Propagation and Maintenance Mechanism of the TC/Submonthly Wave Pattern and TC Feedback in the Western North Pacific

2012 ◽  
Vol 25 (24) ◽  
pp. 8591-8610 ◽  
Author(s):  
Ken-Chung Ko ◽  
Huang-Hsiung Hsu ◽  
Chia Chou
Keyword(s):  
Kinetic Energy ◽  
North Pacific ◽  
Western North Pacific ◽  
Dominant Role ◽  
Intraseasonal Oscillation ◽  
Lower Troposphere ◽  
Wave Pattern ◽  
Monsoon Trough ◽  
Mean Flow ◽  
Circulation Circuit

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.

scholarly journals The Role of Multiscale Interaction in Synoptic-Scale Eddy Kinetic Energy over the Western North Pacific in Autumn

2014 ◽  
Vol 27 (10) ◽  
pp. 3750-3766 ◽  
Author(s):  
Chih-Hua Tsou ◽  
Huang-Hsiung Hsu ◽  
Pang-Chi Hsu
Keyword(s):  
Kinetic Energy ◽  
Energy Conversion ◽  
North Pacific ◽  
Western North Pacific ◽  
Intraseasonal Oscillation ◽  
Lower Troposphere ◽  
Synoptic Scale ◽  
Multiscale Interactions ◽  
Barotropic Energy Conversion ◽  
Mean Circulation

Abstract This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synoptic-mean and synoptic-ISO interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CKS−M) and synoptic-ISO barotropic energy conversion (CKS−ISO) components. The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CKS−M > 0) during the ISO westerly and easterly phases. However, CKS−ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CKS−ISO > 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE–ISO interactions causes further strengthening of SSEs during the ISO westerly phase. By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CKS−ISO < 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part I: Energetics Diagnosis*

2011 ◽  
Vol 24 (3) ◽  
pp. 927-941 ◽  
Author(s):  
Pang-chi Hsu ◽  
Tim Li ◽  
Chih-Hua Tsou
Keyword(s):  
Kinetic Energy ◽  
Energy Conversion ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Intraseasonal Oscillation ◽  
Active Phase ◽  
Boreal Summer ◽  
Synoptic Eddies ◽  
Barotropic Energy Conversion

Abstract The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetics diagnostic tool that separates the effects of ISO and a low-frequency background state (LFBS; with periods longer than 90 days). The LFBS always contributes positively toward the EKE in the boreal summer, regardless of the ISO phases. The synoptic eddies extract energy from the ISO during the ISO active phase. This positive barotropic energy conversion occurs when the synoptic eddies interact with low-level cyclonic and convergent–confluent ISO flows. This contrasts with the ISO suppressed phase during which the synoptic eddies lose kinetic energy to the ISO flow. The anticyclonic and divergent–diffluent ISO flows during the suppressed phase are responsible for the negative barotropic energy conversion. A positive (negative) EKE tendency occurs during the ISO suppressed-to-active (active-to-suppressed) transitional phase. The cause of this asymmetric EKE tendency is attributed to the spatial phase relation among the ISO vorticity, eddy structure, and EKE. The southwest–northeast-tilted synoptic disturbances interacting with cyclonic (anticyclonic) vorticity of ISO lead to a positive (negative) EKE tendency in the northwest region of the maximum EKE center. The genesis number and location and intensification rate of tropical cyclones in the western North Pacific are closely related to the barotropic energy conversion. The enhanced barotropic energy conversion favors the generation and development of synoptic seed disturbances, some of which eventually grow into tropical cyclones.

scholarly journals Influence of the Monsoon Trough on Westward-Propagating Tropical Waves over the Western North Pacific. Part II: Energetics and Numerical Experiments

2015 ◽  
Vol 28 (23) ◽  
pp. 9332-9349 ◽  
Author(s):  
Liang Wu ◽  
Zhiping Wen ◽  
Renguang Wu
Keyword(s):  
North Pacific ◽  
Numerical Experiments ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Water Model ◽  
Mean Flow ◽  
Horizontal Structure ◽  
Rotational Flows ◽  
The Mean ◽  
The Waves

Abstract Part I of this study examined the modulation of the monsoon trough (MT) on tropical depression (TD)-type–mixed Rossby–gravity (MRG) and equatorial Rossby (ER) waves over the western North Pacific based on observations. This part investigates the interaction of these waves with the MT through a diagnostics of energy conversion that separates the effect of the MT on TD–MRG and ER waves. It is found that the barotropic conversion associated with the MT is the most important mechanism for the growth of eddy energy in both TD–MRG and ER waves. The large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower-tropospheric waves through a positive feedback between the wave growth and horizontal structure. The baroclinic conversion process associated with the MT contributes a smaller part for TD–MRG waves, but is of importance comparable to barotropic conversion for ER waves as it can produce the tilted vertical structure. The growth rates of the waves are much larger during strong MT years than during weak MT years. Numerical experiments are conducted for an idealized MRG or ER wave using a linear shallow-water model. The results confirm that the monsoon background flow can lead to an MRG-to-TD transition and the ER wave amplifies along the axis of the MT and is more active in the strong MT state. Those results are consistent with the findings in Part I. This indicates that the mean flow of the MT provides a favorable background condition for the development of the waves and acts as a key energy source.

