Comparison of impacts of intraseasonal oscillation on tropical cyclogenesis over the western North Pacific based on two methods

2019 ◽  
Vol 40 (4) ◽  
pp. 2418-2428
Author(s):  
Xi Cao ◽  
Renguang Wu
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Intraseasonal Oscillation ◽  
Tropical Cyclogenesis

scholarly journals Effects of Monsoon Trough Intraseasonal Oscillation on Tropical Cyclogenesis over the Western North Pacific*

2014 ◽  
Vol 71 (12) ◽  
pp. 4639-4660 ◽  
Author(s):  
Xi Cao ◽  
Tim Li ◽  
Melinda Peng ◽  
Wen Chen ◽  
Guanghua Chen
Keyword(s):  
North Pacific ◽  
Moisture Transport ◽  
Western North Pacific ◽  
Large Scale ◽  
Diabatic Heating ◽  
Intraseasonal Oscillation ◽  
Monsoon Trough ◽  
Specific Humidity ◽  
Tropical Cyclogenesis ◽  
Background Fields

Abstract The effects of intraseasonal oscillation (ISO) of the western North Pacific (WNP) monsoon trough on tropical cyclone (TC) formation were investigated using the Advanced Research Weather Research and Forecasting (ARW) Model. A weak vortex was specified initially and inserted into the background fields containing climatological-mean anomalies associated with active and inactive phases of monsoon trough ISOs. The diagnosis of simulations showed that monsoon trough ISO can modulate TC development through both dynamic and thermodynamic processes. The dynamic impact is attributed to the lower–midtropospheric large-scale vorticity associated with monsoon trough ISO. Interactions between cyclonic vorticity in the lower middle troposphere during the active ISO phase and a vortex lead to the generation of vortex-scale outflow at the midlevel, which promotes the upward penetration of friction-induced ascending motion and thus upward moisture transport. In addition, the low-level convergence associated with active ISO also helps the upward moisture transport. Both processes contribute to stronger diabatic heating and thus promote a positive convection–circulation–moisture feedback. On the other hand, the large-scale flow associated with inactive ISO suppresses upward motion near the core by inducing the midlevel inflow and the divergence forcing within the boundary layer, both inhibiting TC development. The thermodynamic impact comes from greater background specific humidity associated with active ISO that allows a stronger diabatic heating. Experiments that separated the dynamic and thermodynamic impacts of the ISO showed that the thermodynamic anomaly from active ISO contributes more to TC development, while the dynamic anomalies from inactive ISO can inhibit vortex development completely.

Distinct Western North Pacific Tropical Cyclogenesis During Inactive Intraseasonal Phase of August 1996 and 2014

2021 ◽  
Author(s):  
Xi Cao ◽  
Renguang Wu ◽  
Jing Xu ◽  
Yifeng Dai ◽  
Mingyu Bi ◽  
...  
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Wave Train ◽  
Intraseasonal Oscillation ◽  
Tropical Cyclogenesis ◽  
Maritime Continent ◽  
Intraseasonal Variation ◽  
Southwestern Part ◽  
Positive Vorticity ◽  
Inactive Phase

Abstract While intraseasonal oscillation was in the inactive phase over the western North Pacific (WNP) during August of 1996 and 2014, no tropical cyclone (TC) genesis occurred in August of 2014, whereas 9 TCs (average 5.7 TCs) formed in August of 1996 with 5 TCs in the northeastern part (the largest number since 1979) and 4 TCs in the southwestern part. The present analysis reveals an obvious southwest-northeast-oriented lower-level wave train over the WNP associated with anomalous convection around the Maritime Continent in August 1996. This wave train induced anomalous cyclone and enhanced convection over the northeastern WNP, which provided a favorable background for TC genesis. Over the southwestern WNP, although monthly mean anomalies were unfavorable, the intraseasonal variation contributed to positive vorticity anomalies at the time and location of TC genesis. In contrast, both monthly anomalies and daily variations of environment factors were hostile to TC genesis during August 2014.

