氣候系統(tǒng)與氣候變化研究進(jìn)展

氣候系統(tǒng)與氣候變化
Climate System and Climate Change

氣候系統(tǒng)與氣候變化研究進(jìn)展

2012年，氣候系統(tǒng)研究所以“東亞季風(fēng)變異機(jī)理及其對(duì)中國(guó)氣候影響”研究為核心，圍繞東亞季風(fēng)變異機(jī)理和預(yù)測(cè)、海-陸-氣相互作用對(duì)東亞氣候影響、青藏高原熱力和動(dòng)力過(guò)程及其影響、氣候變化機(jī)理與預(yù)測(cè)理論4個(gè)主要方向開(kāi)展研究，在東亞季風(fēng)長(zhǎng)期變化機(jī)理等方面取得了創(chuàng)新性成果。

1 東亞季風(fēng)變異機(jī)理和預(yù)測(cè)研究

研究了中國(guó)東部地區(qū)盛夏降水與溫度變化的關(guān)系，大氣季節(jié)內(nèi)振蕩、ENSO和AO不同位相配置對(duì)中國(guó)氣候?yàn)?zāi)害的影響，東亞冬季風(fēng)預(yù)報(bào)因子的確定，并對(duì)NCEP氣候預(yù)報(bào)系統(tǒng)（CFS）模式進(jìn)行了評(píng)估。

1.1 中國(guó)中東部地區(qū)降水特征的年代際變化

以降水的強(qiáng)度結(jié)構(gòu)為基礎(chǔ)，通過(guò)e指數(shù)擬合和確定e折倍點(diǎn)對(duì)我國(guó)東部地區(qū)盛夏逐時(shí)降水進(jìn)行分類，利用e折倍強(qiáng)度劃分出弱降水和強(qiáng)降水，超過(guò)2倍e折倍強(qiáng)度的降水被視為極端降水。發(fā)現(xiàn)在過(guò)去幾十年間，弱降水和強(qiáng)降水比例發(fā)生了顯著變化，且弱降水百分比變化的空間型與氣溫變化的空間型相似。根據(jù)各站點(diǎn)的氣溫變化特征，劃分了華北地區(qū)（增溫）、中東部地區(qū)（冷卻）和東南沿海地區(qū)（增溫）。在增溫區(qū)，弱降水比例明顯減少；而冷卻區(qū)弱降水比例呈增加趨勢(shì)。各地區(qū)弱（強(qiáng)）降水比例與氣溫變化均存在負(fù)（正）相關(guān)，而極端降水與氣溫的關(guān)系卻具有顯著的區(qū)域差異（圖1）。

1.2 MJO、ENSO和AO對(duì)中國(guó)極端干旱氣候影響

研究發(fā)現(xiàn)，大氣季節(jié)內(nèi)振蕩（MJO）和北極濤動(dòng)（AO）的持續(xù)性異常對(duì)東亞地區(qū)旱澇有重要影響。2009—2010年我國(guó)云南發(fā)生的秋、冬、春3季連旱與不活躍的MJO和弱AO的長(zhǎng)期維持密切相關(guān)；持續(xù)的MJO不活躍也是2011年云南主汛期期間夏季極端干旱的重要成因之一；秋冬季AO的負(fù)位相有利于烏拉爾山阻塞高壓維持和發(fā)展，而貝加爾湖上空出現(xiàn)負(fù)位勢(shì)高度異常，導(dǎo)致東亞中高緯度經(jīng)向環(huán)流加強(qiáng)和冷空氣向南侵襲，進(jìn)而間接導(dǎo)致華北地區(qū)的干冷氣候。

1.3 東亞冬季風(fēng)預(yù)報(bào)因子

研究發(fā)現(xiàn)，前期秋季中緯度東太平洋區(qū)的海溫異常、北極喀拉海海冰密集度和東亞高空氣溫具有很好的持續(xù)性，可以將異常信號(hào)從秋季傳遞到冬季，進(jìn)而影響冬季風(fēng)的強(qiáng)度。利用上述研究成果建立了東亞冬季風(fēng)預(yù)報(bào)模型，并應(yīng)用到2012/2013年冬季風(fēng)預(yù)報(bào)中。

1.4 氣候預(yù)報(bào)系統(tǒng)（CFS）模式結(jié)果評(píng)估

通過(guò)對(duì)多組NCEP 氣候預(yù)報(bào)系統(tǒng)（CFS）60天后報(bào)結(jié)果的分析，比較了3種分辨率（T62，T126 和T254）的大氣模式對(duì)NCEP CFS 進(jìn)行季風(fēng)季節(jié)進(jìn)程預(yù)測(cè)的影響。結(jié)果表明，對(duì)于陸地上的夏季風(fēng)建立及其推進(jìn)，高分辨率模式整體表現(xiàn)比低分辨率模式好，且在南亞夏季風(fēng)區(qū)比東亞夏季風(fēng)區(qū)更明顯。高分辨率模式最大的改進(jìn)是更加真實(shí)地再現(xiàn)了大地形附近的降水分布，特別是在青藏高原附近。然而，提高大氣模式分辨率不能有效提高海洋上空季風(fēng)活動(dòng)的預(yù)測(cè)效果，隨著大氣模式的分辨率從T62提升至T254，模擬的季風(fēng)降水在南海和菲律賓海反而更差。（李建）

2 海-陸-氣相互作用對(duì)東亞氣候影響研究

揭示了北極海冰持續(xù)減少的動(dòng)力成因，研究了歐亞大陸積雪對(duì)東亞夏季風(fēng)異常的影響、兩類ENSO與太平洋次表層海溫的關(guān)系，以及北太平洋副熱帶和副極地流渦的變化等。

2.1 北極地區(qū)地表風(fēng)變化模態(tài)研究

利用1979—2010年NCEP/NCAR月平均地表風(fēng)，采用復(fù)向量經(jīng)驗(yàn)正交函數(shù)（CVEOF）分析了北極地區(qū)地表風(fēng)變化的2個(gè)主要模態(tài)。第1個(gè)空間模態(tài)包含2個(gè)子模態(tài)，分別為拉普帖夫海（NLS）模態(tài)和北極偶極子（AD）模態(tài)；第2個(gè)模態(tài)包括北極喀拉海（NKS）模態(tài)和北極中部（CA）模態(tài)。在過(guò)去的20年里，這2個(gè)模態(tài)的聯(lián)合動(dòng)力強(qiáng)迫是北極地區(qū)9月海冰面積不斷縮小并降至記錄最低值的重要原因。9月海冰面積最低值主要與夏季（7—9月）AD模態(tài)的負(fù)位相和CA模態(tài)的正位相有關(guān)，這2個(gè)位相都顯示出北冰洋上為一個(gè)異常的反氣旋控制。風(fēng)場(chǎng)通過(guò)頻率和強(qiáng)度影響9月海冰面積。過(guò)去20年9月海冰面積的下降趨勢(shì)與海冰融化季節(jié)（4—9月）CA模態(tài)的增多增強(qiáng)趨勢(shì)有關(guān)。因此，不能簡(jiǎn)單地認(rèn)為是AD模態(tài)異常的結(jié)果。CA模態(tài)在1990年代后期表現(xiàn)出年代際變化特征，1997年以前北冰洋上為異常氣旋環(huán)流而以后被異常反氣旋所代替，與9月海冰面積下降的趨勢(shì)一致。

