Twenty-five years of change in scleractinian coral communities of Daya Bay （northern South China Sea） and its response to the 2008 AD extreme cold climate event

Coral reefs worldwide are becoming increasingly and detrimentally impacted upon by a variety of factors including significant climate changes, such as global warming and increased El Nino-Southern Oscillation activity. Generally, the persistence of coral reefs, especially at low-latitudes, is governed, in part, by sea surface temperatures not exceeding the critical limit （-30℃） at which mass mortality can occur. Thus, it is thought that corals living at high-latitudes （i.e., currently cooler sea surface temperatures） will likely respond more favourably to hypothesized future temperature increases than corals living at low-latitudes （i.e., currently warmer sea surface temperatures）. Consequently, high-latitude coral communities may have the potential to act as regions of refugia for many coral species in the face of potential future global warming. The Daya Bay （22°31′--22°50′N）, northern South China Sea, contains several high-latitude non-reefal coral communities and represents one of the most northerly distributions of scleractinian corals within the region. Significantly, Daya Bay has experienced dramatic warming in both air and sea surface temperatures throughout the past 50 years. In this paper, we analyze 25 years of change in the Daya Bay coral communities, based both on historic surveys and our latest 2006--2008 regional ecological surveys. Our results suggest that, contrary to predictions, there have been significant declines in coral cover within the Daya Bay during the past 25 years （i.e., 76.6% coral cover in 1983/1984 to only 15.3% coral cover by 2008）. Such changes also reflect a significant shift in the most abundant coral species, from Acropora pruinosa to Favites abdita. Most of the modern coral communities became established between 15 and 30 years ago, corresponding to a period of increased winter sea surface temperature. However, very few colonies have become established within the last 15 years, despite a more intense period of warming. By taking into account additional factors, we hypothesize that direct anthropogenic impacts, rather than climatic events, have both restricted the development, and drove the decline, of Daya Bay coral communities in the last 15 years. The Daya Bay has also been subjected to occasional extreme cold events during the past 50 years, with the most recent occurring in early 2008 （13 January-13 February）. During the 2008 cold event, the lowest air temperature reaches only 6.6℃, and the mean sea surface temperature for February fall to 〈 14℃, including six continuous days at 12.3℃. Significantly, the sea surface temperatures fall below the hypothesized critical lower temperature threshold （-13℃） that commonly leads to mass mortality in scleractinian coral communities. Surprisingly, our coral community surveys, conducted both before （August 2007） and after （late February 2008） the extreme 2008 cold event, demonstrate that the Daya Bay coral ecosystems are barely impacted upon during the cold period. Those observations suggest that the Daya Bay scleractinian coral communities have developed adaptations to low sea surface temperatures. Overall, our data support the hypothesis that high-latitude coral communities, such as Daya Bay, have the potential to act as areas of refugia for scleractinian corals in the advent of potential future global warming.     

530年来陕北气候变化与东部地区关系分析

目的 研究陕北地区历史旱涝变化、气温变化及与东部地区的关系，对保护和治理陕北地区生态环境提供可靠依据。方法 利用我国500多年旱涝资料及50a气温资料，运用功率谱方法及小波分析法分析陕北及其东部地区历史上发生的旱涝周期及气温变化。结果 不同地区530a来均呈旱涝交替变化，且存在一定的周期性。从上世纪80年代中后期陕北开始进入偏暖少雨期，气温具有13a，2～4a左右的周期。结论 陕北历史旱涝存在明显的阶段性变化，气温变化具有周期性，异于其他地区。     

冬季中东急流时空变化特征及其与中国气候的关系

采用1968—2009年NCEP/NCAR月平均再分析资料和1968—2008年月平均中国160站降水和气温资料,使用EOF分析等方法,研究了冬季中东急流的时空变化特征及其与中国气候的关系。结果表明:中东急流存在3个主要EOF空间模态(其方差贡献率分别为26.9%、20.9%和17.0%),第1模态反映了急流核下游的急流变化特征,第2模态反映了急流区域的南北反相变化特征,第3模态反映了急流核上游的急流变化特征;冬季中东急流指数具有明显的年际变化特征,主要存在2~4年和准8年的周期振荡。冬季中东急流与中国西南地区气温(华南地区降水)呈显著负(正)相关关系;中东急流偏强、偏弱年的中国气温及降水差值显著区域分别位于西南地区(最大差值达3°C)和华南大范围地区(最大差值达180 mm)。     

大湄公河次区域植被覆盖时空变化特征及其与气象因子的关系

【目的】分析大湄公河次区域植被覆盖的时空分布变化规律及其与气象因子之间的关系,为全球变暖环境下大湄公河次区域植被保护及生态环境修复提供理论依据。【方法】以大湄公河次区域为研究区,使用MOD13Q1-NDVI数据,借助Google Earth Engine(GEE)平台反演区域2005—2019年植被覆盖,采用线性回归分析、马尔科夫模型等分析区域植被覆盖的时空分布规律,并利用偏相关分析法探究植被覆盖与气象因子之间的关系。【结果】大湄公河次区域高植被覆盖的面积占总面积的61.9%,空间上呈现北低南高、东高西低的特点;2005—2016年,区域植被以改善为主,主要是中高植被向高植被类型转化;2016—2019年,区域植被发生明显退化,以高覆盖植被类型退化为主;15年来,呈改善趋势的面积占总面积12.7%,呈退化趋势的面积占总面积3.0%,基于Hurst指数分析发现,区域植被未来显著改善面积大于显著退化,南部地区未来会发生退化;年际变化趋势上,归一化植被指数(NDVI)与气温呈显著正相关,相关系数为0.61,与降水相关性较弱;空间上,区域植被NDVI变化受到气温和降水影响,北部与降水显著负相关,南部与气温显著负相关。【结论】大湄公河次区域植被覆盖整体较好,改善趋势大于退化趋势。综合来看,大湄公河次区域植被变化与气温和降水有一定关系,尤其是北部和南部。     

东北夏季降水的时间变化特征及其与太平洋海温的相关关系

利用中国降水逐日站点观测资料以及HadISST海温资料,分析了1961-2010年东北地区70个观测站夏季降水的变化特征以及太平洋海温对东北夏季降水的可能影响。结果表明：东北夏季降水近50a来呈现下降趋势,具有多阶段性,在60年代以及80-90年代存在明显的突变;在1986-1985年以及1986-2010年2个阶段,热带外太平洋与热带太平洋春季和夏季海温都存在与东北夏季降水的相关关键区,两个阶段相关性显著不同甚至相反,是1961-2010年东北夏季降水与太平洋海温总体相关性较弱的原因。东北夏季降水与太平洋海温的相关性可能与海温冷暖背景有关系：在暖的海温背景下,东北降水与热带太平洋海温呈现显著的正相关;而对于热带外太平洋,暖的海温背景下,两者为负相关,冷的海温背景下,两者为正相关,尤其表现在春季,因此太平洋春季海温异常对于东北夏季降水预测有较好的指示意义。