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 Coupling Modes of Climatological Intraseasonal Oscillation in the East Asian Summer Monsoon

2016 ◽  
Vol 29 (17) ◽  
pp. 6363-6382 ◽  
Author(s):  
Zehao Song ◽  
Congwen Zhu ◽  
Jingzhi Su ◽  
Boqi Liu
Keyword(s):  
East Asia ◽  
North Pacific ◽  
East Asian Summer Monsoon ◽  
Western North Pacific ◽  
Diabatic Heating ◽  
Asian Summer Monsoon ◽  
East Asian ◽  
Intraseasonal Oscillation ◽  
Rainfall Anomaly ◽  
Asian Summer

Abstract The present study used harmonic and multivariate empirical orthogonal function (MV-EOF) analyses to identify the existence of climatological intraseasonal oscillation (CISO) in the diabatic heating, precipitation, and circulation of the East Asian summer monsoon (EASM). The strongest CISO signals are found in the north of the western North Pacific, possibly because of the horizontal gradient of diabatic heating induced by the seasonal land–sea thermal contrast. Further, the phase relationship between the diabatic heating components maintains the EASM CISO. The first two coupling modes of EASM CISO in the circulation are robust during May through August, with a period of 40–80 days, and exhibit phase locking to the stepwise establishment of the EASM, which reveals the coaction of the Mongolian cyclone (MC) around Lake Baikal at 850 hPa, the western North Pacific subtropical high (WNPSH) at 500 hPa, and the South Asian high (SAH) over the Tibetan Plateau (TP) at 200 hPa. The first mode shows that the jointly enhanced MC, WNPSH, and SAH correspond to a tripole rainfall anomaly with strong mei-yu and baiu fronts over East Asia. The second mode, however, indicates the eastward and northwestward propagation of MC and WNPSH, respectively, with suppressed SAH, as well as a dipole rainfall anomaly over East Asia. Both the observations and numerical simulation verify the importance of daily diabatic heating and SST in maintaining the CISO modes over the WNP, where the condensation heating related to atmospheric forcing determines the local intraseasonal air–sea interaction.

Tropical Cyclogenesis Associated with Kelvin Waves and the Madden–Julian Oscillation

2011 ◽  
Vol 139 (9) ◽  
pp. 2723-2734 ◽  
Author(s):  
Carl J. Schreck ◽  
John Molinari
Keyword(s):  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
North Pacific ◽  
Western North Pacific ◽  
Diabatic Heating ◽  
Kelvin Waves ◽  
Tropical Cyclogenesis ◽  
Madden Julian Oscillation ◽  
Convective Envelope

The Madden–Julian oscillation (MJO) influences tropical cyclone formation around the globe. Convectively coupled Kelvin waves are often embedded within the MJO, but their role in tropical cyclogenesis remains uncertain. This case study identifies the influences of the MJO and a series of Kelvin waves on the formation of two tropical cyclones. Typhoons Rammasun and Chataan developed in the western North Pacific on 28 June 2002. Two weeks earlier, conditions had been unfavorable for tropical cyclogenesis because of uniform trade easterlies and a lack of organized convection. The easterlies gave way to equatorial westerlies as the convective envelope of the Madden–Julian oscillation moved into the region. A series of three Kelvin waves modulated the development of the westerlies. Cyclonic potential vorticity (PV) developed in a strip between the growing equatorial westerlies and the persistent trade easterlies farther poleward. Rammasun and Chataan emerged from the apparent breakdown of this strip. The cyclonic PV developed in association with diabatic heating from both the MJO and the Kelvin waves. The tropical cyclones also developed during the largest superposition of equatorial westerlies from the MJO and the Kelvin waves. This chain of events suggests that the MJO and the Kelvin waves each played a role in the development of Rammasun and Chataan.

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