【作者】 冯娟；

【导师】 李建平；

【作者基本信息】 兰州大学 ， 气象学， 2010， 博士

【摘要】 伴随着20世纪下半叶的持续增暖,全球陆地大部分地区存在着干旱化的趋势。干旱气候变化引起的沙漠化和生态退化及其对自然环境和人类社会产生的影响等重大科学问题已日益引起国际社会的高度重视和社会公众的广泛关注。对于干旱的现状分析、成因探讨以及未来趋势预测成了许多研究的焦点问题。由于干旱趋势的加剧,对于典型区域的研究不仅可以深入理解区域的干旱成因,并且也将有利于帮助认识其它区域的气候变化。澳大利亚西南部(SWWA)处于印度洋的东海岸,是西澳政治、经济、文化、教育和旅游中心的所在地,同时也是西澳农业的主产区。自上世纪70年代中期以来,澳洲西南部的雨季降水减少了15-20%,降水的减少使得内陆径流量骤减了将近50%。SWWA地区降水的减少受到了社会各界的广泛关注,一时成为了许多计划和研究的焦点和热点问题。本文利用近60年的观测、再分析、以及模式资料,以澳大利亚西南部雨季降水的持续减少为切入点,通过对西南澳洲大尺度大气环流的动力、热力结构,环流季节演化特征,和澳洲西南部降水分布的研究,首次提出了西南澳类季风环流(SWAC)的概念。讨论了此环流的季节、年际、年代际变化特征及其对SWWA地区冬季降水的影响,探讨了此环流的外部驱动因子,建立了SWWA地区季节降水经验模型,分析了SWAC的可预报性,比较了此环流与经典副热带季风系统—东亚夏季风的异同之处。主要结论如下：(1)SWWA冬季降水与已知的影响澳洲降水的气候系统之间的关系系统分析了已知影响澳洲降水的气候系统,如厄尔尼诺-南方涛动(ENSO)、ENSO Modoki(EM)、印度洋偶极子(IOD)以及南半球环状模(SAM)对SWWA地区冬季降水的影响。结果表明,这些气候模态均不能解释SWWA地区雨季降水的长期变化。特别的,本文的研究表明前人工作中指出的SAM与SWWA地区冬季降水之间的显著负相关关系是由于极端事件造成的。(2)西南澳类季风发环流—SWAC概念的提出通过对西南澳洲气候背景下的降水、环流的季节循环、动力、热力结构以及海陆热力差异的分析,发现SWWA地区的环流表现出类似季风环流系统的特征,也即干湿季节的交替出现、风向的季节性反转、强的海陆热力差异以及海陆热力梯度方向的季节性反转。基于此,首次提出了西南澳类季风环流—SWAC的概念。(3)SWAC的季节、年际、年代际变化及其与SWWA冬季降水的关系基于结论(2),分析了西南澳类季风环流的季节、年际、年代际变化特征,定义了表征SWAC爆发早晚、强度的气候指标；研究了SWAC与SWWA地区降水在年际、年代际时间尺度上的关系。结果表明：SWAC的季节推进与SWWA地区的降水发展是紧密联系在一起的。SWAC环流的强度不仅可以解释SWWA地区前冬、后冬降水的年际变化,而且揭示了SWWA地区降水减少的长期趋势与SWAC环流的长期减弱趋势密切相关。(4)SWAC、SAM以及SWWA冬季降水之间的关系,SWAC环流外部驱动因子探讨通过将SWAC环流强度指数分解为与SAM变化相关的部分和独立于SAM变化的部分,发现SAM与SWAC环流之间的关系是与SWWA地区冬季降水线性独立的部分决定的。而影响SWWA地区降水的SWAC环流部分是线性独立于SAM的。关于SWAC环流的外部驱动因子,本文指出行星尺度热对流的季节性移动使得副热带高压的位置产生季节性变动,导致SWWA地区对流层低层的盛行风向发生季节性反转,这是SWAC环流存在的“第一推动力”。而SWWA地区以西的南印度洋海温的变化为SWAC环流提供了“第二推动力”。数值试验的结果进一步证明以上推论的正确性。(5)建立了SWWA地区季节降水经验模型鉴于SWAC环流与SWWA地区降水之间紧密的耦合关系,将SWAC环流作为影响因子,建立了SWWA地区前冬和后冬季节降水的经验模型,并进行了“单点剔除”和“7点剔除”的59年(1948-2006年)后报试验。结果表明,引进单一SWAC环流因子的经验模型较引入其它气候因子的要好。这为分析研究SWWA地区季节降水提供了新的途径。对SWAC环流的可预报期限分析表明,SWAC环流的最大可预报期限大致为6天。(6)IPCC AMIP大气环流模式对于SWAC环流模拟的评估从SWAC环流的季节特征、年循环、对应的环流异常以及年际变化的角度评估了国际现有的大气环流模式对于SWAC环流的模拟能力。结果表明,除了NCAR的CCSM3模式以外,其余模式基本均能再现SWAC环流的季节特征、以及对应的环流异常。但是对于SWAC环流的年际变化、季节演化的模拟能力所有模式的结果均不尽人意,表现出较大差异,尚有不足,还需进一步的完善。(7)SWAC环流与东亚夏季风的类比分析从两个季风系统所处的地理位置,影响区域的降水变化等方面进行了类比分析研究。首先两个季风系统的季节推进及强度均与各自半球的副热带高压的位置、移动密切相关。并且发现SWWA地区前冬降水与华北盛夏降水表现出相似的年际、年代际变化。而SWWA地区的前冬降水提前了华北盛夏降水两个月时间,这为预测华北盛夏降水提供了重要的可预报源泉。更多还原

