Spatial Diversity of Multiple Tropical Cyclone Events over the Western North Pacific and Associated Physical Processes

The intraseasonal variability of multiple tropical cyclone (MTC) events in the western North Pacific (WNP) during 1979–2015 is analyzed using the best-track dataset archived at the Joint Typhoon Warning Center. MTC events are divided into three phases according to the time intervals of the tropical cyclone (TC) genesis, that is, active, normal, and inactive phases. Composite analysis results indicate that MTC events tend to occur in the active phase when the monsoon trough is stronger and located farther north than at other times. Initialized by the data from a 10-year stable running result, a 12-year control experiment is carried out using the hybrid atmosphere–ocean coupled model developed at the University of Hawaii (UH_HCM model) to evaluate its simulation capability. Compared with the climate observations, the model shows good skill in simulating the large-scale environmental conditions in the WNP, especially the subtropical high and the monsoon trough. In addition, the model can well simulate the climate characteristics of TCs in the WNP, as well as the differences in each MTC phase. However, the simulated frequency of TCs is less and their locations are more northeast, compared with the observations. The vorticity and moisture in the model appear to be the two main factors affecting MTC activity based on analyses of the genesis potential index.

The interannual variability of occurrence of multiple tropical cyclone (MTC) events during June–October in the western North Pacific (WNP) was examined for the period 1979–2006. The number of the MTC events ranged from 2 to 9 per year, exhibiting a remarkable year-to-year variation. Seven active and seven inactive MTC years were identified. Compared to the inactive years, tropical cyclone genesis locations extended farther to the east and in the meridional direction during the active MTC years. A composite analysis shows that inactive MTC years were often associated with the El Nino decaying phase, as warm SST anomalies in the equatorial eastern-central Pacific in the preceding winter transitioned into cold sea surface temperature (SST) anomalies in the concurrent summer. Associated with the SST evolution were suppressed low-level cyclonic vorticity and weakened convection in the WNP monsoon region. In addition to the mean flow difference, significant differences between active and inactive MTC years were also found in the strength of the atmospheric intraseasonal oscillation (ISO). Compared with inactive MTC years, ISO activity was much stronger along the equator and in the WNP region during active MTC years. Both westward- and northward-propagating ISO spectrums strengthened during active MTC years compared to inactive years. The combined mean state and ISO activity changes may set up a favorable environment for the generation of MTC events.

The South China Sea (SCS) summer monsoon (SCSSM) and Western North Pacific tropical cyclones (TCs) are both tropical systems that interact with each other on multiple scales. This study examines the differences in TCs activity characteristics between anomalous strong and weak SCSSM years, and explores the possible mechanisms behind these differences through the coupling relationship between tropical atmospheric circulation and oceanic surface conditions. Results show that the destructiveness of TCs over the Western North Pacific is stronger during weak SCSSM years than in strong years, whereas the opposite occurs for TCs over the SCS. The interaction between the tropical Indo-Pacific ocean and atmosphere plays a key role in the relationship between SCSSM intensity and TCs activity. In strong (weak) SCSSM years, the sea surface temperature anomaly (SSTA) in the tropical Pacific Ocean tends to correspond to a La Niña-like (El Niño-like) distribution, whereas the tropical Indian Ocean shows an Indian Ocean dipole-negative (positive) phase distribution. Moreover, Walker circulations in both the Indian and Pacific Oceans are coupled during these years, which creates a seesaw-like relationship in the conditions for TCs formation between the SCS and the Western Pacific Ocean. During weak SCSSM years, the formation and activity of TCs over the SCS are suppressed due to the weakened water vapor transport caused by abnormal easterly winds from the eastern Indian Ocean to the southern SCS. Meanwhile, the higher SSTA in the Western Pacific Ocean enhances the TCs activity. In strong SCSSM years, the enhanced monsoon drives a stronger monsoon trough, improving the convective environment over the SCS, whereas in contrast, the Western Pacific Ocean is covered by colder water, resulting in poorer conditions for TCs genesis.