scholarly journals Break events of the western North Pacific summer monsoon during 1979–2018

2021 ◽  
pp. 1-47
Author(s):  
Ke Xu ◽  
Riyu Lu
Keyword(s):  
Summer Monsoon ◽  
North Pacific ◽  
Western North Pacific ◽  
Lower Troposphere ◽  
Monsoon Trough ◽  
Significant Suppression ◽  
Upper Troposphere ◽  
Subseasonal Variability ◽  
Case Diagnosis ◽  
Break Period

AbstractThe monsoon break is a typical phenomenon representing the monsoon’s subseasonal variability, but its understanding is still limited for the western North Pacific (WNP) area. This study identified all break events of the WNP summer monsoon (WNPSM) from 1979 to 2018. The statistical analysis suggests that break events occur from late June to late October and peak at the end of August. The occurrence frequency of break events decreases as the duration increases, with 74% persisting 3–7 days and merely 26% lasting longer (8–15 days). During the break period, which is characterized by significant suppression of convection, there is an extensive anticyclonic anomaly in the lower troposphere, corresponding to a notable westward retreat of the monsoon trough and a southwestward shift of the subtropical high. Meanwhile, an anomalous cyclone and convergence in the upper troposphere are also conducive to inhibiting convection.The composite results indicate that both 10–25-day and 30–60-day oscillations contribute to the break, with their dry phases explaining 49.6% and 37.5% of the original suppression of convection, respectively. Around the break, the phase alternation of the 10–25-day oscillation causes convection fluctuation, while the 30–60-day oscillation maintains a stable dry phase that favors the establishment and maintenance of the break. A further case-by-case diagnosis suggests that 46 (51) out of the 61 break events occur in dry phases of the 10–25-day (30–60-day) oscillation, while only 10 (4) events occur in wet phases, indicating that the phase of the two oscillations significantly modulates the occurrence of the monsoon break.

scholarly journals Synoptic-Scale Waves in Sheared Background Flow over the Western North Pacific

2016 ◽  
Vol 73 (11) ◽  
pp. 4583-4603 ◽  
Author(s):  
Tao Feng ◽  
Xiu-Qun Yang ◽  
Wen Zhou ◽  
Ronghui Huang ◽  
Liang Wu ◽  
...  
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Lower Troposphere ◽  
Synoptic Scale ◽  
Mean Flow ◽  
Wave Growth ◽  
Lower Upper ◽  
Sheared Flow ◽  
Upper Level ◽  
Level Disturbance

Abstract Tropical depression (TD)-type waves are the dominant mode of synoptic-scale fluctuations over the western North Pacific. By applying spatiotemporal filters to the observed OLR data and the NCEP–DOE AMIP-II reanalysis data for 1979–2013, this study reveals the characteristics and energetics of convectively coupled TD-type waves under the effects of different circulation patterns in association with vertical wind shear. Results exhibit that different ambient sheared flows significantly affect the vertical structure of westward-propagating TD-type waves, with a lower-tropospheric mode in an easterly sheared background and an upper-tropospheric mode in a westerly sheared background. Energetic diagnoses demonstrate that when the disturbance is trapped in the lower (upper) level by easterly (westerly) shear, the horizontal mean flow in the lower (upper) level favors wave growth by converting energy from the shear of the zonal mean flow (from the convergence of the meridional mean flow). During the penetration of a westward-propagating synoptic-scale disturbance from a westerly sheared flow into an easterly sheared flow, the upper-level disturbance decays, and the lower-level disturbance intensifies. Meanwhile, the upper-level kinetic energy is transferred downward, but the effect induces the wave growth only confined to the midlevels. Consequently, the low-level growth of the westward-propagating upper-level synoptic-scale disturbance is mainly attributed to the barotropic conversion of horizontal mean flow in the lower troposphere.

scholarly journals Seasonal Modulation of Tropical Intraseasonal Oscillations on Tropical Cyclone Geneses in the Western North Pacific

2011 ◽  
Vol 24 (24) ◽  
pp. 6339-6352 ◽  
Author(s):  
Ping Huang ◽  
Chia Chou ◽  
Ronghui Huang
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Asian Summer Monsoon ◽  
North Pacific Ocean ◽  
Intraseasonal Oscillation ◽  
Monsoon Trough ◽  
Intraseasonal Oscillations ◽  
Asian Summer ◽  
Primary Contribution

