岁差驱动的全新世热带太平洋—印度洋水汽输送

Precession forcing of the Holocene moisture transfer between tropical western Pacific and Indian Ocean

  • 摘要: 大洋内部蒸发、水汽输送和降水过程构成了全球水文循环的主体,但长久以来对全新世大洋内部水文循环的演变历史和驱动机制缺乏认识。利用全球热带海区98个站位混合层浮游有孔虫壳体的δ18O和Mg/Ca温度记录,计算了全新世以来表层海水δ18O以及海水剩余δ18O的波动历史。发现最显著的特征是全新世以来热带西太平洋和东印度洋表层海水剩余δ18O具有不同的变化趋势。早—中全新世时期(11.5~6.0 kaBP),热带西太平洋表层海水剩余δ18O比东印度洋偏重约0.2‰,而晚全新世(2.0~0 kaBP),两个海区的重建值几乎相同。结合同位素数值模拟结果,发现岁差通过一系列机制控制了两个大洋不同的剩余δ18O变化信号。早全新世较低的岁差值可以驱动西太平洋往印度洋的大气水汽净输送并降低印度洋降水同位素,同时强盛的南亚季风通过河流体系向孟加拉湾倾注了大量陆地冲淡水。这些机制都有利于东印度洋海水剩余δ18O出现相对负偏移信号。但较低的岁差值导致开放的西太平洋净降水量下降,并通过大气传输损失淡水,因此海水剩余δ18O值较为偏重。本次研究结合大空间尺度上的δ18O重建记录和模拟结果,较为可靠地刻画了岁差调控下洋盆之间的水汽转移过程和机理。

     

    Abstract: Precipitation, evaporation and moisture transport within the oceans are the main components of the global hydrological cycle. However, the evolution of the oceanic hydrological cycle over the Holocene remains a knowledge gap. In this study, through compiling paired planktonic foraminiferal δ18O and Mg/Ca sea surface temperature reconstructions from 98 locations in the tropical ocean, we calculate the fluctuation of sea surface δ18O and residual δ18O for the Holocene period. We notice a striking feature that the residual δ18O records of the tropical western Pacific and eastern Indian Ocean show a different change over the Holocene. The mean residual δ18O of the tropical western Pacific was about 0.2‰ heavier than that of the eastern Indian Ocean during the early-mid Holocene (11.5~6.0 kaBP), but they were almost identical over the late Holocene (2.0~0 kaBP). Combined with the transient climate simulations, we suggest that precession forcing is responsible for this different pattern through modulating a set of climate processes. The lower precession over the early Holocene drove a net atmospheric moisture transport from the western Pacific to the eastern Indian Ocean and lowered precipitation δ18O over the eastern Indian Ocean. Moreover, the strengthened South Asian monsoon delivered large amounts of diluted freshwater into the Bay of Bengal via river systems. All these three mechanisms contribute to a relatively negative excursion of residual δ18O in the eastern Indian Ocean. In contrast, the lower precession resulted in a decrease of net precipitation in the open western Pacific and a loss of freshwater via the atmospheric transport, thus generating heavier residual δ18O values. Through combining seawater δ18O reconstructions from a large spatial extent with the isotope-enabled simulations, this study has provided a reliable picture of the moisture transfer between different ocean basins and unveiled the underlying mechanisms regulated by precession.

     

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