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自20世纪90年代李吉均(1990)提出冰期冷湿、间冰期暖干的“西风模式”之后,国内外学者在全新世时间尺度上开展了大量的环境气候演化方面的研究(Chen Fahu et al.,2010, 2016;谢海超,2019)。众所周知,西风控制区与亚洲季风区具有完全不同的气候环境演化特征与模式,明确二者的相互作用机制,对未来气候环境预测具有重要意义。Chen Fahu et al.(2008)标明的亚洲现代季风边界和Feng Song et al.(2014)确立的西风亚洲边界在青藏高原中部重合,表明该区气候变化受西风与季风的双重作用影响,是研究西风与季风演化特征的优选区域。青藏高原湖泊众多,湖泊沉积物包含的环境信息具有分辨率高、连续性好、时间跨度长等特点,在古气候环境研究中发挥了重要的作用,尤其近年来,由于测年技术的突破发展,年—十年尺度的高分辨率得以建立,越来越多的学者倾向于利用湖泊沉积物来重建古气候环境,介形虫作为其中一种重要的环境转换指标也得到了越来越多的关注(Williams et al.,2001;Kim et al.,2015;Li Xiangzhong et al.,2016;Lu Fengyan et al.,2017)。介形虫是一种水生甲壳类微体生物,几乎在各种自然水体中都能生存,且在生长过程中对其周围的环境因子(盐度、温度等)非常敏感,更为重要的是,其壳体的物质成分几乎全部来源于生活的水体,保存了壳体形成时的环境信息,同时壳体的主要成分是低镁方解石,在沉积环境中易于保存,不易受后期改造的影响,因而是研究气候环境变化很好的材料。
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色林错位于青藏高原中部印度季风与西风环流的过渡地区,是研究二者进退变化的理想场所,其古气候变化特征有助于深入认识西风与季风在青藏高原发挥的影响,诸多学者在此开展古气候环境演变特征及其影响机制的研究(Shi Xuhua et al.,2017;杜丁丁等,2019;Hou Yandong et al.,2021)。但目前来看,多数研究主要围绕湖岸阶地利用地层、地貌及色度等指标开展,尚缺少利用连续的湖泊沉积物开展环境指标研究,因而未能深入探讨季风及西风进退对湖泊变化的影响程度。
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本研究利用2012年在色林错深水区钻获的一根完整连续的湖芯,通过分析沉积物中介形虫 Limnocythere inopinata 的丰度变化特征及其壳体微量元素重建了色林错中晚全新世以来的古气候变化特征,并通过与西风区、过渡区和季风区的其他湖泊沉积记录对比,探讨了中晚全新世以来青藏高原中部地区气候环境变化对西风及季风演化的响应。
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1 研究区概况
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色林错位于青藏高原中部(图1),地处西藏自治区申扎、班戈、尼玛三县交界处,海拔约4552m。根据色林错流域气象站数据以及卫星遥感影像资料显示(边多等,2010;杨志刚等,2015;闫立娟,2020),近50年来色林错湖泊面积显著增大,自1999~2008年湖面面积平均上涨率为41km2/a,已成为西藏第一大湖。
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色林错三面环山,地形封闭,流域河系发育,入湖河流主要有扎根藏布、扎加藏布、波曲藏布和阿里藏布,总流域面积约45000km2,是西藏最大的内陆湖水系,流域内较大的湖泊还有错鄂、雅个冬错、吴如错和格仁错等。
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色林错地处高原半干旱地区,太阳辐射强,日照时间长,年日照时数达2910~2970h,年均最高气温5.5~6.9℃,年均最低气温-6.6~6℃,年平均气温0.8~1℃,不同区域表层水温变化幅度约1.3℃,年降水量290~321mm,pH值9.19~9.49,溶氧量4.62~5.12mg/L(杜丁丁等,2019)。
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2 材料与方法
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项目组于2012年冬季在色林错西湖盆水深30m处(31°43.538′ N,88°44.474′ E,图1)利用重力钻获取了一根湖芯(SL-1)。岩芯全长2.78m,取芯率100%。钻探取样时湖芯沉积物-水界面清晰,无扰动。沉积物主要为黑色、灰黑色黏土层夹灰黑色、青灰色中细砂、粉砂层,具体分布特征见王海雷等(2014)岩性描述。
