Characteristics Of Wind Speed And Atmospheric Kinetic Energy Over The Tibetan Plateau In Spring And Their Relationship With Vegetation Coverage

Based on daily observed data of 73 stations in Tibetan Plateau,JRA-55 reanalysis data and NOAA AVHRR-NDVI data,by using correlation analysis,composite methods,year-to-year differences and equation of kinetic energy budget,the variation of wind speed in the Tibetan Plateau in spring during the period of 1971-2012 and its response to global warming is investigated.The spatial and temporal variations of the Asian westerlies in the spring are analyzed using decomposition theory with its barotropic and baroclinic components.The coupling relationship of the kinetic energy and the vertical distribution of the atmospheric apparent heat source over the Tibetan Plateau is revealed via singular value decomposition(SVD).Based on the concept of concentration period and degree,the concentration of gales over the Tibetan Plateau and related circulation during windy period in recent 40 years are analyzed.Finally,the possible effects of mean wind speed and gale days on vegetation coverage over the Tibetan Plateau are discussed.It is indicated that there is a statistically significant decrease of spring wind speed in the Tibetan Plateau as the climate is warming,while the variation tend to be steady as a hiatus of warming occurs the beginning of 21st century.It is suggested that the gales over the Tibetan Plateau showed a declining trend with a reduced rate of-14d/10a from 1971 to 2012,and the annual range of the gale days is also reduced.The gale concentration period(GCP)over the Plateau delayed with latitude and was moving up earlier in the past 42 years,which is from the early April to early March.The gale concentration degree(GCD)has an increase tendency,and it depends on the number of gale days,the more the gale days are,the lower the concentration degree is.The asymmetrical thermal condition of both the Tibetan Plateau and its purlieus make an impact on atmospheric baroclinicity by which the variation of wind speed in the Plateau can be influenced in two ways:one is changing directly the pressure gradient force near surface,another is affecting the transportation of kinetic energy from upper level to middle level.The spring wind speed in the Tibetan Plateau increases when surface temperature increases in the Tibetan Plateau,Indo-China Peninsula and India Peninsula but decreases in the north and east of Asian.A decline trend of spring wind speed in the Plateau at the end of 20th century due to more rapid warming in the North Asia than in the South Asia,and a stable trend of wind speed in the years after followed is accompanied by warming in Central Plateau and South Asia while cooling in the North and East Asia.The gale concentration period(GCP)is affected by the meridional displacement of the jet stream.In February and March,when the subtropical jet stream in North Africa and Western Asia and the low-level westerly belt in April tended to be southerly,along with the lower pressure in the subtropical region,there is more gale days over the Tibetan Plateau and GCP is late;otherwise,GCP is early.The gale concentration degree(GCD)is related to the intensity of subtropical jet stream in Central Asia and Tibetan Plateau from February to April.When the subtropical jet in February and April is weak and the jet in March is strong,the gale days are concentrated in March and the GCD is high,and vice versa.The intensity and structure of the westerlies(or the jet stream)can be characterized by change of the atmospheric kinetic energy.The decomposition of atmospheric kinetic energy show that the center of the barotropic kinetic energy of the subtropical region occurs to the east of the center of the total kinetic energy,while the center of the baroclinic kinetic energy is to the west.In the spring,the intensity of low-level heat sources and high-level cold sources over the Tibetan Plateau have significant interdecadal characteristics,which change the atmospheric baroclinicity by adjusting the baroclinic flow at low and middle latitudes.The enhanced heat source over the plateau will increase the atmospheric kinetic energy of East Asia and decrease the kinetic energy in South Asia,causing the subtropical jet to move northward.Over the Tibetan Plateau,the thickness of the lower-level heat sources shows obvious interannual variability,which mainly affects the upper-level wind shear in the mid-high latitudes.A thinner heat source is favorable for strengthening the East Asia subtropical jet and weakening the temperate jet.Meanwhile,the intensified mean flow over the upper reaches of the Tibetan Plateau is conducive to the downstream transport of heat,which then changes the vertical structure of the apparent heat source over the plateau.Moreover,the heat source over the plateau will change the composition of the atmospheric kinetic energy of the region downstream of the plateau via interactions between barotropic and baroclinic motions.In most parts of Qinghai Tibet Plateau,especially the desert and grassland with sparse vegetation,the stronger the average wind speed,the lower the surface vegetation coverage.The vegetation coverage in these areas experienced a turning point from the increase to the decrease in the middle of the 1990s,which corresponds to the turning of wind speed change from decreasing to the increasing.It is also indicated that the annual maximum vegetation coverage in the plateau increased in the recent 30 years,and desertification has been relieved.Vegetation in desert is more sensitive to surface wind speed in winter and spring,while vegetation in grassland is more sensitive to wind speed in spring and summer.In the southwestern part of the plateau where water and heat conditions are relatively deteriorating,wind speed is the main climatic factor affecting vegetation coverage.In the northwestern part of Sichuan,when the GCP was delayed,the number of gale days increased significantly,and the vegetation coverage decreased in spring and autumn.In the Tanggula Mountain pass,when the GCD is high,the uneven distribution of the gale days is more significant in the year,the vegetation coverage in spring and summer is increased,while the vegetation coverage in autumn and winter is reduced.The increase of the average wind speed and the number of gale days will stabilize the change of vegetation cover,while vegetation coverage functions to makes the average wind speed and the number of gale days more uncertain,and the importance of a flow of information from wind to vegetation is larger than one from the latter to the former.The results of this paper provide a scientific basis for the prevention and control desertification of the Tibetan plateau,and have far-reaching significance for disaster prevention and reduction,the rational use of climate resources and the improvement of the ecological environment.It is also useful for understanding the seasonal transition and regional climate change in East Asia.