Evaluation of Simulation Capability for Multiple Tropical Cyclone Events in the Western North Pacific of the UH_HCM Model

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.

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.

Multiple tropical cyclone (TC) events (MTCEs) can cause disproportionate damages beyond the cumulative impacts of individual TCs, yet their physical processes and driving mechanisms remain poorly understood. This study focuses on spatial diversity in MTCE occurrence and their associated physical processes over the western North Pacific (WNP). Based on spatial features, MTCEs are objectively classified into three clusters: eastern induced (EI) cluster, western induced in the nearshore (WI-N) cluster, and western induced in the open sea (WI-O) cluster. The EI cluster is driven by the strengthened South China Sea summer monsoon, with TCs forming within the monsoon trough and confluence regions. The WI-N cluster primarily arises from the interaction between the monsoon westerlies and easterlies associated with an anomalous anticyclone. The WI-N cluster is characterized by tropical wave trains, potentially linked to TC-induced Rossby wave dispersion and easterly waves. Dynamic genesis potential analysis reveals that enhanced midlevel vertical motion dominates the dynamic factors controlling the MTCE formation across the WNP. Meanwhile, barotropic energy conversions, arising from the convergence and meridional shear of large-scale zonal winds, serve as the primary sources of eddy kinetic energy for MTCE formation. Upper-level baroclinic energy conversions also play a significant role, especially for the WI-N and WI-O clusters. Time decomposition reveals that factors across multiple time scales, including the quasi-biweekly oscillation, intraseasonal oscillations, and low-frequency variability, contribute to WNP-MTCE formation. Our findings offer a comprehensive view to better understand the spatial diversity of MTCE over the WNP.

The different impacts of Pacific meridional mode (PMM) and central Pacific (CP) El Nino on tropical cyclone (TC) genesis over the western North Pacific (WNP) in the period of 1965–2018 were investigated in this study. The results show that PMM is mainly related to the eastern subtropical Pacific warming and CP El Nino is mainly related to the central tropical Pacific warming. They have a similar impact on TC genesis over the WNP and more TCs form in the east of the WNP due to the high correlation between the central tropical Pacific warming and eastern subtropical Pacific warming. In the central tropical Pacific, due to the SST forcing of the atmosphere, SST warming leads to the remarkable atmospheric response. In the eastern subtropical Pacific, the variation of the atmosphere precedes SST to establish the SST warming, and the atmospheric response to this SST warming is limited. During the CP El Nino, the anomalous convention associated with the central tropical Pacific warming produces a poleward extension of anomalous westerlies over the WNP by the Matsuno-Gill-type Rossby wave response. Such anomalous westerlies strengthen the monsoon trough (MT) with an eastward shift. The enhanced MT accompanied by the changed large-scale environmental conditions and synoptic-scale perturbations are beneficial to TC genesis over the WNP. However, the PMM with the eastern subtropical Pacific warming has no significant impact on the tropical circulation over the WNP and only a limited effect on the subtropical high. As a result, it has no direct effect on the MT, and the PMM itself has no significant positive correlation with TC genesis over the WNP. However, the impact of the PMM with the central tropical Pacific warming on TC genesis over the WNP is consistent with that during the CP El Nino. These findings suggest the dominant role of the SST warming in the central tropical Pacific on TC genesis over WNP, and the simultaneous impact of PMM on TC genesis over WNP is mainly realized by it.

During the period 1970–2020, evident interdecadal variability of meridional tropical cyclone (TC) genesis occurs in the western North Pacific (WNP) from September through October (SO). From the periods 1970–1996 (IP1) to 1998–2020 (IP2), TC genesis during SO is enhanced (suppressed) in the region north (south) of 20°N. The large‐scale modulating processes feature a general warming of sea surface temperature (SST) over the WNP and systematic northward displacement in atmospheric circulations. The northward‐shifted Pacific subtropical high and monsoon trough result in an anomalous anticyclone across the subtropical region and an anomalous cyclone across the South China Sea and the northern WNP east of Taiwan. This anomalous cyclone facilitates TC genesis to increase in the northern WNP. Meanwhile, the southeastward‐extending monsoon trough retreats northwestward, leading to an anomalous anticyclone over the tropical eastern WNP and a decrease in TC genesis over the southern region. Correlation analyses indicate that the Pacific Decadal Oscillation (PDO) is more effective than global warming in modulating interdecadal increases of TC genesis in the northern region due to inclusions of both trend and oscillation features. Interdecadal decreases of TC genesis in the southern region exhibit a major oscillation feature and are thus mainly affected by interdecadal oscillation components of PDO. More TCs forming in the oceans east of Taiwan are steered by the overlying anomalous cyclone to recurve northward toward oceans to the northeast and north of Taiwan. TC‐induced rainfall over Taiwan thus increases by 25% from IP1 to IP2, which accounts for about 82% of increased total rainfall in Taiwan, showing the notable impact of interdecadal TC genesis on local climate in the WNP.