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除去顶部3cm的含粉砂淤泥层,对剩下的岩芯样品进行0.5cm的间隔采样,共获得了536个样品。每个样品先用低温烘干,然后称5g于去离子水中浸泡3天,再过200目筛子,把剩余物在室温下晾干并挑选其中的介形虫化石。在显微镜下挑选优势属种 Limnocythere inopinata 的成年壳体用于微量元素实验,用毛笔蘸取去离子水对壳体进行清洗,直至壳体干净无杂质,晾干后用环氧树脂将其固定在载玻片上进行电感耦合等离子体质谱(ICP-MS)测定。壳体Mg、Mn和Ca等元素的含量在国家地质测试中心采用原位法进行测试,利用美国Applied Spectra Inc J-100飞秒激光与Thermo X-Series四级杆质谱联合进行。实验条件下,采取50 μm斑束点剥蚀方式,频率8Hz,能量10J/cm2对壳体进行剥蚀,剥蚀出来的微量样品进入质谱仪进行元素分析。每扫描10个样品点,分析2个MASC-3和2个NIST610标准物质来校正质量歧视和仪器灵敏度漂移。采用无内标-多外标总量归一化进行定量,每个点总分析时间约70s,其中空白采集时间为20s。每个样品分析两次,取平均值作为各微量元素的含量,并计算其比值。
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图1 青藏高原中部色林错地理位置及SL-1钻孔位置示意图
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Fig.1 Location of Selin Co in central Qinghai-Tibetan Plateau and site of core SL-1
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3 结果与分析
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3.1 年代
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色林错SL-1孔岩芯的定年由王海雷等(2014)完成。其中岩芯顶部25cm在中国科学院南京地理与湖泊研究所完成137Cs和210Pb测试,获得岩芯顶部8cm处年龄为185a。8cm以下共取得12个沉积物样品在美国BETA实验室完成AMS14C测年。由于样品量较少,顶部数据混乱,只150cm之后的4个数据比较稳定,连续性较好。利用树轮校正年龄,选取顶部25cm的沉积年龄用于校正14C年代的碳库效应,通过线性内插及外推法最终建立色林错SL-1岩芯的年代框架,并推算出最底层沉积年代大致为5.3ka BP。
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3.2 介形虫种群组合特征
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通过实验分析,SL-1钻孔岩芯536个样品均含有介形虫化石,最大化石丰度出现在32.5~15.5cm(0.6~0.3ka BP)处,该段样品化石丰度均大于500枚/5g,其中28.5cm(0.6ka BP)处化石丰度达到600枚/5g,为SL-1全岩芯样品中介形虫化石丰度最大的层位。介形虫化石的鉴定主要依据《中国介形类化石》(第一卷和第二卷)(侯佑堂等,2002, 2007)。经鉴定,SL-1钻孔岩芯中的介形虫化石共有3属8种,分别为 Limnocythere inopinata、 Limnocytherellina kunlunensis、L.trispinosa、Leucocythere dilatata、 L.parasculpta、L.subculpta、L.tropis 和L.mirabilis,其中L.inopinata(图2)是优势属种,在岩芯中自下而上均有产出,且丰度较大,为利用介形虫壳体及其微量元素研究色林错中晚全新世以来的古气候环境变化提供了良好的材料。由于其他属种的丰度均很小,多为1~2枚/5g,所以本研究主要根据 L.inopinata 的丰度变化特征来探讨古气候变化阶段。图3展示了 L.inopinata 丰度变化及其壳体微量元素的变化规律,由于4.5ka BP之前的 L.inopinata 丰度较小且以幼体和破碎的壳体居多,能用于微量元素实验测试的成体介壳很少,所以壳体微量元素测试结果是从4.5ka BP有连续线性曲线开始。从图中可以明显看出,5.3~3.6ka BP阶段介形虫化石 L.inopinata 丰度最小,从3.6ka BP开始,化石丰度较前一阶段有明显增加直到1.8ka BP。从1.8ka BP开始, L.inopinata 化石丰度逐渐达到整个沉积阶段的最大值并呈现峰值/谷值交替变化的模式,直到钻孔顶部。
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3.3 Limnocythere inopinata 壳体微量元素
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图3展示了SL-1钻孔岩芯中 L.inopinata 壳体Mg/Ca和Mn/Ca摩尔比值的变化曲线。从图3中可以看出,壳体Mg/Ca比值约为0.01~0.38mol/mol,Mn/Ca比值变化于0.0003~0.004mol/mol之间,总体上壳体Mg/Ca比值在2.0ka BP之前较低,从2.0ka BP开始明显增加,尤其从1.5ka BP开始增长更为显著,到0.7ka BP左右达到最大值,之后略有下降但仍为高值直到钻孔顶部。Mn/Ca比值在4.5~2.9ka BP期间处于较高值,其后开始降低直到1.8ka BP,期间在2.4~2.0ka BP阶段呈现较高值。从1.8ka BP开始Mn/Ca比值呈现持续增长的模式,直到钻孔顶部达到最大值。