2.2 歐亞大陸積雪對(duì)華南降水影響數(shù)值模擬

基于歐亞大陸積雪深度資料和中國(guó)臺(tái)站降水資料的奇異值分析（SVD）結(jié)果，使用大氣環(huán)流模式（CAM3.1）分別進(jìn)行3組集合試驗(yàn)，來(lái)研究歐亞大陸積雪的反照率效應(yīng)和水文效應(yīng)對(duì)2010年5—6月華南降水的影響。第1組試驗(yàn)綜合考慮積雪的2種物理效應(yīng)，既有反照率效應(yīng)又有水文效應(yīng)；第2組試驗(yàn)僅考慮積雪反照率效應(yīng)，忽略水文效應(yīng)；第3組試驗(yàn)只考慮積雪水文效應(yīng)，忽略反照率效應(yīng)。試驗(yàn)結(jié)果表明，積雪的2種物理效應(yīng)都會(huì)對(duì)后期華南降水產(chǎn)生影響，但是3組試驗(yàn)中積雪不同物理效應(yīng)所導(dǎo)致的異常幅度和范圍存在較大差異，其中積雪水文效應(yīng)比反照率效應(yīng)引起的變化幅度大。當(dāng)2種效應(yīng)共同作用時(shí)產(chǎn)生的異常與統(tǒng)計(jì)分析結(jié)果最接近，變化幅度也最大，但是并不等于單純反照率效應(yīng)和單純水文效應(yīng)作用之和。

2.3 春季歐亞積雪與東亞夏季風(fēng)關(guān)聯(lián)

研究了春季歐亞大陸融雪量與東亞夏季風(fēng)的關(guān)系，并初步討論了其可能聯(lián)系機(jī)制。研究表明，春季融雪量EOF第1模態(tài)表現(xiàn)出年代際變化特征，這與東亞夏季風(fēng)和中國(guó)夏季降水的年代際轉(zhuǎn)型具有非常好的一致性。而EOF 第2模態(tài)與東亞夏季風(fēng)在年際尺度上具有同位相變化關(guān)系，當(dāng)春季融雪量在東西伯利亞和巴爾喀什湖附近異常偏多時(shí)，后期在東亞地區(qū)容易出現(xiàn)由高緯度至低緯度的“負(fù)-正-負(fù)”經(jīng)向波列結(jié)構(gòu)。融雪量異常偏少時(shí)，情況則相反。初步分析了春季融雪量異常與后期夏季東亞地區(qū)大氣環(huán)流出現(xiàn)經(jīng)向波列結(jié)構(gòu)的可能聯(lián)系機(jī)制，東西伯利亞以及巴爾喀什湖附近異常偏多的春季融雪量能夠在該地區(qū)促使位勢(shì)高度場(chǎng)表現(xiàn)為正異常，隨著時(shí)間的演變，巴爾喀什湖附近地區(qū)的高壓向東移動(dòng)發(fā)展，東西伯利亞地區(qū)的高壓一部分向低緯移動(dòng)，可能造成夏季東亞地區(qū)的經(jīng)向波列結(jié)構(gòu)，進(jìn)而對(duì)東亞的天氣和氣候產(chǎn)生影響。

2.4 表層和次表層海溫耦合模態(tài)和2類ENSO關(guān)系

對(duì)2類ENSO事件時(shí)空變化研究發(fā)現(xiàn)，東部和中部型ENSO分別表現(xiàn)出顯著的年際尺度（27年）和年代際尺度（10～15年）變化特征。在熱帶太平洋海表溫度（SST）和次表層溫度（SOT）變化存在顯著的耦合關(guān)系：第1個(gè)SVD模態(tài)對(duì)應(yīng)的SST和SOT異常場(chǎng)上均表現(xiàn)出緯向偶極型分布，即熱帶中東太平洋海溫異常增暖，熱帶西太平洋海溫異常偏冷；第2個(gè)SVD模態(tài)對(duì)應(yīng)的SST和SOT異常場(chǎng)上均呈現(xiàn)出長(zhǎng)期線性趨勢(shì)的變化；而第3個(gè)SVD模態(tài)所對(duì)應(yīng)的SST和SOT異常場(chǎng)上均呈現(xiàn)出緯向3極型分布，即海溫異常偏高出現(xiàn)于熱帶中太平洋，熱帶西太平洋和東太平洋海溫異常偏冷。此外，20世紀(jì)80年代后SOT在年代際時(shí)間尺度上振幅明顯增強(qiáng)這一事實(shí)說(shuō)明，中部型ENSO事件的頻率和強(qiáng)度均呈現(xiàn)出上升趨勢(shì)，且在過(guò)去的30年里中部型ENSO事件明顯加強(qiáng)了對(duì)全球氣候的影響（圖2）。

2.5 北太平洋副熱帶和副極地流渦年際變率

北太平洋副熱帶和副極地流渦應(yīng)對(duì)風(fēng)應(yīng)力變化的調(diào)整會(huì)產(chǎn)生不同尺度變化，在氣候變化中有重要作用。基于SODA（Simple Ocean Data Assimilation）和GODAS（Global Ocean Data Assimilation System）資料，利用三維海洋環(huán)流診斷方法分析了副熱帶和副極地流渦的環(huán)流強(qiáng)度和中心位置的變化，建立了副熱帶和副極地流渦的強(qiáng)度、經(jīng)向位置和深度位置的3類指數(shù)序列。結(jié)果表明，所建指數(shù)能夠很好地表達(dá)副熱帶和副極地流渦的季節(jié)、年際和年代際變化。在夏季副熱帶流渦強(qiáng)度要遠(yuǎn)小于冬季，副極地流渦冬季最強(qiáng)，秋季最弱。副熱帶流渦從2—3月開(kāi)始北移，到10月左右開(kāi)始南移，1月左右位置最靠南。而副極地流渦是春季北移，夏季南移，秋季又北移，到冬季南移到極值點(diǎn)。2個(gè)流渦的共同特征是1976—1977年前偏弱偏北，之后偏強(qiáng)加深偏南。對(duì)于年代際尺度的變化，副極地流渦的貢獻(xiàn)極其重要。副極地流渦的強(qiáng)度指數(shù)顯示流渦在1976—1977年存在明顯的由弱到強(qiáng)的躍變過(guò)程。強(qiáng)度指數(shù)與太平洋年代際振蕩（PDO）指數(shù)的相關(guān)系數(shù)為0.45，要遠(yuǎn)好于副熱帶環(huán)流的強(qiáng)度指數(shù)與PDO指數(shù)的相關(guān)。檢驗(yàn)證明，受中小尺度渦動(dòng)的影響，大尺度流渦強(qiáng)度量級(jí)對(duì)資料分辨率有很強(qiáng)的依賴性，但指數(shù)所表達(dá)的流渦年際和年代際大尺度變化受模式分辨率影響較小。（祝從文）

