The Indo-Pacific tropical rain belt (TRB) marks the high-rainfall region in the tropics traversed by the seasonal migration of the intertropical convergence zone. In particular, changes in the position of the northward and southward edges of the TRB can have considerable impacts on natural and human systems across the Indo-Pacific warm pool and adjacent land areas. Using state-of-the-art climate model simulations conducted as part of the Last Millennium Ensemble with the Community Earth System Model, we evaluate variations in the width of the Indo-Pacific TRB over the last millennium (AD 850-1850). The climate model results complement a recent reconstruction of late Holocene variability of the Indo-Pacific TRB derived from precisely-dated stalagmites from cave KNI-51 in north-central Australia, whose location make it very sensitive to decadal variations in the southern edge of the TRB over the past 3,000 years.
In the model simulations and proxy-based records, we identify multi-decadal to centennial periods when the Indo-Pacific TRB expanded/contracted during the last millennium, as indicated by symmetric strengthening/weakening of summer monsoons in the Northern and Southern Hemispheres (the East Asian summer monsoon in China and Australian summer monsoon). Prolonged periods of an expanded TRB coincide with characteristic Indo-Pacific sea surface temperatures resembling the negative phase of the Interdecadal Pacific Oscillation, reminiscent of conditions seen during an expanded Hadley Circulation in recent decades. Synthesis of terrestrial and marine palaeo proxy records provides a test of the roles played by internal variability and external forcing for specific TRB expansion/contraction periods observed in the palaeoclimate record.