Climate Characteristics Of The Hadley And Walker Circulations Over The Tropical Pacific Based On The Three-pattern Decom-position Of Global Atmospheric Circulation

The Pacific Walker Circulation(PWC)is one of the most important circulation systems in the tropics,and it plays an important role in sea-air interaction.Changes in PWC are closely related to the monsoon,El Ni?o-Southern Oscillation(ENSO),and precipitation on adjacent region.The Hadley circulation(HC)plays a vital role in the transportation and exchange of various variables around the world.Therefore,the HC and PWC play an important role in regulating the evolution and development of global weather and climate.On the same latitude zone,there are significant differences in the characteristics of the land surface and oceans at different longitudes,resulting in different characteristics of the local HC at different longitudes.Thus,the evolution of different local HC plays a different role in local and global weather and climate.Since China is located in the eastern part of Eurasia,the weather and climate in China are significantly affected by the large-scale atmospheric circulation in the Pacific.Therefore,it is important to study the changes of the HC and PWC in the tropical Pacific.The three-pattern decomposition of the global atmospheric circulation(3P-DGAC)proposed by the Chou Jifan's team can separate the vertical velocity of the meridional and zonal circulations from the vertical velocity,so that the meridional vertical circulation and the zonal vertical circulation can be more accurately described,especially local meridional vertical circulation and zonal vertical circulation.Therefore,the 3P-DGAC method is used to analyze the climatic state and evolution characteristics of the local HC in the tropical Pacific region,and the interdecadal variation of PWC and its relationship with inhomogeneous temperature changes are studied.The main research results obtained here are as follows:(1)Climatic states and evolution characteristics of the local HC in the tropical PacificThe local HC in the western Pacific(WPHC)and that in the eastern Pacific(EPHC)are both the meridional vertical circulations that rise in the north of the equator and sink in the subtropical regions of both hemispheres.However,the climate states of the WPHC and EPHC are different.Specifically,the WPHC is stronger than the EPHC in both hemispheres,and the width of the WPHC in both hemispheres is significantly wider than that of the EPHC.The interannual changes of the WPHC and EPHC intensity derived from different reanalysis data are similar.The long-term trends of the EPHC intensity derived from different reanalysis data are relatively consistent and generally show an increasing trend.However,the long-term trends of the WPHC intensity derived from different reanalysis data,especially the long-term trend of WPHC intensity in the southern hemisphere,are significantly different.The positions of the sinking branches of the WPHC and EPHC in the northern hemisphere both expanded significantly poleward,and the trends of the poleward extension are 0.30 and 0.49 degrees per decade.Although the positions of the sinking branches of the WPHC and EPHC in the southern hemisphere both expanded southward,the trends are weaker than those in the northern hemisphere and are not significant.The expansion of the WPHC and EPHC in the northern hemisphere(southern hemisphere)will result in a significant decrease in precipitation on the north(south)side of the mean position of the HC sinking branch.(2)Interdecadal change of the PWC from 1961 to 2012 and its relationship with inhomogeneous temperature changesIn the 1961–2012 period,the PWC displays interdecadal changes,i.e.,weakening(strengthening)between the periods 1961–1974 and 1979–1997(1979–1997 and 1999–2012).Along with the interdecadal changes of the PWC,the precipitation,SLP,SST,and zonal wind at 1000 hPa also display significant interdecadal changes.Specifically,during the weakening period of the PWC,the SST in the eastern(western)Pacific Ocean significantly increased(decreased),leading to an abnormal SST gradient between the eastern and western Pacific oceans.Changes in the SST causes weakening of the SLP gradient between the eastern Pacific and the western Pacific,which results in a significant weakening of the east wind in the lower troposphere of the tropical Pacific,thus leading to a reduction of the PWC intensity.The weakening of the PWC weakens the ascending(sinking)movement in the western(central and eastern)Pacific Ocean,which significantly reduces(increases)the precipitation in the western(central and eastern)Pacific.During the strengthening period of the PWC,the changes of these meteorological variables are opposite to those during the weakening period of the PWC.The tropical zonal circulation IOC(zonal circulation in the Indian Ocean),AOC(zonal circulation in the Atlantic)and PWC are mutually coupled,and their changes are closely related.Specifically,with the interdecadal change of PWC,there are also obvious interdecadal changes in IOC and AOC.IOC showed an interdecadal change that firstly weakened and then strengthened throughout the time period,while AOC showed a interdecadal change that firstly strengthened and then weakened throughout the time period.The interdecadal change of temperature gradient is an important cause of the interdecadal change of PWC intensity.Specifically,during the weakening period of the PWC,there is warming anomaly from the lower troposphere to the middle and upper troposphere in the middle–east Pacific,which is beneficial to the formation of abnormal ascending movement in the region.In the western Pacific,there exists cold anomaly from the lower troposphere to the upper and middle troposphere,which is beneficial to the formation of abnormal sinking movement in the region.At the top of the troposphere,there are anomalous warm air and cold air centers in the western Pacific and the middle–east Pacific,respectively,leading to anomalous easterly winds.At the bottom of the troposphere,there are cold and warm anomalies in the western Pacific and the middle–east Pacific,leading to anomalous westerly winds.Anomalies in the zonal inhomogeneous temperature of the troposphere cause an abnormal counterclockwise secondary circulation that rises in the warmer east Pacific and sinks in the cooler western Pacific,which weakens the PWC.During the PWC enhancement period,the temperature gradient changes in the opposite direction.The PWC intensity index defined based on the three-dimensional air temperature field can describe the interdecadal variation of PWC intensity.Combining the new index with the dynamic equation of the 3P-DGAC method can provide new ideas and directions for the prediction of the PWC.(3)The relationship between the Walker circulation and the local HC in the tropical PacificThe WPHC,EPHC and PWC are all affected by ENSO.PWC is directly affected by ENSO,while WPHC(EPHC)is indirectly affected by ENSO through the rising branch(sinking branch)of PWC.Therefore,the impact of ENSO on the WPHC and EPHC is much smaller than that on the PWC.WPHC is more obviously affected by ENSO than EPHC for two reasons.Firstly,EPHC itself has a smaller variability than WPHC.Secondly,the position of the rising branch of EPHC is slightly farther north than the position of PWC sinking branch.