3 青藏高原熱力、動(dòng)力過(guò)程及其影響研究

利用衛(wèi)星觀測(cè)資料，重點(diǎn)討論了青藏高原東南地區(qū)降水的日變化特征，以及高原低渦和中國(guó)南方地區(qū)云的日變化特征。

3.1 青藏高原東南地區(qū)降水日變化特征

青藏高原東南地區(qū)地形復(fù)雜，河谷縱橫，其夏季降水表現(xiàn)為傍晚和午夜降水雙峰值并存的特征。衛(wèi)星降水?dāng)?shù)據(jù)顯示出一致的午后降水峰值，而常規(guī)臺(tái)站觀測(cè)降水則為午夜峰值?；谏R拉山加密觀測(cè)試驗(yàn)數(shù)據(jù)的分析表明，高原上降水的日變化特征與地形密切相關(guān)，山坡的降水峰值常出現(xiàn)在傍晚，而山谷的降水峰值常出現(xiàn)在午夜。衛(wèi)星反演降水的日變化特征與色齊拉山的山坡站一致；而常規(guī)臺(tái)站觀測(cè)降水日變化與山谷站一致。衛(wèi)星與常規(guī)臺(tái)站的降水的日變化差異與常規(guī)臺(tái)站位于山谷中有關(guān)，位于山谷中的常規(guī)臺(tái)站觀測(cè)降水?dāng)?shù)據(jù)難以全面反映高原降水的區(qū)域特征。同時(shí)，衛(wèi)星降水對(duì)夜雨的低估對(duì)二者的差異亦有貢獻(xiàn)。上述分析表明，在分析復(fù)雜地形區(qū)的降水性質(zhì)時(shí)，充分利用多種觀測(cè)資料將有助于全面揭示區(qū)域的降水特征，研究結(jié)果也為高原地區(qū)臺(tái)站的后續(xù)建設(shè)提供了參考（圖3）。

3.2 高原低渦和中國(guó)南方地區(qū)云日變化特征

利用美國(guó)國(guó)家環(huán)境預(yù)報(bào)中心（NECP）提供的每日4次再分析資料（水平分辨率為1°×1°）、我國(guó)12 h一次的常規(guī)探空資料和逐時(shí)地面氣候要素觀測(cè)資料，以及中日合作JICA項(xiàng)目提供的大氣可降水量（PWV）資料和改則站每日4次的加密探空資料，統(tǒng)計(jì)了2006—2008年5—9月青藏高原低渦在各時(shí)次的生成頻數(shù)，并對(duì)各時(shí)段大尺度環(huán)流場(chǎng)、相關(guān)要素場(chǎng)和加熱場(chǎng)進(jìn)行了合成分析。研究結(jié)果表明，青藏高原低渦的生成具有日變化特征，其在傍晚到午夜的生成頻數(shù)最高，在清晨到正午產(chǎn)生頻數(shù)最低。高原低渦的產(chǎn)生是大尺度環(huán)流場(chǎng)和高原地區(qū)加熱場(chǎng)共同作用的結(jié)果，降水的凝結(jié)潛熱加熱對(duì)高原低渦的產(chǎn)生有直接的促進(jìn)作用。傍晚，高原中西部500 hPa氣流有較強(qiáng)的輻合，200 hPa高空西風(fēng)急流和南亞高壓的強(qiáng)度較強(qiáng)，并且高原主體上水汽收入最多，層結(jié)較不穩(wěn)定，有利于降水在此時(shí)發(fā)生。降水的凝結(jié)潛熱加熱有利于形成上暖下冷的熱力結(jié)構(gòu)，使500 hPa的氣旋性擾動(dòng)得到發(fā)展，導(dǎo)致大量低渦形成于傍晚后到午夜這個(gè)時(shí)間段，清晨到正午反之。

3.3 夏季亮溫揭示的云變化特征

利用風(fēng)云靜止氣象衛(wèi)星紅外亮溫?cái)?shù)據(jù)，分析了夏季亮溫日變化的氣候特征。根據(jù)云分類產(chǎn)品剔除晴空數(shù)據(jù)后，按照云頂亮溫不同的日變化特征，將云分為3類，即冷云（云頂亮溫低于-30 °C）、中層云（云頂亮溫介于-30～0 °C）和暖云（云頂亮溫高于0 °C），著重關(guān)注中國(guó)南方地區(qū)3類云發(fā)生頻率的日變化特征及其與降水日變化區(qū)域差異的關(guān)系。分析發(fā)現(xiàn)，中國(guó)南方地區(qū)夏季3類云的日變化存在共性特征，即多數(shù)地區(qū)的冷云在傍晚發(fā)生最為頻繁，在高原下游亦存在夜間的峰值。中層云發(fā)生頻率峰值位相多在夜間，西南地區(qū)的峰值多出現(xiàn)在下半夜而東南地區(qū)多在上半夜，在高原東部存在傍晚峰值。暖云在中午前后發(fā)生更頻繁，且區(qū)域差異相對(duì)較小，只在南部沿海地區(qū)存在夜間峰值。同時(shí)，不同云的日變化亦存在鮮明的區(qū)域差異，特別是冷云與中層云的發(fā)生頻率在西南和東南均表現(xiàn)出不同的日變化特征。冷云在西南（東南）地區(qū)的清晨（傍晚）發(fā)生更為頻繁。中層云發(fā)生頻率在西南地區(qū)為午夜至凌晨的峰值，而在東南地區(qū)為上半夜的峰值。此外，冷云和中層云頻率的日變化還存在明顯的季節(jié)變化，而暖云頻次日變化季節(jié)變化小。冷云頻率的傍晚峰值幾乎全年存在，在暖季主要為單峰結(jié)構(gòu)，而在冷季存在夜間至凌晨的次峰值。中層云頻率峰值在夏季出現(xiàn)在上半夜，而在其他季節(jié)多出現(xiàn)在午夜至凌晨。暖云峰值位相在全年均出現(xiàn)在中午前后，其季節(jié)變化主要體現(xiàn)在日變化振幅上，在暖季較小而其他季節(jié)較大。研究結(jié)果表明，充分利用高分辨率的靜止衛(wèi)星亮溫有助于增進(jìn)對(duì)區(qū)域云雨日變化特征的理解。（陳昊明）

