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Ken-Chung Ko
and
Po-Sheng Chiu

Abstract

In this study, the circulation patterns of the summer monsoon associated with monsoon rainfall in southern Taiwan were analyzed and two types of monsoon patterns were defined. The first type was characterized by a broad low pressure area extending northeastward from a low pressure center near southern China to southern Japan. Strong southwesterly flows were observed over the southern flank of the monsoon trough. The second type of monsoon pattern was characterized by a strong westward-extending anticyclone in the area north of Taiwan (including central China, South Korea, and Japan), and a weaker east–west-elongated monsoon trough south of Taiwan, driving the easterly flow to southern Taiwan. The modulating effect of intraseasonal oscillations (ISOs) on these monsoon flow patterns indicates that as ISOs propagate northwestward toward southern China, they create favorable conditions for developing and strengthening southwesterly flows and convection. However, because of the latitudinal limit of northwestward-propagating ISOs, only the edge of the strongest convection over the southern flank of the ISO cyclonic circulation reaches southern Taiwan during the westerly phase. Thus, although the westerly ISO pattern appears to be stronger than the easterly pattern, it brings less rainfall to southern Taiwan. Through the tightening of pressure gradients, the ISO typically generates anomalous cyclones (anticyclones) that can affect the southwesterly and northeasterly flows near its southern (northern) and northern (southern) rims. Therefore, fluctuations in the low-frequency background flow can exert a notable effect on the monsoon rainfall and associated circulation systems near Taiwan.

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Ken-Chung Ko
and
Jyun-Hong Liu

Abstract

This study introduces a modified Pacific–Japan (PJ) index that exhibits a substantial periodicity of 5–16 days in the East Asian summer monsoon region. The quasi-periodic fluctuations of the PJ index can indicate changes in the large-scale circulation systems. In the PJ high phase, the wave pattern propagates northwestward from the western North Pacific tropics to an area near northern Luzon and is then forced to move westward because of a stationary, anomalous high pressure system over southern Japan. The tropical cyclones (TCs) associated with the anomalous low pressure systems tend to follow a straight-moving propagation route through the northern South China Sea. The anomalous cyclonic flow causes heavy rainfall in eastern Taiwan. However, in the PJ low phase, the wave pattern and TCs follow a recurving propagation route toward higher latitudes. The circulation pattern typically brings heavy rainfall to northern Taiwan in the PJ low phase. Therefore, wave patterns under the influence of the quasi-periodic fluctuations of the PJ pattern affect rainfall because of the changing propagation routes of the wave patterns, as well as the TC tracks.

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Ken-Chung Ko
and
Huang-Hsiung Hsu

Abstract

The impact of tropical perturbation on the extratropical wave activity in the North Pacific in the submonthly time scale is demonstrated here. Previous studies identified a tropical cyclone (TC)/submonthly wave pattern, which propagated north-northwestward in the Philippine Sea and recurved in the oceanic region between Japan and Taiwan. This study found that, after the arrival of the TC/submonthly wave pattern at the recurving region, the eastward-propagating wave activity in the extratropical North Pacific was significantly enhanced. It is suggested that the TC/submonthly wave pattern, which is originated in the tropical western North Pacific, enhances the eastward energy propagation of Rossby wave–like perturbation in the extratropical North Pacific and may have an impact on the long-range weather predictability in the eastern North Pacific and western North America.

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Ken-Chung Ko
and
Huang-Hsiung Hsu

Abstract

This study demonstrates the multiscale nature, from synoptic to intraseasonal time scales, of the atmospheric flow in the tropical western North Pacific. The multiscale features include intraseasonal oscillations (ISO), northwestward-propagating submonthly wave patterns, and recurving tropical cyclones (TCs). In the ISO westerly phase, the wave pattern was better organized and the TCs were clustered near the cyclonic circulation of the wave pattern during the genesis, development, and propagation. On the other hand, the wave pattern and TCs were weak and poorly organized in the ISO easterly phase. The distinct characteristics between the westerly and easterly phases could be attributed to the ISO modulation on the monsoon trough and the subtropical anticyclonic ridge. The ISO in the westerly phase provided a favorable background (e.g., enhanced monsoon trough and moisture confluent zone) for the wave–TC pattern development, while the ISO in the easterly phase provided a less favorable environment.

