氣候系統(tǒng)與氣候變化

氣候系統(tǒng)與氣候變化研究進(jìn)展

2010年度氣候系統(tǒng)研究所在準(zhǔn)雙周振蕩結(jié)構(gòu)與起源、東部地區(qū)降水年代際演變特征、日變化機(jī)理、降水評估和古氣候研究等方面取得明顯進(jìn)展，主要研究成果如下。

1. 中國地區(qū)樹輪及千年氣候變化研究

2010年，國家自然科學(xué)基金重大項(xiàng)目“中國地區(qū)樹輪及千年氣候變化研究”在樹輪氣候重建、中國地區(qū)千年氣候及東亞季風(fēng)變化特征與規(guī)律分析、千年氣候變化數(shù)值模式改進(jìn)和數(shù)值模擬試驗(yàn)、中國地區(qū)千年氣候變化機(jī)制以及包含外強(qiáng)迫作用的氣候變化預(yù)測新理論與方法的探索等方面取得了新的研究成果。分別在中國東部地區(qū)、福建及黃土高原等地區(qū)進(jìn)行了野外樹輪采樣及數(shù)據(jù)分析工作，完成了利用樹輪寬度資料重建青藏高原地區(qū)2485年的溫度序列; 研究了近百年來氣候變化的年代際特征和演變規(guī)律，探索氣候代用指標(biāo)年代際空間型分布的重建方法，對近千年來多種氣溫、降水代用資料進(jìn)行綜合集成，綜合分析中國氣溫和降水變化的時(shí)空特征及空間格局的變化規(guī)律，在過去千年尺度上的SLP重建及東亞夏季風(fēng)降水指數(shù)重建方面取得了重大突破；運(yùn)用數(shù)值模擬，對過去中世紀(jì)暖期、小冰期和20世紀(jì)現(xiàn)代暖期3個(gè)特征時(shí)期的氣候耦合模式進(jìn)行比較，并對模式模擬的千年溫度進(jìn)行對比分析，探討過去千年氣候模擬中大洋環(huán)流的貢獻(xiàn)率，以及利用不同模式對模擬結(jié)果進(jìn)行對比，研究分析各模式的模擬性能等; 進(jìn)行了非平穩(wěn)氣候過程的預(yù)測試驗(yàn)和氣候序列的外強(qiáng)迫分離和提取研究，初步建立了一個(gè)包含外強(qiáng)迫因子的氣候變化的非線性動(dòng)力學(xué)預(yù)測方法。

2. 準(zhǔn)雙周振蕩結(jié)構(gòu)與起源研究

對北半球春季熱帶印度洋上空大氣準(zhǔn)雙周振蕩（QBWO，10～20天）的結(jié)構(gòu)和演變特征進(jìn)行了細(xì)致分析，結(jié)果顯示，QBWO對流擾動(dòng)首先出現(xiàn)在西印度洋上空，然后緩慢東移。當(dāng)對流擾動(dòng)到達(dá)印度洋中部后，突然快速躍入東印度洋地區(qū)。同時(shí)，原位于東印度洋地區(qū)的反位相對流擾動(dòng)以Rossby波渦旋對的形式向兩極移動(dòng)。然而對垂直環(huán)流的分析發(fā)現(xiàn)，在東傳的對流擾動(dòng)到達(dá)東印度洋之前，蘇門答臘西側(cè)的邊界層已經(jīng)出現(xiàn)了與對流層中相反的垂直運(yùn)動(dòng)。通過動(dòng)力診斷，提出了東印度洋上空準(zhǔn)雙周振蕩的邊界層激發(fā)機(jī)制。在熱帶東印度洋準(zhǔn)雙周振蕩的弱位相，垂直擾動(dòng)對背景風(fēng)場東風(fēng)動(dòng)量的垂直輸送將在自由大氣中產(chǎn)生正壓東風(fēng)，進(jìn)而導(dǎo)致自由大氣的輻散。而邊界層中大氣的散度和垂直運(yùn)動(dòng)是由自由大氣的散度所決定的，因此自由大氣的輻散將導(dǎo)致邊界層大氣輻合和上升運(yùn)動(dòng)的出現(xiàn)。以上結(jié)果表明，熱帶東印度洋對流QBWO循環(huán)位相轉(zhuǎn)換主要受到局地邊界層過程的影響，而不是西印度洋擾動(dòng)?xùn)|傳的結(jié)果。