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4 讨论
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介形虫生活的环境多种多样,但其单个属种往往偏爱某些特定的环境,在这些环境下,介形虫容易繁殖发育,并达到最大丰度(杨藩等,2006)。除介形虫属种外,介形虫壳体微量元素也是一种良好的环境转换指标,目前应用较多的有Mg/Ca、Sr/Ca和Mn/Ca等。一般认为壳体Sr/Ca能有效指示水体盐度的变化,与水体盐度呈正相关关系(Yang Qichao et al.,2014;Kim et al.,2015),但在有文石沉积的情况下,受矿物结晶条件影响不能精确指示古盐度变化(胡广等,2008),SL-1钻孔不同层位的岩芯矿物中均含有文石,因此本文不利用壳体Sr/Ca比值进行分析。介壳的Mg离子主要来源于寄生水体,一般认为当水体化学成分稳定时壳体Mg/Ca只与水体温度呈正相关关系(Chivas et al,1983;赵泉鸿等,1994),并有多位学者在水体化学成分稳定的情况下基于特定的介形虫属种建立了壳体Mg/Ca与温度之间的定量转换关系(Chivas et al.,1986;Wansard,1996;De Deckker et al.,1999)。Yang Yibo et al.(2013)通过对柴达木盆地西部SG-1钻孔钙质沉积物中Mn元素的分析认为,在干旱区Mn元素含量的长期降低指示气候长期的干旱,并通过与Wang Jiuyi et al.(2012)在SG-1钻孔中划分的9个沉积阶段的湖泊水位变化特征进行比较分析,明确了Mn元素含量对湖泊水位的有效指示作用。Kim et al.(2015)在此基础上利用介形虫壳体的Mn/Ca比值探讨了蒙古北部Hovsgol湖泊末次间冰期以来的水位变化特征,与其他微量元素的指示结果一致。综合介形虫 Limnocythere inopinata 丰度及其壳体微量元素(图3)变化特征,本研究将色林错湖区5.3ka BP以来的古气候环境变化划分为3个阶段。
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图2 青藏高原中部色林错 L.inopinata 扫描电镜照片
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Fig.2 SEM photos of L.inopinata in Selin Co, central Qinghai-Tibetan Plateau
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(a)— L.inopinata 右壳;(b)— L.inopinata 双壳背视图
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(a)—Right lateral view of L.inopinata;(b)—dorsal view of L.inopinata
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阶段1:5.3~2.9ka BP,此阶段 Limnocythere inopinata 壳体的Mg/Ca比值均较低,尤其是3.6ka BP之前更低,指示水体温度较低。而壳体Mn/Ca比值从4.5ka BP开始呈现增长的趋势,直到2.9ka BP,表明色林错在此阶段水位上涨。 L.inopinata的丰度变化可以分为两个亚阶段,第一亚阶段为5.3~3.6ka BP,此阶段 L.inopinata 的丰度最低,且呈逐渐降低的趋势。第二亚阶段为3.6~2.9ka BP,此阶段介形虫的丰度较第一亚阶段有了明显的增长,但在整个沉积序列中仍处于低值。Wang Can et al.(2021)通过室内培养实验证明 L.inopinata 的耐受温度为11~17℃,最适生长温度为11~14℃,偏喜温暖环境,因此本研究认为色林错在5.3~3.6ka BP介形虫丰度很小可能与水体温度过低有关,而在3.6~2.9ka BP时期壳体Mg/Ca比值略有增长,指示温度较之前有所升高,介形虫丰度的增加正是对此的响应。