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Ken-Chung Ko
and
Dayton G. Vincent

Abstract

The behavior of subtropical jet streaks over the Australian–South Pacific region is investigated for four consecutive 6-month summer seasons, November to April, during the period 1985–89. The study is an extension of a recent one by Ko and Vincent (hereafter referred to as KV), in which jet streaks during the summer season of 1984–85 were examined. Ko and Vincent found that the upper-tropospheric zonal wind behaved in a quasi-periodic manner, with maxima recurring over the western South Pacific at intervals between one and two weeks. In the present study, wind maxima are found to exhibit a 1–2-week periodic oscillation over the South Pacific in all four years, thus substantiating the finding by KV. In addition, a second set of jet streaks, with a similar periodicity, is found over the southern Australian region. It is also seen that the mean summertime position of the South Pacific jet and its accompanying jet streaks are much farther cast during the El Niño event of 1986–87 than they are in the other three years. The jet is also strongest in 1986–87.

Based on point-to-point correlations of the 5–20-day filtered winds, it appears that the South Pacific jet streak form over eastern and sometimes northern Australia, and propagate east-southeastward at about 10° longitude per day (12 m s−1). The jet streaks over southern Australia originate over the eastern Indian Ocean and propagate eastward at about the same phase speed as the South Pacific jets. These two tracks of wind maxima appear to repeat their patterns approximately every 12 days. In some years, the South Pacific and southern Australian jet streaks appear to be distinctly separate, while in other years it seems as though the South Pacific jets are a continuation of the jets that originated over the Indian Ocean and propagated across southern Australia. Finally, it is found that the jet streaks in both regions are advected eastward in close proximity to upper-tropospheric midlatitude troughs, which also have a statistically significant period between one and two weeks. Consequently, these results could impact on medium-range, as well as short-term intraseasonal, forecasting.

In the concluding remarks, the similarity is noted between the results found in this study and those found by other authors who have investigated jet streak activity in the Northern Hemisphere. A hypothesis is suggested for the behavior of the jet streaks diagnosed in this study.

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Ken-Chung Ko
and
Dayton G. Vincent

Abstract

A composite of 10 cases of zonal wind maxima at 200 hPa over the subtropical region stretching from Australia to the central Pacific is examined for the six-month period, November 1984-April 1985. This region is unique in that distinct westerly jets frequently form and propagate eastward at latitudes between 20° and 35°S in the summer season. Some statistical tests were applied and suggest that the flow patterns are quasi periodic, consisting of a tendency for new jet streaks to develop over the eastern Australian region approximately every one to two weeks. These jets then take about 10 days to propagate across the western Pacific before dissipating or, perhaps, moving toward higher latitudes. Their average propagation speed is approximately 4 m s−1. An examination of the case-to-case variability of the jets provides additional evidence that they are significant features. A diagnosis of the trough/ridge systems at 200 and 850 hPa, together with calculations of the vertically integrated mean and shear kinetic energies suggests that baroclinic processes dominate in the entrance and center regions of the jet, whereas barotropic processes dominate in the exit and downstream regions.

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Ken-Chung Ko
and
Jyun-Hong Liu

Abstract

In this study, submonthly wave patterns under the periodically fluctuating Japan–South China Sea (JSCS) pattern were separated into intraseasonal oscillation (ISO) westerly and easterly phases. The composite results showed that the wave patterns in the westerly JSCS high and low phases were more effectively organized and situated in a more intense monsoon trough than their counterparts in the easterly phase. Tropical cyclones (TCs) tended to occur over cyclonic systems of the wave patterns where minimal vertical shear axes were also located. More TCs formed in the westerly JSCS phases near areas where maximal moisture convergence was located. Taiwan experienced heavier rainfall during the westerly JSCS phases because the ISO enhanced the monsoon troughs for the cyclonic system of the wave pattern to develop near Taiwan in the westerly JSCS high phase. Additionally, in the westerly JSCS low phase, a stronger southeasterly mean flow generated by the ISO steered the wave pattern and TCs farther west and thus closer to Taiwan. Therefore, the ISO influenced the mean state circulation systems and then steered TCs and the wave patterns to affect Taiwan’s weather.