This study reveals a significant in-phase relationship between the South China Sea summer monsoon (SCSSM) withdrawal date and tropical cyclone (TC) genesis over the western North Pacific (WNP). The number of TCs generated over the WNP from mid-September to mid-October is positively correlated with the SCSSM withdrawal date during the period of 1979–2016. The decreased (increased) number of TCs generated during early (late) SCSSM withdrawal years is attributed to both internal atmospheric dynamics and external sea surface temperature (SST) forcing. Through dynamic (Rossby wave response) and thermodynamic (increased moisture) processes, the warm SST anomalies during late withdrawal years over the tropical WNP contribute to maintaining the monsoon trough (MT) in the boreal autumn and moisturizing the mid-level atmosphere, providing a favorable environment for TC genesis. The remaining MT can facilitate the conversion of mean kinetic energy into eddy kinetic energy (EKE) and enhance synoptic-scale waves. In addition, upper-level baroclinic energy conversion also contributes to EKE development. Both barotropic and baroclinic processes favor TC genesis over the WNP. In contrast, colder SSTs during early withdrawal years induce the early withdrawal of the MT, leading to depressed enhancement of the EKE and weakening the northwestward propagation of synoptic-scale waves. Hence, fewer (more) TCs tend to be generated during early (late) withdrawal years.

The present study reveals the impact of the interannual variability of the South China Sea monsoon trough (SCSMT) on tropical cyclone (TC) activity over the South China Sea (SCS) and the western North Pacific (WNP) from June to August during the period of 1958–2019. The SCSMT can influence TC genesis and occurrence through affecting the large‐scale environmental conditions and barotropic energy conversion. The intensification of the SCSMT can lead to enhanced low‐level vorticity and upper‐level divergence, decreased vertical wind shear, and increased midlevel humidity over the northern SCS and subtropical WNP. The strengthening of the SCSMT is accompanied by an intensification and eastward extension of the WNP monsoon trough and eastward retreat of the WNP subtropical high. All these changes associated with the SCSMT are favourable for TC genesis and occurrence over the north SCS and WNP. More TCs form over the WNP around 20° N, while higher TC occurrence frequency appears over the north SCS and WNP north of 20° N. Diagnostic results of eddy kinetic energy distribution and barotropic energy conversion provide a possible explanation for the impact of the SCSMT on TC activity. The synoptic‐scale disturbances are more active in the WNP region north of 20° N which might lead to an increase in TC occurrence frequency. While the barotropic energy conversion is intensified in the northern SCS and east of Philippines, more eddy energy converted from the mean flow is favourable for TC genesis. The meridional shear of mean zonal wind and the meridional convergence of mean meridional wind, which are related with the interannual variability of the SCSMT, primarily contribute to the barotropic energy conversion.

The statistical feature of occurrence of multiple tropical cyclone (MTC) events in the western North Pacific (WNP) is examined during summer (June–September) for the period of 1979–2006. The number of MTC events ranged from one to eight per year, experiencing a marked interannual variation. The spatial distance between the TCs associated with MTC events is mostly less than 3000 km, which accounts for 73% of total samples. The longest active phase of an MTC event lasts for nine days, and about 80% of the MTC events last for five days or less. A composite analysis of active and inactive MTC phases reveals that positive low-level (negative upper-level) vorticity anomalies and enhanced convection and midtropospheric relative humidity are the favorable large-scale conditions for MTC genesis. About 77% of the MTC events occurred in the region where either the atmospheric intraseasonal (25–70 day) oscillation (ISO) or biweekly (10–20 day) oscillation (BWO) is in a wet phase. The overall occurrence of the MTC events is greatly regulated by the combined large-scale impact of BWO, ISO, and the lower-frequency (90 days or longer) oscillation. On the interannual time scale, the MTC frequency is closely related to the seasonal mean anomalies of 850-hPa vorticity, outgoing longwave radiation (OLR), and 500-hPa humidity fields. The combined ISO and BWO activity is greatly strengthened (weakened) in the WNP region during the MTC active (inactive) years.

