Transition Characteristics Of Surface Sensible Heat Flux In The Mid-eastern Tibetan Plateau And Its Relationship With SSTA At Different Stages

Based on satellite remote(NDVI)data and station data,the daily surface sensible heat flux sequence of 70 stations on the plateau was calculated from 1982 to 2018.Firstly,the rotation empirical orthogonal function(REOF)decomposition was divided into different zones on the plateau,and then the Piecewise Linear Fitting Model(PLFIM)was used to analyze the annual mean sensible heat flux transition characteristics of each zone and 70 stations.The relative contribution of temperature difference and wind speed to sensible heat change was quantitatively studied by multiple linear regression analysis of variance method.Then,the spatial distribution and transition characteristics of the sensible heat over the four seasons are compared and analyzed,the relative contribution of temperature difference and wind speed to the change of sensible heat in the plateau and the seasonal differences are studied.Finally,the possible influence of the climate background field on the transition of sensible heat trend is discussed from the perspective of atmospheric circulation.The main conclusions are:(1)According to the decomposition of REOF will plateau mean sensible heat can be divided into four climate zones: ?zone as the northern plateau area;? zone as the eastern plateau area;? zone as plateau southwest area;?zone as southeastern plateau area.On the whole,the average sensible heat in winter,spring,summer and annual-mean on the plateau changed in 2001,and it was 2000 earlier in autumn;on the regional level,the sensible heat in different seasons was the earliest in zone II,the latest in Zone III;on the seasonal level,the turning time in zone II was the earliest in spring(1997),and the turning time in Zone III in winter(1997).(2)Before the transition of sensible heat trend,the air-surface temperature difference has a major contribution to the change of sensible heat in winter,and the ground wind speed has a major contribution to the change of sensible heat in summer.The change of average sensible heat in spring,autumn and annual is affected by both the air-surface temperature difference and the ground wind speed.After the transition,the contribution rate of air-surface temperature difference to the change of sensibleheat in each season is enhanced.The key areas of the trend transition of sensible heat flux in the plateau are distributed in zone II and zone III.(3)Before 2000,in the mid latitude region of the northern hemisphere(25-40°N),the annual mean westerly jet was weak and the jet axis was South.At the same time,the temperature from the North(South)to the high(low)latitude region of the plateau was abnormally high(low),and the meridional temperature gradient and pressure gradient decreased,which made the plateau wind speed continue to decrease in this period.After 2000,the situation is exactly the opposite,the meridional temperature gradient and pressure gradient increase,which makes the decrease trend of the plateau wind speed ease in this period,and gradually become an increasing trend.(4)Before the transition of sensible heat trend,the first mode of the sensible heat flux is mainly caused by the modulation of ENSO,especially the Nino3 SSTA.The most consistent spring enhancement is the El Nino development stage.After the transition,the second mode of "North-South Inversion" in the plateau is mainly affected by the North Atlantic three-pole SSTA(NAT).When the north is strong and the south is weak in spring,the North Atlantic NAT is in a negative phase and increases gradually from winter to spring.