Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation （OLR） data, and pentad NOAA CMAP precipitation from 1979 to 2006, the variation of the western Pacific subtropical h...Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation （OLR） data, and pentad NOAA CMAP precipitation from 1979 to 2006, the variation of the western Pacific subtropical high （WPSH） ridge during late spring and early summer （LSES） and its relationship with the onset of the Asian summer monsoon is discussed from a climatological perspective. It is found that a remarkable southward retreat process （SRP） of the WPSH during LSES appears at both lower and higher levels of the troposphere, with a lifespan of approximate two weeks. Afterwards, the first northward jump of the WPSH occurs. The end date of the WPSH SRP in the upper troposphere is about 10 days earlier than the beginning of the WPSH SRP in the lower troposphere, showing a meaningful leading signal for predicting the WPSH SRP in the lower troposphere and the subsequent northward jump of the WPSH. The WPSH SRP at lower levels happens simultaneously with a notable eastward shift of the WPSH. After the WPSH SRP at lower levels comes to the southernmost position around the end of May, the WPSH ridge axis inclines northward rather than southward with altitude due to the change of the meridional gradient of air temperature. The Asian summer monsoon onset and associated variations in strong convection and rainfall in Asia are closely related to the variations of W＇PSH SRP during LSES. In the mid-late period of the higher-level WPSH SRP, around the end of April, the summer monsoon onset takes place in the Andaman Sea and the Bay of Bengal. Following the start of the lower-level WPSH SRP, the South China Sea （SCS） summer monsoon breaks out （May 14-15）. By the end of the lower-level WPSH SRP, in the beginning of June, the Indian summer monsoon kicks off. Upon the end of the lower-level WPSH return stage, the East Asian summer monsoon begins. The commencement of each component of the Asian summer monsoon system corresponds nicely to a particular stage of the WPSH SRP in the lower or higher troposphere. This offers valuable information for monsoon onset prediction in different sectors of Asia. In addition, it is found that there is a typical wet-dry-wet sandwich precipitation pattern, with two rainfall belts in the regions south and north to the WPSH main body, and a dry belt under it. The variation of this rainfall pattern is related to the shift of the WPSH ridge.展开更多
Using monthly mean atmospheric re- analysis data and oceanic assimilation data product,the winter ocean-atmosphere interaction in middle and high latitude in Northwest Pacific was studied. Key regions were chosen to s...Using monthly mean atmospheric re- analysis data and oceanic assimilation data product,the winter ocean-atmosphere interaction in middle and high latitude in Northwest Pacific was studied. Key regions were chosen to study the variations of Aleutian Low,the 500hPa westerly,the meriodional sea surface temperature (SST) gradient with three indices defined as Aleutian Low index,zonal index and meridional SST gradient between Kuroshio- influenced region and Oyashio-influenced region. The results show that when there is a deeper Aleu- tian Low accompanied by a stronger northerly wind in the western part of the Aleutian Low,the subpolar gyre of North Pacific is strengthened,the SST in the Oyashio-influenced region is decreased and the me- ridional SST gradient between Kuroshio-influ- enced region and Oyashio-influenced region is increased,which,in turn,will enhance the westerly jet in the upper troposphere due to thermal-wind relation. The strengthened westerly jet makes a favorable condi- tion for the deeper Aleutian Low over North Pacific beneath the left part of the jet exit region. Conse- quently,a positive feedback forms there.展开更多
基金the Ministry of Science and Technology of China under Grant No.2006CB403600the National Natural Science Foundation of China under Grant Nos.40821092 and 40523001.
文摘Based on the climatological daily mean NCEP/NCAR reanalysis data, NOAA outgoing longwave radiation （OLR） data, and pentad NOAA CMAP precipitation from 1979 to 2006, the variation of the western Pacific subtropical high （WPSH） ridge during late spring and early summer （LSES） and its relationship with the onset of the Asian summer monsoon is discussed from a climatological perspective. It is found that a remarkable southward retreat process （SRP） of the WPSH during LSES appears at both lower and higher levels of the troposphere, with a lifespan of approximate two weeks. Afterwards, the first northward jump of the WPSH occurs. The end date of the WPSH SRP in the upper troposphere is about 10 days earlier than the beginning of the WPSH SRP in the lower troposphere, showing a meaningful leading signal for predicting the WPSH SRP in the lower troposphere and the subsequent northward jump of the WPSH. The WPSH SRP at lower levels happens simultaneously with a notable eastward shift of the WPSH. After the WPSH SRP at lower levels comes to the southernmost position around the end of May, the WPSH ridge axis inclines northward rather than southward with altitude due to the change of the meridional gradient of air temperature. The Asian summer monsoon onset and associated variations in strong convection and rainfall in Asia are closely related to the variations of W＇PSH SRP during LSES. In the mid-late period of the higher-level WPSH SRP, around the end of April, the summer monsoon onset takes place in the Andaman Sea and the Bay of Bengal. Following the start of the lower-level WPSH SRP, the South China Sea （SCS） summer monsoon breaks out （May 14-15）. By the end of the lower-level WPSH SRP, in the beginning of June, the Indian summer monsoon kicks off. Upon the end of the lower-level WPSH return stage, the East Asian summer monsoon begins. The commencement of each component of the Asian summer monsoon system corresponds nicely to a particular stage of the WPSH SRP in the lower or higher troposphere. This offers valuable information for monsoon onset prediction in different sectors of Asia. In addition, it is found that there is a typical wet-dry-wet sandwich precipitation pattern, with two rainfall belts in the regions south and north to the WPSH main body, and a dry belt under it. The variation of this rainfall pattern is related to the shift of the WPSH ridge.
基金the National Natural Science Foundation of China （Grant No. 40136010） National Basic Research Program of China （Grant No. 2005CB422301 ）.
文摘Using monthly mean atmospheric re- analysis data and oceanic assimilation data product,the winter ocean-atmosphere interaction in middle and high latitude in Northwest Pacific was studied. Key regions were chosen to study the variations of Aleutian Low,the 500hPa westerly,the meriodional sea surface temperature (SST) gradient with three indices defined as Aleutian Low index,zonal index and meridional SST gradient between Kuroshio- influenced region and Oyashio-influenced region. The results show that when there is a deeper Aleu- tian Low accompanied by a stronger northerly wind in the western part of the Aleutian Low,the subpolar gyre of North Pacific is strengthened,the SST in the Oyashio-influenced region is decreased and the me- ridional SST gradient between Kuroshio-influ- enced region and Oyashio-influenced region is increased,which,in turn,will enhance the westerly jet in the upper troposphere due to thermal-wind relation. The strengthened westerly jet makes a favorable condi- tion for the deeper Aleutian Low over North Pacific beneath the left part of the jet exit region. Conse- quently,a positive feedback forms there.