The present study investigates modulation of western North Pacific (WNP) tropical cyclone (TC) genesis in relation to different phases of the intraseasonal oscillation (ISO) of ITCZ convection during May to October in the period 1979–2008. The phases of the ITCZ ISO were determined based on 30–80-day filtered OLR anomalies averaged over the region (5°–20°N, 120°–150°E). The number of TCs during the active phases was nearly three times more than during the inactive phases. The active (inactive) phases of ISO were characterized by low-level cyclonic (anticyclonic) circulation anomalies, higher (lower) midlevel relative humidity anomalies, and larger (smaller) vertical gradient anomalies of relative vorticity associated with enhanced (weakened) ITCZ convection anomalies. During the active phases, TCs tended to form in the center of the ITCZ region.

The numerical simulations of tropical cyclone (TC) genesis during the strong and weak monsoon trough (MT) years, in which meteorological fields are composited, are conducted using advanced research weather research and forecasting model. The simulation results show that both tropical disturbances tend to form in the east of the western North Pacific (WNP) near 160°–170°E during the strong and weak MT years. During the strong MT years, there is a faster formation rate of TC. The eastward-extending MT gradually evolves into a closed monsoon gyre over the WNP during the early stage. The following rapid development of TC can be attributed to the enhanced lower-level southwesterly flows induced by the cross-equatorial currents, enhanced easterly winds, and weak vertical wind shear, which provide a favorable environment for TC genesis. The strengthened large-scale circulation spawns abundant convective updrafts resulting in the aggregation of cyclonic vorticity. In contrast, during the weak MT years, the westward-retreated MT gradually evolves into expansive easterly winds over the WNP. Two episodes of convective updrafts are triggered with a longer interval, and thus lead to a slower TC genesis compared with that during the strong MT years.

In autumn 2024 (late October to mid‐November), an unprecedented multiple tropical cyclone (MTC) event occurred over the western North Pacific (WNP). Six storms formed within a month, surpassing the climatological mean by 4 standard deviations. Four storms were simultaneously active—an event unmatched in 45 years. This anomaly was linked to exceptionally strong Madden–Julian Oscillation (MJO)‐related suppressed convection over the Indo‐Pacific warm pool, triggering a Kelvin wave response that enhanced cyclonic circulation and moisture convergence, and reduced vertical wind shear over the WNP. All subseasonal‐to‐seasonal models, including the ECMWF, deep learning FuXi‐S2S, and WNP TC hybrid models, predicted the active TC period beyond 4 weeks. Although ECMWF model struggled to predict the inactive‐to‐active transition, the other two models maintained predictive skill up to 4–5 weeks. A comparison of the ECMWF ensemble members that did and did not capture the MTC event showed that accurate MJO prediction is key to subseasonal MTC forecasting.

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.

A daily East Asia–Pacific teleconnection (EAP) index was constructed to investigate the impact of the intraseasonal variability (ISV) of this index on the genesis of multiple tropical cyclones (MTC) in boreal summer over the western North Pacific (WNP). The result indicates that the EAP index has dominant intraseasonal periods of 10–20 days, 20–40 days and 50–70 days, respectively. The ISV of the EAP during 1979–2019 can be classified into three types, a single-period-domination type (37%), a multiple period coexistence type (24%) and a transition type (39%). It is found that during El Niño (La Niña) summers, the ISV of the EAP is dominated by a higher-frequency (lower-frequency) oscillation with a period of around 20–30 (50–70) days. The distinctive ISV characteristics during the different ENSO years were accompanied with different dynamic and thermodynamic background conditions over the WNP and the South China Sea, which modulated the frequency and location of MTC genesis. By examining the relative contributions of individual environmental variables of the Genesis Potential Index, we found that the low-level absolute vorticity and mid-level relative humidity are two important environmental factors modulating MTC genesis. However, the relative role of these variables tends to change with the EAP ISV phase. The environmental condition over the SCS appears less influenced by ENSO. A more southern location of MTC genesis during El Niño is attributed to the change of the environmental humidity.

The role of the 50- to 200-hPa zonal wind shear in modulating tropical cyclone (TC) genesis over the western North Pacific (WNP) in May is investigated in this study. Concurrent with the strong cross-tropopause shear over the key region (0°–5°N, 160°–180°E), suppressed convection was observed over the tropical WNP, especially over the South China Sea and the Philippines. The monsoon trough (MT) was confined westward. However, enhanced convection occurred in the weak shear years and the MT extended eastward. This cross-tropopause wind shear is negatively correlated with TC genesis in May, with a decreased (increased) number of TCs corresponding to strong (weak) cross-tropopause wind shear. This cross-tropopause wind shear can be treated as the combined impacts of the El Nino-Southern Oscillation (ENSO) events and the stratospheric quasi-biennial oscillation (QBO). When decaying El Nino events coupled with the easterly phase of the QBO were noted, the cross-tropopause wind shear was stronger with weakened convection, and an enhanced western Pacific subtropical high was observed. TCs are rarely generated during these years. In contrast, the modulation of the QBO westerly phase on decaying La Nina events is limited. Affected by the QBO westerly phase, TC genesis in the May following La Nina events is only slightly enhanced. The energy analysis indicates that the combined impacts of the decaying El Nino events and the QBO easterly phase might suppress the barotropic eddy kinetic energy conversion in May, whereas the decaying La Nina events and the QBO west phase act in an opposite manner.

