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Abstract
The nonstationarity of the intraseasonal oscillations (ISOs) associated with the western North Pacific summer monsoon (WNPSM) is examined using a wavelet analysis of outgoing longwave radiation (OLR). Both the 10–20- and 30–60-day ISOs are found to display significant interannual modulations, and their relative strengths vary with time. The variation of OLR associated with a strong ISO, either 10–20- or 30–60-day, could be as large as 20 W m−2 in magnitude. Case studies showed that the mechanism for development of low OLR may differ in individual years, and that the 10–20-day ISO, the 30–60-day ISO, and the seasonal cycle may each become dominant in different years.
Abstract
The nonstationarity of the intraseasonal oscillations (ISOs) associated with the western North Pacific summer monsoon (WNPSM) is examined using a wavelet analysis of outgoing longwave radiation (OLR). Both the 10–20- and 30–60-day ISOs are found to display significant interannual modulations, and their relative strengths vary with time. The variation of OLR associated with a strong ISO, either 10–20- or 30–60-day, could be as large as 20 W m−2 in magnitude. Case studies showed that the mechanism for development of low OLR may differ in individual years, and that the 10–20-day ISO, the 30–60-day ISO, and the seasonal cycle may each become dominant in different years.
Abstract
This paper presents the results of an investigation on the variations of tropical cyclone (TC) activity over the western North Pacific (WNP) associated with both El Niño (EN) and La Niña (LN) events. The study is based on the monthly number of TCs that occurred during the period 1959–97. Anomalies within each 5° lat × 5° long box from the year before (EN−1 and LN−1) to the year after (EN+1 and LN+1) are examined.
During an EN−1 year, more (less) TCs are found in September and October over the South China Sea (southeast of Japan). In an EN year, TC activity is below normal during these two months over the South China Sea (SCS) but above normal especially in the late season in the eastern part of the WNP. After the mature phase of the warm event (i.e., during an EN+1 year), TC activity over the entire ocean basin tends to be below normal.
No significant anomalies are found during an LN−1 year. However, in an LN year, the SCS tends to have more TCs in September and October, but for the rest of the WNP, TC activity tends to be below normal from August to November. During the year after an LN event, the entire basin generally has more TCs. Such a situation is especially true over the SCS from May to July.
All these anomalous activities are apparently linked to anomalies in the large-scale flow patterns at 850 and 500 hPa. Because the 850-hPa flow is related to TC genesis and development, areas with anomalous cyclonic (anticyclonic) flow are generally found to be associated with above- (below-) normal TC activity. Anomalous 500-hPa flow is identified as responsible for steering TCs toward or away from a region, thus rendering the TC activity in that region above or below normal.
Abstract
This paper presents the results of an investigation on the variations of tropical cyclone (TC) activity over the western North Pacific (WNP) associated with both El Niño (EN) and La Niña (LN) events. The study is based on the monthly number of TCs that occurred during the period 1959–97. Anomalies within each 5° lat × 5° long box from the year before (EN−1 and LN−1) to the year after (EN+1 and LN+1) are examined.
During an EN−1 year, more (less) TCs are found in September and October over the South China Sea (southeast of Japan). In an EN year, TC activity is below normal during these two months over the South China Sea (SCS) but above normal especially in the late season in the eastern part of the WNP. After the mature phase of the warm event (i.e., during an EN+1 year), TC activity over the entire ocean basin tends to be below normal.
No significant anomalies are found during an LN−1 year. However, in an LN year, the SCS tends to have more TCs in September and October, but for the rest of the WNP, TC activity tends to be below normal from August to November. During the year after an LN event, the entire basin generally has more TCs. Such a situation is especially true over the SCS from May to July.
All these anomalous activities are apparently linked to anomalies in the large-scale flow patterns at 850 and 500 hPa. Because the 850-hPa flow is related to TC genesis and development, areas with anomalous cyclonic (anticyclonic) flow are generally found to be associated with above- (below-) normal TC activity. Anomalous 500-hPa flow is identified as responsible for steering TCs toward or away from a region, thus rendering the TC activity in that region above or below normal.