This paper addresses whether a tropical cyclone can trigger the onset of the South China Sea (SCS) summer monsoon (SM). We conducted a statistical analysis of tropical cyclones (TCs) generated over the western North Pacific (WNP) between late-April and May. The results showed that there were cases in which TCs were generated before the onset of the SCSSM, accounting for 43.2% of the TCs generated during this season. This study examined a representative case, Super Typhoon Chanchu (0601), which was determined to be influential in the onset of the SCSSM. With a northwestward track, Chanchu brought strong convection and westerly winds to the SCS on 12 May, which triggered the intrusion of the southwesterly winds from the Bay of Bengal and the eastward retreat of the western Pacific subtropical high. Super Typhoon Chanchu provides an example in which a TC triggered the onset of the SCSSM. The negative correlation between the onset date of the SCSSM and the number of TCs generated over the WNP used to be interpreted as the influence of the monsoon trough on TC genesis. This work provides a supplementary illustration that this relationship also includes the impact of TCs on the onset of the SCSSM.

ABSTRACTThis study investigated the relationship between the onset date of the South China Sea summer monsoon (SCSSM) and tropical cyclone (TC) genesis over the western North Pacific (WNP). The inter‐annual WNP TC genesis numbers in May is significantly negatively correlated with the onset date of the SCSSM for the epoch of 1979–2015. In the early (late) onset SCSSM years, more (less) TCs tend to be generated. The monsoon trough would enter the WNP earlier in the early SCSSM years and its easternmost location would extend to the east of 140°E. This process would enhance the barotropic eddy energy conversions, which would convert more mean kinetic energy into eddy kinetic energy and supply more energy for TC genesis in the WNP in May. A predicted early (late) onset of the SCSSM might correlate with more WNP TC genesis in May.

In this study, a significant interdecadal change in the South China Sea (SCS) summer monsoon (SCSSM) withdrawal, which occurred around the mid-2000s, is revealed using NCEP-DOE reanalysis, CMAP rainfall, and OLR data. The withdrawal of the SCSSM occurred much later (about 2 weeks) after the mid-2000s. The westerlies and rainfall are stronger around the SCS during the period after the mid-2000s compared to those that occurred before the mid-2000s, which is consistent with the delayed SCSSM withdrawal. The robust and significant increases in rainfall and convection around the SCS and Philippine Sea are dynamically associated with the appearance of an anomalous low-level cyclone around the northern SCS, and the anomalous westerlies at approximately 10°N extend eastward from the Indo-China Peninsula to the western North Pacific (WNP). Anomalous mid-level ascending motion and upper-level divergence were also observed around the Philippine Sea, together with anomalous descending motion and upper-level convergence over the equatorial eastern Indian Ocean. Correspondingly, an anomalous zonal circulation formed between the WNP (upward motion) and eastern Indian Ocean (downward motion). Further analysis indicates that the increasing number and frequent visits to the SCS by the tropical cyclones and enhanced quasi-biweekly oscillation activities may both contribute to the delayed SCSSM withdrawal around the mid-2000s.

The onset of the South China Sea (SCS) summer monsoon (SCSSM) has considerable impacts on the weather and climate in East Asia and beyond. Southern China often experiences persistent heavy rainfall during the SCSSM onset, and this rainfall can be attributed to the 10–30-day intraseasonal oscillation (ISO) originating from the equatorial western Pacific and propagating northwestward. Before the monsoon onset, the SCS is controlled by a strong negative phase of ISO featured by an anomalous low-level anticyclone while active convection dominates the Bay of Bengal (BOB). In this condition, anomalous southwesterlies at the northwestern flank of the SCS anticyclone carry abundant water vapor to southern China. Meanwhile, the convective forcing over the BOB moves to the southern Tibetan Plateau and triggers an anomalous upper-tropospheric anticyclone. This anticyclone extends downstream with the background westerlies and brings northerly anomalies to southern China, inducing anomalous ascending motions through altering potential vorticity advection. Thus, the negative phase of ISO induces heavy rainfall in southern China through enhancing both moisture supply and ascending motions. When the ISO evolves into a positive phase, convective activity and low-level westerlies intensify over the SCS, indicating the onset of the SCSSM. Afterward, the above dynamic and thermodynamic conditions over southern China are reversed and then the heavy rainfall recedes. We highlight that the ISO not only plays a significant role in triggering the onset of the SCSSM but also causes a persistent heavy rainfall process in southern China during the monsoon onset. Significance Statement Persistent heavy rainfall in southern China can have significant societal impacts, including severe floods, agricultural damage, and human health. It is found that during the onset of SCSSM, southern China often experiences persistent heavy rainfall lasting nearly 1 week, characterized by a significant increase before monsoon onset but a quick reduction afterward. Why is this heavy rainfall process generated during the SCSSM onset and how does it quickly decay afterward? We emphasize the key role of the 10–30-day intraseasonal oscillation (ISO) originating from the equatorial western Pacific in triggering this persistent heavy rainfall process. This study helps to improve the understanding of the SCSSM onset and its relationship with the weather and climate in southern China.

