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Abstract
The flow patterns at various levels in the atmosphere around northwest Pacific tropical cyclones are studied using objectively-analyzed wind fields produced by the United States Navy. The results show large differences in the flow fields among groups of cyclones moving in different directions. Appreciable baroclinity is found in cyclones moving northward or northeastward. The results also demonstrate the impotence of stratifying cyclones by their characteristics and synoptic environment in the study and prediction of tropical cyclone motion
The relationships between tropical cyclone motion and the midtropospheric flow averaged around the 5–7° latitude radial band are also investigated using both the composite and individual cases. The composite results are generally consistent with those obtained from individual cases. In most cases, these relationships also agree with those derived in previous studies from rawinsonde composites and objectively-analyzed height fields. Since the objectively-analyzed wind fields used in this study are available for individual cases, the results suggest possible application of these fields to additional research studies of tropical cyclone motion as well as to development of short-term prediction techniques.
Abstract
The flow patterns at various levels in the atmosphere around northwest Pacific tropical cyclones are studied using objectively-analyzed wind fields produced by the United States Navy. The results show large differences in the flow fields among groups of cyclones moving in different directions. Appreciable baroclinity is found in cyclones moving northward or northeastward. The results also demonstrate the impotence of stratifying cyclones by their characteristics and synoptic environment in the study and prediction of tropical cyclone motion
The relationships between tropical cyclone motion and the midtropospheric flow averaged around the 5–7° latitude radial band are also investigated using both the composite and individual cases. The composite results are generally consistent with those obtained from individual cases. In most cases, these relationships also agree with those derived in previous studies from rawinsonde composites and objectively-analyzed height fields. Since the objectively-analyzed wind fields used in this study are available for individual cases, the results suggest possible application of these fields to additional research studies of tropical cyclone motion as well as to development of short-term prediction techniques.
Abstract
The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of ∼3 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is ∼180° out of phase with the SOI series at the Southern Oscillation frequency.
It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.
Abstract
The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, defined as the sea-level pressure difference between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series. One corresponds to the generally accepted Southern Oscillation with a period of ∼3 to 3.5 years and another at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI leading typhoon activity by almost a year. At the QBO frequency, the two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is ∼180° out of phase with the SOI series at the Southern Oscillation frequency.
It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods of low SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.
Abstract
This paper presents an observational study of the physics of tropical cyclone motion. Analyses of the vorticity budget using both aircraft and rawinsonde composite data were performed. As expected, the results show a definite link between the local change in relative vorticity and tropical cyclone movement. The main contributor to this local change, at least in the middle troposphere, is the horizontal advection of absolute vorticity with the divergence term usually playing a secondary but not necessarily negligible role. The vertical advection and tilting terms are generally much smaller.
The contribution of the divergence term as an extra component in determining the movement of tropical cyclone is discussed. The mass to wind adjustment as a result of the increase in vorticity is viewed as a combination of the advection of temperature (or mass) and subsidence. Substantiating evidence of this viewpoint is presented for cyclones undergoing turning motion.
Abstract
This paper presents an observational study of the physics of tropical cyclone motion. Analyses of the vorticity budget using both aircraft and rawinsonde composite data were performed. As expected, the results show a definite link between the local change in relative vorticity and tropical cyclone movement. The main contributor to this local change, at least in the middle troposphere, is the horizontal advection of absolute vorticity with the divergence term usually playing a secondary but not necessarily negligible role. The vertical advection and tilting terms are generally much smaller.
The contribution of the divergence term as an extra component in determining the movement of tropical cyclone is discussed. The mass to wind adjustment as a result of the increase in vorticity is viewed as a combination of the advection of temperature (or mass) and subsidence. Substantiating evidence of this viewpoint is presented for cyclones undergoing turning motion.
Abstract
In most dynamical studies of synoptic-scale phenomena, only the components of the Coriolis force contributed by the horizontal motion are considered, and only in the horizontal momentum equation. The other components are neglected based on a scale analysis. However, it is shown that such an analysis may not be fully valid in a tropical cyclone (TC) and that these terms should be included. The two neglected terms are 1) ew , the Coriolis force in the x-momentum equation due to vertical motion, and 2) we , the Coriolis force in the vertical equation of motion due to the zonal wind. In this paper, effects of the first term (i.e., ew ) on the structure and motion of a TC are investigated through numerical simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5).
