Tropical rainfall over the last two millennia: evidence for a low-latitude hydrologic seesaw
Precipitation in tropical regions is governed by the Intertropical Convergence Zone (ITCZ), a band of high rainfall encircling the globe near the equator. The ITCZ migrates seasonally between the hemispheres, following the thermal equator of the Earth. In this study, we used a number of published high resolution and precisely dated paleoclimate reconstructions distributed across the tropics and subtropics of the globe and covering the last 2000 years. The reconstructions are extracted from stalagmites, lake records, ice cores, and tree rings. We focused on records that reflect changes in ITCZ-driven precipitation, to track past changes in this important driver of low-latitude climate.
We find that changes in global temperature gradients caused the ITCZ to migrate South for prolonged periods of time, most notably during the Little Ice Age (ca. 1300-1800 AD), where the Northern Hemisphere tropics became drier, while the Southern Hemisphere experienced wetter conditions. This disturbance in tropical atmospheric circulation patterns can be tracked up to the mid-latitudes, where changes in the North Atlantic Oscillation (NAO) are apparent at the same time. Similar observations have previously been made over glacial-interglacial cycles and Dansgaard-Oeschger events, when cooling of the Northern Hemisphere during full glacial conditions and Heinrich events pushed the ITCZ southward, while warmer interglacials were characterized by a more northerly position of the tropical rainfall belt. Our results however show that even much smaller changes in global temperatures can trigger a notable shift in the distribution of low latitude rainfall patterns. The global extent of this seesaw pattern highlights the importance of tropical energy fluxes on climate.
Researchers crossing a stream in Belize during the rainy season
Postdoc Franziska Lechleitner, first author on the paper, describes her work: “We did extensive fieldwork at Yok Balum Cave in southern Belize over the past seven years, to study and characterize the evolution of tropical Atlantic rainfall patterns and climate over the last 2000 years. Apart from the collection of stalagmite samples, we installed an extensive network of monitoring equipment, to understand how weather conditions affect the cave, and collected water samples from inside the cave and the overlying soil, to identify processes occurring between the surface and the cave.
“The cave is located right at the northern extent of the present-day ITCZ and therefore is extremely sensitive to even small shifts in the latter, as we found in a previous study (Ridley et al., 2015, Nature Geoscience).”
Main image: One of the stalagmites used in the study from Yok Balum Cave in southern Belize (image credit to Izabela Walczak, Durham University).
Paper: Tropical rainfall over the last two millennia: evidence for a low-latitude hydrologic seesaw, by Franziska A. Lechleitner, Sebastian F. M. Breitenbach, Kira Rehfeld, Harriet E. Ridley, Yemane Asmerom, Keith M. Prufer, Norbert Marwan, Bedartha Goswami, Douglas J. Kennett, Valorie V. Aquino, Victor Polyak, Gerald H. Haug, Timothy I. Eglinton & James U. L. Baldini, is published in Scientific Reports doi:10.1038/srep45809