Study On The Hot-spots Of Land-atmosphere Interaction And Its Relationship With General Circulation In The Northern Hemisphere

Land-atmosphere interaction is an important physical process of climate system,and soil moisture(SM)is the key variable in land-atmosphere interaction.Whether there are hot-spots of land-atmosphere interaction in the Northern hemisphere(NH),and what is the relationship between SM in hot-spots and general circulation,has long been achieved much attentions.This thesis analyzed the characteristics of temporal and spatial variations of SM in the NH,and revealed that the seasonal frozen-thawing zone(SFTZ)was the hot-spot of land-atmosphere interaction in each season in the NH based on ERA5?GLEAM and GLDAS2.1 Noah reanalysis datasets.As the transition season from winter to summer,the spring is the most sensitive period because of the distinct variation of SM in the SFTZ in the NH.On this basis,the spring SM memory in the SFTZ was analyzed,and the relationship between SM anomaly in spring,and SM anomaly in preceding autumn and subsequent summer in the SFTZ were analyzed.By correcting the spring SM biases in the SFTZ in the numerical model,impacts of spring SM in the SFTZ on the general circulation in spring and summer was revealed.The relationship between spring SM anomaly and general circulation in contemporary spring and subsequent summer was diagnosed and analyzed in the perspective of dynamics and energy conversion.The persistence of spring SM anomalies and its influence mechanism on general circulation in contemporary spring and subsequent summer was also examined by numerical sensitivity experiments.The main research contents and conclusions are as follows:1.The SFTZ is the hot-spot of land-atmosphere interaction in the NH.The most significant region of interannual variation of SM in spring,summer,autumn and winter in the NH is consistent with the SFTZ in the NH based on SM data from ERA5,GLEAM and GLDAS 2.1 Noah datasets.There are two main regions in the SFZT in the NH,that are,central North America(NA),and Europe and Central Asia(EA).The interannual variation of spring SM anomalies between NA and EA is opposite.The spring is the transition season from winter to summer circulation.Therefore,this thesis focuses on the study of spring SM in the SFTZ.2.The SM anomalies in the SFTZ in the NH in preceding autumn can sustain until the subsequent spring through the water storage effect of SM in the freeze—thaw process,and the SM anomalies in spring can sustain itself until the subsequent summer.The SM anomalies in preceding autumn caused by contemporary precipitation can sustain until the subsequent spring through the freeze-thaw process,which is the reason why opposite SM anomalies exist between NA and EA in spring and its preceding autumn.In addition,the spring SM anomalies with inverse phases between NA and EA can sustain until the subsequent summer.The interannual SM anomalies with inverse phases between NA and EA in spring and autumn are related to the trough and ridge of westerly flow,i.e.stationary wave.In spring and autumn,NA is in front of ridge and behind the trough at 500 h Pa,while the EA is controlled by the trough at 500 h Pa.It means that the circulation situations with inverse phases between NA and EA.While NA is between the positive and negative interannual anomalies of zonal 3-wave,EA is controlled by the negative interannual anomalies of zonal 3-wave.That means the interannual anomalies of zonal 3-wave leads to the intensity of trough ridge in the two regions has opposite variation.This is related to the persistence of SM anomalies with inverse phases between NA and EA from preceding autumn to the subsequent spring.3.The spring SM anomalies in the SFTZ cause the anomalous surface diabatic heating,and then cause the anomalous atmospheric circulation in the middle and high latitudes in spring and summer.Because of the biases of SM in the reanalysis data and initial and boundary conditions for numerical models,thus,correcting the SM biases in the SFTZ can presumably correct the simulation biases of SM in spring and summer in the SFTZ,as well as the simulation improvement of surface diabatic heating in spring and summer,especially for surface sensible heat(SH).Subsequently,the simulation of potential height in spring and summer,and the precipitation in summer is also improved.It indicates that SM anomalies in the SFTZ have a significant impact on atmospheric circulation in the middle and high latitudes in the NH.These conclusions are also obtained from the statistical results.4.The spring SM anomalies in the SFTZ will cause the baroclinicity anomalies in NA and EA,and subsequently cause the wave source anomaly in the NH.The dry(wet)anomalies in NA and wet(dry)anomalies in EA of spring SM will cause stronger(weaker)SH in NA and the weaker(stronger)SH in EA in spring,which leads to baroclinicity and baroclinic growth rate weakening(enhancing)in southern NA(EA)and enhancing(weakening)in northern NA(EA),while opposite distribution of baroclinicity and baroclinic growth rate anomalies locate in southern and northern EA(NA).The inconsistent distribution of baroclinicity and baroclinic growth rate anomalies in the northern and southern NA/EA in spring leads to the inconsistent positive or negative anomaly of wave source anomalies in the middle troposphere in these two regions.The spring SM anomalies in NA and EA continuously cause the stronger(weaker)SH and weaker(stronger)surface latent heat(LH)in NA,and the weaker(stronger)SH and stronger(weaker)LH in EA in summer.These anomalies of surface diabatic heating result in the baroclinicity and baroclinic growth rate weakening(enhancing)in the middle and lower troposphere in NA and EA in summer,which subsequently cause the wave sources weakening(enhancing)in the middle troposphere in NA and EA in summer.The wave source anomalies in NA and EA will cause the wave source anomalies in the middle and high latitudes in the whole NH by energy dispersion,and then maintain the wave train around the NH in spring and summer.In the perspective of energy conversion,conversion from mean available potential energy(APE)to eddy APE(conversion from eddy APE to mean APE)induced by spring SM anomalies in NA and EA,cause the enhancing(weakening)of eddy,which is beneficial to enhancing(weakening)of wave source.5.Simulation results demonstrate that the persistence of SM anomalies in the SFTZ in the NH,and its climate effect and mechanism.Through the numerical experiment with different configuration of SM anomalies in NA and EA regions,it is verified that the SM anomalies in preceding autumn in the SFTZ can sustain until the subsequent spring,and can induce the corresponding wave source and wave train in summer.Meanwhile,the simulated wave source and wave train are basically consistent with the statistical results.These results indicate that the "signal" of SM anomalies in preceding autumn in the SFTZ can be used as a predictor of the summer climate in the next year.