Impact Of The Fake Below-ground Meridional Wind On Spatial Pattern And Long-term Trends Of Hadley Circulation

The Hadley circulation(HC)is the strongest and largest vertical circulation in the global atmospheric circulation,which plays an important role in the process of physical quantity and mass transportation and exchange between the low latitude region and the mid-high latitude region.Since the subtropical arid and semi-arid zones are closely connected with the sinking branches of the HC,the poleward shifts of the sinking branch can lead to an expansion of dry zones,affecting global climate change,natural ecology,agriculture,and water resources.Based on monthly data of wind field from 12 Coupled Model Intercomparison Projection Phase 5(CMIP5)models and 3 reanalysis datasets,we investigate impact of the fake below-ground meridional wind(FBGMW)on spatial pattern of the global zonal mean HC and the regional HC over the East Asia by using stream function of meridional circulation derived from the three-pattern decomposition global atmospheric circulation method(3P-DGAC).The main research results obtained here are as follows:(1)The effect of the FBGMW on global zonal mean HCResults derived from both model and reanalysis datasets show that,although the wind field difference(mainly characterized as the south wind in the low latitude and the north wind in the middle latitude)caused by not considering the FBGMW only exists in the lower troposphere,not considering the FBGMW can affect the HC stream function of the whole layer(mainly characterized as the low-latitude counterclockwise anomalous circulation exists south of 30°N and the mid-latitude clockwise anomalous circulation exists north of 30°N)through the vertical integration process;therefore,the spatial pattern of HC is affected when the FBGMW is not considered.It should be noted that the influence of wind field differences in the lower troposphere on HC spatial is only obvious during boreal summer.In other seasons and in the southern hemisphere,although there are still wind field differences in the lower troposphere,the impact of the FBGMW in southern/northern hemisphere is weak.Because the intensity of the global zonal mean HC is weakest in the northern hemisphere in summer,and the magnitude of the stream function error caused by not considering the FBGMW is equivalent to the magnitude of the climatology of the HC stream function.The calculation error caused by not considering the FBGMW could lead to poleward shift of HC sinking branch location and width in the northern hemisphere.In some years,due to this large calculation error,an anomalous "minor circulation" structure embedded within the spatial pattern of HC during summer would appear around 30°N.(2)Influences of the FBGMW on the spatial pattern of the HC and future estimation of the sinking branch derived from CMIP5 datasets in boreal summerDuring 1979-2005,the differences between corrected and uncorrected spatial pattern of HC in boreal summer derived from 12 CMIP5 datasets by using 3P-DGAC method show that,do not correct the calculation error caused by not considering the FBGMW could lead to an underestimation of HC intensity in northern hemisphere,and the location of the sinking branch and the total width of HC will be overestimated.In July,the calculation error of the location of the sinking branch of HC in the northern hemisphere reached 6.80°,as the calculation error of the total width reached 6.86°.The calculation of HC total width increased by about 11.53%without considering the FBGMW.The linear trends of HC sinking branch in boreal summer derived from 12 CMIP5 datasets are inconsistent.However,the trends before and after correction are quite different,which indicates that the FBGMW has a significant influence on the linear trend of HC sinking branch in northern hemisphere.The quantitative analysis shows that in the 2040-2099 period under Representative Concentration Pathway 8.5(RCP8.5)scenarios,the future poleward trend of the northern hemisphere HC sinking branch in summer decreased by 1.07° per decade after the error correction.(3)Influences of the FBGMW on the sinking branch of the HC and the relationship between the HC sinking branch with precipitation during boreal summer derived from reanalysis datasetsSimilar to the results in CMIP5 models,the impact of the FBGMW,derived from 3 reanalysis datasets by using 3P-DGAC,leading to an underestimation of HC intensity and an overestimation of the location of HC sinking branch and width in northern hemisphere during summer(from June to September).The linear trends of HC sinking branch in boreal summer derived from 3 reanalysis datasets are consistent.The poleward trend of HC sinking branch will be overestimated when the error caused by not considering the FBGMW is not corrected.This suggests the previous conclusion that the HC has obviously poleward expanded over the past few decades may be disturbed by the calculation error caused by not considering the FBGMW.After correction,the expansion of the HC in the northern hemisphere will result in a significant decrease in precipitation over Middle East,northern China,Japan,north Pacific,western North America and the Atlantic Ocean on the north side of the mean position of the HC sinking branch.(4)The impact of the FBGMW on annual and seasonal mean regional HC over East AsiaThe location of the sinking branch of the northern hemisphere HC,the trend of its poleward expansion and the total width of annual mean,spring and summer mean regional HC over East Asia would be overestimated if the error due to the FBGMW not be corrected.However,the impact of the FBGMW on regional HC over East Asia has obvious seasonal difference,the intensity of annual mean regional HC over East Asia increased in the northern hemisphere after correction but spring and summer mean regional HC intensity was decreased.In spring and summer,the seasonal mean regional HC is very weak,and the FBGMW has a great influence on the HC,the relative error of regional HC intensity in summer is 94.48%.Thus,it is necessary to correct the calculation error caused by the FBGMW.In the northern hemisphere during spring time,there is a significant positive correlation between the expansion of the regional HC and precipitation over the region of central China,north China and central Inner Mongolia after correction.In the northern hemisphere summer,the expansion of the regional HC results in a significant decrease in precipitation over North China and central Inner Mongolia,as well as a significant increase in precipitation over Shandong,Hebei,Taiwan,and south China sea.