| Compared with average climate changes,climate extremes which are considered as low-probability events,have greater impacts on natural and social systems due to their suddenness,unpredictability and strong destructiveness.Located in the typical arid-semiarid climate transition zone,the Mongolian Plateau,dominated by herbage animal husbandry,is greatly affected by climate extremes,and its ecosystem is extremely fragile.In the context of global warming,the spatial-temporal variation research about climate extremes and their impacts on vegetation is not only beneficial to the protection of the grassland ecological environment in the Mongolian Plateau,but also of great significance to formulate an effective meteorological disaster risk management.Based on daily highest temperature,daily minimum temperature and daily precipitation dataset of 111 meteorological stations over Mongolian Plateau from 1961 to 2014.Firstly,sixteen extreme temperature indices and eleven extreme precipitation indices were calculated.Then multi-spatial-temporal characteristics of climate extremes in the scope of intensity,duration and frequency were analyzed.At the same time,the vegetation variations were analyzed based on GIMMS NDVI3 g data.Thirdly,correlation analysis and multiple stepwise regression analysis were used to analyze the relationships between extreme climate indices and NDVI in different spatial scales from the perspective of single factor and multiple factors,respectively.Finally,the future change of climate extremes in duration were explored based on the climate model data.At the same time,the vulnerable areas of grassland and forest were also identified,and adaptive options of different scales were proposed.Main conclusions were drawn as followings: 1.Variation characters of temperature extremes in spatial-temporally and its impact factorsThe spatial-temporal features of temperature extremes are as follows.Temporally,extreme warming occurs in Mongolian Plateau during the recent 54 years.Among them,the duration and frequency of extreme temperature events related to low temperature are significantly reduced,while,the intensity,duration and frequency of extreme temperature events related to high temperature are significantly increased,and the change was most obvious at night in summer.Spatially,temperature extremes in intensity(except Daily Temperature Range: DTR)?Warm Spell Duration(WSDI)?Growing Season Length(GSL)increased as a whole,while DTR was mainly decreased,and the change was most obvious in typical grassland.Cold Spell Duration(CSDI)decreased mostly,but in forest grassland and typical grassland still exist some increases.Temperature extremes in frequency decreased in cold indices,but there were some increases mainly located in Mongolia.The research about impact factors of temperature extremes shows that:(1)After the average temperature abrupt in Mongolia Plateau,changes of geopotential height field and wind field at 850 hPa resulted in,the appearance of anti-cyclonic circulation over the Mongolia Plateau in summer,the strengthening anti-cyclonic circulations in winter between the southwest centered of Mongolian Plateau and the areas near 50 ° E and 40 ° N,which increased the warm days in Mongolian Plateau;Less(more)cloud cover during the day(night)resulted in more warm day(night)days and less cold day(night)days.(2)Multivariate ENSO(MEI),Arctic Oscillation(AO)and North Atlantic Oscillation(NAO)are significantly negatively(positively)correlated with extreme cold(warm)temperature indices.(3)Temperature extremes are mostly positively correlated with longitude,and negatively correlated with latitude and altitude.2.Variation characters of precipitation extremes in spatial-temporally and its impact factorsThe spatial-temporal features of precipitation extremes are as follows: Annually,precipitation extremes over Mongolian Plateau generally decreased in intensity,duration and frequency in recent 54 years and decreased most obviously in the forest grassland and typical grassland,but significantly increased in the desert area.On the seasonal scale,in spring,precipitation extremes in intensity generally decreased,and the number of stations increased in Inner Mongolia was significantly more than that in Mongolia,but there was a significant decrease trend in the typical grassland of Inner Mongolia and in the forest grassland of Mongolia.In summer,precipitation extremes in intensity decreased in most sites,and the change was most obvious in typical grassland.The research about impact factors of precipitation extremes shows that:(1)After the average precipitation abrupt in Mongolian Plateau,changes of geopotential height field and wind field at 850 hPa can lead to the intensification of anti-cyclones in the Mongolian Plateau in summer,weakening the summer monsoon,which decreased precipitation extremes;In winter,East Asian winter wind is weak,which is conducive to the increase of extreme precipitation events in the Mongolian Plateau.(2)Wet extreme precipitation indices and atmospheric circulation indices generally positively correlated relationships.Continuous Dry Days(CDD)had a negative correlation with MEI,NAO,AO and Pacific Decadal Oscillation(PDO),while it had a positive correlation with sunspot number(SN)and the Indian Ocean dipole(IOD),East Asian Summer Monsoon(EASM)and South Asia Summer Monsoon(SASM)(3)Longitude and altitude have a great influence on precipitation extremes in intensity and frequency,while latitude has a great influence on the precipitation extremes in duration.3.Influence on vegetation of climate extremes over Mongolian PlateauThe temporal variation of NDVI are as follows: NDVI decreased in both plateau scale and regional scale in recent 33 years,and changes in Mongolia are obvious.NDVI in cultivated land,typical grassland and desert grassland showed trends of increase,but in meadow grassland,high mountain belt,desert,forest and forest grassland showed decrease.However,the seasonal variation trend was consistent.NDVI generally increased significantly in spring and autumn,and decreased in summer and winter.NDVI at plateau scale and regional scale increased significantly in April and October,and decreased significantly in December,while NDVI of all vegetation types decreased significantly in December.The spatial variation of NDVI are as follows: the increasing area of NDVI in Mongolian Plateau is slightly lower than that of degraded area.The increased NDVI are mainly distributed in desert,desert grassland,typical grassland and part of cultivated land in eastern Inner Mongolia,as well as typical grassland,meadow grassland and forest in Mongolia.However,NDVI in the forests,forest steppes,meadow steppes and western deserts in Inner Mongolia,as well as deserts,meadow steppes and central forest steppes in Mongolia have decreased significantly.The vegetation coverage has obvious seasonal variation characteristics in Mongolian Plateau,with the largest coverage area in summer,followed by autumn and spring,and at least in winter.The study on the influence of climate extremes on NDVI shows that the response time of vegetation to climate extremes was related to the space-time scales and vegetation types.Compared with precipitation extremes,vegetation was more sensitive to and has a longer lag response time to temperature extremes in intensity.4.Assessment of future scenarios under climate extremes over Mongolian PlateauVariation of climate extremes in the future shows that: In the case of warming 1.5 ? or 2? globally,there are extreme warm-and-wet over Mongolian Plateau,and desert region in the southwestern areas is most obvious.If the global average temperature is not controlled,the continuous warming and wetting phenomenon will be further extreme,which poses a new challenge to the growth of vegetation on the Mongolian Plateau.There are great scientific significance and wide applications in this research.Its results can supplement and improve the theoretical system of natural disaster risk assessment.In addition,the impact factors and spatial distribution patterns of climate extremes on the Mongolian Plateau can be clarified,which is conducive to the sustainable development of the social economy and ecological environment of the Mongolian Plateau. |
PDF Full Text Request