Influence Of The BSISO And Tibetan Plateau Summer Monsoon On Precipitation In Southwest China

Based on CPC daily precipitation data,reanalysis data from NCEP,boreal summer intraseasonal oscillation(BSISO)index and Qinghai-Tibetan Plateau Monsoon Index(QTPMI),using sliding average method,power spectrum,Lanczos filtering,synthetic phase analysis,we illustrate that two modes of climate variability--the BSISO and the Tibetan Plateau Summer Monsoon(TPSM)—have distinct effects on Precipitation in Southwest China(SW).In addition,we discuss the joint influence and compared differences from that of the individual influence.The results are as follows:(1)The two modes of BSISO have different significant effects on SW in different phases.The probability of extreme precipitation in SW decreases when BSISO1 and BSISO2 are located in the dry phases.The circulation anomaly shows that there is a northerly wind anomaly over the SW region,which inhibits the water vapor transport in the lower layer.At the same time,there is a subsidence anomaly in the middle layer.The negative anomaly area of precipitation expands,and the high value area is located in the east of the SW,so the precipitation decreases.The probability of extreme precipitation in SW increases when BSISO1 and BSISO2 are divided into wet phase.The circulation anomaly shows that there is a southerly wind anomaly over SW,which enhances the water vapor transport in the lower layer,and the upward movement in the middle layer,which expands the range of positive anomaly of precipitation and increases precipitation.(2)TPSM has significant low-frequency oscillation characteristics dominated by quasi biweekly oscillation,and its intensity is inversely correlated with the probability of extreme precipitation in SW,the stronger(weaker)the TPSM,the smaller(greater)the probability of extreme precipitation in SW.When TPSM is in the strongest phase3,the circulation indicates there is easterly wind over the SW,the middle layer is accompanied by subsidence anomaly,and there is a wide range of water vapor divergence area in the middle of the SW.The range of negative anomaly of precipitation is expanded,which decreases precipitation.On the contrary,when TPSM is located in the weakest phase 7,westerly winds prevail over the SW,strengthening the water vapor transport in the lower layer,accompanied by enhanced upward movement in the middle layer.There is a large-scale water vapor convergence area in the middle of the SW,and the range of precipitation anomaly is expanding which increases precipitation.(3)The joint influence is different from the single factor effect of BSISO or TPSM.The results show that dry phases of the BSISO2 with the TPSM has the greatest impact on precipitation in SW,and the positive anomaly distribution range of precipitation in SW is the widest.Within BSISO phases,when AQ change to IAQ,the large value area of positive precipitation anomaly in SW shifts from north to south.In AQ combined phases,there is a strong southwest wind in the SW in 500 h Pa.,corresponding to the abnormal upward movement in the northern region.Concurrently,on both sides of the SW are configured with a high pressure and a low pressure in 850 h Pa.So that the water vapor from the Indian Ocean is transported upward from the south and the water vapor from the western Pacific enters the north to form a water vapor convergence area.By contrast,in IAQ combined phases,there is a northerly wind in the SW in 500 h Pa,corresponding to the strong upward movement anomaly in the south,the lower layer is covered by strong cyclone clusters,leading to the water vapor from the Indian Ocean continues to transport into the south of SW,forming a water vapor convergence zone.