The results suggest that after the ew term has been included, the structure of a TC even on an f plane is changed. A southwestward displacement of a TC center with a speed of ∼1 km h−1 is found in the f-plane experiment. On a β plane, inclusion of the ew term gives a vortex track that is generally west to southwest of the inherent northwestward track (due to the β effect). A scale analysis suggests that the ew term can be as large as half the magnitude of the horizontal acceleration. This term generates an asymmetric wind structure with a generally easterly flow near the center, which therefore causes the vortex to displace toward the southwest. A rainfall asymmetry consistent with the convergence associated with the wind asymmetry is also found and accounts for 10%–20% of the symmetric parts.
Abstract
In most dynamical studies of synoptic-scale phenomena, only the components of the Coriolis force contributed by the horizontal motion are considered, and only in the horizontal momentum equation. The other components are neglected based on a scale analysis. However, it is shown that such an analysis may not be fully valid in a tropical cyclone (TC) and that these terms should be included. The two neglected terms are 1) ew , the Coriolis force in the x-momentum equation due to vertical motion, and 2) we , the Coriolis force in the vertical equation of motion due to the zonal wind. In this paper, effects of the first term (i.e., ew ) on the structure and motion of a TC are investigated through numerical simulations using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5).
The results suggest that after the ew term has been included, the structure of a TC even on an f plane is changed. A southwestward displacement of a TC center with a speed of ∼1 km h−1 is found in the f-plane experiment. On a β plane, inclusion of the ew term gives a vortex track that is generally west to southwest of the inherent northwestward track (due to the β effect). A scale analysis suggests that the ew term can be as large as half the magnitude of the horizontal acceleration. This term generates an asymmetric wind structure with a generally easterly flow near the center, which therefore causes the vortex to displace toward the southwest. A rainfall asymmetry consistent with the convergence associated with the wind asymmetry is also found and accounts for 10%–20% of the symmetric parts.
Abstract
This study investigates the physical processes associated with changes in the convective structure of a tropical cyclone (TC) during landfall using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, version 3 (MM5). The land surface is moved toward a spunup vortex at a constant zonal speed on an f plane. Four experiments are carried out with the following fluxes modified over land: turning off sensible heat flux, turning off moisture flux, setting a higher surface roughness, and combining the last two processes.
The results suggest that sensible heat flux appears to show no appreciable effect while moisture supply is the dominant factor in modifying the convective structure. Prior to landfall, maximum precipitation is found to the front and left quadrants of the TC but to the front and right quadrants after landfall when moisture is turned off and surface roughness increased.
To understand the physical processes involved, a conceptual experiment is carried out in which moisture supply only occurs over the ocean and at the lowest level of the atmosphere, and such supply is transported around by the averaged circulation of the TC. It is shown that the dry air over land is being advected up and around so that at some locations the stability of the atmosphere is reduced. Analyses of the data from the more realistic numerical experiments demonstrate that convective instability is indeed largest just upstream of where the maximum rainfall occurs. In other words, the effect of the change in moisture supply on the convection distribution during TC landfall is through the modification of the moist static stability of the atmosphere.
Abstract
This study investigates the physical processes associated with changes in the convective structure of a tropical cyclone (TC) during landfall using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, version 3 (MM5). The land surface is moved toward a spunup vortex at a constant zonal speed on an f plane. Four experiments are carried out with the following fluxes modified over land: turning off sensible heat flux, turning off moisture flux, setting a higher surface roughness, and combining the last two processes.
The results suggest that sensible heat flux appears to show no appreciable effect while moisture supply is the dominant factor in modifying the convective structure. Prior to landfall, maximum precipitation is found to the front and left quadrants of the TC but to the front and right quadrants after landfall when moisture is turned off and surface roughness increased.
To understand the physical processes involved, a conceptual experiment is carried out in which moisture supply only occurs over the ocean and at the lowest level of the atmosphere, and such supply is transported around by the averaged circulation of the TC. It is shown that the dry air over land is being advected up and around so that at some locations the stability of the atmosphere is reduced. Analyses of the data from the more realistic numerical experiments demonstrate that convective instability is indeed largest just upstream of where the maximum rainfall occurs. In other words, the effect of the change in moisture supply on the convection distribution during TC landfall is through the modification of the moist static stability of the atmosphere.
Abstract
The objective of this study is to explore, based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed.
The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced.
Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode.
Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies.