4 氣候變化機(jī)理與預(yù)測(cè)理論研究

結(jié)合觀測(cè)資料和數(shù)值模擬，討論了東亞季風(fēng)年代際變化和過(guò)去千年變化的可能成因。

4.1 亞洲大陸地表溫度變化對(duì)東亞季風(fēng)的影響

采用觀測(cè)資料和美國(guó)國(guó)家環(huán)境預(yù)報(bào)中心CFS模式輸出的大氣分量，揭示了近半個(gè)世紀(jì)以來(lái)在全球變暖背景下亞洲大陸地表增溫幅度小于其他地區(qū)的特征，盡管亞洲表面溫度增加，但是由于世界其他地方的增溫更大，亞洲大陸相對(duì)而言仍然是一個(gè)熱沉降區(qū)。與亞洲之外的區(qū)域不同，近10年整層積分的亞洲對(duì)流層溫度比前幾十年偏冷。由此提出了亞洲大陸的這種“相對(duì)變冷”可能是導(dǎo)致亞洲季風(fēng)變?nèi)醯闹匾蛑弧?/p>

4.2 全球變暖對(duì)東亞夏季風(fēng)的影響

利用觀測(cè)資料和數(shù)值模擬，研究了東亞夏季風(fēng)減弱的可能成因。發(fā)現(xiàn)東亞夏季風(fēng)的持續(xù)性減弱與環(huán)貝加爾湖地區(qū)的變暖存在密切聯(lián)系。該地區(qū)的變暖可以在東亞地區(qū)上空激發(fā)出對(duì)流層異常反氣旋，從而抑制了夏季西南季風(fēng)向北輸送，導(dǎo)致亞洲北部干旱和南方降水偏多現(xiàn)象。采用CAM3數(shù)值模式考慮溫室氣體、海表溫度、太陽(yáng)輻照度和火山活動(dòng)的模擬結(jié)果可以再現(xiàn)貝加爾湖變暖和相關(guān)的大氣環(huán)流，然而不包含溫室氣體的模擬不能再現(xiàn)貝加爾湖地區(qū)1970年代的變暖。這意味著全球變暖可能導(dǎo)致了貝加爾湖地區(qū)的局地變暖，進(jìn)而導(dǎo)致了東亞夏季風(fēng)近幾十年來(lái)的減弱。

4.3 中國(guó)夏季氣候長(zhǎng)期變化模式模擬研究

利用國(guó)家氣候中心大氣環(huán)流模式（BCC_AGCM），初步診斷分析并評(píng)估了1955—2000年?yáng)|亞夏季風(fēng)年代際變化的整體結(jié)構(gòu)。東亞夏季風(fēng)在觀測(cè)海溫強(qiáng)迫下，合理再現(xiàn)了降水、氣溫和環(huán)流的年代際變化，模式再現(xiàn)了中國(guó)降水南澇北旱的主要特征，進(jìn)一步增加了對(duì)南澇北旱成因的認(rèn)識(shí)。同樣，對(duì)流層中上層變冷、高層西風(fēng)急流南移和低層西南季風(fēng)環(huán)流減弱以及他們和降水變化的關(guān)系也被模式再現(xiàn)。模式模擬的一個(gè)最主要的缺陷是對(duì)流層變冷和大尺度環(huán)流變化的幅度比再分析資料的小，他們和降水變化的關(guān)系亦是如此。還對(duì)模式中對(duì)流層中上層變冷的中心和上層西風(fēng)急流軸西移減弱的現(xiàn)象和觀測(cè)進(jìn)行了比較。總體而言，盡管BCC_AGCM模式在模擬東亞氣候的年代際變化存在一些問(wèn)題，尤其是在變化中心和幅度上，但是該模式能夠合理地再現(xiàn)觀測(cè)中降水變化的配置及其相關(guān)的氣溫和環(huán)流變化。因此，該模式可以用來(lái)進(jìn)一步探討東亞年代際變化的機(jī)制。同時(shí)，利用SST強(qiáng)迫的數(shù)值模式合理再現(xiàn)降水變化的配置及其相關(guān)大氣環(huán)流結(jié)果表明，東亞氣候的年代際變化可能是對(duì)全球氣候變化的區(qū)域響應(yīng)。

4.4 千年氣候變化數(shù)值模擬研究

采用中等復(fù)雜程度的UVic地球系統(tǒng)氣候模式模擬了氣候強(qiáng)迫因子(太陽(yáng)輻射、火山灰、太陽(yáng)軌道、陸表植被變化、溫室氣體和人為排放的硫酸鹽氣溶膠)對(duì)中國(guó)東部地區(qū)過(guò)去千年氣候變化的貢獻(xiàn)。結(jié)果表明，考慮所有氣候強(qiáng)迫因子的數(shù)值試驗(yàn)可以較好地再現(xiàn)北半球和中國(guó)東部地區(qū)中世紀(jì)暖期、小冰期和20世紀(jì)暖期這3個(gè)特征時(shí)期，與重建的氣溫在百年尺度上也具有較好的一致性。模擬結(jié)果很好地反映了中國(guó)東部氣溫異常在中世紀(jì)暖期和20世紀(jì)上半葉比全球氣溫異常偏高，以及小冰期氣溫異常比全球偏低的事實(shí)。根據(jù)中國(guó)東部氣溫的冷暖程度和氣候強(qiáng)迫因子的相對(duì)貢獻(xiàn)大小，將過(guò)去千年中國(guó)東部地區(qū)氣溫分為8個(gè)階段：中世紀(jì)暖期3個(gè)、小冰期4個(gè)和20世紀(jì)暖期1個(gè)，并揭示了氣候強(qiáng)迫因子對(duì)這些子階段維持及其轉(zhuǎn)換的貢獻(xiàn)。結(jié)果表明，中國(guó)東部地區(qū)中世紀(jì)暖期的主要貢獻(xiàn)來(lái)自于太陽(yáng)輻射，火山灰次之；小冰期各個(gè)子階段的轉(zhuǎn)換過(guò)程中，主要取決于溫室氣體、火山灰和太陽(yáng)輻射的相對(duì)貢獻(xiàn)大小。溫室氣體和火山灰分別為小冰期的最后2個(gè)階段中最主要的貢獻(xiàn)因子。本研究發(fā)現(xiàn)了不同自然氣候強(qiáng)迫因子之間和不同人為氣候強(qiáng)迫因子之間的非線性響應(yīng)，太陽(yáng)軌道變化和火山灰氣溶膠強(qiáng)迫的非線性響應(yīng)與溫室氣體和陸表植被(或者硫酸鹽氣溶膠)強(qiáng)迫的非線性響應(yīng)對(duì)中國(guó)東部地區(qū)20世紀(jì)末的增溫貢獻(xiàn)都達(dá)到了約0.2 °C，而自然氣候強(qiáng)迫因子和人為氣候強(qiáng)迫因子之間不存在明顯的非線性響應(yīng)。自然氣候強(qiáng)迫因子之間和人為氣候強(qiáng)迫因子之間的非線性響應(yīng)對(duì)中國(guó)東部地區(qū)20世紀(jì)變暖大約分別貢獻(xiàn)了0.09 °C和0.18 °C，二者之和約占中國(guó)東部地區(qū)20世紀(jì)末增暖的50%（圖4，紅線減去灰線），其余的增溫來(lái)自于氣候強(qiáng)迫因子的線性貢獻(xiàn)(即線性響應(yīng))。氣候強(qiáng)迫因子之間的非線性響應(yīng)為認(rèn)識(shí)氣候變化提供了一個(gè)新視角（圖4）。（肖棟）

