ENSO-related droughts and ISM variations during the last millennium in tropical southwest China

Abstract

Severe El Nino-Southern Oscillation (ENSO) droughts that occurred in southwest China during the last millennium have exerted important impacts on natural ecosystems and human society. Fossil phytoliths in high-resolution (~ 7 year) fluvial sediments in the monsoon rainforest are studied to reveal the influence of ENSO activity on tropical southwest China. The phytolith records together with multiproxy analysis indicate that three phases of distinctively warm-dry and cool-wet climate pattern have exerted profound influences on the sedimentary environment and tropical monsoon rainforest, which is sensitive to the occurrences of severe droughts and fires. Integrated comparisons demonstrate that climatic variations rebuilt by Ic (temperature proxy) are consistent with the temperature change in the Northern Hemisphere, Indian Summer Monsoon (ISM) variations in the northern Indian Ocean and southwest China, and aridity index (Iph) is linked to ENSO activity in the equatorial Pacific Ocean. At decadal timescale, there is an internal connection between ISM variability and ENSO activity in the tropical southwest China. Due to the modulation of ENSO activity, there is an antiphase relationship between temperature and ISM intensity, with a remarkably dry Medieval Warm Period (MWP), Modern Warming (MW) and a mostly wet Little Ice Age (LIA) during the last millennium. Spectral analysis of the climatic indices (Ic and Iph) indicated that the dominant cycle (~ 88 year) is a significant component of periodic solar activity variations. Our results confirmed that the possible forcing mechanism of such a “warm-dry and cool-wet” climate pattern is related to the coupling of solar periodic variations and ENSO variance spanning the tropical Indian Ocean and the “Indian subcontinent-southwest China”. This is significant of understanding the relationship among droughts, ENSO variance and ISM variation in the tropical southwest China under the background of global warming.