2001 Vol. 7, No. 1
2001, 7(1): 1-10.
Abstract:
The region of south China is sometimes subject to major climatic catastrophes in winter. To have a clear understanding, the time in which extremely severe cold months occur in the south China wintertime over the past 45 years are determined and characteristics of the 500-hPa geopotential fields and SST fields are studied for the simultaneous and preceding 6-month periods. Similarity exists in the 500-hPa geopotential fields between each current severely cold month, with the geopotential pattern of being high in the north, but low in the south, of Asian-Pacific region and meridional circulation developing. The work presents anomalies of the months with significant differences in the 500-hPa geopotential field of the previous periods. The SSTA is continuous in the distribution from each extremely severe cold winter month back to the 6 months leading up to it for the region of south China while the SST pattern is of El Niño in January and the preceding 1 ~ 6 months for equatorial eastern Pacific but of La Niña in February and December. It is concluded that the prediction of severely cold winter months are possible with the use of the geopotential field at 500 hPa and the SST fields for the months ahead of the target time.
The region of south China is sometimes subject to major climatic catastrophes in winter. To have a clear understanding, the time in which extremely severe cold months occur in the south China wintertime over the past 45 years are determined and characteristics of the 500-hPa geopotential fields and SST fields are studied for the simultaneous and preceding 6-month periods. Similarity exists in the 500-hPa geopotential fields between each current severely cold month, with the geopotential pattern of being high in the north, but low in the south, of Asian-Pacific region and meridional circulation developing. The work presents anomalies of the months with significant differences in the 500-hPa geopotential field of the previous periods. The SSTA is continuous in the distribution from each extremely severe cold winter month back to the 6 months leading up to it for the region of south China while the SST pattern is of El Niño in January and the preceding 1 ~ 6 months for equatorial eastern Pacific but of La Niña in February and December. It is concluded that the prediction of severely cold winter months are possible with the use of the geopotential field at 500 hPa and the SST fields for the months ahead of the target time.
2001, 7(1): 11-18.
Abstract:
The work has made a statistic study of the variations of extremely severe cold winter months in the south of China and general circulation and external forcing factors in preceding periods. The result shows that from the current month to the preceding March the subtropical high in the west Pacific is persistently weak or located more to the east and south. When the summer monsoon is weak in East Asia in the year before, the winter monsoon will be strong in the current year in which the extremely severe cold month occurs. The Asian polar vortex expands in the preceding July, August and September and the current winter. The Tibetan Plateau has fewer days of snow cover in the November and December before the cold month occurs. There is less snow in the Tibetan Plateau in the preceding winter / spring of each extremely severe cold month. There are more polar ice in the polar Region for the 11 months before the current February, especially the previous March through August, and in Region in January ~ November before the current cold month of December but less ice in Region in March ~ August.
The work has made a statistic study of the variations of extremely severe cold winter months in the south of China and general circulation and external forcing factors in preceding periods. The result shows that from the current month to the preceding March the subtropical high in the west Pacific is persistently weak or located more to the east and south. When the summer monsoon is weak in East Asia in the year before, the winter monsoon will be strong in the current year in which the extremely severe cold month occurs. The Asian polar vortex expands in the preceding July, August and September and the current winter. The Tibetan Plateau has fewer days of snow cover in the November and December before the cold month occurs. There is less snow in the Tibetan Plateau in the preceding winter / spring of each extremely severe cold month. There are more polar ice in the polar Region for the 11 months before the current February, especially the previous March through August, and in Region in January ~ November before the current cold month of December but less ice in Region in March ~ August.
2001, 7(1): 19-28.
Abstract:
The South China Sea warm pool interacts vigorously with the summer monsoon which is active in the region. However, there has not been a definition concerning the former warm pool which is as specific as that for the latter. The seasonal and inter-annual variability of the South China Sea warm pool and its relations to the South China Sea monsoon onset were analyzed using Levitus and NCEP/NCAR OISST data. The results show that, the seasonal variability of the South China Sea warm pool is obvious, which is weak in winter, develops rapidly in spring, becomes strong and extensive in summer and early autumn, and quickly decays from mid-autumn. The South China Sea warm pool is 55 m in thickness in the strongest period and its axis is oriented from southwest to northeast with the main section locating along the western offshore steep slope of northern Kalimantan-Palawan Island. For the warm pools in the South China Sea, west Pacific and Indian Ocean, the oscillation, which is within the same large scale air-sea coupling system, is periodic around 5 years. There are additional oscillations of about 2.5 years and simultaneous inter-annual variations for the latter two warm pools. The intensity of the South China Sea warm pool varies by a lag of about 5 months as compared to the west Pacific one. The result also indicates that the inter-annual variation of the intensity index is closely related with the onset time of the South China Sea monsoon. When the former is persistently warmer (colder) in preceding winter and spring, the monsoon in the South China Sea usually sets in on a later (earlier) date in early summer. The relation is associated with the activity of the high pressure over the sea in early summer. An oceanic background is given for the prediction of the South China Sea summer monsoon, though the mechanism through which the warm pool and eventually the monsoon are affected remains unclear.