圖1 不同強(qiáng)度降水百分比變化(1986—2005年減1966—1985年)(實(shí)線、虛線和點(diǎn)線分別代表華北、中東部和華南沿海地區(qū)，粗線為7點(diǎn)平滑曲線)Fig1 The 20-yr mean changes (1986—2005 minus 1966—1985) in the percentage of rainfall amount as a function of rainfall intensity. (The solid, dashed, and dotted lines are for North China (sienna), central eastern China (green), and southeastern coastal area of China (blue), respectively. Thick lines denote the seven point smoothing series)

圖2 1958—2007年熱帶太平洋地區(qū)海表面溫度(SST)(a)與赤道(5°S-5°N 平均)次表層海溫(SOT)的SVD 第3模態(tài)(b)及相對(duì)應(yīng)的時(shí)間系數(shù)分布(c)(綠色粗體曲線是氣候平均的20 °C等溫線, SST時(shí)間序列(藍(lán)色實(shí)線)與SOT 時(shí)間序列(紅色實(shí)線)的相關(guān)系數(shù)為0.76）Fig2 The third SVD mode of the SST (a) and the SOT anomalous felds (b) and their expansion coeffcients (c) during 1958—2007. The expansion coeffcient for SST and SOT is indicated by the blue solid line and the red solid line, respectively. In (a) and (b), units are arbitrary, and the green bold curve in (b) is the climatological mean of the 20 °C isotherm. Variance fraction in the SST and SOT is expressed as a percentage value on the upper right corner above each panel, and the correlation coeffcient between the expansion coeffcients is on the top right corner above (c)

圖3 臺(tái)站觀測(cè)(a)與TRMM 3B42反演降水(b)得到的1998—2004年暖季降水峰值時(shí)間分布(箭頭指向表示當(dāng)?shù)貢r(shí)間，時(shí)鐘如(a)左上角所示，填色為地形高度，(c)為臺(tái)站觀測(cè)(黑虛線)與TRMM反演(灰實(shí)線)降水(單位：mm/h)平均的日變化曲線)Fig3 Precipitation from station observation (a) and TRMM 3B42 (b) in warm seasons during 1998—2004. Vectors denote the local solar time (LST) when the rainfall reaches the diurnal maxima (The clock is shown in the upper-left of the plot. The shading denotes the topography. (c) The diurnal curves of average rainfall amount (unit∶ mm/h) from station observations (black dashed line) and TRMM 3B42 (grey solid line)

圖4 氣候強(qiáng)迫因子對(duì)中國(guó)東部表面氣溫變化的貢獻(xiàn)(氣候強(qiáng)迫因子包括溫室氣體(CO2和附加溫室氣體)、太陽(yáng)入射輻射變化、太陽(yáng)軌道變化、異常火山灰氣溶膠、陸表植被變化、人為排放的硫酸鹽氣溶膠和所有氣候強(qiáng)迫因子, 灰色表示氣候強(qiáng)迫因子的單獨(dú)貢獻(xiàn)之和)Fig4 Contributions of climate forcing factors to the temperature variation of East China (transient run minus equilibrium run). The climate forcing factors are greenhouse gases (CO2and additional greenhouse gases), solar insolation variability, solar orbit change, anomalous volcano aerosols, land cover changes, and sulfate aerosols. The red and grey lines represent the contributions of all climate forcing factors and the sum of contributions of individual climate forcing factor, respectively

Progress in Climate and Climate Change Research

In 2012, the Institute of Climate System of CAMS focused their researches on variability and mechanisms of the East Asian monsoon (EAM) and its effects on the climate of China, explored (1) variability, mechanisms, and prediction of the EAM, (2) influences of the air-sea-land interactions on the East Asian climate, (3) atmospheric thermodynamic and dynamic processes over the Tibetan Plateau and their impacts, and (4) mechanisms and projection theories of climate change. Some new observational phenomena have been unveiled, and innovative research results have been achieved.

1 Variability, mechanisms and prediction of the East Asian monsoon

Researches in this feld include the relation between late summer precipitation and temperature in eastern China; infuences of MJO, ENSO, and AO on the climatic disasters in China; identifcation of the predictors for the East Asian winter monsoon; and evaluation of the NCEP Climate Forecast System.

1.1 Decadal variability of climate characteristics in China

The late summer rainfall over eastern contiguous China is classifed into moderate, intense, and extreme precipitation according to hourly rainfall intensity structure. The e-folding decay intensity derived from the exponential distribution of rainfall amount is defned as the threshold that partitions rainfall into moderate and intense rainfall, and the double e-folding decay intensity is used as the threshold to pick out extreme cases. In the last several decades, the ratio between moderate and intense rainfall has experienced signifcant changes. Moreover, the spatial pattern of changes in the percentage of moderate rainfall presents a direct relation with that of the surface air temperature. Based on temperature changes, three regimes, i.e. northern China (warming), central eastern China (cooling), and southeastern coastal area of China (warming), are defned. In the warming regimes, the percentage of moderate rainfall exhibits a decreasing trend. In central eastern China, where the temperature has fallen, the percentage of moderate rainfall increased prominently. In all the three regimes, signifcant negative (positive) correlations between the percentage of moderate (intense) rainfall and the change of temperature are found. The relation between the extreme rainfall and the surface air temperature is different in different regions (Fig1).

