The Seasonal Evolution And Intra-seasonal Variability Of Summer Asian-Pacific Oscillation

Using the daily reanalysis data from1981to2007, this study investigated theteleconnection over the Asian-Pacific region in summer during1May to30September and itsrelationship with the Asian-Pacific atmospheric circulation and the Asian monsoon rainfall.Secondly, the climatic intra-seasonal oscillation (CISO) of the Asian-Pacific Oscillation isstudied based on climatic averaged data. The periodicity of the APO index, temporal andspacial distribution, evolution and propagation of relating circulation and rainfall associatedwith the CISO of the APO are discussed. The major conclusions are as follows:(1)The variability of the daily upper tropospheric teddy temperature between Asia andthe North Pacific reflects an out-of-phase relationship which is similar to the Asian-PacificOscillation (APO). When the troposphere is warming in the midlatitudes of the AsianContinent, it is cooling in the midlatitudes of the North Pacific; vice versa. The APO alsoappears in the stratosphere, but with a reversed phase.(2)The daily APO index not only reflects the zonal thermal contrast between the Eurasiaand the North Pacific but also indicates the meridional thermal contrast between the Asia andits southen Indian Ocean. The APO index is generally big during the mid July to the earlyAugust for each year, and from1981to2007the appearance of the maximal value gets earlier.The daily APO has significant low-frequency oscillation at30-60days, two weeks, and oneweek.(3)With higher daily APO index conditions, the summer South Asian High and the Asianlow are stronger, the North Pacific subtropical high strengthens too and move northward. Inthe Asian-Africa region, the tropical easterly jet stream in the upper troposphere and thesouthwesterly or southerly winds in the lower troposphere are stronger, accompanied by asummer rainfall increases in India, South China, North China and the northeast Asia.(4) The climatic averaged daily upper tropospheric teddy temperature between Asia andthe North Pacific during1May to30September also reflects an out-of-phase relationship.The daily APO index shows a30-60d oscillation period, and the features of the teleconnectionbetween Asia and the Pacific also exists on this intra-seasonal timescale.(5)During the evolution of the climatic intra-seasonal oscillation of the APO, the upper tropospheric teddy temperature shows pronouncedly eastward propagation from90°E to theeastern Pacific accompanied by the eastward propagation of the high-pressure system in theupper troposphere and the low-pressure system in SLP. During the cycle of the intra-seasonaloscillation of the APO, OLR and atmospheric circulation also shows a cycle from negative topositive. Besides, the30-60d oscillation of OLR and wind shows pronouncedly propagationfrom south to north in the East-Asian monsoon and the Indian monsoon region. Thedistribution of low-frequency rainfall corresponds to the variability of OLR and circulationfield. In the0phase, rainfall in the south of the Yangtze River and India Peninsula(20°-30°N)shows maximal positive anomalies.