Mid-Late Holocene Sub-Millennial Scale Inverse Trends of South Asian Summer and Winter Monsoons in Sri Lanka

Kasun Gayantha,Peter Frenzel,Rohana Chandrajith,Florian Ott,Gerd Gleixner,Joyanto Routh,Patrick Roberts,Oshan Wedage

doi:10.3389/feart.2021.789291

Abstract

The South Asian Monsoon (SAM) brings precipitation crucial for agriculture across the densely populated region of South Asia. Identifying the key long-term drivers of the SAM is essential to improve the predictability of future monsoonal trends in the context of current global climate scenarios and increasingly frequent drought and flooding events in this part of the world. Here, we reconstruct ∼6000 years of climatic and environmental history of the South Asian summer monsoon-fed Bolgoda South Lake and the Horton Plains, and the winter monsoon-fed Panama lagoon, in Sri Lanka to better understand monsoonal operation over this island and its connection to broader climate systems. Multiple proxies (diagnostic biomarkers, hydrogen and carbon isotopes of individual n-alkane, grain size, and Zr/Rb elemental ratio) indicate a sub-millennial scale decreasing trend of summer monsoon rainfall in the wet zone of Sri Lanka alongside an increasing trend of winter monsoon rainfall in the dry zone during the last ∼6000 years. We also observed multi-centennial scale arid events in the Bolgoda South Lake and Horton Plains records at ∼3,500 and ∼1,000 cal years BP. Inverse monsoonal behavior during the mid- and late Holocene seems to be led by the southward migration of the mean latitudinal position of ITCZ, induced by varying solar energy distribution between the Northern and Southern hemispheres due to Earth’s processional cycle. Our observations are broadly supported by existing paleoclimatic records from the Indian sub-continent, but abrupt arid phases are asynchronous in the regional records. In addition, these short-term arid conditions do not show systematic correlations with the different modes of climate variables known to have teleconnections with the Indian Ocean monsoon.

Highlights

The South Asian Monsoon (SAM) is a major component of the Asian monsoon system that plays a critical role in global climate and the socio-economic conditions of the nearly one quarter of the world’s human population that lives in in South Asia (Gupta et al, 2020)
This sand layer deposited between 5.5—2 cm showed characteristics of a high energy depositional event that is most likely linked to the 2004 tsunami, which is known to have significantly impacted the region (Wijetunge, 2006)
For the Bolgoda South Lake (BGSL) core, ages ranged between 5620 cal yr BP and 1410 cal yr BP according to the BACON age-depth model constructed based on 14C ages in bulk sediments (Figure 2A)

Summary

Introduction

The South Asian Monsoon (SAM) is a major component of the Asian monsoon system that plays a critical role in global climate and the socio-economic conditions of the nearly one quarter of the world’s human population that lives in in South Asia (Gupta et al, 2020). The seasonal migration of the equatorial low-pressure trough commonly called Inter Tropical Convergence Zone (ITCZ), determines the timing, intensity, and direction of tropical monsoon winds and the accompanying rainfall (An et al, 2015; Mohtadi et al, 2016) This migration is thought to be mainly controlled by the annual solar cycle causing variations in the solar induced cross-equatorial pressure gradient. The ITCZ retreats southward and reaches its southern-most position around January This retreat results in a reversal of wind direction and provides the winter monsoon rainfall (Northeast monsoon; NEM) to the southern tip of India and the northern and eastern parts of Sri Lanka (Figure 1; Banerji et al, 2020; Gupta et al, 2020). Large variations in this general pattern are known, especially for regions more distant to the equator, which makes it difficult to identify key patterns and driving forces of change (Achyuthan et al, 2016; Misra et al, 2019)

Methods

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Discussion

Conclusion