Abstract
The objective of this study is to explore, based on the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data, the intraseasonal variability of the South China Sea (SCS) summer monsoon (SM) in terms of its structure and propagation, as well as interannual variations. A possible mechanism that is responsible for the origin of the 10–20-day oscillation of the SCS SM is also proposed.
The 30–60-day (hereafter the 3/6 mode) and 10–20-day (hereafter the 1/2 mode) oscillations are found to be the two intraseasonal modes that control the behavior of the SCSSM activities for most of the years. Both the 3/6 and 1/2 modes are distinct, but may not always exist simultaneously in a particular year, and their contributions to the overall variations differ among different years. Thus, the interannual variability in the intraseasonal oscillation activity of the SCS SM may be categorized as follows: the 3/6 category, in which the 3/6 mode is more significant (in terms of the percentage of variance explained) than the 1/2 mode; the 1/2 category, in which the 1/2 mode is dominant; and the dual category, in which both the 3/6 and 1/2 modes are pronounced.
Composite analyses of the 3/6 category cases indicate that the 30–60-day oscillation of the SCS SM exhibits a trough–ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the SCS, with anomalous cyclones (anticyclones), along with enhanced (suppressed) convection, migrating northward from the equator to the midlatitudes. The northward-migrating 3/6-mode monsoon trough–ridge in the lower troposphere is coupled with the eastward-propagating 3/6-mode divergence–convergence in the upper troposphere. It is also found that, for the years in the dual category, the SCS SM activities are basically controlled by the 3/6 mode, but modified by the 1/2 mode.
Composite results of the 1/2-mode category cases show that the 10–20-day oscillation is manifest as an anticyclone–cyclone system over the western tropical Pacific, propagating northwestward into the SCS. A close coupling also exists between the upper-level convergence (divergence) and the low-level anticyclone (cyclone). It is found that the 1/2 mode of the SCS SM mainly originates from the equatorial central Pacific, although a disturbance from the northeast of the SCS also contributes to this mode. The flow patterns from an inactive to an active period resemble those associated with a mixed Rossby–gravity wave observed in previous studies.
Abstract
A unified index for both the summer and winter monsoons over south China (SC) is proposed for the purpose of studying their interannual variability. By examining the monthly distribution of the meridional flow υ over the Asia–Pacific region from 20 yr (1976–95) of the reanalysis data of the National Centers for Environmental Prediction, the area of the South China Sea (SCS) is identified as an important segment of the planetary-scale east Asia monsoon circulation. The monthly υ fields at 1000 and 200 hPa over the SCS show the most significant reversal in direction between summer and winter. The summer rainfall over SC is found to correlate well with these two fields as well as their differences averaged over the northern part of the SCS (7.5°–20°N, 107.5°–120°E). Winter temperatures over SC are, however, only related to the υ field at 1000 hPa within the same region. It is therefore proposed to define a unified monsoon index for SC as the value of υ at 1000 hPa averaged over this region within the period of June through August for summer and November through February for winter.
Abstract
A unified index for both the summer and winter monsoons over south China (SC) is proposed for the purpose of studying their interannual variability. By examining the monthly distribution of the meridional flow υ over the Asia–Pacific region from 20 yr (1976–95) of the reanalysis data of the National Centers for Environmental Prediction, the area of the South China Sea (SCS) is identified as an important segment of the planetary-scale east Asia monsoon circulation. The monthly υ fields at 1000 and 200 hPa over the SCS show the most significant reversal in direction between summer and winter. The summer rainfall over SC is found to correlate well with these two fields as well as their differences averaged over the northern part of the SCS (7.5°–20°N, 107.5°–120°E). Winter temperatures over SC are, however, only related to the υ field at 1000 hPa within the same region. It is therefore proposed to define a unified monsoon index for SC as the value of υ at 1000 hPa averaged over this region within the period of June through August for summer and November through February for winter.
Abstract
Based on the switch of a significant sea surface temperature anomaly (SSTA) over the central equatorial Pacific (the Niño-3.4 region) from ≥0.5°C to ⩽−0.5°C, three types of transitions from the warm (El Niño) to the cold (La Niña) phase of the El Niño–Southern Oscillation can be identified. They are the spring occurrence (SP) type, in which the SSTA first falls below −0.5°C in April or May after the termination of an El Niño event; the summer occurrence (SU) type, in which the SSTA does not reach this threshold until July or later; and the nonoccurrence (NON) type, in which the SSTA never reaches the threshold. Of the 12 El Niño episodes that occurred during the period of 1951–97, the number in each type is 3, 4, and 5, respectively.
