Institute for Space Studies and Center for Climate Systems Research, Columbia University, 2880 Broadway, New York, NY 10034

Figure 1. Difference in precipitation source region to southern Indonesia between 6 kyr and modern GISS ModelE-R simulations.


Water isotope records collectively provide some of the most extensive proxy evidence for past climate. However, a known or assumed relationship between water isotopes and climate is required for the interpretation of these records. Climate variability on annual to orbital timescales impacts the hydrologic cycle and influences water isotope distribution, with varying impacts on individual climate variables and water isotopes. As such, the relationship between water isotopes and climate may not remain constant through time. The goal of this study is to examine the relationship between climate and water isotope variability in southern Indonesia to facilitate understanding of late Quaternary changes in monsoon rainfall recorded by oxygen isotopes in speleothems.

We assess the relationship between water isotopes and climate and infer the primary mechanisms controlling water isotope variability on various time scales using multiple simulations of current and past climate (Holocene through glacial). The GISS ModelE-R, a fully coupled atmosphere-ocean GCM equipped with water isotope as well as other tracers, is ideal for tracing the source of water isotope variability. We investigate the mechanisms controlling water isotope variability through the addition of isotopic tracers that allow us to explicitly track water vapour and precipitation sources to a region.

We find that the relationship between water isotopes and climatic variables is different at various timescales and that this relationship can change during abrupt climate excursions.  Model results support the interpretation of isotopic variability in tropical speleothem records and allow a greater understanding of late Quaternary changes in precipitation.  For example, model simulations of precipitation variability in southern Indonesia show a significant northward shift in precipitation source region during the middle Holocene (Figure 1). This change in source area of precipitation likely results from a southward shift in the position of the Intertropical Convergence Zone through the Holocene (Wanner et al., 2008), which would have altered the isotopic composition of rainfall recorded in speleothems in southern Indonesia.

Ultimately, model outputs indicate that the isotopic composition of rainfall delivered to a speleothem site is dependent on numerous dynamic variables, including rainfall amount, precipitation source region and transport trajectory from source to rainout. Model simulations of water isotope variability greatly assist in interpreting oxygen isotopes variability within speleothem records of the palaeomonsoon.

 


Wanner, H., Beer, J., Bütikofer, J., Crowley, T. J., Cubasch, U., Flückiger, J., Goosse, H., Grosjean, M., Joos, F., Kaplan, J. O., Küttel, M., Müller, S. A., Prentice, I. C., Solomina, O., Stocker, T. F., Tarasov, P., Wagner, M. & Widmann, M. (2008) Mid- to Late Holocene climate change: an overview. Quaternary Science Reviews, 27, 1791-1828.