The temporal clustering of the western North Pacific tropical cyclogenesis and its modulation by the Madden–Julian oscillation (MJO) during the 1991 summer were examined based on the tropical cyclone best track, outgoing longwave radiation, and NCEP/NCAR reanalysis datasets. The wavelet analysis shows that convective activities around the monsoon trough in the western North Pacific possessed a distinct MJO with a period of 20–60 days. Two or more tropical cyclones were observed to form successively during each active phase of the MJO, and tropical cyclones tended to generate around the southeastern part of the maximum vorticity of the low-frequency cyclonic circulation during the developing and peak stages of the active MJO phase. But tropical cyclogenesis scarcely occurred during inactive MJO phases. Thus the MJO was a major agent in modulating repeated development of tropical cyclones in the western North Pacific during the 1991 summer. The MJO in circulation was characterized by a huge anomalous cyclone (anticyclone) in the lower troposphere existing alternately over the western North Pacific, leading to an enhanced (weakened) monsoon trough. An examination of the meridional gradient of absolute vorticity associated with the zonal flow indicates that the zonal flow in the monsoon trough region satisfied the necessary conditions for barotropic instability, with both zonal flow and the meridional gradient of absolute vorticity varying on the similar MJO timescale. The intraseasonal oscillation of such an unstable zonal flow might thus be an important mechanism for temporal clustering of tropical cyclogenesis in the western North Pacific. The barotropic conversion could provide a major energy source for the formation and growth of tropical cyclones in the western North Pacific during active MJO phases, with the eddy kinetic energy generation being dominated by both terms of eddies interacting with zonal and meridional gradients of the basic zonal flow.

ABSTRACTThe interdecadal changes in the frequency of tropical cyclone (TC) genesis in the western North Pacific (WNP) were examined for the epoch 1979–2013. There was an interdecadal increase in the frequency of TC genesis in the WNP in May during 1999–2013 than during 1979–1998, and such changes differed from those occurring between June and December. This interdecadal change is attributed to the early onset of the South China Sea summer monsoon (SCSSM) during 1999–2013, when the southwesterly winds spread eastwards and the monsoon trough (MT) moved into the WNP earlier. The MT favoured enhanced convection and anomalous low‐level cyclonic vorticity, facilitating more TC genesis. During 1979–1998, in contrast, the onset of the SCSSM was delayed and the MT was confined to the west of the South China Sea. As a result, weakened convection and anomalous anticyclonic low‐level vorticity are observed over the WNP, facilitating less TC genesis.

ABSTRACTThis study explores the relationship between summer‐developed ENSOs and tropical cyclone (TC) genesis over the western North Pacific (WNP) in distinguishing two types of summer ENSOs with distinct developing evolutions: so‐called ‘continuing ENSOs’ and ‘emerging ENSOs’. The summer equatorial Pacific sea surface temperature anomalies (SSTAs) during continuing ENSOs persist from the preceding winter, whereas emerging ENSOs feature a rapid decline in SSTAs from the preceding winter and newly develop from the following spring. The impact of summer ENSOs on WNP TC genesis is characterised by a shift in genesis location, as revealed in previous studies. The shift in genesis location is characterised by a zonal pattern with increased (decreased) TCs east (west) of 140° E in May to June (MJ) and a northwest–southeast dipole pattern with increased (decreased) TC genesis in the southeast (northwest) part of the WNP in July to September (JAS). The present study reveals that this impact of summer ENSOs is mainly contributed by emerging ENSOs, whereas the continuing type weakens ENSO's impact. During emerging ENSOs, more intense cyclonic anomalies over the central Pacific significantly intensify the monsoon trough east of 140° E, strengthening the impact of summer ENSOs on the location of TC genesis. The reduced vertical wind shear and enhanced absolute vorticity brought by the monsoon trough are important for the shift in TC genesis. The present study indicates that the classification of these two types of summer ENSOs with different temporal evolutions is a crucial factor in studying the impact of summer ENSOs on WNP TC activity.

In this study, the formation of an idealized multiple tropical cyclones (MTCs) event within a monsoon trough (MT) region is investigated by using the weather forecasting model (WRF_ARW). Sensitivity experiments are conducted by specifying different initial conditions. It is revealed that both dynamical and thermodynamical conditions associated with the MT are important in triggering MTCs event over the western North Pacific basin. In the composite active years, which have a strong MT with higher ambient moisture, an MTCs event is produced. In contrast, no MTCs event occurs in the inactive years, which confirms the previous observational study. The possible pathway for the formation of MTCs event is proposed. In the active years, under favourable moist environments, the first TC is generated faster through the greater barotropic kinetic energy conversion. Once the first tropical cyclone (TC) is generated, the energy dispersion induced low‐level Rossby wave train acts as a precursor to the second TC. Furthermore, the upper‐level asymmetric outflow jet acts as a dynamical forcing to induce vertical motion, which builds up a favourable environmental condition for a second TC development. This work provides some insights into the formation of MTCs event.