Abstract
The seasonal north-south migration of the intertropical convergence zone (ITCZ) defines the tropical rain belt (TRB), a region of enormous terrestrial and marine biodiversity and home to 40% of people on Earth. The TRB is dynamic and has been shown to shift south as a coherent system during periods of Northern Hemisphere cooling. However, recent studies of Indo-Pacific hydroclimate suggest that during the Little Ice Age (LIA; AD 1400–1850), the TRB in this region contracted rather than being displaced uniformly southward. This behaviour is not well understood, particularly during climatic fluctuations less pronounced than those of the LIA, the largest centennial-scale cool period of the last millennium. Here we show that the Indo-Pacific TRB expanded and contracted numerous times over multi-decadal to centennial scales during the last 3,000 yr. By integrating precisely-dated stalagmite records of tropical hydroclimate from southern China with a newly enhanced stalagmite time series from northern Australia, our study reveals a previously unidentified coherence between the austral and boreal summer monsoon. State-of-the-art climate model simulations of the last millennium suggest these are linked to changes in the structure of the regional manifestation of the atmosphere’s meridional circulation.
Highlights
The intertropical convergence zone (ITCZ) represents the region of intense rainfall associated with the rising branch of the atmospheric meridional overturning circulation, a primary driver of energy and moisture exchange between the tropics and higher latitudes[1,2] (Fig. 1)
While numerous late Holocene stalagmite records have been developed for the East Asian Summer Monsoon (EASM), fewer well-resolved stalagmite records have been constructed for the southern margin of the Indo-Australian Summer Monsoon (IASM)[27,28,29]
We investigated late Holocene dynamics of Indo-Pacific hydroclimate by comparing the newly enhanced oxygen isotope record from cave KNI-51 to stalagmite time series located along a north-south transect through the EASM region of China (Fig. 1): Wanxiang cave (33.3°N, 105.0°E)[30]; Heshang cave (30.5°N, 110.4°E)[31]; and Dongge cave (25.3°N, 108.1°E)[14]
Summary
Indo-Pacific Tropical Rain Belt over the Last Three Millennia received: 30 March 2016 accepted: 14 September 2016 Published: 29 September 2016. Composite precipitation anomalies in the LME simulations reflect wetter conditions across southern China and across northern Australia; substantial decreases in rainfall mark the central and eastern core of the TRB (Fig. 4b) This pattern is apparent in the zonal mean plots for precipitation averaged across the Austral-Asian sector (longitude range 100°–140°E; Fig. 4b), and is consistent with an expansion of the regional TRB via a more poleward positioning of the rising branch of the tropical meridional circulation. No clear sign of tropical Pacific zonal SST anomalies such as those associated with ENSO in the equatorial eastern Pacific, are manifest in the model simulations at either time (Figure S3) These results suggest that broad-scale changes to the meridional atmospheric circulation as reflected in the LME precipitation response occur through a suite of integrated forcings and represent a likely mechanism for TRB expansion and contraction. The contribution of methane from the Australian tropics during periods of TRB expansion should be carefully considered in models of Holocene climate
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