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4.2 ka气候事件发生于世界各地文明的前夜,对人类社会造成了深远的影响(deMenocal,2001;Arz et al.,2006)。这一突发的气候冷事件在青藏高原众多区域得到了验证(吴中海等,2003;沈吉等,2004;Ji Junfeng et al.,2005;吴艳宏等,2006),在本文SL-1钻孔岩芯中也有显示。不同于前人研究结果的是,SL-1岩芯中 Limnocythere inopinata 的丰度从4.5 ka BP左右就开始降低,直到4.2 ka BP达到最低,仅为5枚/5g,这说明在色林错湖区4.2 ka BP冷事件并不是突发的,而是一个缓慢降温的过程。4 ka冷事件也是一次全球性的气候突变,在地中海、东海、中原地区及青藏高原等多个区域均有显示(Staubwasser et al.,2003, 2006;Zhao Meixun et al.,2014),尤其在低纬度地区,对农业及社会发展产生了重要的影响(Wu Wenxiang et al.,2004;Drysdale et al.,2006)。SL-1孔在4.0 ka BP时L.inopinata的丰度极低,应是对此次气候冷干突变事件的响应。
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图3 青藏高原中部色林错 L.inopinata 丰度及壳体微量元素变化图
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Fig.3 Abundance and trace elements in shells of L.inopinata in Selin Co, central Qinghai-Tibetan Plateau
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(a)—壳体Mg/Ca比值变化图;(b)—壳体Mn/Ca比值变化图;(c)— L.inopinata 丰度图
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(a)—Mg/Ca ratios in the shells;(b)—Mn/Ca ratios in the shells;(c)—abundance of L.inopinata
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阶段2:2.9~1.8ka BP,总体上本阶段 Limnocythere inopinata 的丰度较前一阶段明显增加,只在本阶段末约2.2~1.8ka BP介形虫丰度突然降低。壳体Mn/Ca比值较低,而壳体Mg/Ca比值虽然较前一阶段略有增长,但仍为低值,指示该时期色林错水温仍然较低且湖泊水位下降。在本阶段后期2.0ka BP开始,壳体Mg/Ca比值开始明显上升,但此时期壳体Mn/Ca比值较低指示湖泊水位下降,约2.2ka BP开始突降的介形虫丰度表明此阶段气候环境恶劣,不适宜介形虫繁殖生长,因此壳体Mg/Ca比值的增长可能与水体Mg/Ca比值发生变化有关。壳体Mn/Ca在2.3ka BP、2.2ka BP和2.1ka BP三个时期出现高值,对应的三个时期的Mg/Ca比值也较高,应是指示三个短暂的温暖湿润期。
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2.8 ka BP冷事件在北大西洋(Bond et al.,1997;Bond,2001)以及我国湖北神农架地区(李伟等,2014)和内蒙古地区(和雅杰等,2020)均得到了印证,色林错湖区在2.8 ka BP时,介形虫丰度突降且趋近于0,壳体Mg/Ca和Mn/Ca比值在此时期均处于低值,表明色林错湖区的2.8 ka BP事件是一次突发事件,气候寒冷干旱。
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阶段3:1.8ka BP至今,此阶段 Limnocythere inopinata 丰度达到整个钻孔的最大值,表明此阶段的湖水环境最适宜 L.inopinata 生长繁殖。 L.inopinata壳体的Mg/Ca比值从1.8ka BP开始显著增长,尤其在1.5~0.7ka BP期间是整个钻孔壳体Mg/Ca比值最高的阶段。从0.7ka BP开始,壳体Mg/Ca比值较之前有较明显降低,表明气温有所下降,但从整个钻孔来看仍处于高值阶段,指示气温仍较温暖。壳体Mn/Ca比值在此阶段一直处于持续增长趋势,尤其在钻孔顶部0.7ka BP以来增长更为显著,表明此阶段色林错处于温暖湿润的环境中,虽然在后期温度略有降低,但总体来看介形虫丰度并未受到影响,且湖泊水位持续增长直到近代,与流域气象站数据以及卫星遥感影像资料显示的色林错近50年来不断扩张的结果一致。从图3c可以看出, L.inopinata 的丰度在0.9~0.7ka BP和0.3~0.2ka BP两个阶段明显降低且持续时间较长,指示两个时期的气候环境发生了较大的转变。0.9~0.7ka BP, L.inopinata 壳体Mg/Ca比值处于较高状态,而壳体的Mn/Ca比值在此期间出现较明显的下降,表明此阶段温度较高,湖泊水位不断下降,介形虫丰度的减少可能与短期气温上升、蒸发强烈、气候干旱、湖盆萎缩有关。 L.inopinata 壳体Mg/Ca和Mn/Ca比值均在0.3ka BP出现峰值,指示色林错在0.3ka BP时气候极度温暖湿润,但在0.3ka BP之后直到0.2ka BP,壳体Mg/Ca和Mn/Ca比值均处于下降趋势,表明气温及水位在此期间均有所降低,介形虫丰度的降低应是对这一气候转变的响应。