Influence of monsoon trough length on interannual variability of Northwest Pacific multiple tropical cyclone events

It has been observed that the percentage of tropical cyclones originating from easterly waves is much higher in the North Atlantic (∼60%) than in the western North Pacific (10%–20%). This disparity between the two ocean basins exists because the majority (71%) of tropical cyclogeneses in the western North Pacific occur in the favorable synoptic environments evolved from monsoon gyres. Because the North Atlantic does not have a monsoon trough similar to the western North Pacific that stimulates monsoon gyre formation, a much larger portion of tropical cyclogeneses than in the western North Pacific are caused directly by easterly waves. This study also analyzed the percentage of easterly waves that form tropical cyclones in the western North Pacific. By carefully separating easterly waves from the lower-tropospheric disturbances generated by upper-level vortices that originate from the tropical upper-tropospheric trough (TUTT), it is observed that 25% of easterly waves form tropical cyclones in this region. Because TUTT-induced lower-tropospheric disturbances often become embedded in the trade easterlies and resemble easterly waves, they have likely been mistakenly identified as easterly waves. Inclusion of these “false” easterly waves in the “true” easterly wave population would result in an underestimation of the percentage of easterly waves that form tropical cyclones, because the TUTT-induced disturbances rarely stimulate tropical cyclogenesis. However, an analysis of monsoon gyre formation mechanisms over the western North Pacific reveals that 82% of monsoon gyres develop through a monsoon trough–easterly wave interaction. Thus, it can be inferred that 58% (i.e., 82% × 71%) of tropical cyclones in this region are an indirect result of easterly waves. Including the percentage of tropical cyclones that form directly from easterly waves (∼25%), it is found that tropical cyclones formed directly and indirectly from easterly waves account for over 80% of tropical cyclogeneses in the western North Pacific. This is more than the percentage that has been documented by previous studies in the North Atlantic.

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.

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.

This study investigates the energy conversion processes and their relation to convection (circulation) during the South China Sea summer monsoon (SCSSM) years from the viewpoint of atmospheric perturbation potential energy (PPE). An atmospheric PPE dipole pattern associated with the SCSSM develops over the western North Pacific (WNP) and southern Maritime Continent (SMC) in the boreal summer, serving as a link between the SCSSM and diabatic heating. Actually, the conversion between the energy variations and the convection over the WNP is distinctly different with that over the SMC. The precipitation leads the PPE over the WNP, while similar situation is reversed over the SMC. During strong SCSSM years, the higher PPE over the WNP, controlled primarily by the latent heat released from condensation related to surplus precipitation, is corresponding to the negative energy conversion ( text{C}_{k} ) over there. This indicates that more PPE is converted to perturbation kinetic energy and further intensifying ascending motion over the WNP. Consequently, the descending movement reduces the PPE and is corresponding to positive text{C}_{k} over the SMC, suggesting that the less PPE converts into the perturbation kinetic energy and in turn favors the descending movement and deficit precipitation there. The enhanced southwesterly induced by this SCSSM Hadley circulation, superimposed on the mean southwesterly wind, further favors the intensification of the SCSSM, implying that the SCSSM can maintain development through the positive convection–PPE–circulation feedback.