1.2 Impacts of MJO, ENSO, and AO on the climatic disasters in China

The persistent anomalies of Madden-Julian Oscillation (MJO) and Arctic Oscillation (AO) have important implications for the drought and food in eastern Asia. The three-season drought in autumn, winter, and spring during the period of 2009—2010 in Yunnan Province was caused by the extreme anomalies of MJO and AO∶MJO was inactive while AO was unusually weak. The persistent inactive MJO was also responsible for the extreme summer drought in Yunnan during the main food season in 2011. During the negative phase of the AO, geopotential height at 500 hPa decreases around the Lake Baikal, the Ural blocking high develops, and meridional circulation anomalies prevail over East Asia, indirectly causing a cold and dry climate in North China.

1.3 Identifcation of the predictors for the East Asian winter monsoon

The SST anomalies at the midlatitude eastern Pacifc, the sea ice concentration in the Kara Sea, and the upper-level temperature over East Asia are revealed to be persistent. These abnormal signals can maintain from autumn to winter and infuence the intensity of the winter monsoon. A predictive model for the East Asian winter monsoon has been set up based on this fnding and has been applied in prediction of the 2012/2013 winter monsoon.

1.4 Evaluation of the NCEP Climate Forecast System

A series of 60-day hindcasts by the Climate Forecast System of the US National Centers for Environmental Prediction is analyzed to understand the impacts of atmospheric model resolutions (T62, T126, and T254) on predictions of the Asian summer monsoon. It is found that, in predicting the magnitude and timing of monsoon rainfall over lands, high model resolutions overall perform better than lower model resolutions. The increase in prediction skills with model resolution is more apparent over South Asia than over Southeast Asia. The largest improvement is seen over the Tibetan Plateau, at least for precipitation. However, the increase in model resolution does not enhance the skill of the predictions over oceans. Over the South China Sea and the Philippine Sea, simulations of monsoon rainfall even become worse from T62 to T254. (Li Jian)

2 Infuences of the air-sea-land interactions on the East Asian climate

The dynamic cause of the declining Arctic sea ice extent are revealed. The influences of the Eurasian snow on the East Asian monsoon, the relationship between two types of ENSO events and Pacifc subsurface SST, and the variations of the North Pacifc subtropical and subpolar gyres are discussed.

2.1 Dominant modes in the Arctic wind felds

Monthly mean surface wind data from the National Centers for Environmental Prediction/National Centers for Atmospheric Research (NCEP/NCAR) reanalysis dataset during the period 1979—2010 are used to describe the frst two patterns of Arctic surface wind variability by means of the complex vector empirical orthogonal function (CVEOF) analysis. The leading pattern consists of two sub-patterns∶ the northern Laptev Sea (NLS) pattern and the Arctic dipole (AD) pattern. The second pattern contains the northern Kara Sea (NKS) pattern and the central Arctic (CA) pattern. Over the past two decades, the combined dynamical forcing of the frst two patterns has contributed to the Arctic September sea ice extent (SIE) minima and its declining trend. The September SIE minima are mainly associated with the negative phase of the AD pattern and the positive phase of the CA pattern during the summer season (July to September), and both phases coherently show an anomalous anticyclone over the Arctic Ocean. Wind patterns affect the September SIE through their frequency and intensity. The negative trend in the September SIE over the past two decades is associated with increased frequency and enhanced intensity of the CA pattern during the melting season from April to September; thus, it cannot be simply attributed to the AD anomaly characterized by the second empirical orthogonal function mode of sea level pressure north of 70°N. The CA pattern exhibits interdecadal variability in the late 1990s, and an anomalous cyclone prevails before 1997 and is then replaced by an anomalous anticyclone over the Arctic Ocean that is consistent with the rapid decline trend in the September SIE.

2.2 Simulation of the infuences of the Eurasian snow on the East Asian monsoon

The snow albedo and hydrological effects on the precipitation in South China in 2010 was investigated based on the singular value decomposition (SVD) analysis of the Eurasian snow depth dataset and the 160-station rainfall dataset in China. Three ensemble simulations were conducted by using the community atmosphere model 3.1 (CAM3.1). The frst ensemble simulation includes both the snow albedo and the snow hydrological effect. The second ensemble simulation considers only the snow albedo effect but ignores the hydrological effect. The third ensemble simulation considers only the snow hydrological effect but ignores the albedo effect. The results indicate that both effects could have impacts on the South China rainfall. However, there are great differences in amplitudes and ranges of abnormities induced by these two effects. The magnitude of abnormities caused by the snow hydrological effect is larger than that by the albedo effect. When those two effects work together, the result agrees well with observations and the magnitude is the largest. However, the magnitude is not equal to the sum of that caused by the snow albedo and hydrological effects.

2.3 Impacts of spring Eurasian snowmelt on the East Asian summer monsoon

The effects of spring Eurasian snowmelt on the East Asian summer monsoon (EASM) and summer precipitation in China, in addition to a possibly related physical mechanism, were investigated by using observation data. The leading mode of spring Eurasian snowmelt shows a decadal variation, which agrees well with the EASM interdecadal transition and the China summer rainfall. In addition, the second mode of the snowmelt variability correlates positively to the EASM variability. When the snowmelt in East Siberia near the Balkhash Lake increases in spring, the EASM tends to appear in a negative–positive–negative meridional wave train from high to low latitudes in summer. When the snowmelt decreases, opposite conditions appear. The link mechanism of the spring snowmelt and summer atmospheric circulation in East Asia may be attributed to a positive feedback between abnormal snow and atmospheric thickness anomalies in the same region during the same period, which promotes two high-pressure systems. Subsequently, the Balkhash Lake high-pressure system develops eastward, and part of the East Siberian high-pressure system moves to lower latitudes. As a result, the atmospheric circulations over East Asia may form the meridional wave train in summer, which may lead to an anomalous change in the East Asian summer weather and climate.

2.4 Linkage between two types of ENSO events and Pacifc subsurface SST

The variations of subsurface ocean temperature (SOT) were investigated to disclose their linkage to the eastern and central Pacifc ENSO (EP and CP-ENSO) events. The wavelet analyses suggest that the variation of the EP (CP-ENSO) events shows a 2–7- (10–15-) yr oscillation in the tropical sea surface temperature (SST), and is coupled with a zonal dipole mode and a tripole mode in the SOT anomalous feld revealed by the singular value decomposition (SVD) analysis. During the mature phase of CP-ENSO, the positive center of SOT at the subsurface layer locates in the west of dateline, which results in the increase of SOT in the Ni?o4 region and causes the CP-ENSO event. Statistical analysis implies that, the eastern and central Pacifc subsurface indices defned by the expansion coeffcients of the frst and third SVD mode for SOT have shown their capabilities in distinguishing the EP and CP-ENSO events. In addition, corresponding to the increase of the SOT amplitude on the 10–15-yr timescale, we fnd that the frequency and intensity of CP-El Ni?o events have exhibited an upward trend after the 1980s, which suggests that the CP-ENSO event had an enhanced impact on the global climate in the past decades (Fig2).