Abstract
Based on the time of first occurrence of a significant sea surface temperature anomaly (SSTA) in the Niño-3.4 area (5°S–5°N, 170°–120°W), two types of El Niño episodes can be identified: the spring (SP) type in which the SSTA first increased to greater than 0.5°C in April or May, and the summer (SU) type in which this threshold is first reached in July or August. Composites of the SSTAs for these two types of events during the period 1950–97 show that the SP (SU) event is generally a stronger (weaker) warm episode in terms of the SSTA amplitude, and longer (shorter) in terms of the period during which the SSTA is greater than 0.5°C.
Before the occurrence of both types of El Niño episodes, the zonal wind anomalies over the western equatorial Pacific are always westerly. The east Asian winter monsoon is also strong. The difference between the two types is mainly in the timing of the occurrence of the westerly anomalies. For the SP (SU) events, these anomalies extend to the date line by January (May) of the El Niño year. A third component found in both types of El Niño episodes is anomalous southerlies over the northeastern coast of Australia during the El Niño year, which appear earlier in SP events. The difference between the two types of El Niño episodes is apparently phase locked to the annual variation in SST over the western equatorial Pacific.
A stronger east Asian winter monsoon and westerly anomalies in the previous summer are also found in some non–El Niño years. However, in these cases, no anomalous southerlies occur over the northeast of Australia. Therefore, it appears that only when anomalous northerlies from the east Asian winter monsoon converge with anomalous southerlies associated with the transition of Australian monsoon can sufficiently strong westerly anomalies form over the western equatorial Pacific to cause an El Niño event to occur. The presence of a strong south Asian summer monsoon in the previous year is also necessary. The timing of occurrence of southerlies over northeastern Australia apparently determines the onset time of an El Niño event.
Abstract
Based on the time of first occurrence of a significant sea surface temperature anomaly (SSTA) in the Niño-3.4 area (5°S–5°N, 170°–120°W), two types of El Niño episodes can be identified: the spring (SP) type in which the SSTA first increased to greater than 0.5°C in April or May, and the summer (SU) type in which this threshold is first reached in July or August. Composites of the SSTAs for these two types of events during the period 1950–97 show that the SP (SU) event is generally a stronger (weaker) warm episode in terms of the SSTA amplitude, and longer (shorter) in terms of the period during which the SSTA is greater than 0.5°C.
Before the occurrence of both types of El Niño episodes, the zonal wind anomalies over the western equatorial Pacific are always westerly. The east Asian winter monsoon is also strong. The difference between the two types is mainly in the timing of the occurrence of the westerly anomalies. For the SP (SU) events, these anomalies extend to the date line by January (May) of the El Niño year. A third component found in both types of El Niño episodes is anomalous southerlies over the northeastern coast of Australia during the El Niño year, which appear earlier in SP events. The difference between the two types of El Niño episodes is apparently phase locked to the annual variation in SST over the western equatorial Pacific.
A stronger east Asian winter monsoon and westerly anomalies in the previous summer are also found in some non–El Niño years. However, in these cases, no anomalous southerlies occur over the northeast of Australia. Therefore, it appears that only when anomalous northerlies from the east Asian winter monsoon converge with anomalous southerlies associated with the transition of Australian monsoon can sufficiently strong westerly anomalies form over the western equatorial Pacific to cause an El Niño event to occur. The presence of a strong south Asian summer monsoon in the previous year is also necessary. The timing of occurrence of southerlies over northeastern Australia apparently determines the onset time of an El Niño event.
