Tropical Cyclone Intensity Change Research Articles

Lightning activity and its relationship with typhoon intensity and vertical wind shear for Super Typhoon Haiyan (1330)

Super Typhoon Haiyan (1330), which occurred in 2013, is the most powerful typhoon during landfall in the meteorological record. In this study, the temporal and spatial distributions of lightning activity of Haiyan were analyzed by using the lightning data from the World Wide Lightning Location Network, typhoon intensity and position data from the China Meteorological Administration, and horizontal wind data from the ECMWF. Three distinct regions were identified in the spatial distribution of daily average lightning density, with the maxima in the inner core and the minima in the inner rainband. The lightning density in the intensifying stage of Haiyan was greater than that in its weakening stage. During the time when the typhoon intensity measured with maximum sustained wind speed was between 32.7 and 41.4 ms−1, the storm had the largest lightning density in the inner core, compared with other intensity stages. In contrast to earlier typhoon studies, the eyewall lightning burst out three times. The first two eyewall lightning outbreaks occurred during the period of rapid intensification and before the maximum intensity of the storm, suggesting that the eyewall lightning activity could be used to identify the change in tropical cyclone intensity. The flashes frequently occurred in the inner core, and in the outer rainbands with the black body temperature below 220 K. Combined with the ECMWF wind data, the influences of vertical wind shear (VWS) on the azimuthal distribution of flashes were also analyzed, showing that strong VWS produced downshear left asymmetry of lightning activity in the inner core and downshear right asymmetry in the rainbands.

Satellite Air–Sea Enthalpy Flux and Intensity Change of Tropical Cyclones over the Western North Pacific

AbstractAn improved high-resolution satellite enthalpy flux dataset is employed to study the composites of initial (i.e., t = 0 h) latent heat flux (LHF), sensible heat flux (SHF), and their bulk variables associated with four intensity-change categories of tropical cyclones (TCs) over the western North Pacific Ocean—rapidly intensifying (RI), slowly intensifying, neutral, and weakening—in a vertical wind shear–relative coordinate system with horizontal dimensions normalized by the radius of maximum wind. Results show that RI TCs are associated with significantly higher LHF and SHF in all TC environments than non-RI TCs, which are mainly attributable to the air–sea humidity difference (DQ) and the air–sea temperature difference (DT), respectively. Higher DQ and DT are primarily due to significantly higher sea surface temperature (SST) underlying RI TCs, emphasizing the crucial role of SST in supplying more energy to TCs that undergo rapid intensification, in which LHF plays a more important role than SHF. Relative to non-RI TCs, LHF and SHF for RI TCs show a more symmetric pattern. The magnitude and pattern of air–sea enthalpy flux could serve as potential predictors for rapid intensification of TCs.

A Horizontal Index for the Influence of Upper-Level Environmental Flow on Tropical Cyclone Intensity

Abstract A horizontal map of the upper-level forcing index (ULFI) is constructed to show the possible influence of upper-level large-scale environmental flow on the intensity change of tropical cyclones (TCs). The ULFI includes three commonly used diagnostics, that is, 200-hPa eddy flux convergences of both relative (REFC) and planetary angular momentum (PEFC), as well as axisymmetric absolute vorticity as a denominator that rescales the strength of the eddy forcings similar to the outflow-layer inertial stability. A simple procedure is adopted to convert these storm-relative components and the ULFI into Eulerian horizontal maps. Applications of this index map to three selected TC cases clearly demonstrate the process of upper-level TC–environment interaction: when a TC moves into a region of high (low) index, significant upper-level asymmetric forcing is exerted on the TC, leading to the strengthening (weakening) of the TC’s axisymmetric outflow and then possibly its intensity. As such, the horizontal map of ULFI not only provides a quantitative way of estimating the strength of upper-level asymmetric forcing at each grid point, but also serves as an indicator showing where the possible intensity change of a TC would occur under the influence of upper-level environmental flow. The index is thus recommended to be used in future studies of TC–environment interaction.

Precipitation Properties Observed during Tropical Cyclone Intensity Change

Abstract Using a 15-yr (1998–2012) multiplatform dataset of passive microwave satellite data [tropical cyclone–passive microwave (TC-PMW)] for Atlantic and east Pacific storms, this study examines the relative importance of various precipitation properties, specifically convective intensity, symmetry, and area, to the spectrum of intensity changes observed in tropical cyclones. Analyses are presented not only spatially in shear-relative quadrants around the center, but also every 6 h during a 42-h period encompassing 18 h prior to onset of intensification to 24 h after. Compared to those with slower intensification rates, storms with higher intensification rates (including rapid intensification) have more symmetric distributions of precipitation prior to onset of intensification, as well as a greater overall areal coverage of precipitation. The rate of symmetrization prior to, and during, intensification increases with increasing intensity change as rapidly intensifying storms are more symmetric than slowly intensifying storms. While results also clearly show important contributions from strong convection, it is concluded that intensification is more closely related to the evolution of the areal, radial, and symmetric distribution of precipitation that is not necessarily intense.