2.5 Variations of the North Pacifc subtropical and subpolar gyres

The adjustment of the North Pacific Subtropical and Subpolar Gyres towards changes in wind stress leads to variability on different timescales, which plays a significant role in climate changes. Based on the Simple Ocean Data Assimilation (SODA) and Global Ocean Data Assimilation System (GODAS) datasets, we diagnose variations of the Subtropical and Subpolar Gyres by applying the “three-dimension Ocean Circulation Diagnostic Method”, and establish three types of index series consisting of the strength, meridional and depth centers of the Subtropical and Subpolar Gyres. The results show that the indices can well present the seasonal, interannual, and interdecadal variability of the Subtropical and Subpolar Gyres. The Gyres are both strongest in winter, but the Subtropical Gyre is weakest in summer and the Subpolar Gyre is weakest in autumn. The subtropical gyre starts moving northward from February to March, southward in October, and to the southernmost around January. In contrast, the Subpolar Gyre moves northward in spring, southward in summer, northward again in autumn, and reaches the extreme southern point in winter. The common feature in their interannual and interdecadal variability is that the two gyres are weaker and more northward before 1976—1977, while stronger and more southward after 1976—1977. The Subpolar Gyre has made a paramount contribution to variability on interdecadal scales. As is indicated by the Subpolar Gyre strength index, there is an important shift from weak to strong around 1976—1977, and its correlation coeffcient with the North Pacifc Decadal Oscillation (PDO) is 0.45, which is far better than that between the subtropical gyre strength and PDO. Experimental results show that infuenced by small and mesoscale eddies, the magnitude of largescale gyres is strongly dependent on data resolution, but seasonal, interannual, and interdecadal large scale variability of the two gyres as manifested by the indices is less affected by model resolution. (Zhu Congwen)

3 Atmospheric thermodynamic and dynamic processes over the Tibetan Plateau and their impacts

Based on the satellite observation data, the diurnal variation of rainfall over the southeastern Tibetan Plateau, the Tibetan Plateau vortices, and the diurnal variation of clouds over southern China are studied.

3.1 Diurnal variation of rainfall over the southeastern Tibetan Plateau

The terrain of the Tibetan Plateau is complex, and it buckles into a series of ridges with a succession of gorges that carve their way through the mountains. The complex topography, as well as the substantial inhomogeneity in landscape and landcover on the Tibetan Plateau, may therefore produce some unique regional diurnal rainfall characteristics. The results show that the diurnal variation of summer rainfall over the Tibetan Plateau has evident double peaks. The prevailing nocturnal rainfall peak in observations at routine stations can be largely attributed to the relatively lower location of the stations, which are mostly situated in valleys. The records from a 3-yr intensifed observational experiment at eight stations along the hillside of Seqilashan over the southeastern Tibetan Plateau revealed an evident afternoon peak of warm season rainfall, similar to that indicated by the TRMM data. The different diurnal phases between valley and hillside station precipitations are closely related to the orographically induced regional circulations. The results of this study indicate that the prevailing nocturnal rainfall associated with the relatively lower location of routine observation stations can partially explain the diurnal rainfall variations between observation station records and TRMM data (Fig3).

3.2 Tibetan Plateau vortices and the diurnal variation of clouds over southern China

Using the NCEP global fnal analysis data (FNL), the observational radiosonde data, the hourly surface climatic elements dataset, the atmosphere precipitable water vapor (PWV) and 6-h radiosonde data at Gerze station, the generation frequency of the Tibetan Plateau vortices (TPVs) in 4 periods of a day (00∶00—06∶00 UTC, 06∶00—12∶00 UTC, 12∶00—18∶00 UTC, and 18∶00—00∶00 UTC), during May—September of 2006—2008, is analyzed. The effects of the large-scale circulation, the related meteorological elements, and the heating fields on the diurnal variation of the TPVs’ formation frequency are discussed. The generation frequency of the TPVs shows a robust diurnal variation, depending on both the large-scale circulation and the latent heat. The peak of the generation frequency of the TPVs tends to reach the maximum during evening to midnight (18∶00—00∶00 LT), and the minimum during early morning to noon (06∶00—12∶00 LT). The condensation latent heat induced by precipitation exerts a direct promotion effect on the generation of the TPVs. In the evening (at 18∶00 LT), there is notable convergence at 500 hPa over the central-western plateau, the jet stream and the South Asian high at 200 hPa are strong, the water vapor flows to the main body of the Tibetan Plateau, and the stratification is unstable at this time. All of the above conditions are helpful for triggering precipitation, which induces the condensation latent heat release. The upper-level heating is favorable for the development of the cyclonic disturbance at 500 hPa during 18∶00—00 ∶00 LT. The reverse situation occurs druing 06∶00—12∶00 LT.

