Tropical Circulation Systems and Monsoons

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It is generally believed that decreasing insolation led to a decreased positive energy influx into the atmosphere over the central Asian continent, thus diminishing the thermal land-sea gradient and relocating the ITCZ further south This relocation is also 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 , Hence, a variable boundary of monsoonal precipitation is the only mechanism capable to explain the observed paleo-moisture evolution over the Tibetan Plateau.

However, the maximum northward ISM extent remains unknown. 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. The mountain ranges delineate the boundary between the Tibetan Plateau and the internally-drained Qaidam Basin and adjacent central Asian deserts in the North.

At this latitude, archives of paleoenvironmental changes become sparse because of the prevailing aridity and high altitude. One of the few archives is Lake Heihai, situated in an intermontane basin in between the two mountain ranges. It constitutes a sink of two sediment cascades originating in the Kunlun range in the south and the Burhan Buda range in the north. Areas with similar or higher orographic forcing on southerly air masses as compared to the study area are indicated by colours as factor of higher forcing supplementary information.

Monsoon Systems

Circles indicate sample location. 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. Cluster 2, 3 and 4 represent fluvial sediments originating from foothills of the Kunlun range Cluster C2 and Burhan Buda range C3 and C4.

Due to the lack of morphological evidences for former glaciation we assume predominantly precipitation generated runoff for these sub-catchments. To estimate the contribution of each mentioned sediment source in the basin to the present-day sediment supply of Lake Heihai we sampled the recent lake floor at 28 positions. Surface and core sample locations are shown in the center of the figure. Lake sketch indicates the position circle and membership degree circle size of individual samples to cluster center C1 to C4.

Sample memberships are plotted against depth in a sediment core. For a temporal analysis, a sediment core was recovered from the deepest part of the lake basin. Our lake record extends back into the Late Pleistocene supplementary information. The asymmetric sediment supply between the southern and northern range is persistent throughout the temporal coverage of the sediment record Fig.

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The decrease is asynchronous to the moisture evolution of the East Asian Summer Monsoon 6 , 12 and balanced by a compositional change to glacio-fluvial sediment sources at Lake Heihai Fig. We thus assume a dominant link of summer rainfall on the Kunlun range to ISM precipitation.

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We attribute the low intensity of fluvial sediment supply from the Burhan Buda range to a barrier effect of the Kunlun range. The orography of the Kunlun range forces moisture bearing air masses migrating on ISM trajectories to a sudden ascend, leading to intensified orographic precipitation. Although the Burhan Buda range imposes a similar orographic forcing on southerly air masses supplementary information , the remaining atmospheric moisture is insufficient for orographic precipitation to occur.

The orography of the Kunlun range thus delineates the northern boundary of ISM precipitation in the central Kunlun fault system. There are no additional atmospheric moisture sources in the central Asian desert belt north of the barrier which could enable a reactivation of summer monsoonal precipitation. Furthermore, an intensified uplift associated with the Kunlun Range in the Mid-Pleistocene caused an intensification of arid conditions in the Asian inland While ISM precipitation dominates in the central parts, uncertainty remains about the influence of the East Asian Monsoon on precipitation patterns in the east of the barrier.

Based on our results we suggest that ISM precipitation continuously prevails between the Himalayan and Kunlun range in a low intensity branch. The branch is persistent throughout the Holocene. 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. The out-of-phase relationship is ascribed to a predominant westerly influence on the moisture evolution of arid central Asia 23 , reaffirming the delineation of major circulation systems at the northern Tibetan barrier.

Thus, our lake record mirrors the northernmost intensity variations of dominant ISM influence. If the thermal land-sea contrast is responsible for the ocean-to-continent advection of high moist static energy, which forcing mechanism continues to set the inner-continental poleward advection of ISM air masses in motion? Monsoonal rainfalls can occur in high latitudes under idealized conditions as suggested by climate models which exclude impeding mechanisms such as the ventilation effect and Rossby-Wave dynamics 4.

This suggests that in the absence of impeding mechanisms, radiative forcing drives the spatial extent of monsoonal rainfalls. This phase lag of monsoonal strength to orbital forcing is partly attributed to a delayed maximum latent heat export from the Southern Indian Ocean resulting from differentiated northern and southern hemispheric insolation forcing 24 , 25 , Absolute intensities converge proportionally between the latitudes from the Early to Late Holocene Fig.

We assume that the impact of differentiated radiative forcing on the inner-continental thermal regime 27 drives the inner-continental northward migration of the Tibetan ISM branch on centennial to millennial time scales. 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.

Our results have great implications for future climate scenarios and modelling approaches in central and high Asia.

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