The South China Sea warm pool interacts vigorously with the summer monsoon which is active in the region. However, there has not been a definition concerning the former warm pool which is as specific as that for the latter. The seasonal and inter-annual variability of the South China Sea warm pool and its relations to the South China Sea monsoon onset were analyzed using Levitus and NCEP/NCAR OISST data. The results show that, the seasonal variability of the South China Sea warm pool is obvious, which is weak in winter, develops rapidly in spring, becomes strong and extensive in summer and early autumn, and quickly decays from mid-autumn. The South China Sea warm pool is 55 m in thickness in the strongest period and its axis is oriented from southwest to northeast with the main section locating along the western offshore steep slope of northern Kalimantan-Palawan Island. For the warm pools in the South China Sea, west Pacific and Indian Ocean, the oscillation, which is within the same large scale air-sea coupling system, is periodic around 5 years. There are additional oscillations of about 2.5 years and simultaneous inter-annual variations for the latter two warm pools. The intensity of the South China Sea warm pool varies by a lag of about 5 months as compared to the west Pacific one. The result also indicates that the inter-annual variation of the intensity index is closely related with the onset time of the South China Sea monsoon. When the former is persistently warmer (colder) in preceding winter and spring, the monsoon in the South China Sea usually sets in on a later (earlier) date in early summer. The relation is associated with the activity of the high pressure over the sea in early summer. An oceanic background is given for the prediction of the South China Sea summer monsoon, though the mechanism through which the warm pool and eventually the monsoon are affected remains unclear.
2001, 7(1): 29-40.
Abstract:
Based on the method of composite analysis, the onset process and preceding signs of summer monsoon over the South China Sea (SCS) is investigated. The result indicates that convection activities appear first over the Indo-China Peninsula prior to the onset of the monsoon, then around the Philippines just at the point of onset, implying that the convection activities around the Philippines serve as one of the reasons leading to the SCS monsoon onset. Before the SCS monsoon onset, the equatorial westerly over the Indian Ocean (75°E 95°E ) experiences noticeable enhancement and plays an important role on the SCS monsoon onset. It propagates eastward rapidly and causes the establishment and strengthening of equatorial westerly in the southern SCS, on the one hand, it results in the migration southward of the westerly on south side of the south-China stationary front by means of shift northeastward of the westerly and convection over the Bay of Bengal, on the other. Further study also shows that the intensification of equatorial westerly in the Indian Ocean (75°E 95°E) and the southern SCS is closely related to the reinforcement of the Southern-Hemisphere Mascarene high and Australian high, and cross-equatorial flow northward around Somali, at 85°E and 105°E, respectively.
Based on the method of composite analysis, the onset process and preceding signs of summer monsoon over the South China Sea (SCS) is investigated. The result indicates that convection activities appear first over the Indo-China Peninsula prior to the onset of the monsoon, then around the Philippines just at the point of onset, implying that the convection activities around the Philippines serve as one of the reasons leading to the SCS monsoon onset. Before the SCS monsoon onset, the equatorial westerly over the Indian Ocean (75°E 95°E ) experiences noticeable enhancement and plays an important role on the SCS monsoon onset. It propagates eastward rapidly and causes the establishment and strengthening of equatorial westerly in the southern SCS, on the one hand, it results in the migration southward of the westerly on south side of the south-China stationary front by means of shift northeastward of the westerly and convection over the Bay of Bengal, on the other. Further study also shows that the intensification of equatorial westerly in the Indian Ocean (75°E 95°E) and the southern SCS is closely related to the reinforcement of the Southern-Hemisphere Mascarene high and Australian high, and cross-equatorial flow northward around Somali, at 85°E and 105°E, respectively.
