Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response
Dipole Response of Millennial Variability in Tropical South American Precipitation and δ18Op during the Last Deglaciation. Part I: Rainfall Response
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DOI：
10.1175/jcli-d-22-0172.1
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发表时间：
2023-07-01
影响因子：
4.9
通讯作者：
He, Chengfei
中科院分区：
文献类型：
--
作者：
Bao, Yuntao;Liu, Zhengyu;He, Chengfei
氧同位素洞穴沉积物已被广泛用于推断热带南美洲（TSA）过去的气候变化。然而，千年降水的空间格局、降水对818 O（818 Op）的响应一直存在争议，其响应机制尚不清楚。特别是，目前还不清楚区域降水是否代表千年南美夏季风（SASM）的强度。在这里，我们研究TSA水文气候变率在最后一次冰消期（20-11万年前）相结合的同位素启用社区地球系统模型（iCESM）和洞穴记录在低地TSA的瞬态模拟。我们的模型合理地模拟了TSA上水文气候变量和水同位素的冰消演变，尽管低估了变异的幅度。北大西洋融水排放是推动TSA千年水文气候变化的主要因素。降水和818 Op的空间格局显示与热带辐合带的纬向迁移相关的西北-东南偶极子，而不是在许多以前的洞穴沉积记录中推断的大陆范围内的连贯变化。偶极响应得到了多尺度古气候代用指标的支持。由于融水强迫增加，热带风暴减弱（其特征是低层东风减弱），导致亚马逊西部降雨量减少，巴西东部降雨量增加。类似的偶极子响应也产生了太阳辐射，冰盖，温室气体，这表明了固有的稳定性的空间特征的SASM不管外部强迫和时间尺度。最后，我们讨论了模型-代理差异的潜在原因，并提出了在亚马逊中西部建立更多古气候代理数据的必要性。我们希望调和关于南美洲热带地区千年水文气候是否存在一致或异质响应的争议，并清楚地了解其背后的强迫机制。启用瞬态模拟填补了洞穴沉积物重建中的差距，以捕捉千年降水/818 Op和季风强度变化的全貌。我们强调了在降水和818 Op千年和轨道时间尺度上的异质偶极子响应。增加融水排放ITCZ向南移动，有利于在巴西沿海潮湿的条件。与此同时，低层东风和夏季风强度减弱，导致亚马逊中西部干旱。然而，在我们的模型中，水文气候响应的千年变化被低估了，再加上亚马逊中西部缺乏直接的古气候代理，使特定古气候事件变化的解释复杂化，并对限制偶极子的空间范围提出了挑战。因此，我们强调有必要增加代理的来源，加强代理解释，提高气候模式的性能在未来。
Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation , precipitation 818O (818Op) response have remained controversial , their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20-11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reason-ably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA's millennial hydrocli-mate variability. The spatial pattern of both precipitation and 818Op show a northwest-southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteris-tics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model- proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon.SIGNIFICANCE STATEMENT: We want to reconcile the controversy on whether there is a coherent or heteroge-neous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mecha-nisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/818Op and monsoon intensity change. We highlight a heterogeneous dipole response in precipitation and 818Op on millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydro-climate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central -west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future.