3. 我國東部地區(qū)降水年代際變化特征研究

在過去幾十年間，中國東部地區(qū)盛夏降水呈現(xiàn)出“南澇北旱”的年代際變化特征。前人對這一現(xiàn)象的研究多基于逐日降水資料，而24 h累積降水往往不能精確反映每次降水事件的演變特征。利用逐時(shí)降水資料，對與“南澇北旱”相關(guān)的降水頻率、強(qiáng)度變化進(jìn)行了細(xì)致分析。主要結(jié)論如下：在過去40年間，我國華北地區(qū)降水量、降水頻率減少，但降水強(qiáng)度增強(qiáng)，而長江中下游地區(qū)降水量、降水頻率增加，降水強(qiáng)度呈現(xiàn)減弱趨勢；“南澇北旱”型降水變化主要來自于中低強(qiáng)度降水（≤10 mm/h）的貢獻(xiàn)；強(qiáng)降水（＞ 20 mm/h）的頻率和降水量變化都未呈現(xiàn)出“南澇北旱”型特征。

中國東部地區(qū)7—8月份弱降水(＜10 mm/h)的降水量(a)和降水頻率(b)在近40年間的年代際變化(1986—2005年與1966—1985年兩時(shí)段之差)，(c)、(d)為相應(yīng)的強(qiáng)降水變化(降水量＞20 mm/h)，三角(正值)和矩形(負(fù)值)為通過5%顯著性檢驗(yàn)的站點(diǎn)The 20-yr mean changes (1986—2005 mean minus 1966—1985 mean) of rainfall amount (mm) from July to August (a) and hourly rainfall frequency (b) of rainfall with hourly intensity 0.1-10 mm/h. (c),(d) as in (a), (b) but for rainfall with hourly intensity more than 20 mm/h. The stations statistically signif cant at the 5% conf dence level according to the t-test are marked by solid triangles (squares) in the positive (negative) region

4. 降水日變化機(jī)理研究

長江流域夏季降水主要表現(xiàn)為持續(xù)性夜雨，此類降水事件多在夜間開始，且開始時(shí)間沿長江流域自西向東滯后。由于持續(xù)性降水開始后經(jīng)過一段時(shí)間累積才能達(dá)到峰值，因而長江流域降水峰值時(shí)間也表現(xiàn)出一致的向東滯后信號。長江流域持續(xù)性夜雨與大尺度強(qiáng)迫密切相關(guān)，特別是長江以南低空西南急流的加強(qiáng)不僅增強(qiáng)長江流域低層輻合，同時(shí)也向該區(qū)域輸送大量水汽，有利于該區(qū)域降水的發(fā)生與維持?；贘RA再分析資料的分析表明，對流層低層風(fēng)場日變化明顯，且自夜間至午后，異常風(fēng)場順時(shí)針旋轉(zhuǎn)。低層西南風(fēng)的日變化可引起低層大尺度輻合輻散和垂直運(yùn)動(dòng)的變化，從而影響長江流域持續(xù)性夜雨的開始時(shí)間。傍晚吹向高原的異常東風(fēng)引起高原下游的輻合上升，使長江上游持續(xù)性降水多在傍晚開始而在午夜達(dá)峰值。午夜加強(qiáng)的南風(fēng)氣流增強(qiáng)向長江中游的水汽輸送，有利于中游地區(qū)降水在午夜開始。清晨，加強(qiáng)的西南風(fēng)氣流引起長江中下游地區(qū)低層強(qiáng)輻合，從而使下游持續(xù)性降水易在上午開始。

長江中下游地區(qū)（27°－33°N）平均的夏季長持續(xù)性降水事件標(biāo)準(zhǔn)化降水量的日變化Time-longitude distribution of normalized rainfall diurnal variation averaged between 27°N and 33°N for averaged summer precipitation amount of long-duration rainfall events