2001, 7(1): 41-52.
Abstract:
Identification of key SST zones is essential in predicting the weather / climate systems in East Asia. With the SST data by the U.K. Meteorological Office and 40-year geopotential height and wind fields by NCAR / NCEP, the relationship between the East Asian summer monsoon and north Pacific SSTA is studied, which reveals their interactions are of interdecadal variation. Before mid-1970's, the north Pacific SSTA acts upon the summer monsoon in East Asia through a great circle wavetrain and results in more rainfall in the summer of the northern part of China. After 1976, the SSTA weakens the wavetrain and no longer influences the precipitation in North China due to loosened links with the East Asian summer monsoon. It can be drawn that the key SST zones having potential effects on the weather / climate systems in East Asia do not stay in one particular area of the ocean but rather shift elsewhere as governed by the interdecadal variations of the air-sea interactions. It is hoped that the study would help shed light on the prediction of drought / flood spans in China.
Identification of key SST zones is essential in predicting the weather / climate systems in East Asia. With the SST data by the U.K. Meteorological Office and 40-year geopotential height and wind fields by NCAR / NCEP, the relationship between the East Asian summer monsoon and north Pacific SSTA is studied, which reveals their interactions are of interdecadal variation. Before mid-1970's, the north Pacific SSTA acts upon the summer monsoon in East Asia through a great circle wavetrain and results in more rainfall in the summer of the northern part of China. After 1976, the SSTA weakens the wavetrain and no longer influences the precipitation in North China due to loosened links with the East Asian summer monsoon. It can be drawn that the key SST zones having potential effects on the weather / climate systems in East Asia do not stay in one particular area of the ocean but rather shift elsewhere as governed by the interdecadal variations of the air-sea interactions. It is hoped that the study would help shed light on the prediction of drought / flood spans in China.
2001, 7(1): 53-62.
Abstract:
To have a clearer picture of mechanisms responsible for the deviation of tropical cyclone (to be simplified as TC hereafter) tracks, the current work assumes the TC as a circular vortex with a radius of R. A general motion equation of TC is then determined by averaging its horizontal motion equation over the sentire region of TC. In the meantime, with the moving track of TC assumed as a characteristic arc, the curvature equation is derived for the track of movement and patterns of its deviation due to TC structure and variation are discussed. The result shows that the scale, size, maximum wind speed and radius are factors causing the deviation of TC tracks. In addition, asymmetric structure of TC is also important for the deviation of tracks. The results, achieved with hypothesis, agree with facts in some cases but disagree with them in others, which are to be verified with more observations or numerical simulations.
To have a clearer picture of mechanisms responsible for the deviation of tropical cyclone (to be simplified as TC hereafter) tracks, the current work assumes the TC as a circular vortex with a radius of R. A general motion equation of TC is then determined by averaging its horizontal motion equation over the sentire region of TC. In the meantime, with the moving track of TC assumed as a characteristic arc, the curvature equation is derived for the track of movement and patterns of its deviation due to TC structure and variation are discussed. The result shows that the scale, size, maximum wind speed and radius are factors causing the deviation of TC tracks. In addition, asymmetric structure of TC is also important for the deviation of tracks. The results, achieved with hypothesis, agree with facts in some cases but disagree with them in others, which are to be verified with more observations or numerical simulations.
2001, 7(1): 63-68.
Abstract:
Better correlation exists between the activity of tropical cyclones affecting East China and Shanghai and the concurrent signals of SSTA in tropical Pacific. In an attempt to justify this statistic finding, a four-dimensional variational data assimilation system is established to optimize the initial fields of a hybrid air-sea coupled model. The prediction skill of tropical SSTA is improved. Long-term statistical models for predicting annual TC frequency affecting East China area and Shanghai city are developed based on 37-year products of this model and the forecast trials have achieved satisfactory results in 1998 and 1999.
Better correlation exists between the activity of tropical cyclones affecting East China and Shanghai and the concurrent signals of SSTA in tropical Pacific. In an attempt to justify this statistic finding, a four-dimensional variational data assimilation system is established to optimize the initial fields of a hybrid air-sea coupled model. The prediction skill of tropical SSTA is improved. Long-term statistical models for predicting annual TC frequency affecting East China area and Shanghai city are developed based on 37-year products of this model and the forecast trials have achieved satisfactory results in 1998 and 1999.
