Climatological characteristics of subtropical anticyclone structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data.The ridge-surface of subtropical anticyclone is defined by the boundar...Climatological characteristics of subtropical anticyclone structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data.The ridge-surface of subtropical anticyclone is defined by the boundary surface between westerly to the north and easterly to the south (WEB in brief).In Afro-Asian monsoon area,the subtropical high in troposphere whose ridgelines are consecutive in wintertime takes on relatively symmetrical and zonal structure,the WEB tilts southward with increasing height.In summer,the subtropical high ridgelines are discontinuous at low levels and continuous at upper levels,the WEB tilts northward from the bottom up.Under the constraint of thermal wind relation,the WEB usually tilts toward warmer zone.May is the period when subtropical high modality most significantly varies.The structure and properties of subtropical high during seasonal transition are different from area to area.A new concept 'seasonal transition axis' is proposed based on formation and variation of the vertical ridge axis of subtropical anticyclone.The subtropical high of summer pattern firstly occurs over eastern Bay of Bengal in the beginning of May.then stabilizes over eastern Bay of Bengal,Indo-China,and western South China Sea in the 3rd pentad of May,it exists over the South China Sea in the 4th- 5th pentad of May and establishes over central India in the 1st-2nd pentad of June.The three consequential stages when summer modal subtropical high occurs correspond to that of Asian summer monsoon onset,respectively.To a great extent,the summer monsoon onset over the Bay of Bengal depends on the reversal of meridional temperature gradient in vicinity of the WEB in upper troposphere.The meridional temperature gradient at middle-upper levels in troposphere can be used as a good indicator for measuring the seasonal transition and Asian monsoon onset.展开更多
Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal ...Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal orientation in mid-upper troposphere.During the Asian monsoon bursting consequentially over the Bay of Bengal,South China Sea,and South Asia,in addition to the reversal of winds in the lower and upper troposphere and deep convection before and after the onset,the atmospheric meridional temperature gradient (MTG) in the vicinity of the ridge-surface of subtropical high (WEB defined in Part Ⅰ) exhibits a significant reversal.Since the establishment of temperature structure with higher over north than over south of the WEB in the mid-upper troposphere (200-500 hPa) characterizes the collective essential that the Asian summer monsoon bursts over different areas,the MTG in mid-upper troposphere,based on the thermodynamics associated with the seasonal transition,should be a reasonable index to measure the Asian monsoon onset.The definition for onset date is proposed,and the time series of onset date for different sections are determined.As compared with the onset dates determined by other indices such as 850-hPa zonal wind and OLR.correlation analyses indicate that the 850-hPa zonal wind is only regional index,but the MTG index is applicable universally to the Asian monsoon regime.展开更多
基金National Key Program for Developing Basic Sciences(G1998040900)National Natural Science Foundation of China(40135020,49905002)+1 种基金Chinese Academy of Sciences(ZKCX2-SW-210)LASG Foundation(40023001)
文摘Climatological characteristics of subtropical anticyclone structure during seasonal transition are investigated based on NCEP/NCAR reanalysis data.The ridge-surface of subtropical anticyclone is defined by the boundary surface between westerly to the north and easterly to the south (WEB in brief).In Afro-Asian monsoon area,the subtropical high in troposphere whose ridgelines are consecutive in wintertime takes on relatively symmetrical and zonal structure,the WEB tilts southward with increasing height.In summer,the subtropical high ridgelines are discontinuous at low levels and continuous at upper levels,the WEB tilts northward from the bottom up.Under the constraint of thermal wind relation,the WEB usually tilts toward warmer zone.May is the period when subtropical high modality most significantly varies.The structure and properties of subtropical high during seasonal transition are different from area to area.A new concept 'seasonal transition axis' is proposed based on formation and variation of the vertical ridge axis of subtropical anticyclone.The subtropical high of summer pattern firstly occurs over eastern Bay of Bengal in the beginning of May.then stabilizes over eastern Bay of Bengal,Indo-China,and western South China Sea in the 3rd pentad of May,it exists over the South China Sea in the 4th- 5th pentad of May and establishes over central India in the 1st-2nd pentad of June.The three consequential stages when summer modal subtropical high occurs correspond to that of Asian summer monsoon onset,respectively.To a great extent,the summer monsoon onset over the Bay of Bengal depends on the reversal of meridional temperature gradient in vicinity of the WEB in upper troposphere.The meridional temperature gradient at middle-upper levels in troposphere can be used as a good indicator for measuring the seasonal transition and Asian monsoon onset.
基金the National Natural Science Foundation of China (40375022,40135020)Chinese Academy of Sciences (ZKCX2-SW-210)LASG Foundation (40023001)
文摘Based on the studies in Part Ⅰ (see Mao et al.2003),this paper further examines the relationship between the Asian summer monsoon onset and variation in meridional position of the warm temperature ridge with a zonal orientation in mid-upper troposphere.During the Asian monsoon bursting consequentially over the Bay of Bengal,South China Sea,and South Asia,in addition to the reversal of winds in the lower and upper troposphere and deep convection before and after the onset,the atmospheric meridional temperature gradient (MTG) in the vicinity of the ridge-surface of subtropical high (WEB defined in Part Ⅰ) exhibits a significant reversal.Since the establishment of temperature structure with higher over north than over south of the WEB in the mid-upper troposphere (200-500 hPa) characterizes the collective essential that the Asian summer monsoon bursts over different areas,the MTG in mid-upper troposphere,based on the thermodynamics associated with the seasonal transition,should be a reasonable index to measure the Asian monsoon onset.The definition for onset date is proposed,and the time series of onset date for different sections are determined.As compared with the onset dates determined by other indices such as 850-hPa zonal wind and OLR.correlation analyses indicate that the 850-hPa zonal wind is only regional index,but the MTG index is applicable universally to the Asian monsoon regime.