No significant difference in the SSTA composites can be found among the three types prior to the termination of the El Niño; however, the subsurface ocean temperatures have very different structures and temporal evolutions. Over the eastern equatorial Pacific, the thermocline depth is the smallest in the SP events in the spring following the El Niño event. The decrease in the mixed layer depth also propagates eastward in both types of cold events but with different speeds. When and if a La Niña event will occur appears to depend on the timing of the enhancement of the central and eastern Pacific trades off the equator. A strengthening of the Pacific subtropical highs in both the Northern and Southern Hemispheres is apparently responsible for such an enhancement. Once the strengthening of the trades occurs, the SST and near-equatorial zonal wind anomalies will follow to initiate the onset of the La Niña.
In the SP type, the subtropical highs in both hemispheres in the eastern and central Pacific strengthen starting at around October of the El Niño year, which then enhances the northeast and southeast trades off the equatorial Pacific east of the date line. Due to Ekman forcing, the enhanced easterlies will cause surface water to drift poleward, which then reduces the depth of the thermocline. This upwelling sets up Rossby waves that propagate westward. By the following January, the negative anomalies in mixed-layer depth have reached the western boundary of the Pacific. They are then reflected and propagate eastward as a slow, coupled air–sea mode, which reduces the thermocline depth in the equatorial region. This results in a cooling of the ocean, which then induces equatorial easterly anomalies. The eastward-propagating wave reaches the central equatorial Pacific by spring so that the SSTA over the Niño-3.4 falls below −0.5°C, and hence the onset of the SP-type La Niña.
In the SU type, the subtropical high in the South Pacific does not strengthen until spring of the year following the El Niño. The above process is therefore delayed so that the onset does not occur until July. For the NON type, the subtropical highs never strengthened, and so no switch in the zonal wind anomalies, and hence no La Niña, takes place.
Abstract
Based on the switch of a significant sea surface temperature anomaly (SSTA) over the central equatorial Pacific (the Niño-3.4 region) from ≥0.5°C to ⩽−0.5°C, three types of transitions from the warm (El Niño) to the cold (La Niña) phase of the El Niño–Southern Oscillation can be identified. They are the spring occurrence (SP) type, in which the SSTA first falls below −0.5°C in April or May after the termination of an El Niño event; the summer occurrence (SU) type, in which the SSTA does not reach this threshold until July or later; and the nonoccurrence (NON) type, in which the SSTA never reaches the threshold. Of the 12 El Niño episodes that occurred during the period of 1951–97, the number in each type is 3, 4, and 5, respectively.
No significant difference in the SSTA composites can be found among the three types prior to the termination of the El Niño; however, the subsurface ocean temperatures have very different structures and temporal evolutions. Over the eastern equatorial Pacific, the thermocline depth is the smallest in the SP events in the spring following the El Niño event. The decrease in the mixed layer depth also propagates eastward in both types of cold events but with different speeds. When and if a La Niña event will occur appears to depend on the timing of the enhancement of the central and eastern Pacific trades off the equator. A strengthening of the Pacific subtropical highs in both the Northern and Southern Hemispheres is apparently responsible for such an enhancement. Once the strengthening of the trades occurs, the SST and near-equatorial zonal wind anomalies will follow to initiate the onset of the La Niña.
In the SP type, the subtropical highs in both hemispheres in the eastern and central Pacific strengthen starting at around October of the El Niño year, which then enhances the northeast and southeast trades off the equatorial Pacific east of the date line. Due to Ekman forcing, the enhanced easterlies will cause surface water to drift poleward, which then reduces the depth of the thermocline. This upwelling sets up Rossby waves that propagate westward. By the following January, the negative anomalies in mixed-layer depth have reached the western boundary of the Pacific. They are then reflected and propagate eastward as a slow, coupled air–sea mode, which reduces the thermocline depth in the equatorial region. This results in a cooling of the ocean, which then induces equatorial easterly anomalies. The eastward-propagating wave reaches the central equatorial Pacific by spring so that the SSTA over the Niño-3.4 falls below −0.5°C, and hence the onset of the SP-type La Niña.
In the SU type, the subtropical high in the South Pacific does not strengthen until spring of the year following the El Niño. The above process is therefore delayed so that the onset does not occur until July. For the NON type, the subtropical highs never strengthened, and so no switch in the zonal wind anomalies, and hence no La Niña, takes place.
