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.

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 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.

scholarly journals Roles of interdecadal variability of the western North Pacific monsoon trough in shifting tropical cyclone formation

2021 ◽  
Author(s):  
Xiaofang Feng ◽  
Liguang Wu
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Interdecadal Variability ◽  
Cold Tongue ◽  
Tropical Eastern Pacific ◽  
Poleward Shift ◽  
The Mean ◽  
East West

AbstractConsistent with the northward migration of the annual mean latitude of tropical cyclone (TC) lifetime maximum intensity (LMI), the basin-wide mean location of TC formation shifted northward in the western North Pacific (WNP) basin over the past four decades. Whether such a shift was related to the anthropogenic influence is important to understanding the response of TC activity to climate change. Instead of detecting the effects of individual environmental factors on this shift, here we focus on the interdecadal variability of the monsoon trough (MT), within which most TCs in the WNP basin occur, and its roles in the shift of the basin-wide mean location of TC formation using 60-year reanalysis data. Interdecadal variations of the MT exhibit two main modes: one associated with the Pacific decadal oscillation (PDO) and the other associated with the interdecadal Pacific oscillation (IPO). In addition, the north–south shift of the mean latitude of TC formation is accompanied by east–west extension of the tropical upper tropospheric trough (TUTT) and the tropical eastern Pacific cold tongue indicated by the east–west contrast of sea surface temperature (SST) anomalies. The poleward shift of the mean TC formation latitude is closely associated with the IPO mode of the MT. The westward retreat of the northwest-to-southeast-oriented MT and the accompanied westward extension of the TUTT reduced TC formation in the eastern part of the WNP basin when the cold tongue shifted westward. It is indicated that the observed poleward shift of TC formation was mainly attributed to natural variability in recent decades.

Interannual Variations in Mixed Rossby–Gravity Waves and Their Impacts on Tropical Cyclogenesis over the Western North Pacific

2009 ◽  
Vol 22 (3) ◽  
pp. 535-549 ◽  
Author(s):  
Guanghua Chen ◽  
Ronghui Huang
Keyword(s):  
Transition Zone ◽  
North Pacific ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Tropical Cyclogenesis ◽  
Convective Heating ◽  
Monsoon Circulation ◽  
Interannual Variations ◽  
Mean Flow ◽  
High Resolution Data

Abstract The present study investigates the transition from mixed Rossby–gravity (MRG) waves to tropical depression (TD)-type disturbances and its interannual variations over the western North Pacific (WNP), using ECMWF high-resolution data for the years of 1980–2001. As the equatorially trapped MRG waves propagate westward into the WNP, the MRG waves transit to TD-type disturbances because of background flow change. Interannual variations in the transition of MRG waves are related to monsoon circulation change in response to tropical convective heating over the warm pool (WP) region. When the WP is in a warm state, convective heating is enhanced in the western part of the WNP and the monsoon trough retreats westward, which induces a westward shift of the wave transition zone. In contrast, when the WP is in a relatively cold state, the eastward penetration of convection and monsoon trough shifts the wave transition to the eastern part of the WNP. The zonal wind convergence and shear in the monsoon trough region provide a favorable condition for MRG waves to asymptote to Rossby waves. The asymmetric basic flow contributes to MRG waves moving off the equator toward the northwest. The northeast–southwest-oriented axis of TD-type disturbances in collaboration with the monsoonal environment is favorable for the conversion of eddy kinetic energy from the mean flow. The intensification of the amplitude and shortening of the wavelength during wave transition, to a certain extent, is associated with tropical cyclogenesis over the WNP. Therefore, interannual variations in the longitudinal location of tropical cyclone formation may be interpreted partly by displacement of the wave transition zone. Moreover, this phenomenon of cyclogenesis induced by the wave transition is more common during the cold years in which the monsoon trough penetrates eastward and equatorward.

scholarly journals Mesoscale Features Associated with Tropical Cyclone Formations in the Western North Pacific

2008 ◽  
Vol 136 (6) ◽  
pp. 2006-2022 ◽  
Author(s):  
Cheng-Shang Lee ◽  
Kevin K. W. Cheung ◽  
Jenny S. N. Hui ◽  
Russell L. Elsberry
Keyword(s):  
Tropical Cyclone ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
North Pacific Ocean ◽  
Deep Convection ◽  
Mesoscale Convective System ◽  
Convective System ◽  
Synoptic Patterns ◽  
The Mean

