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Johnny C-L. Chan

Abstract

Supertyphoon Abby (1983), although not one of the most destructive on record, received a great deal of attention from the typhoon forecasters in Guam. For a large part of Abby's lifetime, nearly all objectively predicted tracks were almost 90° to the left of the actual track of the cyclone. This study is an attempt to understand the reasons for the failure of the forecast models.

The intensity and size (horizontal extent) of the supertyphoon are hypothesized to be the main factors contributing to such a forecast failure. After intensifying to a maximum wind speed of 75 m s−1 (145 kt), Abby continued to grow, with the radius of 15 m s−1 (30 kt) winds extending beyond 600 km. Abby's circulation, which can be readily identified on synoptic charts, apparently affected the performance of the dynamical models. The “steering flow” vector as estimated from the operational analyses is found to be almost normal to the motion vector of Abby, which might provide a partial explanation of the forecasts by the objective methods.

These results suggest the need to analyze the performance of forecast models under different synoptic as well as storm-related factors. They also suggest the importance of studying the interaction between the tropical cyclone circulation and its environment.

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Johnny C. L. Chan
and
Xudong Liang

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.

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Johnny C. L. Chan

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.

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Jianjun Xu
and
Johnny C. L. Chan

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.

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Bin Wang
and
Johnny C. L. Chan

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.

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Johnny C-L. Chan

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.

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Johnny C. L. Chan
and
Jianjun Xu

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.

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Er Lu
and
Johnny C. L. Chan

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.

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Jiangyu Mao
and
Johnny C. L. Chan

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.

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Johnny C. L. Chan

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.

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