Interannual And Interdecadal Variability Of Meridional And Zonal Atmospheric Circulation Cells Over The Tropical Pacific

The meridional and zonal mass stream functions on every grid are calculated incharacterizing the atmospheric vertical circulation, by the monthly mean NCEP/NCAR reanalysis data from January1951to December2010. The main features ofthe regional Hadley circulation and the Walker circulation in the tropical Pacific, andthe zonal mean Hadley circulation are discussed on seasonal, interannual andinterdecadal scales. According to the spatial pattern of the interannual main mode ofthe Hadley circulation, the Western Pacific (WP:100°E~140°E) and Central Pacific(CP:170°E~150°W) are selected as the regions, as well as the tropical Walkercirculation is studied, closed relationship is found between the Hadley circulation andthe Walker circulation. The main conclusions are as follows:1. Main features of the zonal mean Hadley circulationHadley circulation has significant seasonal evolution: with cross-equatorial lapcirculation in winter and summer, and the equatorial quasi-symmetric double ringcirculation in spring and autumn; Interannual variability of the Hadley circulationoccurs mainly in the Pacific Ocean, closely related to the ENSO phenomenon; Astrong equatorial quasi-symmetric double ring anomaly occurs in the mature of ElNi o (La Ni a); Cross-equatorial lap circulation strengthening Hadley circulation inthe previous summer, while weakening the Hadley circulation in the summer afterstrong El Ni o, causing quasi-biennial oscillation of the Hadley circulation; TheHadley circulation enhanced linearly with cross-equatorial lap circulation which islike the Hadley circulation in winter in the last60years; Cross-equatorial lap wintertype abnormal circulation from mid-1970s to2000has been substituted by ofsummer type abnormalities circulation in recent10years. Africa and South America play important roles on the long-term linear trends and interdecadal anomalies.2. The main features of the regional Hadley circulation and the Walker circulationin the Pacific(1) Seasonal variability: heating by the warm pool, the ascending in the tropicalis strong and wide for the WP Hadley circulation, with cross-equatorial lap circulationin winter and summer, especially in summer with significant characteristic of summermonsoon, and with equatorial quasi-symmetric double ring circulation in spring andautumn; CP region is affected by the Western Pacific Warm Pool and the easternPacific cold tongue, the regional Hadley circulation is different from WP region, withthe ascending area lies on the both sides of the equator and slightly cross-equatorialcharacteristic; the center position of the Walker circulation moves about10longitudeswith the seasons in the west-east direction, most west in summer and east in winterand autumn; the Walker circulation is strongest in summer and weakest in spring.(2) Interannual variability: the regional Hadley circulation in WP and CP and theWalker circulation are all closely related with ENSO, during the mature of El Ni o,with symmetric double ring abnormal circulation for the Hadley circulation, the CPHadley circulation is strengthened by the ascending abnormally above the equator, theWP Hadley circulation is weakened by the descending abnormally, the Walkercirculation is weakened also; Contrary during the mature of La Ni a; Cross-equatorialanticlockwise abnormal circulation occurs for the WP and CP regional Hadleycirculation in the previous spring and summer of strong El Ni o, and cross-equatorialclockwise abnormal circulation occurs after strong El Ni o, and abnormal ascending(descending) movement in the central Pacific for the Walker circulation before (after)strong El Ni o, indicating significant quasi-biennial oscillation during strong El Ni o;Quasi-biennial oscillation is weaker during La Ni a and common El Ni o.(3) Interdecadal change: In the nearly60years, the northern branch of WPregional Hadley circulation has increases significantly, while the southern branch hasthe tendency to weaken; The subsidence trend anomalies over the equatorial Pacificmakes the CP regional Hadley circulation and Walker circulation is weakenedobviously; After2000, the Walker circulation and the WP Hadley circulation both have been stronger than that in1980-1999, while for the CP regional Hadleycirculation, the northern branch has been stronger, and the southern one has beenrheological weakened.3. Main characteristics of vertical motion and precipitationThe vertical velocity can be decomposed into the zonal vertical velocity zand the meridional vertical velocity m; Vertical velocity and precipitationdistributions have a very good matching relation in climatology, interannual andinterdecadal variations; mand its abnormal distribution region is like the west-eastbelt, while zand its abnormal distribution is like the north-south belt, what may beassociated with the distribution of the lands and oceans; The center of mover thetropical Western (Eastern) Pacific significantly (not significantly) moves north-southby the seasons; The main mode of interannual variability is the ENSO signal; Intropical areas, the abnormal symbols of mand zare consistent, but the strengthof mis significantly larger than z; In mid-latitude areas, their magnitudedifference is not big, but the ascending and descending distributions are opposite insome areas, reflecting that the meridional circulation and the zonal circulation may beweakened each other; The roles of zonal and meridional circulation are both importanton precipitation in the extratropical areas, through the relationship betweeninterannual variability of precipitation with zand m.