2001, 7(1): 69-79.
Abstract:
To add to the growing mature research on the tropical 30-50 day oscillations from a new prospective, the current work bases on dynamic analysis of baroclinic quasi-geostrophic models to discuss dynamic mechanisms for the generation and propagation of CISK-Rossby waves, and to understand restraints and effects of different wave structures and thermodynamic forcing on the 30-50 day oscillations in the tropical atmosphere. Some important properties of the oscillation propagation have been explained and, in detail, with respect to its meridional propagation and vertical “baroclinic” structure. The work has come up with some new opinions and viewpoints. New opinions about the propagation and energy dispersion are to be proved by more observations and study.
To add to the growing mature research on the tropical 30-50 day oscillations from a new prospective, the current work bases on dynamic analysis of baroclinic quasi-geostrophic models to discuss dynamic mechanisms for the generation and propagation of CISK-Rossby waves, and to understand restraints and effects of different wave structures and thermodynamic forcing on the 30-50 day oscillations in the tropical atmosphere. Some important properties of the oscillation propagation have been explained and, in detail, with respect to its meridional propagation and vertical “baroclinic” structure. The work has come up with some new opinions and viewpoints. New opinions about the propagation and energy dispersion are to be proved by more observations and study.
2001, 7(1): 80-85.
Abstract:
An algorithm for computation of cloud motion winds has been developed at the National Satellite Meteorological Center in China. Since 1997, it has been applied to calculate the cloud motion winds for a 1.25 lat. 1.25 long. mesh over the northwest Pacific region with the satellite data from GMS-5. The development of the tropical cyclones is studied. It shows that the tropical cyclone is usually intrigued by the westerly jet streams at the upper levels of the troposphere, which may be caused by mid-latitude troughs well extending into the tropics. During the prime season of summer, the westerly flowing equatorward of the TUTT may also be a cause for the generation of typhoons.
An algorithm for computation of cloud motion winds has been developed at the National Satellite Meteorological Center in China. Since 1997, it has been applied to calculate the cloud motion winds for a 1.25 lat. 1.25 long. mesh over the northwest Pacific region with the satellite data from GMS-5. The development of the tropical cyclones is studied. It shows that the tropical cyclone is usually intrigued by the westerly jet streams at the upper levels of the troposphere, which may be caused by mid-latitude troughs well extending into the tropics. During the prime season of summer, the westerly flowing equatorward of the TUTT may also be a cause for the generation of typhoons.
2001, 7(1): 86-92.
Abstract:
Outgoing Longwave Radiation (OLR) has been shown to play an important role in climatic diagnosis and long-term prediction and research. With the OLR data 1974 ~ 1997 as observed by satellites, the characteristics are computed. The results are used to depict the location and intensity of the subtropical high in the study of the relationship between the annual frequency of tropical cyclones affecting the Fujian province and ITCZ / subtropical high. It is shown that in years of fewer (more) tropical cyclones, the ITCZ is southward (northward) located with weaker (stronger) intensity, and the subtropical high is southward (northward). As shown in the relationship between the anomalous years of tropical cyclones and characteristics of preceding OLR fields, the OLR anomalies are just oppositely distributed in the Pacific Ocean for years of more (fewer) tropical cyclones. In other words, the years of fewer (more) tropical cyclones are associated with positive anomalies of OLR in the tropical west Pacific but negative (positive) anomalies in the equatorial central and eastern Pacific. It is hoped that our study be setting foundation for short-term climatological prediction of tropical cyclones.
Outgoing Longwave Radiation (OLR) has been shown to play an important role in climatic diagnosis and long-term prediction and research. With the OLR data 1974 ~ 1997 as observed by satellites, the characteristics are computed. The results are used to depict the location and intensity of the subtropical high in the study of the relationship between the annual frequency of tropical cyclones affecting the Fujian province and ITCZ / subtropical high. It is shown that in years of fewer (more) tropical cyclones, the ITCZ is southward (northward) located with weaker (stronger) intensity, and the subtropical high is southward (northward). As shown in the relationship between the anomalous years of tropical cyclones and characteristics of preceding OLR fields, the OLR anomalies are just oppositely distributed in the Pacific Ocean for years of more (fewer) tropical cyclones. In other words, the years of fewer (more) tropical cyclones are associated with positive anomalies of OLR in the tropical west Pacific but negative (positive) anomalies in the equatorial central and eastern Pacific. It is hoped that our study be setting foundation for short-term climatological prediction of tropical cyclones.
