The Characteristics Of Land Surface Heat Fluxes And Their Responses To Monsoon Humidity Index In Summer Monsoon Transition Zone Of China

Land surface heat exchanges have significant impacts on climate changes.They are the energy source for atmospheric circulations over land,key link between the land and the atmosphere,and thus an important aspect of understanding weather and climate change.The East Asian summer monsoon transition zone(EASMTZ)is a narrow belt-shape region where summer monsoon system transitions to mid-latitude westerlies from southeast to northwest.It is where the cold,dry air masses and warm,humid air masses visit alternatively or simultaneously.Its influencing factors of climate change are complex.However,summer monsoon is undoubtedly one of the most important factors.In recent decades,the East Asian summer monsoon has been in a continuous weakening phase.In this context,the EASMTZ is facing the pressure of aridity and desertification,which is also driving significant changes in land surface heat exchanges.Therefore,it is necessary to study systematically the response of land surface heat exchanges to summer monsoon.Based on previous researches,a humidity index for summer monsoon strength in the EASMTZ was constructed,and the spatiotemporal characteristics of the land surface heat fluxes in this region were analyzed.The responses of the land surface heat fluxes to the summer monsoon and the corresponding response mechanism are investigated.The main points for the paper are summarized as following:(1)Quantitative description of summer monsoon activity in the EASMTZ.Based on the specific humidity threshold that can describe the seasonal migration of the East Asian summer monsoon,a humidity index(HI)is constructed.The HI index performs well in characterizing the changes in rainfall and the atmospheric humidity related to the East Asian summer monsoon in the EASMTZ,and can also capture the primary mode of summer rainfall in China.Therefore,HI provides reliable measurement for the summer monsoon activity in the EASMTZ.The summer monsoon was weak in the EASMTZ during the 1980s(that is,the HI was low),and then strengthened in the 1990 s.However,after 2000,the HI was again in a low state.HI is closely related to the significant anomalies in western Pacific subtropical high and East Asian subtropical westerly jet which control the summer monsoon moisture transported into the EASMTZ.These circulation anomalies are linked to ENSO.Correlation analyses show that a high(low)HI tends to occur during the cold(warm)phase of ENSO,accompanied by a warm-to-cold(cold-to-warm)phase transition.There are two possible mechanisms that are responsible for the linkage between HI and ENSO.One is the upper-level zonal wind anomaly wave trains which transfers ENSO-related zonal wind anomaly northwards into EASMTZ through thermal wind balance and eddy-driven mean meridional circulation.The other involves the Indian summer monsoon and mid-latitude circumglobal teleconnection.ENSO may exert influences on the humidity in EASMTZ via Indian summer monsoon and circumglobal teleconnection when the interaction between Indian summer monsoon and ENSO is active.(2)The temporal and spatial characteristics of land surface heat exchanges in the EASMTZ.Firstly,latent heat fluxes show obvious transition characteristics in space,changing from the relatively equilibrium state outside the EASMTZ to the ‘abrupt transition’ within the EASMTZ.Moreover,the transition feature becomes more prominent in summer.In contrast,the spatial gradient for sensible heat flux is much weaker.Secondly,land surface heat fluxes features a "staircase" spatial pattern in both the east-west direction and the north-south direction,which is directly related to the regional differences in heat fluxes among the Loess Plateau,North China,and Northeast China.The latent heat flux shows an upward trend,while the sensible heat flux has undergone a decreasing trend in the past three decades.The land surface heat fluxes experienced significant interdecadal fluctuations in the late 20 th century.The changes in land surface heat flux are closely related to the intensity of the summer monsoon activity before the fluctuations,but they may be more affected by human activities afterwards.(3)Response of land surface heat fluxes to summer monsoon.The spatial pattern of the response of sensible heat flux to HI is "+-+" type or "-+-" type,which describes the spatial difference in latitude direction,that is,the difference between the three climate regions of the Loess Plateau,North China and Northeast China.The response of sensible heat flux to summer monsoon or HI is branded with climate background.The spatial characteristics of the response of latent heat flux to HI is "±" type,which describes the spatial difference of the meridian,that is,the difference between the north and south of the transition zone.This is consistent with the spatial distribution of the degree of influence of the summer monsoon.These spatial patterns are consistent with the dominant EOF spatial patterns of land surface heat fluxes and their differences for strong-minus-weak summer monsoon.(4)The main mechanism responsible for responses of land surface heat fluxes to the summer monsoon index HI involves three aspects,including net radiation and soil moisture,the near-surface temperature and specific humidity vertical gradients dt and dq,and biophysical characteristics parameters.HI adjusts the limiting factors of land surface heat exchanges by affecting the soil moisture and the canopy coupling index,which further exerts influence on the relationship between land surface energy exchanges and the near-surface vertical temperature and humidity gradient dt and dq.Given net radiation remains constant,vegetation ecophysical characteristics are significantly related to soil moisture and dt and dq in the case of weak summer monsoon,that is,changes in soil moisture will cause changes in vegetation ecophysical characteristics which further affect dt and dq.However,given soil moisture remains constant,the relationship between vegetation biophysical properties and dt and dq is very weak;that is,changes in net radiation cannot cause regular changes in canopy physical properties.However,the degree of interpretation of soil moisture on changes in vegetation ecophysical properties and the interpretation of vegetation ecophysical properties on dq are regulated by the strength of the summer monsoon(ie,HI).The greater the HI,the lower the degree of interpretation.In the case of strong summer monsoon,precipitation in the summer monsoon transition area increases and soil moisture is high;at this time,the canopy coupling index Ω is large,making the land surface heat exchange more dependent on net radiation.The land surface heat fluxes are constrained by energy and the relationship between fluxes and the vegetation characteristic parameters is weakened.Conversely,in the case of weak summer monsoon,precipitation reduced and the soil is dry.The canopy coupling index Ω is small and the dependence of land surface heat fluxes on the water vapor pressure deficit(soil moisture)increases.The land surface heat exchanges are soil moisture-limited,and the relationship between fluxes with vegetation characteristic parameters enhances.