【Abstract】 Along with the continued warming trend during the second half of the 20th century, drought is existed over most parts of terrestrial in the global. The great impacts of drought on natural environment and human society, such as desertification and ecological degradation have already attached increasing attentions of the international scientific community and the public. Studies of analyzing the statues, attributions and projecting the future trend of drought have been the focused and hot issues in many projects and researches. As the drought becomes severer, research for a typical region is not only helpful for further understanding the causes of the drought in the certain region, also would be useful for improving the understanding and providing case study to other similar regions.Southwest Western Australia (SWWA) locates in the southwest corner of the Western Australia. It is not only the political, economic, cultural, educational, and tourism center of the Western Australia, and also the wine region and wheat belt of the Western Australia. The observed rainy season precipitation of the SWWA has significantly declined since 1950s. The reduction in winter rainfall resulted in an even sharper fall in the stream flow in the southwestern Australia and heavily impacted the water resources in the state. The attribution of rainfall variation becomes the focus of many researches and has drawn much attention.Based on the observation data, reanalysis datasets, and model data in recent 6 decades, the variability and circulation features influencing the SWWA winter rainfall (SWR) are investigated. It is found that the climate of southwest Australia bears a strong seasonality in the annual cycle and exhibits a monsoon-like atmospheric circulation, which is termed as the southwest Australian circulation (SWAC). The seasonal march, interannual and interdecadal variability of the SWAC, and its relationship with the SWR is discussed. The external forcing and predictability of the SWAC are also explored. Based on the well-coupled linkage between the SWAC and SWR, empirical models are established to predict the SWWA seasonal rainfall by the SWAC. Finally, the comparison study between the SWAC and the classical subtropical monsoon system-East Asian summer monsoon (EASM) is carried out to further comprehend the SWAC. Major conclusion can be summarized as follows:(1) The relationship between the SWR and the known climatic systems that influencing the Australian rainfallThe impacts of the known modes (e.g. El Nino-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), ENSO Modoki (EM) and the Southern Hemisphere Annular Mode (SAM)) that influencing the Australian rainfall on the SWR are analyzed. It is found that they can not explain the rainfall variation of the SWWA neither in early (May to July, MJJ) or late winter. Particularly, the reported significant inverse relationship between the SAM and SWR is caused by an exceptional extreme year.(2) The advance of the SWAC conceptionBased on the investigation on rainfall distribution, seasonal march of the circulation features, dynamic and thermal structures of the atmospheric circulation, and thermal contrast in land and sea in the southwest Australia, we find that the circulation over SWWA shows strong seasonality, and exhibits monsoon-like characteristics. That is, alternate wet and dry seasons, seasonal reversal of winds, and an evident land-sea thermal contrast. Taking account of these monsoonal characteristics, we proposed the conception of monsoon-like SWAC by the first time.(3) The seasonal march, interannual, interdecadal variation of the SWAC and its relationship with the SWWA rainfallBased on Conclusion (2), the seasonal, interannual, and interdecadal variation of the SWAC is discussed, the onset and strength indices of the SWAC are defined, and the relationship between the SWAC and SWR is analyzed. Result indicates that the SWAC explains not only a large portion of the interannual variability of SWR in both early and late winter, but also the long-term drying trend over SWWA in early winter is attributed to the weakened of the SWAC. The well-coupled SWAC-SWR relationship seems to be largely independent of the known effects of ENSO, IOD, SAM and EM, nor the responds of them. The result offers qualified support for the argument that the SWAC may contribute to the rainfall decrease in the SWWA early winter rainfall.(4) The relationship between the SWAC, SAM and SWWA rainfall, and the external forcing of the SWACWe find that the SAM activity does not produce modulation to the SWAC signal which has a relation with the SWR. With regard to the external forcing of the SWAC, we suggest two possible drivers. The first may be the shift of the planetary-scale thermal convection that inducs the seasonal shift of the subtropical high and results the reversal winds over SWWA. Second, the Indian Ocean SSTs westward of SWWA may provide another external forcing to the SWAC. The result is further established by numerical experiments.(5) The establishment of the empirical model to reckon seasonal SWWA rainfallBased on the well-coupled linkage between the SWAC and SWR, the empirical models are built to hindcast the seasonal SWWA rainfall in both early and late winter by the SWAC, respectively. Hindcast is performed for the 1948-2006 period, which shows hindcast prediction skill based on the single SWAC is better than the combination of other climatic modes. The empirical models provide perspective tool to forecast the SWWA seasonal rainfall.(6) Assessment of the simulation skills of the SWAC by the IPCC AMIP modelsThe seasonal features, annual cycle, and corresponding circulation anomalies of the SWAC are evaluated using the IPCC AMIP models. Result shows that except the NCAR CCSM3.0, the else models can capture the basic seasonal features and corresponding circulation anomalous characteristics of the SWAC, but not for the seasonal march procession and interannual variability. Thus the relevant part of the models should be improved to well simulate the SWWA climate.(7) Analogy the SWAC and the East Asian Summer Monson (EASM)Since the SWAC and EASM both are located at subtropical regions, and in the same longitude scale, we analogy the similarlities and dissimilarities of the two monsoon system. The seasonal march and strength of the two monsoon systems are both closely attached with the position and shift of the subtropical high in each hemisphere. We find that the early winter rainfall over SWWA has a closely linkage with the midsummer rainfall of the north China in both interannual and interdecadal timescale, especially strong in the interdecadal timescale. Further, the early winter rainfall over SWWA and midsummer rainfall in the north China both are closely attached with the mean position of the subtropical high ridge. One important issue is that the early winter rainfall of the SWWA is two months advanced the north China midsummer rainfall, which provides a potential predictability source for the north China midsummer rainfall.更多还原