Abstract
An analysis of 35-yr (1965–99) data reveals vital impacts of strong (but not moderate) El Niño and La Niña events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Niño summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°–17°N, 140°E–180°) and decreases in the northwest quadrant (17°–30°N, 120°–140°E). The July–September mean location of TS formation is 6° latitude lower, while that in October–December is 18° longitude eastward in the strong warm versus strong cold years. After the El Niño (La Niña), the early season (January–July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35°N, is 2.5 times more than during strong cold years. This implies that El Niño substantially enhances poleward transport of heat–moisture and impacts high latitudes through changing TS formation and tracks.
The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Niño–induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Niño forcing. The WNP TS activities in July–December are noticeably predictable using preceding winter–spring Niño-3.4 SST anomalies, while the TS formation in March–July is exceedingly predictable using preceding October–December Niño-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere–ocean interaction.
Abstract
An analysis of 35-yr (1965–99) data reveals vital impacts of strong (but not moderate) El Niño and La Niña events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Niño summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0°–17°N, 140°E–180°) and decreases in the northwest quadrant (17°–30°N, 120°–140°E). The July–September mean location of TS formation is 6° latitude lower, while that in October–December is 18° longitude eastward in the strong warm versus strong cold years. After the El Niño (La Niña), the early season (January–July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35°N, is 2.5 times more than during strong cold years. This implies that El Niño substantially enhances poleward transport of heat–moisture and impacts high latitudes through changing TS formation and tracks.
The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Niño–induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Niño forcing. The WNP TS activities in July–December are noticeably predictable using preceding winter–spring Niño-3.4 SST anomalies, while the TS formation in March–July is exceedingly predictable using preceding October–December Niño-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere–ocean interaction.
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.
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
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
Based on results from climate model simulations, many researchers have suggested that because of global warming, the sea surface temperature (SST) will likely increase, which will then lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed typhoon activity (which is used as a proxy for the intensity of TCs averaged over a season) over the western North Pacific (WNP) for the past 40 yr. The average typhoon activity over a season is found to have no significant relationship with SST in the WNP but increases when the SST over the equatorial eastern Pacific Ocean is above normal. The mean annual typhoon activity is generally higher (lower) during an El Niño (La Niña) year. Such interannual variations of typhoon activity appear to be largely constrained by the large-scale atmospheric factors that are closely related to the El Niño–Southern Oscillation (ENSO) phenomenon. These large-scale dynamic and thermodynamic factors include low-level relative vorticity, vertical wind shear, and moist static energy. Such results are shown to be physically consistent with one another and with those from previous studies on the interannual variations of TC activity. The results emphasize the danger of drawing conclusions about future TC intensity based on current climate model simulations that are not designed to make such predictions.
Abstract
Based on results from climate model simulations, many researchers have suggested that because of global warming, the sea surface temperature (SST) will likely increase, which will then lead to an increase in the intensity of tropical cyclones (TCs). This paper reports results of a study of the relationship between SST and observed typhoon activity (which is used as a proxy for the intensity of TCs averaged over a season) over the western North Pacific (WNP) for the past 40 yr. The average typhoon activity over a season is found to have no significant relationship with SST in the WNP but increases when the SST over the equatorial eastern Pacific Ocean is above normal. The mean annual typhoon activity is generally higher (lower) during an El Niño (La Niña) year. Such interannual variations of typhoon activity appear to be largely constrained by the large-scale atmospheric factors that are closely related to the El Niño–Southern Oscillation (ENSO) phenomenon. These large-scale dynamic and thermodynamic factors include low-level relative vorticity, vertical wind shear, and moist static energy. Such results are shown to be physically consistent with one another and with those from previous studies on the interannual variations of TC activity. The results emphasize the danger of drawing conclusions about future TC intensity based on current climate model simulations that are not designed to make such predictions.
Abstract
Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960–2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998–2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.
The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multidecadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200- and 850-hPa horizontal zonal winds (10°–17.5°N, 150°E–180°) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880-gpm line of the June–October 500-hPa geopotential height (0°–40°N, 100°E–180°), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a stronger-than-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998–2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.