3.3 FY-2C derived diurnal features of clouds in the southern contiguous China

Hourly infrared (IR) brightness temperature (BT) derived from China’s first operational geostationary meteorological satellite Feng-Yun (FY)-2C is analyzed over the southern contiguous China (20°–33°N, 100°–122°E) during 2005—2008. The focus is to investigate the diurnal variation of clouds and compare their different features between the southwestern and southeastern China. According to the diurnal features in summer, the clouds are frst divided into three categories by the cloud top temperature (CTT, derived from IR BT)∶ cold cloud (CC, defined as CTT lower than -30 °C), middle cloud (MC, defined as CTT between-30 °C and 0 °C), and warm cloud (WC, defned as CTT warmer than 0 °C). The CC occurs most frequently in the late afternoon in most regions over southern China. The summer mean frequency of CC, MC, and WC shows different diurnal variations in the southern contiguous China. The CC exhibits a large diurnal amplitude in frequency and occurs most frequently in the late afternoon in most regions. The frequency of MC shows a relatively weaker diurnal amplitude and presents a dominant nocturnal maximum during the day. The WC occurs more (less) frequently in the daytime (nocturnal hours) and reaches the peak around noon. Despite the similarity, the diurnal features of summer mean CC and MC frequency in the southwestern and southeastern China present remarkable regional differences, whereas the diurnal variation of WC shows no obvious zonal contrasts over most of southern China. The CC occurs more frequently in the early morning (late afternoon) in the west (east) region. In the west region, the frequency of CC also has a weak secondary diurnal peak. The frequency of MC shows midnight to early morning maxima in the west region and evening to midnight maxima in the east region. The diurnal variations of CC and MC frequency exhibit evident seasonal changes, whereas the frequency of WC reaches the diurnal maxima around noon all over the year. The late afternoon peak of CC frequency appears in almost all months. In the warmer season, the late afternoon diurnal maximum dominates; whereas in the cold seasons, there is also a midnight to late evening secondary peak. The MC frequency reaches the diurnal peak in the early evening in summer and in the late evening in other seasons. The seasonal changes of WC frequency only present in its diurnal amplitude, which is smaller in warmer seasons and larger in cold seasons. It is noted that the distribution of diurnal variation of the CC (MC) shows great similarity with that of convective (stratiform) rainfall detected by TRMM PR, especially the diurnal variation of precipitation profles. The similarity indicates that the BT from FY-2 series is potentially useful for the study of the diurnal variation of clouds and rainfall in the southern contiguous China, which may further extend our knowledge of the evolution of the regional cloud and rainfall, as the geostationary orbit satellites can provide much more information than the polar orbit satellites such as TRMM PR. (Chen Haoming)

4 Mechanisms and projection theories of climate change

Using the observed data and numerical simulation, possible causes of the decadal variations and past millennial variations of the East Asian monsoon are discussed. The main results are as follows.

4.1 Impacts of the change of surface air temperature over the Asian continent on the East Asian summer monsoon

Using observed data and output from the atmospheric component of the NCEP Climate Forecast System, we fnd a relatively smaller warming in Asia compared to the surrounding regions. Although the surface air temperature over Asia has increased, the landmass has become a relative ‘heat sink’ because of the larger warming in other regions of the world. Indeed, over Asia, the vertically integrated tropospheric temperature in the most recent decade is colder than that in the earlier decades, a feature different from the characteristics outside Asia. Therefore, the “heat sink” over the Asian landmass may be a plausible reason for the weakening of the Asian monsoon.

4.2 Possible impacts of global warming on the East Asian summer monsoon

The possible causes of the weakening of the East Asian summer monsoon (EASM) were investigated. We found that the decreased intensity of the EASM is signifcantly correlated with the increase of the surface air temperature (SAT) averaged over the Lake Baikal region defned as SATI. Corresponding to the increasing SATI, an anomalous low-level anticyclone occurs with northeasterly prevailing over northern East Asia, resulting in a weakened southwesterly monsoon and drier climate in this region. Numerical experiments with the community atmosphere model version 3 (CAM3) show that the joint forcing induced by greenhouse gases (GHG), sea surface temperature (SST), solar radiance (SR), and volcano activity (VC) can replicate the observed trend of SATI and its related circulation anomalies, but without GHG forcing the model failed tosimulate the warming trend of SATI after the 1970s. This implies that the global warming is likely responsible for the local warming around the Lake Baikal, which in turn weakens the EASM in recent decades.

4.3 Modeling of the climate change in China by BCC_AGCM

Primary diagnostic metrics are proposed to evaluate the integrated structure of interdecadal changes of the East Asian climate in midsummer (July—August) over the recent half-century (1955—2000) in the BCC_ AGCM models. When forced by historical sea surface temperatures (SST), the ensemble simulation with the BCC_AGCM reasonably has reproduced the coherent interdecadal changes of rainfall, temperature, and circulation. The main feature of the ‘southern-fooding-and-northern-drought’ in rainfall change is captured by the model. Correspondingly, the tropospheric cooling in the upper and middle troposphere, the southward shift of the upper level westerly jet, and the weakening of the low-level southwesterly monsoon fow are also reproduced, as well as their relationships with rainfall changes. One of the main defciencies of the simulation is that the amplitudes of the changes of the tropospheric cooling and large-scale circulation are both much weaker than those in reanalysis, and they are consistent with the rainfall defciency. Also, the upper and middle troposphere cooling center and the decreasing of upper-level westerly jet axis shift westward in the model simulations compared with the observations. Overall, although BCC_AGCM shows problems in simulating the interdecadal changes of the East Asian climate, especially the amplitude and locations of change centers, it reasonably represents the observed confguration of rainfall variation and the associated coherent temperature and circulation changes. Therefore, it could be used to further discuss the mechanisms of the interdecadal variations in East Asia. Meanwhile, the reasonably reproduced confguration of rainfall and its associated largescale circulation by SST-forced runs indicate that the interdecadal variations in East Asia could mostly arise from the regional response to the global climate change.

4.4 Modeling of global climate change in the past millennium

The UVic Earth System Climate Model (UVic Model), an earth system model of intermediate complexity, was employed to investigate the contributions of climate forcings (e.g., solar insolation variability, anomalous volcanic aerosols, greenhouse gas, solar orbital change, land cover changes, and anthropogenic sulfate aerosols) to surface air temperature over East China in the past millennium. The simulation of the UVic Model could reproduce the three main characteristic periods (e.g., the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the 20th Century Warming Period (20CWP)) of the Northern Hemisphere and East China, which were consistent with the corresponding reconstructed air temperatures on century scales. The simulation results reflected that the air temperature anomalies of East China were larger than those of the global air temperature during the MWP and the frst half of 20CWP and were lower than those during the LIA. According to the surface air temperature of East China, the past millennium has been further divided into three periods in the MWP, four in the LIA, and one in the 20CWP. The MWP of East China was caused primarily by solar insolation and secondarily by volcanic aerosols. The variation of the LIA was dominated by the relative contributions of solar insolation variability, greenhouse gas, and volcano aerosols. Greenhouse gas and volcano aerosols were the main forcing of the third and fourth periods of the LIA, respectively. We examined the nonlinear responses among the natural and anthropogenic forcings in terms of surface air temperature over East China. The nonlinear responses between the solar orbit change and anomalous volcano aerosols and those between the greenhouse gases and land cover change (or anthropogenic sulfate aerosols) all contributed approximately 0.2 °C by the end of the 20th century. However, the output of the energy-moisture balance atmospheric model from UVic showed no obvious nonlinear responses between anthropogenic and natural forcings. The nonlinear responses among all the climate forcings (both anthropogenic and natural) contributed to a temperature increase of approximately 0.27 °C at the end of the 20th century, accounting for approximately half of the warming during this period (Fig4, red line minus grey line); the remainder was due to the linear response of the climate forcings themselves. The nonlinear responses of the climate forcing factors offer a new way to understand the climate change (Fig4). (Xiao Dong)