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青藏高原中部地区位于季风和西风带的边界过渡区域,同时受西风环流和季风系统控制,是研究西风环流与季风相互作用的理想场所。在中全新世以后,随着北半球太阳辐射的继续减少,海陆热力差的持续减小使得季风强度持续衰退,季风区气候逐渐干旱。Hurrell et al.(2003)指出北大西洋涛动(NAO)指数约在4.3ka BP之后逐渐由正相位转变为负相位,导致西风强度逐渐增加,强劲的西风带动西风环流带向南扩张进而带动更多的水汽进入内陆,使得气候更加湿润。位于西风区的古里雅冰芯分析显示,气温从5000a BP左右开始激烈下降(姚檀栋等,1997);位于过渡区的纳木错介形虫记录表明湖区温度从5.7ka BP开始下降,但湖泊水位在5.7~2.9ka BP期间呈上升趋势(Zhu Liping et al.,2010);位于青藏高原中部的赤布张错介形虫记录显示从全新世早期到中期,湖区具有温度下降、湿度增加、湖泊扩张的演化特征(董楠等,2021);而位于季风区的普莫雍错介形虫属种组合记录表明从6~3ka BP,湖泊基本处于低水位状态(Peng Ping et al.,2013),综合比较,色林错在5.3~2.9ka BP,较低的 Limnocythere inopinata 丰度和低位的壳体Mg/Ca比值均表明气温较低,但较高的壳体Mn/Ca比值表明湖泊水位较高,冷湿的气候特征与西风区及过渡区的湖泊演化特征相近,表明此阶段青藏高原中部地区主要受西风环流影响。
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从2.9~1.8ka BP,色林错基本处于低水位状态,气温略有增长但仍较低,介形虫丰度较前一阶段增加。Chen Fahu et al.(2014)通过云南星云湖沉积物中的孢粉研究明确了8500a BP以来印度季风的演化特征,并指出季风区年降水量自中全新世以来呈逐渐降低的趋势;位于青藏高原中部的赤布张错介形虫记录表明该区在2.9~1.5ka BP为相对低温时期(董楠等,2021);位于季风区的普莫雍错介形虫组合显示从2.5ka BP开始,湖泊水位呈下降趋势直至1.3ka BP(Peng Ping et al.,2013);但位于西风区的库赛湖沉积物中矿物组成结果显示,湖区在2.6ka BP气候向冷干发展(姚波,2011)。色林错虽然在2.3~2.0ka BP期间出现过短暂的温暖湿润时期,但总体上气温不高、水面较低,指示此阶段季风北进,西风环流虽然北撤但仍能影响到色林错,青藏高原中部地区处于西风与季风的共同作用影响下,气候偏凉干。
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晚全新世尤其1.5ka BP以来,色林错地区温度和水位都有极大的提升,处于高值的 Limnocythere inopinata 丰度及其壳体的Mg/Ca和Mn/Ca比值均表明此阶段色林错气候温暖湿润。该特征与青藏高原过渡区和季风区的其他湖泊记录基本一致,如过渡区的纳木错介形虫记录表明从1.5ka BP至今湖水深度增加(Zhu Liping et al.,2010);赤布张错介形虫组合及壳体微量元素也表明1.5ka BP至今湖泊水温上升、湖面扩张(董楠等,2021);季风区的普莫雍错介形虫记录表明1.3ka BP之后湖泊水位上升(Peng Ping et al.,2013);而位于西风区的库赛湖记录表明2.2ka BP之后气候更加冷干(姚波,2011)。因此推断这一时期色林错的气候变化可能受季风影响更大。除了季风带来的水汽之外,色林错三面环山,温度升高导致的冰川融水增加也是色林错水面上涨的重要因素(Hou Yandong et al.,2021)。
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5 结论
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(1)色林错地区中全新世以来的气候演变主要分为3个阶段, 5.3~2.9ka BP,气温较低,但湖泊呈扩张趋势;2.9~1.8ka BP,气温略回升,而水位却处于持续降低的状态,期间2.3~2.0ka BP出现短暂的温暖湿润阶段;1.8ka BP至今,气温和水位均呈明显的上涨趋势,虽然气温在0.7ka BP略有下降,但湖泊水位增长却更为显著。
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(2)色林错5.3ka BP以来的环境记录反映了青藏高原中部地区受西风环流和季风影响程度的变化。全新世中期,色林错的气候环境变化与西风区和过渡区的其他湖泊环境记录一致,显示气温偏低、湖面扩张的特征,说明色林错受西风环流的影响更为明显。全新世晚期2.9~1.8ka BP,色林错气温略有回升、湖泊水位下降,阶段性出现温暖湿润气候,与季风区和西风区湖泊环境记录均有相近之处,表明此阶段色林错受西风和季风的共同作用影响。1.8ka BP以来,色林错与过渡区的纳木错、赤布张错及季风区的普莫雍错均呈现相同的变化趋势,指示青藏高原中部地区在晚全新世湖泊扩张与季风增强带来的降水和气温升高导致的冰川融水增加有密切的关系。
