The seasonal migration of the intertropical convergence zone (ITCZ) across latitudes brings rains to the billions of people living in (sub)tropical regions around the globe. Positioning of the ITCZ is impacted by meridional temperature gradients, with the boreal summer ITCZ having been shown in numerous studies to have shifted south during periods of northern hemisphere cooling. Owing to the few high resolution, hydroclimate-sensitive proxies in the southern hemisphere, concomitant changes in the position of the austral summer ITCZ are not well understood. Some evidence suggests that both the boreal and austral summer ITCZs drift north or south in concert, thereby maintaining a quasi-constant width in the tropical rain belt (TRB), the region spanned by the annual migration of the ITCZ. This creates a precipitation dipole at the northern and southern margins of the TRB (i.e., wetter at the northern edge of the TRB and drier at the southern edge during a southward shift). Some recent studies instead argue that the boreal and austral summer ITCZs shifted poleward or equatorward in tandem, causing the TRB to expand (poleward movement) or contract (equatorward movement), creating wetter or drier conditions at the TRB margins. The goals of this study were to investigate the nature of Indo-Pacific TRB dynamics by (i) reconstructing monsoon rainfall using stalagmites from the southern margin of the ITCZ in the central Australian tropics, and (ii) determining the drivers of the observed behavior using climate model simulations.

As part of this study, a total of seven aragonite stalagmites were collected from cave KNI-51, tropical Western Australia. These samples were bisected vertically, precisely dated using uranium-thorium disequilibrium methods, and analyzed for oxygen isotope ratios, a well-established proxy for monsoon rainfall amount. The results were integrated with nine KNI-51 stalagmites that had been previously collected and similarly analyzed. Together, this composite data set spans the majority of the last ~4000 years. In addition, overlap of multiple stalagmites allows for unprecedented levels of replication (Figure 1). Two stalagmites (one newly collected and one previously collected) were offset in isotope values relative to coeval samples.

Apparent in the data are multidecadal to centennial-scale periods of above or below average rainfall. When compared to high resolution stalagmite reconstructions of monsoon rainfall from the northern TRB margin in China, expansion/contraction of the TRB is apparent. The origins of this behavior were investigated using the Last Millennium Ensemble, a high-resolution suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model, which allow assessment of individual or coupled effects of the following forcings: solar irradiance, land cover, volcanic activity, and greenhouse gases. Internal (not externally forced) variability can also be assessed by comparing individual ensemble members. Periods of an expanded or contracted TRB were identified, combined with records of ocean surface temperature derived from published hydroclimate-sensitive coral and tree ring records, and investigated in the model. The results, while still being refined, suggest that expansion of the Indo-Pacific TRB is driven not by external forcing (e.g., variations in solar output or volcanic eruptions) but instead by internal variability in the climate system. TRB expansion/contraction behavior is closely tied to the Interdecadal Pacific Oscillation, which describes multidecadal ocean surface temperature variations in the tropical and northern Pacific (Figure 2). While not definitive, this outcome provides a mechanism for testing this hypothesis through development of high-resolution, rainfall-sensitive proxy records from other sites that appear in the model output as responding clearly to expansion and contraction of the TRB.

Dynamics of the austral summer ITCZ identified from KNI-51 stalagmites also have implications for northwest Australian tropical cyclone (TC) activity. The majority of Australian TCs are formed in association with the ITCZ and thus its positioning impacts the number and location of such storms that make landfall across Western Australia. A 1500 year-long, annually-resolved stalagmite oxygen isotope record of TC activity from Cape Range, subtropical Western Australia (Haig et al., 2014, Nature, 505, 667) covaries with the KNI-51 record over multi-decadal to centennial scales for the last millennium. One hypothesis for this behavior is that during times when the ITCZ moved southward, more monsoon rain fell at KNI-51 and more TCs passed near Cape Range. To test this hypothesis, collaborators Dr. Francesco Pausata and Mr. Roberto Ingrosso (University of Quebec) assessed ITCZ position over Australia using reanalysis data for the last 40 years and the Max Planck Institute Earth System Model Past-1000 Year simulation for the last millennium. These data allowed downscaled TC simulations by collaborator Dr. Kerry Emanuel (MIT). Together, these analyses support a causal link between positioning of the ITCZ and rainfall at both sites (Figure 3), a finding with important implications for moisture budgets in Australia's drought-sensitive subtropical regions.

Last Modified: 09/08/2022
Modified by: Rhawn F Denniston