Changes Of The Hadley Circulation And The Influence Of Orography On Them Derived From The Three-pattern Decomposition Of Global Atmospheric Circulation

The Hadley circulation(HC)is an important circulation member of the monsoon system.Since the exchanges of energy,quality,angular momentum,water vapor,and other variables between the tropics and extratropics are closely related to the HC,the HC is fundamentally important to the mean state and variability of global weather and climate systems.Chou Jifan's team proposed a novel method of three-dimentional(3D)circulation decomposition that can describe the actual atmosphere from a global-wide perspective,i.e.,the global atmospheric circulation is decomposed into three orthogonal horizontal,meridional,and zonal circulations,which is called the three-pattern decomposition of the global atmospheric circulation(3P-DGAC).In this study,firstly,we give a brief introduction of the 3P-DGAC method and propose a high-stability algorithm of the 3P-DGAC method.The advantage of the 3P-DGAC method in representing the global atmospheric circulation is also provided.Then,we investigate the differences and connections between the 3P-DGAC method,the representation of the horizontal vortex circulation in the middle–high latitudes,and the local partitioning method of the overturning circulation in the tropics.Additionally,we explore the changes of the HC and the influence of orography on them.According to the research and analysis in this study,we can obtain the following conclusions:(1)The 3P-DGAC method and its high-stability algorithmThe 3D horizontal,meridional,and zonal circulations can be considered as the global generalization of the Rossby wave at middle–high latitudes and Hadley and Walker circulations at low latitudes,and the main characteristics of the traditional circlations can be well captured by the three-pattern circulations(i.e.,the horizontal,meridional,and zonal circulations).The kinetic energy of the horizontal circulation is mainly located at middle–high latitudes and that of the meirdional and zonal circulations is mainly located at low latitudes,while the kinetic energy of the traditional Rossby wave,Hadley circulation,and Walker circulation are all located at middle–high latitudes,implying that the definitation of the kinetic energy of the three-pattern circulations is more accurate than that of the traditional circulations.In addition,the vertical velocity can be decomposed into two components by using the 3P-DGAC method,one is caused by the meridional circulation and the other one is caused by the zonal circulation.Thus,the meridional and zonal circulations can be more accurately described by using the 3P-DGAC method.(2)Comparison of the 3P-DGAC method and the traditional two-dimensional(2D)decomposition methodThe representation method of the horizontal vortex circulation in the middle–high latitudes and the local partitioning method of the overturning circulation in the tropics are based on the traditional 2D decomposition of the vortex and divergent circulations in the fluid dynamics.The traditional 2D decomposition method cannot separate the contributions of the divergent motions from the total vertical vorticity.Different from the traditional 2D decomposition method,the 3P-DGAC method can quantitatively decompose the vertical vorticity into two components: one part is caused by the horizontal circulation,whereas the other one is induced by the vertical circulations.If we let the vertical vorticity induced by the vertical circulations equal zero,then the 3D horizontal circulation must be equal to the traditional 2D vortex circulation and the overturning circulations derived from the traditional and 3P-DGAC methods are also equal.Namely,3P-DGAC method reduces to the traditional 2D decomposition method.Thus,the 3P-DGAC method is not a simple superposition of the representation methods of the horizontal vortex circulation at middle–high latitudes and the overturning circulations in the tropics.Indeed,the 3P-DGAC method can be considered as a generalization of the traditional 2D decomposition method of the vortex and divergent circulations.(3)On the discrepancies in the changes of the annual mean HC among different regions and between CMIP5 models and reanalysesIn the historical time period,for every reanalysis trend in the HC intensity and poleward edge in a specific region,there exists at least one model trend that is not statistically significantly different from the reanalysis trend at the 95% confidence level.In other words,for any regional HC,none of the reanalysis trends in the HC intensity and poleward edge are statistically significantly different from every CMIP5 historical simulation's trend.Therefore,there is no discrepancy in the long-term trends in the HC intensity and poleward edge estimated in CMIP5 models and the reanalyses over the historical period.The correlations of the HC intensities(poleward edges)among different regions derived from the reanalyses are generally different from(similar to)those obtained from CMIP5 historical simulations,and the difference(similarity)may be caused by the difference(similarity)of the internal variabilities between the reanalyses and piControl simulations.In the future time period,the correlations of the variations in the HC intensities and poleward edges among the six selected regions do not change with global warming since the initial fields for the historical and RCP simulations are both provided by the piControl simulations.(4)Significant role of orography in shaping the northern HC and its poleward expansion during boreal summerBased on the mass stream function(MSF)without considering orography(i.e.,surface pressure),the “minor circulation” is evident in the Northern Hemisphere(NH)HC during boreal summer and characterized as a southward circulation embedded within the conventional northward circulation.However,after considering orography,although the minor southward circulation in the NH HC around 30°N still occurs,the northward circulation around 40°N unexpectedly changes into a southward circulation;therefore,the NH HC is characterized as one single northward circulation at low latitudes.The results can be obtained regardless of the method and reanalysis data set used;hence,we further concluded that the “minor circulation” concept is introduced by not considering orography.Although the difference in the meridional wind before and after considering orography in the NH during boreal summer is characterized as northward(southward)wind anomalies at low(middle)latitudes in the lower troposphere,an anomalous southward(northward)circulation at low(middle)latitudes with a magnitude comparable to the strength of the NH HC is evident throughout the latitude-pressure cross section.This finding implies that regional biases in the meridional wind can result in considerable biases in the entire NH HC via the process of vertical integration.After considering orography,the magnitude and significance of the poleward expansion of the NH HC during boreal summer are both reduced regardless of the method used.(5)Significant role of orography in influencing the mean state and evolution of the HCNot considering orography leads to obvious biases in the mean state of the HC,including the strength,location of the rising and sinking branches,and the meridional width of the HC,in all months and hemispheres.During boreal summer,the strength of the NH HC is relatively weak,and the bias in the NH HC caused by not considering orography is characterized as an anomalous southward(northward)circulation at low(middle)latitudes with a magnitude comparable to the strength of the NH HC.Thus,the “minor circulation” exists only in the NH HC during boreal summer.In addition,the differences in the NH HC before and after considering orography during boreal summer derived from the traditional,3P-DGAC,and local partitioning methods are quite similar;thus,the “minor circulation” can be observed regardless of the method used.The influence of not considering orography on the evolution of the HC intensity is small in all months and hemispheres.During boreal summer,especially in June and July,not considering orography has vital influence on the evolution of the rising branch and sinking branch of the NH HC,while the influence of not considering orography on the evolution of the rising branch and sinking branch of the NH HC in other months and that on the sinking branch of the SH HC in all months are negligible.