Characteristics Of Surface Heat Content Variation In Eurasia And Its Possible Relationship With East Asian Summer Monsoon Rainfall

Land surface conditions constitute a complicated system that cannot be objectively described using only one factor.Based on soil temperature,soil moisture(soil liquid water and soil ice)achieved from off-line simulation of the Community Land Model version 4.0(CLM4.0)and soil texture dataset supported by the International Geosphere-Biosphere Programme(IGBP),soil enthalpy is derived and calculated to represent land surface heat content.Then,its spatiotemporal characteristics in Eurasia and sensitivities to soil moisture and soil temperature are investigated.In addition,the use of soil enthalpy as a metric for monitoring land surface heating is explored from different perspectives(e.g.,long-term trend,land memory and rainfall forecast).Finally,impacts of soil enthalpy on East Asia summer monsoon rainfall are analyzed.Main conclusions are as follows.(1)Soil enthalpy is a distinct variable that accounts for variations in soil temperature,soil liquid water,soil ice and soil solid matter(including organic matter,sand and clay),and it can directly reflect the land surface thermal condition in units of energy.Soil enthalpy in Eurasia has the characteristics of decreasing with latitude,value of which is generally less than zero at mid-high latitudes in winter.The long-term trend of soil enthalpy is mainly to increase with the most obvious increase about 60 cm from the surface.The inter-annual variability of soil enthalpy over Eastern Europe is very prominent,while summer is the weakest season.For the spatial-temporal evolution,it exhibits a meridional dipole distribution pattern along 45 °N on an inter-annual time scale.The persistence of soil enthalpy in the wintertime is much better when the anomaly duration can reach 9 months,and the seasonal variations in the persistence of soil solid matter enthalpy and soil liquid water enthalpy change with latitude.(2)The relationships between soil temperature,soil moisture and soil enthalpy were compared in details.The spatial distribution of soil enthalpy climatology is similar to soil temperature,while the spatial distribution for inter-annual variability is quite consistent with soil moisture.There is obvious differences between the long-term trends of soil enthalpy with the two others,while the anomaly of soil enthalpy persistence is mainly the average of the two others.In addition,soil moisture has a negative influence on soil enthalpy when the soil contains high concentrations of ice and a positive influence after the soil ice has melted,whereas soil temperature has a positive influence on soil enthalpy all the year around.Soil enthalpy is most sensitive to soil ice at mid-high latitudes but to soil temperature at mid-low latitudes such as North Africa and Arabia peninsula.With the increase of soil depth,soil enthalpy become more and more sensitive to the change in soil temperature.(3)During the water phase change process of melting in Eastern Europe in winter,soil enthalpy can describe the phenomenon of increasing latent heat accumulated in soil,while this cannot be achieved through measurements of soil moisture and soil temperature.Soil enthalpy increasing is notably related to the soil ice deceasing in Eastern Mongolia,the soil temperature increasing in India River Plain,and both the increasing of soil temperature and soil liquid water in Central Africa.Additionally,soil enthalpy in East China has a successive and longer memory in early spring,and is better suited for precipitation forecasting than soil temperature and soil moisture,indicating the application of soil enthalpy as a metric for monitoring land surface heating and designing seasonal prediction systems.(4)The summer precipitation in East China is closely connected to antecedent spring soil enthalpy on the Eurasian continent with two significant key regions,one located in Central Asia and the other located in East European Plain.When soil enthalpy in May is abnormally high(low)in Central Asia but abnormally low(high)in the East European Plain,soil heat causes geopotential height anomalies by altering longwave radiation emitted upward from the land surface.The geopotential height anomalies,guided by basic westerly currents,propagate eastward.In June,the abnormally high(low)pressure over northeastern China and the anomalous easterly(westerly)winds on its south side favor the weakening(strengthening)and northward shifting(southward retreat)of westerly jet streams,which in turn result in a northerly(southerly)location of the subtropical high.In July,the region north of the Yangtze-Huaihe River Plain,controlled by an anomalous anticyclone(cyclone)in the lower troposphere,receives little(abundant)precipitation,whereas southern China,controlled by an anomalous cyclone(anticyclone),receives abundant(little)precipitation.