Abstract The mesoscale features of 124 tropical cyclone formations in the western North Pacific Ocean during 1999–2004 are investigated through large-scale analyses, satellite infrared brightness temperature (TB), and Quick Scatterometer (QuikSCAT) oceanic wind data. Based on low-level wind flow and surge direction, the formation cases are classified into six synoptic patterns: easterly wave (EW), northeasterly flow (NE), coexistence of northeasterly and southwesterly flow (NE–SW), southwesterly flow (SW), monsoon confluence (MC), and monsoon shear (MS). Then the general convection characteristics and mesoscale convective system (MCS) activities associated with these formation cases are studied under this classification scheme. Convection processes in the EW cases are distinguished from the monsoon-related formations in that the convection is less deep and closer to the formation center. Five characteristic temporal evolutions of the deep convection are identified: (i) single convection event, (ii) two convection events, (iii) three convection events, (iv) gradual decrease in TB, and (v) fluctuating TB, or a slight increase in TB before formation. Although no dominant temporal evolution differentiates cases in the six synoptic patterns, evolutions ii and iii seem to be the common routes taken by the monsoon-related formations. The overall percentage of cases with MCS activity at multiple times is 63%, and in 35% of cases more than one MCS coexisted. Most of the MC and MS cases develop multiple MCSs that lead to several episodes of deep convection. These two patterns have the highest percentage of coexisting MCSs such that potential interaction between these systems may play a role in the formation process. The MCSs in the monsoon-related formations are distributed around the center, except in the NE–SW cases in which clustering of MCSs is found about 100–200 km east of the center during the 12 h before formation. On average only one MCS occurs during an EW formation, whereas the mean value is around two for the other monsoon-related patterns. Both the mean lifetime and time of first appearance of MCS in EW are much shorter than those developed in other synoptic patterns, which indicates that the overall formation evolution in the EW case is faster. Moreover, this MCS is most likely to be found within 100 km east of the center 12 h before formation. The implications of these results to internal mechanisms of tropical cyclone formation are discussed in light of other recent mesoscale studies.

scholarly journals Observations of Upper-Tropospheric Influence on a Monsoon Trough over the Western North Pacific

2014 ◽  
Vol 142 (4) ◽  
pp. 1472-1488 ◽  
Author(s):  
Biao Geng ◽  
Kunio Yoneyama ◽  
Ryuichi Shirooka
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
North Pacific Ocean ◽  
Low Pressure ◽  
Monsoon Trough ◽  
Pressure Cell ◽  
Intensive Observations ◽  
Upper Level ◽  
Synoptic Observations ◽  
Eastern Edge

Abstract This study examined the synoptic evolution and internal structure of a monsoon trough in association with the deep equatorward intrusion of a midlatitude upper trough in the western North Pacific Ocean in June 2008. The study was based on data from routine synoptic observations and intensive observations conducted on board the research vessel Mirai at 12°N, 135°E. The monsoon trough was first observed to extend southeastward from the center of a tropical depression. It then moved northward, with its eastern edge moving faster and approaching a surface low pressure cell induced by the upper trough. The distinct northward migration caused the monsoon trough to become oriented from the southwest to the northeast. The monsoon trough merged with the surface low pressure cell and extended broadly northeastward. The passage of the monsoon trough over the Mirai was accompanied by lower pressure, higher air and sea surface temperature, and minimal rainfall. The monsoon trough extended upward to nearly 500 hPa and sloped southward with height. It was overlain by northwesterly winds, negative geopotential height and temperature anomalies, and extremely dry air in the upper troposphere. Precipitation systems were weak and scattered near the monsoon trough but were intense and extensive south of the surface monsoon trough, where intense low-level convergence and upper-level divergence caused deep and vigorous upward motion. It appears that the upper trough exerted important impacts on the development of both the monsoon trough and associated precipitation, which are discussed according to the observational results.

scholarly journals Influence of the Monsoon Trough on Westward-Propagating Tropical Waves over the Western North Pacific. Part I: Observations

2015 ◽  
Vol 28 (18) ◽  
pp. 7108-7127 ◽  
Author(s):  
Liang Wu ◽  
Zhiping Wen ◽  
Renguang Wu
Keyword(s):  
Interannual Variability ◽  
North Pacific ◽  
Western North Pacific ◽  
Three Dimensional ◽  
Marked Change ◽  
Monsoon Trough ◽  
Dimensional Structure ◽  
Tropical Depression ◽  
Tropical Waves ◽  
East West

Abstract The present study investigates the possible linkage between the monsoon trough and the interannual variability of the activity of westward-propagating tropical waves (WTW) over the western North Pacific (WNP) during July–November for the period 1979–2007. It is shown that the interannual variability of WTW activity is closely related to the location of the monsoon trough. During the years when the enhanced (weakened) monsoon trough extends eastward (retreats westward), the lower-tropospheric WTW activity is above (below) normal within the southeastern quadrant of the WNP. Furthermore, this study evaluates different wave structures and dynamics of two types of WTWs, equatorial Rossby (ER) waves and mixed Rossby–gravity (MRG)–tropical depression (TD)-type waves, in strong monsoon trough (S-MT) and weak monsoon trough (W-MT) years over the WNP. There is a significant change in the three-dimensional structure as those waves propagate westward to the east of the monsoon trough. For the TD–MRG waves, an apparent transition from MRG waves to off-equatorial TD disturbances is identified in the region of the monsoon trough. For the ER waves, their amplitudes have a faster growth, but their structures and propagation characters have no marked change. Differences in the location of the monsoon trough may lead to an east–west contrast in the WTWs. In a companion study (Part II), diagnostics of energetics and numerical experiments are conducted to explain the observed results in the present study.

scholarly journals Effects of monsoon trough interannual variation on tropical cyclogenesis over the western North Pacific

2014 ◽  
Vol 41 (12) ◽  
pp. 4332-4339 ◽  
Author(s):  
Xi Cao ◽  
Tim Li ◽  
Melinda Peng ◽  
Wen Chen ◽  
Guanghua Chen
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Interannual Variation ◽  
Monsoon Trough ◽  
Tropical Cyclogenesis
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