5. 對再分析資料中國大陸地區(qū)夏季降水量的評估分析研究

以中國臺(tái)站降水為參照，評估分析了3套再分析資料(NCEP/DOE，ERA和JRA)對中國夏季降水場的再現(xiàn)能力。結(jié)果顯示，3套資料均能基本再現(xiàn)夏季降水量分布的主要特征，亦能較好刻畫大部分地區(qū)降水的年際和季節(jié)內(nèi)變化特征。但同時(shí)也存在諸多問題：NCEP資料在青藏高原東坡存在一個(gè)虛假降水中心，且對中西部和西南地區(qū)夏季降水的季節(jié)內(nèi)演變特征再現(xiàn)能力較差；ERA資料降水量系統(tǒng)性偏小，且不能合理描述中西部地區(qū)降水的年際變化；JRA資料高估了華南沿海地區(qū)的降水量。3套資料均存在高估弱降水、低估強(qiáng)降水的問題。對日降水量≥0.1 mm的降水事件進(jìn)行TS、BS評分，3套資料的TS評分在0.6左右，BS評分在1.5左右；隨著參與評分的降水等級的提升，TS、BS評分都迅速下降，3套資料對暴雨的預(yù)報(bào)評分都非常低。通過分析可了解當(dāng)前較先進(jìn)的數(shù)值預(yù)報(bào)模式在基本真實(shí)的環(huán)流場強(qiáng)迫下對我國夏季降水的預(yù)報(bào)能力，認(rèn)識模式中物理參數(shù)化過程導(dǎo)致的模擬偏差，為模式發(fā)展和評估提供參照。

NCEP/DOE、ERA和JRA 3套再分析資料的TS,BS評分結(jié)果（TS值用圖中每個(gè)點(diǎn)與橫軸的夾角表示，BS值用點(diǎn)距原點(diǎn)的直線距離表示；數(shù)字代表3種再分析資料，不同標(biāo)識表示不同的降水等級）TS and BS scores of rainfall in three reanalysis datasets. （TS score is shown by separation angle between each point and the X-axis, and BS score is proportional to the radial distance from the origin. Different numbers represent different data sources. Different marks represent different rainfall grades.）

6. PMIP2耦合模式模擬的全新世中期和末次冰盛期的北極濤動(dòng)

利用古氣候模式比較計(jì)劃第2階段（PMIP2）的4個(gè)海氣耦合模式（CCSM、HadCM3、IPSL以及MIROC3.2）比較了全新世中期和末次冰盛期的北極濤動(dòng)(AO)相對于工業(yè)化前的變化。結(jié)果表明，全新世中期AO的強(qiáng)度比工業(yè)化前略減弱。盡管AO的分布型和垂直結(jié)構(gòu)與工業(yè)化前相似，極區(qū)的西風(fēng)稍微減弱并且向下移動(dòng)到平流層下層，伴隨著極渦的減弱以及冷的極區(qū)變暖。在末次冰盛期，北半球經(jīng)歷了嚴(yán)寒，AO指數(shù)的標(biāo)準(zhǔn)偏差在所有模式中都較小，AO的強(qiáng)度與全新世中期相比有較大減弱。而且，AO空間分布型正、負(fù)值中心的值減小并且極渦和西風(fēng)的強(qiáng)度進(jìn)一步減弱，西風(fēng)中心移動(dòng)到中緯度對流層上層。平流層極區(qū)變得異常偏暖，而對流層極區(qū)仍然保持偏冷。AO似乎表現(xiàn)出對氣候態(tài)背景的敏感性。同時(shí)發(fā)現(xiàn)，在全新世中期和末次冰盛期向上傳播的靜止Rossby波比工業(yè)化前強(qiáng)。這種行星波活動(dòng)的增強(qiáng)導(dǎo)致了全新世中期和末次冰盛期AO的減弱。最近的研究表明，在歐亞秋季積雪和冬季AO之間有著顯著的相關(guān)。進(jìn)一步用Rossby波的向上傳播來解釋聯(lián)系A(chǔ)O和積雪厚度之間的物理過程，結(jié)果顯示，與工業(yè)化前相比，末次冰盛期秋季積雪厚度的大量增加，加強(qiáng)了向上傳播的Rossby波。

Rossby波在全新世中期(a,b)和末次冰盛期(c,d)的垂直傳播相對于工業(yè)化前期的差值（陰影區(qū)表示通過99%的顯著性檢驗(yàn)）Changes in the vertical propagation of Rossby waves during the MH (a, b) and the LGM (c, d) compared to the PI in CCSM (a, c) and MIROC (b, d) simulation. (Shaded areas indicate signif cant changes at the 99% conf dence level, passing the t-test)

