Severe Frosts in Western Australia in September 2016
Severe Frosts in Western Australia in September 2016
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DOI：
10.1175/bams-d-17-0088.1
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发表时间：
2018
影响因子：
8
通讯作者：
M. Grose;M. Black;J. Risbey;P. Uhe;P. Hope;K. Haustein;D. Mitchell
中科院分区：
文献类型：
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作者：
M. Grose;M. Black;J. Risbey;P. Uhe;P. Hope;K. Haustein;D. Mitchell
导论. 2016年9月，西澳大利亚西南部的小麦种植带在收获前经历了几次严重霜冻，导致100万吨粮食作物损失（GIWA 2016）。使用Jones等人（2009）的网格化观测数据集，整个月在谷物带的某个地方有18个霜冻风险夜（Tmin <2°C），9月霜冻面积和频率很大（图29.1a），是1956年以来的最高值。任何网格单元的最高计数是13个霜冻风险夜晚，其中9个严重霜冻风险夜晚（Tmin <0°C）。自1910年有可靠记录以来，许多地方的9月霜夜数量最多，该地区的大部分地区都在前五年（图29.1 b）。SWWA还看到低于平均降雨量和湿度，偏南的每月风异常，以及凉爽的海面温度（SST）紧邻SWWA在9月。9月期间有弱拉尼娜现象和负印度洋偶极子条件。人类对极端寒冷天气的影响是两种影响的净结果：气候平均状态的温度上升和强迫环流变化。自1910年以来，SWWA地区已经变暖了约1°C，这表明霜冻风险降低（BOM和CSIRO 2016）。然而，温室气体强迫可能会通过对环流特征的影响，抵消或抵消平均气温上升的影响，从而增加某些极端寒冷天气的频率或强度。在气候变化和与北方半球极端寒冷增加有关的环流变化之间存在假设的联系（例如，Cohen et al. 2014; Zhang et al. 2016; Mann et al. 2017）。在澳大利亚南部的一些地区，尽管所有季节的平均温度都在上升，但霜冻频率和霜冻季节的长度一直在增加（Crimp等人，2016）。造成这一增长的原因尚不完全清楚，但可能与温室气体迫使的循环变化有关。中纬度地区气压的增加归因于温室气体（例如，吉莱特等人，2013年）。这个趋势包括副热带高压脊的增强，但副热带高压脊通过促进晴朗的天空与霜冻风险只有微弱的联系。与霜冻的联系可能更多的是一个特定的平均海平面气压（MSLP）异常的函数。西南偏西地区的寒冷爆发和霜冻风险通常与澳大利亚西部印度洋的正MSLP异常以及澳大利亚南部和东南部的负MSLP异常有关，来自澳大利亚南部的冷空气平流进入西南偏西地区（Ashcroft等人，2009年; Pook等人，2011年）。2016年9月的许多天都显示了这种MSLP特征，表现为印度洋地区约40°S的缓慢移动阻塞高压。因此，一个重要的问题是，这种环流异常是否更有可能是由于温室效应。预计冬季澳大利亚东南部地区的阻塞高峰将减弱并向东移动（Grose et al. 2017）。然而，春季印度洋地区的阻塞可能会有不同的反应。事实上，2014年8月澳大利亚南部异常高的MSLP更有可能是由于人类感染（Grose et al. 2015），这与阻塞高压有关。附属机构：GRose和RisBe-CSIRO海洋和大气，澳大利亚塔斯马尼亚州霍巴特; Black-ARC气候系统科学卓越中心和墨尔本大学，澳大利亚维多利亚州墨尔本; Uhe-环境变化研究所，牛津大学，牛津大学，牛津电子研究中心，联合王国牛津; hoPe-Bureau of Meteorology，澳大利亚维多利亚州墨尔本;牛津大学haUstein-Environmental Change Institute，牛津，联合王国;米切尔-环境变化研究所，牛津大学，牛津，和布里斯托大学地理科学学院，布里斯托，联合王国
Introduction. The wheat belt of southwest Western Australia (SWWA) experienced several severe frosts just before harvest in September 2016, leading to a loss of one million tonnes of grain crops (GIWA 2016). Using the Jones et al. (2009) gridded observation dataset, there were 18 frost-risk nights (Tmin <2°C) somewhere in the grain belt through the month and the September frost area and frequency was extensive (Fig. 29.1a), the highest since 1956. The highest count at any grid cell was 13 frost-risk nights, with 9 severe frost-risk nights (Tmin <0°C). Many places saw the highest number of September frost nights since reliable records began in 1910, with most of the region in the top five years (Fig. 29.1b). SWWA also saw belowaverage rainfall and humidity, southerly monthly wind anomalies, and cool sea surface temperatures (SSTs) immediately adjacent to SWWA in September. There were weak La Niña and negative Indian Ocean Dipole conditions during September. The effect of human influence on cold extremes is the net result of two influences: rising temperatures of the climate mean state and forced changes to circulation. The SWWA region has warmed by around 1°C since 1910, suggesting a reduction in frost risk (BOM and CSIRO 2016). However, greenhouse gas forcing may drive an increase in the frequency or intensity of some cold extremes through an effect on circulation features, offsetting or countering the effect of the rising mean temperature. There is a hypothesized link between climate change and a shift in circulation linked to increased cold extremes in the northern hemisphere (e.g., Cohen et al. 2014; Zhang et al. 2016; Mann et al. 2017). In some regions of southern Australia, frost frequency and the length of the frost season has been increasing despite an increase in mean temperature in all seasons (Crimp et al. 2016). The driver of the increase is not completely clear but may be linked to circulation changes forced by greenhouse gases. An increase in pressure around the midlatitudes has been attributed to greenhouse gases (e.g., Gillett et al. 2013). This trend has included an intensification of the subtropical ridge, but the ridge has only a weak connection to frost risk through promoting clear skies. The link to frosts may be more a function of the particular mean sea level pressure (MSLP) anomalies. Cold outbreaks and frost risk in SWWA are often associated with a positive MSLP anomaly over the Indian Ocean west of Australia and a negative MSLP anomaly across southern and southeastern Australia, advecting cold air from the south of Australia over SWWA (Ashcroft et al. 2009; Pook et al. 2011). Numerous days in September 2016 showed this MSLP signature, expressed as slow moving blocking highs in the Indian Ocean sector at ~40°S. An important question, therefore, is whether this circulation anomaly was made more likely due to greenhouse forcing. The peak of blocking in the southeast Australian sector in winter is projected to weaken and move eastward (Grose et al. 2017). However, blocking in the Indian Ocean sector in spring may have a different response. Indeed, exceptionally high MSLP south of Australia in August 2014 was more likely due to human inf luence (Grose et al. 2015), and this was linked to blocking highs. AFFILIATIONS: GRose and RisBe—CSIRO Ocean and Atmosphere, Hobart, Tasmania, Australia; Black—ARC Centre of Excellence for Climate System Science and University of Melbourne, Melbourne, Victoria, Australia; Uhe—Environmental Change Institute, University of Oxford, and Oxford e-Research Centre, University of Oxford, Oxford, United Kingdom; hoPe—Bureau of Meteorology, Melbourne, Victoria, Australia; haUstein—Environmental Change Institute, University of Oxford, Oxford, United Kingdom; Mitchell—Environmental Change Institute, University of Oxford, Oxford, and School of Geographical Sciences, University of Bristol, Bristol, United Kingdom