2001, 7(1): 93-101.
Abstract:
By using the simulation results of an AGCM, which had been run from 1945 to 1993 forced by COADS SST, the interdecadal variability of the model atmosphere was investigated and compared with that of NCEP reanalysis data. It was found that, interdecadal variability exists significantly in both the tropical Pacific wind fields and the mid-high latitude atmospheric circulation of the model atmosphere. The tendency of time variation and spatial distributions of the interdecadal variability of the model atmosphere are basically consistent with observation. Relative to the mid-high latitude atmospheric circulation, the simulation of tropical Pacific wind is more satisfying, which suggests that anomalous variation of SST is still the main factor for the interdecadal variability of tropical Pacific wind. It might have more significant influence on the tropical wind than on the mid-high latitude atmosphere. However, there is still obvious difference between the simulation and observation. They could be attributed to both the simulation capability of the model and absence of other factors in the model which are important for the interdecadal climate variation.
By using the simulation results of an AGCM, which had been run from 1945 to 1993 forced by COADS SST, the interdecadal variability of the model atmosphere was investigated and compared with that of NCEP reanalysis data. It was found that, interdecadal variability exists significantly in both the tropical Pacific wind fields and the mid-high latitude atmospheric circulation of the model atmosphere. The tendency of time variation and spatial distributions of the interdecadal variability of the model atmosphere are basically consistent with observation. Relative to the mid-high latitude atmospheric circulation, the simulation of tropical Pacific wind is more satisfying, which suggests that anomalous variation of SST is still the main factor for the interdecadal variability of tropical Pacific wind. It might have more significant influence on the tropical wind than on the mid-high latitude atmosphere. However, there is still obvious difference between the simulation and observation. They could be attributed to both the simulation capability of the model and absence of other factors in the model which are important for the interdecadal climate variation.
2001, 7(1): 102-112.
Abstract:
Typhoon-induced heavy rains are mostly studied from the viewpoint of upper-level westerly troughs. It is worthwhile to probe into a case where the rain is caused by tropical cyclone system, which is much heavier. During August 3 ~ 5, 1996, an unusually heavy rainstorm happened in the southwest of Hebei province. It was caused by 3 mesoscale convective cloud clusters on the periphery of a tropical cyclone other than the direct effects of a westerly trough. Generating in a weak baroclinic environment that is unstable with high energy, the cloud clusters were triggered off for development by unstable ageostrophic gravity waves in the low-level southeast jet stream on the periphery of the typhoon. There was a vertical circulation cell with horizontal scale close to 1000 km between the rainstorm area and westerly trough in northeast China. As shown in a computation of the Q vector of frontogenesis function, the circulation cell forms a mechanism of transforming energy between the area of interest and the westerly trough system farther away in northeast China. Study of water vapor chart indicates that high-latitude troughs in the northeast portion of the rain migrate to the southeast to enhance anti-cyclonic divergence in upper-level convection over the area of heavy rain and cause rain clusters, short-lived otherwise, to develop vigorously. It is acting as an amplifier in this case of unusually strong process of rain.
Typhoon-induced heavy rains are mostly studied from the viewpoint of upper-level westerly troughs. It is worthwhile to probe into a case where the rain is caused by tropical cyclone system, which is much heavier. During August 3 ~ 5, 1996, an unusually heavy rainstorm happened in the southwest of Hebei province. It was caused by 3 mesoscale convective cloud clusters on the periphery of a tropical cyclone other than the direct effects of a westerly trough. Generating in a weak baroclinic environment that is unstable with high energy, the cloud clusters were triggered off for development by unstable ageostrophic gravity waves in the low-level southeast jet stream on the periphery of the typhoon. There was a vertical circulation cell with horizontal scale close to 1000 km between the rainstorm area and westerly trough in northeast China. As shown in a computation of the Q vector of frontogenesis function, the circulation cell forms a mechanism of transforming energy between the area of interest and the westerly trough system farther away in northeast China. Study of water vapor chart indicates that high-latitude troughs in the northeast portion of the rain migrate to the southeast to enhance anti-cyclonic divergence in upper-level convection over the area of heavy rain and cause rain clusters, short-lived otherwise, to develop vigorously. It is acting as an amplifier in this case of unusually strong process of rain.
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