Abstract
Tropical cyclone (TC) activity over the western North Pacific (WNP) exhibits a significant interdecadal variation during 1960–2011, with two distinct active and inactive periods each. This study examines changes in TC activity and atmospheric conditions in the recent inactive period (1998–2011). The overall TC activity shows a significant decrease, which is partly related to the decadal variation of TC genesis frequency in the southeastern part of the WNP and the downward trend of TC genesis frequency in the main development region.
The investigation on the factors responsible for the low TC activity mainly focuses on the effect of vertical wind shear and subtropical high on multidecadal time scales. A vertical wind shear index, defined as the mean magnitude of the difference of the 200- and 850-hPa horizontal zonal winds (10°–17.5°N, 150°E–180°) averaged between June and October, is highly correlated with the annual TC number and shows a significant interdecadal variation. Positive anomalies of vertical wind shear are generally found in the eastern part of the tropical WNP during this inactive period. A subtropical high area index, calculated as the area enclosed by the 5880-gpm line of the June–October 500-hPa geopotential height (0°–40°N, 100°E–180°), shows a significant upward trend. A high correlation is also found between this index and the annual TC number, and a stronger-than-normal subtropical high is generally observed during this inactive period. The strong vertical wind shear and strong subtropical high observed during 1998–2011 together apparently lead to unfavorable atmospheric conditions for TC genesis and hence the low TC activity during the period.
Abstract
This study examines the interdecadal variability of the tropical cyclone (TC) tracks over the western North Pacific (WNP) during the 1960–2005 period. An empirical orthogonal function analysis of the 10-yr Gaussian-filtered annual frequency of TC occurrence shows three leading modes of TC occurrence patterns. The first mode is related to the variation of TC activity in the areas near Japan and its east. The second mode is characterized by a northeast–southwest dipole of TC occurrence anomalies along the southeast coast of China and an east–west dipole near Japan and its east. The third mode is similar to the second mode, except for the absence of the east–west dipole. These patterns are shown to be related to the decadal changes in the prevailing TC tracks.
Two characteristic flow patterns related to the first and third modes of TC occurrence pattern are identified. The first pattern is characterized by a north–south dipole of 500-hPa geopotential anomalies over the WNP. Such a pattern may affect the strength and westward extension of the subtropical high and the midlevel steering flow and hence the TC occurrence pattern. The Pacific decadal oscillation (PDO) is found to display a similar dipole-like structure. The decadal variability of TC tracks may therefore be partly attributed to the PDO signal. The second characteristic pattern shows a series of anomalous midlevel atmospheric circulations extending from the sea east of Japan to the south coast of China, which may explain the other part of the decadal variations.
Abstract
This study examines the interdecadal variability of the tropical cyclone (TC) tracks over the western North Pacific (WNP) during the 1960–2005 period. An empirical orthogonal function analysis of the 10-yr Gaussian-filtered annual frequency of TC occurrence shows three leading modes of TC occurrence patterns. The first mode is related to the variation of TC activity in the areas near Japan and its east. The second mode is characterized by a northeast–southwest dipole of TC occurrence anomalies along the southeast coast of China and an east–west dipole near Japan and its east. The third mode is similar to the second mode, except for the absence of the east–west dipole. These patterns are shown to be related to the decadal changes in the prevailing TC tracks.
Two characteristic flow patterns related to the first and third modes of TC occurrence pattern are identified. The first pattern is characterized by a north–south dipole of 500-hPa geopotential anomalies over the WNP. Such a pattern may affect the strength and westward extension of the subtropical high and the midlevel steering flow and hence the TC occurrence pattern. The Pacific decadal oscillation (PDO) is found to display a similar dipole-like structure. The decadal variability of TC tracks may therefore be partly attributed to the PDO signal. The second characteristic pattern shows a series of anomalous midlevel atmospheric circulations extending from the sea east of Japan to the south coast of China, which may explain the other part of the decadal variations.