Climate System and Climate Change

Research Progress in Climate System and Climate Change

Significant progress has been made in 2010 at the Institute of Climate System, including the structure and origin of the Quasi-Biweekly Oscillation, the decadal changes in precipitation over eastern China, the physical mechanisms of diurnal variation of rainfall, the evaluation of rainfall in reanalysis data, and paleoclimate, etc. Major f ndings are summarized as follows:

1. Research on tree rings and millennium climate change in China

During 2010, several new advances of the major program of NSFC “Research on tree rings and millennium climate change in China” were performed in tree ring climate reconstruction, characteristics and rules of East Asia monsoon and millenary climate change over China, numerical simulations and improvement of millenary climate change, mechanisms of climate change over China in the past millennium and new forecasting theories and methods of climate change under the external forcings. First, tree ring samplings over East China, Fujian province and the Loess Plateau and data analyses were accomplished. Especially, a 2485-yr temperature series was reconstructed by the tree ring width data over the Qingzang Plateau. Second, we investigated the structures and rules of the decadalvariation in the past century, explored the methods for reconstructing the spatial pattern of decadal variation, integrated the proxies synthetically in the past millennium such as air temperature, precipitation and so on, and analyzed the spatial structures of China temperature and precipitation and its rules. Great progress was made to reconstruct the sea level pressure and precipitation index of East Asian summer monsoon. Third, comparisons of characteristics among the medieval warm period, little ice age and 20th century warming period and that of the simulated millenary temperatures were made by coupled numerical simulation. The oceanic current contribution in the millenary climate simulation and the abilities of the different models were also evaluated. Last, forecasting experiments of non-stable climate process and separation and extraction of the external forcings were explored. A nonlinear dynamic forecast method including the external forcing was established elementarily.

2. Study on the structure and origin of the quasi-biweekly oscillation

The structure and evolution features of the quasi-biweekly (10–20 day) oscillation (QBWO) in boreal spring over the tropical Indian Ocean (IO) are investigated. It is found that a convective disturbance is initiated over the western IO and moves slowly eastward. After passing the central IO, it abruptly jumps into the eastern IO. Meanwhile, the preexisting suppressed convective anomaly in the eastern IO moves poleward in the form of doublecell Rossby gyres. The analysis of vertical circulation shows that a few days prior to the onset of local convection in the eastern equatorial IO an ascending motion appears in the boundary layer. Based on the diagnosis of the zonal momentum equation, a possible boundary layer–triggering mechanism over the eastern equatorial IO is proposed. The cause of the boundary layer convergence and vertical motion is attributed to the free-atmospheric divergence in association with the development of the barotropic wind. It is the downward transport of the background mean easterly momentum by perturbation vertical motion during the suppressed convective phase of the QBWO that leads to the generation of a barotropic wind easterly—the latter of which further causes the free-atmospheric divergence and, thus, the boundary layer convergence. The result suggests that the local process, rather than the eastward propagation of the disturbance from the western IO, is essential for the phase transition of the QBWO convection over the eastern equatorial IO.

3. Decadal changes in precipitation over eastern China

In recent decades, the late-summer (July–August) rainfall has exhibited a so-called southern fooding and northern drought (SFND) pattern over eastern China. Previous studies focusing on this phenomenon were based on daily rainfall data, which cannot present the detailed process of each rainfall event. Using hourly station rain gauge data, it is found that although the rainfall amount and frequency have signifcantly increased (decreased) in the mid–lower reaches of the Yangtze River valley (North China) during this period, the rainfall intensity has decreased (increased). Results also show that the SFND pattern is mostly attributed to changes in precipitation with moderate and low intensity (less than10 mm/h). Neither frequency nor amount of strong intensity (larger than 20 mm/h) rainfall exhibits the SFND pattern.