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摘要
色林错位于青藏高原中部印度季风和西风环流的过渡地带,同时受西风环流和印度季风系统控制,是研究二者进退变化特征的理想场所。本文利用色林错SL-1钻孔中介形虫 Limnocythere inopinata 的丰度及其壳体微量元素Mg/Ca和Mn/Ca比值重建了色林错5.3 ka BP以来的古气候环境变化特征。5.3~2.9 ka BP, L.inopinata 丰度较小,壳体的低Mg/Ca比值和高Mn/Ca比值表明此阶段气候偏冷湿;2.9~1.8 ka BP, L.inopinata 丰度较前一阶段增加,壳体Mg/Ca比值略有增长但仍为低值表明气温虽然有所回升但仍然较低,Mn/Ca比值较前一阶段明显降低,指示湖泊水位下降;1.8 ka BP至今, L.inopinata 丰度达到最大,壳体的高Mg/Ca和Mn/Ca比值指示湖泊温度和水位均呈显著的上升趋势。通过与西风区、过渡区以及印度季风区其他湖泊的环境沉积记录对比,本文认为青藏高原中部地区在中全新世晚期主要受西风环流影响,气温较低,西风带来大量水汽使得湖面呈扩张趋势;而到晚全新世西风环流逐渐北撤,色林错受季风影响更大,季风带来的降水和气温升高导致的冰川融水增加与色林错水位上升有密切关系。
Abstract
Selin Co, located at the transitional region of Indian monsoon and westerlies circulation in the central Qinghai-Tibetan Plateau (QTP) and controlled by both, is an ideal place for studying the change characteristics of their advance-retreat history. Based on the abundance and trace elements (Mg/Ca and Mn/Ca) in shells of Limnocythere inopinata, we reconstructed the paleoclimate changes since 5.3 ka BP in Selin Co. 5.3~2.9 ka BP, the low abundance and decreased Mg/Ca ratios in shells of L. inopinata indicated cold and humid climate. 2.9~1.8 ka BP, the temperature rose slightly implied by the increase of abundance and little rise in Mg/Ca ratio of L. inopinata, but the reduced Mn/Ca ratios indicated that the lake levels kept decrease compared with the previous stage. The abundance of L. inopinata reached the maximum since 1.8 ka BP, the high values of Mg/Ca and Mn/Ca ratios showed the significant increase of temperature and lake levels. Comparing with the sedimentary records of other lakes in westerlies region, transition region and Indian monsoon region, we think that the central QTP was affected by westerlies circulation in the late stage of Middle Holocene, the temperature was low, but the abundant moisture brought by the westerlies made the lake expand, while in the Late Holocene, the westerlies circulation gradually withdrew northward and Selin Co was more affected by the Indian monsoon, the increased lake water level of Selin Co was closely related to the increase in precipitation and glacial-melt water caused by the rise of temperature under the enhancing of the Indian monsoon.
Keywords
Selin Co ； ostracodes ； westerlies circulation ； Indian monsoon ； climate change
