Indo-Pacific tropical rain belt expanded and contracted in the past

October 5, 2016
Composite map of precipitation anomaly during the AD 1220-1280
Composite map of precipitation anomaly (mm/month) during the AD 1220-1280 pluvial period, the wettest section of a longer pluvial period in both the North Australian and Chinese cave stalagmite records (see yellow stars for locations). Dashed contours delimit areas significant at the 90% level according to a two-tailed t-test. Note expansion of tropical rain belt (blue shading) and concomitant decrease of rainfall in the core equatorial region (red shading). [Figure adapted from Denniston et al. (2016).]

 The seasonal migration of summer monsoon rains across the equator defines the tropical rain belt, a zone of enormous biodiversity and the home of approximately 40% of people on Earth. Reconstructing past dynamics of the tropical rain belt is important for understanding how the width of the tropics may respond to future climate change.

The Austral-Asian monsoon spans the Indo-Pacific, from northern Australia to southern China, and is the largest component of the planetary monsoon. Numerous stalagmite studies have examined variations in monsoon rainfall over previous millennia in China at the northern margin of the Indo-Pacific tropical rain belt, but only a handful of studies have focused on its southern margin. A new study by Denniston, Ummenhofer, and colleagues integrated a high-resolution stalagmite record from Australia with cave sites in southern China to reconstruct the behavior of the tropical rain belt over the last 3000 years. Their analysis revealed a close coupling of monsoon rainfall on both continents, with numerous synchronous pluvial and drought periods, suggesting that the tropical rain belt expanded and contracted numerous times at multidecadal to centennial scales. In order to examine the drivers of this behavior, the team examined rainfall over the Austral-Asian sector using the Last Millennium Ensemble climate model simulations, conducted by NCAR with the Community Earth System Model. The simulations captured a good deal of the expansion/contraction behavior (see figure) and suggested that the tropical rain belt appears particularly susceptible to Pacific ocean temperatures and volcanism.

Written by 
Rhawn Denniston, Cornell College

Rhawn F. Denniston1, Caroline C. Ummenhofer2, Alan D. Wanamaker, Jr.3, Matthew S. Lachniet4, Gabriele Villarini5, Yemane Asmerom6, Victor J. Polyak6, Kristian J. Passaro1, John Cugley7, David Woods8, William F. Humphreys9

1Cornell College

2Woods Hole Oceanographic Institution

3Iowa State University

4University of Nevada, Las Vegas

5University of Iowa

6University of New Mexico

7Australian Speleological Federation

8Department of Parks and Wildlife, Australia

9University of Adelaide