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Ken-Chung Ko
and
Jyun-Hong Liu

Abstract

In this study, intraseasonal oscillations (ISOs) and submonthly wave patterns were separated into maximal variance (MaxV) and minimal variance (MinV) years on the basis of ISO variance from July to October. The mean-state 850-hPa streamfunction for submonthly cases indicated that, in the MinV years, tropical cyclones (TCs) formed near areas southeast of those in the MaxV years. ISOs propagated northward in the MaxV years, whereas a weaker westward-propagating tendency was observed in the MinV years. Track analysis of the centers of the submonthly cyclonic anomalies suggested that the background flow dictated the propagation routes of the easterly cyclonic anomalies in the MaxV years. However, the propagation routes of the westerly cyclonic anomalies were barely affected by the background flow. Further analysis of the ISO mean flow patterns showed that in the MaxV years, the propagation routes of the westerly cyclonic anomalies were more likely controlled by the anomalous easterly flow generated by the ISO westerly cyclonic anomalies. Moreover, rainfall was heavier in Taiwan in the MaxV years because the background flow in the MinV years caused the submonthly cyclonic anomaly tracks to shift away from Taiwan. Therefore, low-frequency large-scale circulations can affect smaller-scale phenomena and local weather.

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Dayton G. Vincent
,
Ken-Chung Ko
, and
Jon M. Schrage

Abstract

The main objective of this study is to obtain a better understanding of the upper-tropospheric subtropical westerly wind maxima over the Australian–South Pacific region in the summer half of the year, which have been documented in previous papers to occur with a periodicity of 1–2 weeks. The focus of the study is to quantify the relative importance of tropical versus nontropical forcing during the acceleration phase of the aforementioned westerly wind maxima. Outgoing longwave radiation, wind data, and kinetic energy budgets, partitioned into rotational and divergent components, are used to examine the significance of the forcing mechanisms during the 6-month summer periods from 1985 to 1989. Criteria are developed to identify strong episodes of zonal wind accelerations. In all, 40 cases were found that met these criteria, or approximately 10 cases per year.

In summary, 17 of the 40 cases suggested that tropical forcing was primarily responsible for the observed increase in the rotational kinetic energy of the jet streaks. In contrast, in 13 cases it appeared that little or no connection occurred between tropical convective heat sources and the accelerating jets. In fact, it seemed that midlatitude wave activity was the important factor during the acceleration phase of most of these 13 cases. For the remaining 10 cases, it was difficult to conclude whether tropical forcing was more important than middle latitude forcing; however, it appeared that tropical forcing, albeit weaker than the 17 aforementioned cases, did play a forcing role.

An examination of the case composites in each of these three categories revealed that the energy cycle for the tropically forced cases consisted of a generation of divergent kinetic energy, a conversion of divergent to rotational kinetic energy, and a loss of rotational kinetic energy due to horizontal export and frictional dissipation. Except for the loss of rotational kinetic energy by dissipation, the main energy cycle for the nontropically forced accelerations was the reverse of that for tropically forced jets. Finally, for those 10 cases where the primary region of forcing was uncertain, the composited energy cycle generally consisted of a compromise between the tropically and nontropically forced composites, although there was a significant generation of divergent kinetic energy, as well as a conversion of divergent to rotational kinetic energy, as for all tropically forced cases.

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Ken-Chung Ko
,
Huang-Hsiung Hsu
, and
Chia Chou

Abstract

Propagation and maintenance mechanisms of the tropical cyclone/submonthly wave pattern in the western North Pacific are explored. The wave pattern exhibited an equivalent barotropic structure with maximum vorticity and kinetic energy in the lower troposphere and propagated northwestward in the Philippine Sea in the intraseasonal oscillation (ISO) westerly phase and north-northeastward near the East Asian coast in the easterly phase. The mean flow advection played a dominant role in the propagation in both phases.

Barotropic energy conversion is the dominant process in maintaining the kinetic energy of the pattern. The wave pattern tended to occur in the confluent zone between the monsoon trough and the anticyclonic ridge, where the kinetic energy could be efficiently extracted from the westerly mean flow associated with the monsoon trough. The individual circulation circuit embedded in the pattern was oriented northeast–southwest (east–west) to have optimal growth and propagation during the ISO westerly (easterly) phase.

When tropical cyclones (TCs) developed in a development-favorable background flow provided by the submonthly wave pattern, they in turn enhanced the amplitudes of the vorticity and kinetic energy of the submonthly wave pattern by more than 50% and helped extract significantly more energy from the background ISO circulation. This TC feedback was much more significant in the ISO westerly phase because of the stronger clustering effect on TCs by the enhanced monsoon trough.

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