4. Physical mechanisms of diurnal variation of rainfall

Long-duration rainfall events dominate the summer rainfall along the Yangtze River (YR). These events tend to start during the night and to peak after several hours of development. The eastward-delayed initiation of the nocturnal long-duration rainfall events is thought to be due to the diurnal clockwise rotation of the low- tropospheric circulation, especially the accelerated nocturnal southwesterlies. In the early evening, the anomalous easterly fow toward the Tibetan Plateau (TP) causes low-level convergence over the Plateau’s eastern slope that induces the formation of rainfall in the upper YR valley. The anomalous wind sequentially rotates clockwise to a southerly fow at midnight and accelerates the meridional wind in the middle valley, resulting in the initiation of rainfall between 23:00 and 03:00 LST. In the early morning, the accelerated southwesterlies in southern China, when combined with decelerated winds in the north of the YR, causes a strong convergence along the YR and contributes to the early morning rainfall in the lower valley.

5. Evaluation and analyses of summer rainfall over the Chinese mainland in three reanalysis datasets

The summer rainfall data in China represented by three reanalysis datasets (NCEP/DOE, ERA and JRA) are evaluated by comparing with the station rainfall data in this study. Results show that the main characteritics of summer rainfall distribution in China are reasonably reproduced in three datasets, and most of the interannual and intraseasonal variations are captured. However, there are still some def ciencies in all of the three reanalysis rainfall products. In the NCEP product, an artif cial rainfall center is located over the east periphery of the Tibetan Plateau, and the intraseasonal evolution of summer rainfall in midwestern and southwestern regions is poorly represented. In the ERA product, the rainfall amount is systematically underestimated and the interannual variation of rainfall over the midwestern regions is not well simulated. In the JRA product, the rainfall over coastal regions of South China is overestimated. The overestimation of weak rainfall and underestimation of heavy rainfall can be found in all of the three products. Concerning the TS and BS scores of the rainfall events with daily amount larger than 0.1 mm, the TS scrore is around 0.6 and the BS scrore is about 1.5 for all the reanalysis rainfall products. However, with the upgrade of rainfall rates, both of the scores decrease rapidly. The scores of torrential rain are very low for all reanalysis products. The analyses in this work not only indicate the capability of current advanced numerical models in simulating summer rainfall in China under the realistic circulation forcings, but also help to understand the simulation biases resulting from physical parameterization. Results would potentially contribute to the model development and evaluation in future.

6. Arctic oscillation during the mid-Holocene and last Glacial Maximum from PMIP2 coupled model simulations

Changes in the Arctic Oscillation (AO) during the mid-Holocene and the last Glacial Maximum were compared to preindustrial (PI) simulations using four coupled ocean–atmosphere models [i.e., Community Climate System Model (CCSM), third climate conf guration of the Met Off ce Unif ed Model (HadCM3) Met Off ce Surface Exchanges Scheme, version 2 (MOSES2), L’Institut Pierre-Simon Laplace (IPSL), and Model for Interdisciplinary Research on Climate 3.2 (MIROC3.2)] from the second phase of the Paleoclimate Modeling Intercomparison Project. Results show that the amplitude of the simulated AO during the mid-Holocene is a little smaller than that of the preindustrial simulation. Although the AO pattern and vertical structures are similar to those in the preindustrial simulation, the polar westerlies are slightly weakened and displaced downward to the lower stratosphere, accompanied by the weakening of the polar vortex and the warming of the cold polar cap region. During the Last Glacial Maximum, when the Northern Hemisphere experiences severe cooling, the intensity of the AO decreases substantially compared to the mid-Holocene, with smaller standard deviations of the AO indices in all models. Furthermore, the magnitude of positive and negative centers of the AO spatial pattern decreases and the strength of the polar vortex and westerlies weakens further with the center of westerlies displaced into the midlatitude upper troposphere. The polar cap region becomes anomalously warm in the stratosphere, whereas it remains cold in the troposphere. The AO appears to be sensitive to background climate state. Upward-propagating stationary Rossby waves are found to be stronger during the mid-Holocene and Last Glacial Maximum than in the preindustrial simulation. This increase in planetary wave activity might be responsible for the simulated weakening of the AO during the mid-Holocene and Last Glacial Maximum. Recent studies have shown that there is a significant correlation between Eurasian fall snow cover and the winter AO. The upward propagation of Rossby waves was further proposed to explain the physical process linking the AO with the snow depth. It is suggested that a large increase in fall snow depth during the Last Glacial Maximum strengthens the upward-propagating stationary Rossby waves relative to the PI.