Abstract The seasonal modulation of tropical intraseasonal oscillation (TISO) on tropical cyclone (TC) geneses over the western North Pacific Ocean (WNP) is investigated in three periods of the WNP TC season: May–June (MJ), July–September (JAS), and October–December (OND). The modulation of the TISO–TC geneses over the WNP is strong in MJ, while it appears weaker in JAS and OND. In MJ, TISO propagates northward via two routes, the west route through the South China Sea and the east route through the WNP monsoon trough region, which are two clustering locations of TC geneses. TISO can synchronously influence most TC geneses over these two regions. In JAS, however, the modulation is out of phase between the monsoon trough region and the East Asian summer monsoon region, as well as the WNP subtropical high region, as a result of further northward propagation of TISO and scattered TC geneses. The TISO–TC genesis modulation in each individual region is comparable to that in MJ, although the modulation over the entire WNP in JAS appears weaker. In OND, TISO has a stronger influence on TC geneses west than east of 150°E because TISO decays and its convection center located at the equator is out of the TC genesis region when propagating eastward into east of 150°E. Midlevel relative humidity is the primary contribution to the modulations of TISO on the genesis environment, while vorticity could contribute to the modulation over the subtropics in JAS.

scholarly journals Possible Linkage between the Monsoon Trough Variability and the Tropical Cyclone Activity over the Western North Pacific

2012 ◽  
Vol 140 (1) ◽  
pp. 140-150 ◽  
Author(s):  
Liang Wu ◽  
Zhiping Wen ◽  
Ronghui Huang ◽  
Renguang Wu
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Monsoon Trough ◽  
Vertical Shear ◽  
Mean Flow ◽  
Tropical Eastern Pacific ◽  
Barotropic Energy Conversion ◽  
Upper Level

Abstract The present study investigates the influence of the monsoon trough (MT) on the interannual variability of tropical cyclone (TC) activity over the western North Pacific during July–November for the period 1979–2007. It is shown that the TC activity is closely related to the MT location. During the years when the MT extends eastward (retreats westward), more (less) TCs form within the southeastern quadrant of the western North Pacific. Such a relationship can be explained by the changes in large-scale environmental factors associated with the movement of the MT. An eastward extension of the MT coincides with warmed ocean surface, enhanced convection, increased relative humidity in the lower and midtroposphere, reduced vertical shear of zonal wind, intensified upper-level divergence, and low-level anomalous cyclonic vorticity over the southeast quadrant of the western North Pacific. These conditions associated with the eastern extension of the MT are favorable for TC genesis, while those associated with the westward retreat of the MT are not. Diagnosis of the barotropic energy conversion indicates that synoptic-scale disturbances moving westward from tropical eastern Pacific will gain the energy from the mean flow when they meet with the eastward-extending MT. This is an important reason for the linkage between MT variability and TC genesis over the western North Pacific.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part II: Apparent Heat and Moisture Sources and Eddy Momentum Transport*

2011 ◽  
Vol 24 (3) ◽  
pp. 942-961 ◽  
Author(s):  
Pang-Chi Hsu ◽  
Tim Li
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Wave Train ◽  
Intraseasonal Oscillation ◽  
Momentum Transport ◽  
Boreal Summer ◽  
Synoptic Scale ◽  
Mean Flow ◽  
The North ◽  
Intraseasonal Oscillations

Abstract The interactions between the boreal summer intraseasonal oscillation (ISO) and synoptic-scale variability (SSV) are investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. It is found that the synoptic Q1 and Q2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast–northwest-oriented wave train pattern over the western North Pacific (WNP). The wave train is well organized and strengthened (loosely organized and weakened) during the ISO active (suppressed) phase. The nonlinearly rectified Q1 and Q2 fields due to the eddy–mean flow interaction account for 10%–30% of the total intraseasonal Q1 and Q2 variabilities over the WNP. During the ISO active (suppressed) phase, the nonlinearly rectified intraseasonal Q1 and Q2 heating (cooling) appear to the northwest of the ISO enhanced (suppressed) convection center, favoring the northwestward propagation of the ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO active (suppressed) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.

scholarly journals Tropical Cyclone Footprints in Long-Term Mean State and Multiscale Climate Variability in the Western North Pacific as Seen in the JRA-55 Reanalysis

2021 ◽  
pp. 1-46
Author(s):  
Sho Arakane ◽  
Huang-Hsiung Hsu
Keyword(s):  
Climate Variability ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Lower Troposphere ◽  
Monsoon Trough ◽  
Climate Projections ◽  
Anticyclonic Circulation ◽  
Mean State

AbstractThe monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of TCs in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity.Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

scholarly journals ISO Modulation on the Submonthly Wave Pattern and Recurving Tropical Cyclones in the Tropical Western North Pacific

2009 ◽  
Vol 22 (3) ◽  
pp. 582-599 ◽  
Author(s):  
Ken-Chung Ko ◽  
Huang-Hsiung Hsu
Keyword(s):  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Wave Pattern ◽  
Monsoon Trough ◽  
Atmospheric Flow ◽  
Wave Patterns ◽  
Intraseasonal Oscillations ◽  
Pattern Development ◽  
Tropical Western North Pacific

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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