| The soil moisture-temperature coupling process is an important physical process in the Earth's climate system,affecting other physical processes in the climate system at different time and spatial scales.However,due to the existence of many coupling indices to diagnose such strength,there are less consensus in understanding the strong coupling regions,that is,“Hot Spots”.On the other hand,the intra-seasonal,interannual,and long-term trends of soil moisture have a significant effect on the nearsurface temperature of these “Hot Spots.Considering of this reason,we compare the “Hot Spot” regions diagnosed by different indices using observations,ERAInterim/Land and MERRA-2/Land reanalysis as well as GLACE-CMIP5 model simulations and analyze the future change of soil moisture-temperature coupling strength.Based on this,we also analyze the impact of inter-annual variability and longterm trends of soil moisture on the near-surface temperature.The main conclusions of thesis are as follows:(1)The spatial distribution of soil moisture-temperature coupling “Hot Spots” based on different data sets and indices was consistent.Similar to the previous studies,the “Hot Spots” were mainly located in dry-wet transition and monsoonal regions such as Central-Southern of North America,Sahel,the Indian Peninsula,and Southern Africa and Northern Australia in the Southern Hemisphere.The simulated distribution of “Hot Spots” by each model of GLACE-CMIP5 is relatively uniform,but the intensities showed significant differences.The future trend of soil moisture can significantly affect soil moisture-temperature coupling strength.In Central-Southern part of North America and Sahel,with the drying trends of soil moisture,the coupling strength in these areas will increase in the future.While in the Indian peninsula and North-Eastern part of South America,the coupling strength of these regions showed weakening trends due to the wetting soil moisture in the future.(2)The inter-annual variability of soil moisture can not only significantly increase the mean and variability of temperature,but also affect the shape of the probability density distribution(PDF)of the near-surface temperature,which will change the PDF of near-surface temperature from a quasi-Gaussian distribution to a positively skewed distribution.Soil moisture variability can increase the means of near-surface temperature in “Hot Spot” up to 1-1.5 K,increase standard deviation about 0.5 K,and skewness about 0.2-0.3.The effects of inter-annual variability of soil moisture on interannual scale temperature variability in “Hot Spots” were significantly higher than that on the synoptic scale,increase about 0.4-0.5 K of inter-annual standard deviation in these regions,while only 0.1 K on synoptic temperature variability.Near-surface temperature had a significant asymmetric response to soil moisture variability,i.e.the response of the maximum temperature to soil moisture changes is most sensitive,followed by the mean temperature and the minimum temperature were the most insensitive.In “Hot Spots”,changes in one standard deviation of latent heat flux caused by soil moisture variability can lead to increase 0.110 K for maximum temperature,0.090 K for mean temperature and 0.050 K for minimum temperature.(3)The long-term trends of soil moisture can contribute to the warming of land near-surface temperature in the future.Under the RCP8.5 emission scenario,the contribution of long-term trends of soil moisture to temperatures over the global land,Sahel,Indian Peninsula,North-Eastern part of South America,and Central-Southern part of North America will be 17.09%,0.92%,4.14%,20.22% and 23.15%,respectively.In the future,near-surface temperature changes still have asymmetric responses to soil moisture long-term changes,especially in “Hot Spots”.The maximum temperature will be most sensitive to long-term trends in soil moisture,followed by the mean temperature,and the minimum temperature will be the most insensitive.The contribution of soil moisture long-term trend to the increase of maximum temperature over global land,the Sahel,the Indian Peninsula,North-Eastern part of North America,and Central-Southern part of North America,will be 15.60%,5.08%,11.56%,25.05%,and 23.01%,respectively.The contribution to increase of minimum temperature increase in the above regions will be 10.01%,2.52%,9.20%,12.29% and 14.06%,respectively.(4)The impact of soil moisture variation on the near-surface temperature are caused by the combination of direct and indirect effects.When considering the variation of soil moisture,the surface evapotranspiration of the “Hot Spot” is mainly constrained by the soil moisture.The net radiation absorbed by the land surface is more partitioned to the sensible heat flux,thus the latent heat flux will decrease,which will directly increase the near-surface temperature.When the soil moisture variations are suppressed,the evapotranspiration is mainly constrained by the net radiation,resulting in a small change in the mean and variability of near-surface temperature.In addition to direct effects,the indirect effects of increased net surface short-wave radiation and decreased net long-wave radiation will also affect near-surface temperature changes due to reductions of latent heat flux and increased sensible heat flux.In general,the impact of soil moisture variation on the near-surface temperature are dominated by direct effects,and the contribution of indirect effects are relatively small. |
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