Abstract
Extra-tropical circulation systems impede poleward moisture advection by the Indian Summer Monsoon. In this context, the Himalayan range is believed to insulate the south Asian circulation from extra-tropical influences and to delineate the northern extent of the Indian Summer Monsoon in central Asia. Paleoclimatic evidence, however, suggests increased moisture availability in the Early Holocene north of the Himalayan range which is attributed to an intensification of the Indian Summer Monsoon. Nevertheless, mechanisms leading to a surpassing of the Himalayan range and the northern maximum extent of summer monsoonal influence remain unknown. Here we show that the Kunlun barrier on the northern Tibetan Plateau [~36°N] delimits Indian Summer Monsoon precipitation during the Holocene. The presence of the barrier relocates the insulation effect 1,000 km further north, allowing a continental low intensity branch of the Indian Summer Monsoon which is persistent throughout the Holocene. Precipitation intensities at its northern extent seem to be driven by differentiated solar heating of the Northern Hemisphere indicating dependency on energy-gradients rather than absolute radiation intensities. The identified spatial constraints of monsoonal precipitation will facilitate the prediction of future monsoonal precipitation patterns in Central Asia under varying climatic conditions.
Highlights
Decreased positive energy influx into the atmosphere over the central Asian continent, diminishing the thermal land-sea gradient and relocating the inner tropical convergence zone (ITCZ) further south[10]
Decreased positive energy influx into the atmosphere over the central Asian continent, diminishing the thermal land-sea gradient and relocating the ITCZ further south[10]. This relocation is observed to the west of the Tibetan Plateau in speleothem records from the Arabian Peninsula[11] and to the east in the East Asian Summer Monsoon branch as recorded by lacustrine sediments from eastern China[12,13]
To reconstruct monsoonal precipitation variations north of the Himalayan barrier we analysed sediment dynamics on two mountain ranges at the northern fringe of the Tibetan Plateau: The Kunlun and Burhan Buda mountain range (Fig. 2a), both normal to Indian Summer Monsoon (ISM) trajectories
Summary
The application of a fuzzy c-means cluster analysis on the minerogenic content of the reference samples reveals 4 dominant sources of fluvial sediment supply (Fig. 2b, see supplementary information). The persistency implies that the actual insulation between air masses of high and low moist static energy, formerly attributed to the Himalayans[2,5], occurs at ~36°N This assumption is reinforced by the asynchronous Holocene moisture evolution south and north of the North Tibetan barrier as evidenced by paleoenvironmental archives throughout monsoonal Asia[6,7] and arid central Asia[21,22,23] south and north of the barrier respectively (see Fig. 4e). The decrease in thermal poleward forcing led to decreased moist static energy advection, limiting, but not excluding convergence in higher latitudes as evidenced by continuous but decreasing Holocene precipitation intensities at the northern ISM barrier. We suggest that besides inter-hemispheric gradients[30] and absolute forcing mechanisms like global temperatures or insolation, their inner-hemispheric distribution must be taken into account for predicting future inner-continental ISM changes
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