This study analyzes the influence of El Niño-Southern Oscillation (ENSO) on the activity of tropical cloud clusters (TCCs) in the western North Pacific (WNP). A TCC must have at least one embedded mesoscale convective system and it must persist for more than 24 hours. In all, 2,248 TCCs were identified during July-October 1981-2009. While more (less) TCCs form in the eastern (western) part of the WNP during El Niño years than during normal years, the converse is true during La Niña years. The ratio of tropical cyclone (TC) numbers to TCC numbers (genesis productivity) was 27.3%, higher than found in previous study. TC genesis productivity does not correlate with the Oceanic Niño Index even in subregions of the WNP. The influence of ENSO on TC numbers in each subregion of the WNP was mainly due to changes in TCC number, not changes in TC genesis productivity.

The development of tropical cloud clusters (TCCs) to tropical cyclones (TCs) is the process of TC formation. This study identifies five main environmental transitions for the development of TCCs to TCs in the western North Pacific by using a cluster analysis method. Of these, three transitions indicate TCCs that develop in monsoon environments and two in easterly environments. Their numbers, distributions, and interannual variability differ. On average, the development time, defined as the period from the TCC forming to it developing into a TC, for TCCs that develop in easterly environments is shorter than that in monsoon environments. For the development of TCC to TC in easterly environments, TCCs have fewer embedded mesoscale convective systems (MCSs), which are located closer to the TCC center. Moreover, there is a stronger inward short-term (less than 10 days) angular momentum flux (AMF) at middle levels (800–500 hPa) before TCC formation. Conversely, in monsoon environments, TCCs have more MCSs, which are located farther from the TCC center. A stronger inward short-term AMF at low levels (1000–850 hPa) is observed before TCC formation and develops upward during the development of TCC to TC. The characteristics of MCS and AMF are significantly correlated with the development time of TCC to TC. In summary, large-scale easterly and monsoon environments cause TCCs to have different MCS and AMF characteristics, leading to higher efficiency for TCCs developing into TCs in easterly environments compared to monsoon environments.

This study analyses the impacts of tropical cyclones (TCs) that formed in monsoon and easterly environments on the major coastlines of the western North Pacific (WNP) basin during 1981–2009. The TC formation processes, defined as the development of tropical cloud cluster to TC, associated with monsoon environments (monsoon trough, monsoon confluence, and north of monsoon trough) are categorized into monsoon‐type TCs (monsoon‐TCs). Similarly, those associated with easterly flow environments (easterly flow west and southwest of subtropical high) are categorized into easterly‐type TCs (easterly‐TCs). Monsoon‐TCs form farther westward and at lower latitudes than easterly‐TCs, contributing to higher landfall proportion on the coastal countries for monsoon‐TCs. Monsoon‐TCs have a higher probability of affecting southern China, Taiwan and Vietnam, while easterly‐TCs tend to affect eastern China, southern Japan and the Philippines. Monsoon‐TCs have more widely dispersed rainfall and slower translation speed during landfall than easterly‐TCs. These characteristics are consistent with stronger environmental moisture transport and weaker steering flow in monsoon environments. Landfalling TC intensity and size are not different between easterly‐ and monsoon‐TCs. Both easterly‐ and monsoon‐TCs have interannual (1–4 years) and interdecadal (8–11 years) variability, which are related to variability of the large‐scale monsoon trough. El Niño–Southern Oscillation is significantly correlated with the interannual variability of easterly‐ and monsoon‐TCs, and changes in the monsoon‐TC landfall proportion and easterly‐TC landfall intensity. The interdecadal variability mainly affects the background vorticity and cyclonic circulation, leading to changes in the formation number of easterly‐ and monsoon‐TCs. In summary, this study provides evidence for connections between multiscale variability of the large‐scale monsoon and easterly patterns, TC formation environments, and TC impacts on the WNP coasts.

This study explores the importance of mid-level moisture for tropical cyclone (TC) formation in monsoon and easterly environments over the western North Pacific in regional simulations (15-km resolution). The Weather Research and Forecasting (WRF) model is used to simulate 22 TCs that form in monsoon environments (MTCs) and 13 TCs that form in easterly environments (ETCs) over the period 2006–2010. To characterize the moisture contribution, simulations with mid-level moisture improved through assimilation of global positioning system (GPS) radio occultation (RO) data (labeled as EPH) are compared to those without (labeled as GTS). In general, the probability of TC formation being detected in the simulations is higher for MTCs than ETCs, regardless of GPS RO assimilation, especially for the monsoon trough environment. Fifty-four percent of ETC formations are sensitive to the mid-level moisture patterns, while only 18% for MTC formations are sensitive, indicating the importance of mid-level moisture is higher for ETC formations. Because of a model dry bias, the simulation of TC formation in an observed environment with lower vorticity but higher moisture is sensitive to the moisture increase through GPS RO data. Sensitivity experiments show that if the moisture in GTS is replaced by that in EPH, the TC formation can be detected in the GTS simulations. In turn, the TC formation cannot be detected in the EPH simulations with GTS moisture. The mechanism causing the difference in simulation performance of TC formation is attributed to more diabatic heating release and stronger positive potential vorticity tendency at mid-levels around the disturbance center caused by the higher moisture magnitudes.

This study uses a nonhierarchical cluster analysis to identify the major environmental circulation patterns associated with tropical cloud cluster (TCC) formation in the western North Pacific. All TCCs that formed in July–October 1981–2009 are examined based on their 850-hPa wind field around TCC centers. Eight types of environmental circulation patterns are identified. Of these, four are related to monsoon systems (trough, confluence, north of trough, and south of trough), three are related to easterly systems (low-latitude zone, west of subtropical high, and southwest of subtropical high), and one is associated with low-latitude cross-equatorial flow. The genesis potential index (GPI) is analyzed to compare how favorable the environmental conditions are for tropical cyclone (TC) formation when TCCs form. Excluding three cluster types with the GPI lower than the climatology of all samples, TCCs formed in monsoon environments have larger sizes, lower brightness temperatures, longer lifetimes, and higher GPIs than those of TCCs formed in easterly environments. However, for TCCs formed in easterly environments, the average GPI for those TCCs that later develop into TCs (developing TCCs) is higher than that for other TCCs (nondeveloping TCCs). This difference is nonsignificant for TCCs formed in monsoon environments. Conversely, the average magnitudes of GPI are similar for developing TCCs, regardless of whether TCCs form in easterly or monsoon environments. In summary, the probability of a TCC to develop into a TC is more sensitive to the environmental conditions for TCCs formed in easterly environments than those formed in monsoon environments.