Effects of Horizontal Diffusion on Tropical Cyclone Intensity Change and Structure in Idealized Three-Dimensional Numerical Simulations

Abstract This study examines the effects of horizontal diffusion on tropical cyclone (TC) intensity change and structure using idealized simulations of the Hurricane Weather Research and Forecasting Model (HWRF). A series of sensitivity experiments were conducted with varying horizontal mixing lengths (Lh), but kept the vertical diffusion coefficient and other physical parameterizations unchanged. The results show that both simulated maximum intensity and intensity change are sensitive to the Lh used in the parameterization of the horizontal turbulent flux, in particular, for Lh less than the model’s horizontal resolution. The results also show that simulated storm structures such as storm size, kinematic boundary layer height, and eyewall slope are sensitive to Lh as well. However, Lh has little impact on the magnitude of the surface inflow angle and thermodynamic mixed layer height. Angular momentum budget analyses indicate that the effect of Lh is to mainly spin down a TC vortex. Both mean and eddy advection terms in the angular momentum budget are affected by the magnitude of Lh. For smaller Lh, the convergence of angular momentum is larger in the boundary layer, which leads to a faster spinup of the vortex. The resolved eddy advection of angular momentum plays an important role in the spinup of the low-level vortex inward from the radius of the maximum wind speed when Lh is small.

Impact assessment of coastal hazards due to future changes of tropical cyclones in the North Pacific Ocean

Abstract Tropical cyclones generate severe hazards in the middle latitudes. A brief review and applications of dynamical and statistical downscaling of tropical cyclone (TC) are described targeting extreme storm surge and storm wave hazard assessment. First, a review of the current understanding of the changes in the characteristics of TCs in the past and in the future is shown. Then, a review and ongoing research about impact assessment of tropical cyclones both dynamical downscaling and statistical model are described for Typhoon Vera in 1959 and Typhoon Haiyan in 2013. Finally, several examples of impact assessment of storm surge and extreme wave changes are presented. Changes in both TC intensity and track are linked to future changes in extreme storm surge and wave climate in middle latitude.

Secondary Circulation of Tropical Cyclones in Vertical Wind Shear: Lagrangian Diagnostic and Pathways of Environmental Interaction

Abstract This study introduces a Lagrangian diagnostic of the secondary circulation of tropical cyclones (TCs), here defined by those trajectories that contribute to latent heat release in the region of high inertial stability of the TC core. This definition accounts for prominent asymmetries and transient flow features. Trajectories are mapped from the three-dimensional physical space to the (two dimensional) entropy–temperature space. The mass flux vector in this space subsumes the thermodynamic characteristics of the secondary circulation. The Lagrangian diagnostic is then employed to further analyze the impact of vertical wind shear on TCs in previously published idealized numerical experiments. One focus of this analysis is the classification and quantitative depiction of different pathways of environmental interaction based on thermodynamic properties of trajectories at initial and end times. Confirming results from previous work, vertical shear significantly increases the intrusion of low–equivalent potential temperature () air into the eyewall through the frictional inflow layer. In contrast to previous ideas, vertical shear decreases midlevel ventilation in these experiments. Consequently, the difference in eyewall between the no-shear and shear experiments is largest at low levels. Vertical shear, however, significantly increases detrainment from the eyewall and modifies the thermodynamic signature of the outflow layer. Finally, vertical shear promotes the occurrence of a novel class of trajectories that has not been described previously. These trajectories lose entropy at cold temperatures by detraining from the outflow layer and subsequently warm by 10–15 K. Further work is needed to investigate in more detail the relative importance of the different pathways for TC intensity change and to extend this study to real atmospheric TCs.

A Statistical Analysis of the Effects of Vertical Wind Shear on Tropical Cyclone Intensity Change over the Western North Pacific

Abstract The effect of vertical wind shear (VWS) between different pressure levels on TC intensity change is statistically analyzed based on the best track data of tropical cyclones (TCs) in the western North Pacific (WNP) from the Joint Typhoon Warning Center (JTWC) and the ECMWF interim reanalysis (ERA-Interim) data during 1981–2013. Results show that the commonly used VWS measure between 200 and 850 hPa is less representative of the attenuating deep-layer shear effect than that between 300 and 1000 hPa. Moreover, the authors find that the low-level shear between 850 (or 700) and 1000 hPa is more negatively correlated with TC intensity change than any deep-layer shear during the active typhoon season, whereas deep-layer shear turns out to be more influential than low-level shear during the remaining less active seasons. Further analysis covering all seasons exhibits that a TC has a better chance to intensify than to decay when the deep-layer shear is lower than 7–9 m s−1 and the low-level shear is below 2.5 m s−1. The probability for TCs to intensify and undergo rapid intensification (RI) increases with decreasing VWS and increasing sea surface temperature (SST). TCs moving at slow translational speeds (less than 3 m s−1) intensify under relatively weaker VWS than TCs moving at intermediate translational speeds (3–8 m s−1). The probability of RI becomes lower than that of rapid decaying (RD) when the translational speed is larger than 8 m s−1. Most TCs tend to decay when the translational speed is larger than 12 m s−1 regardless of the shear condition.

Relationship between lightning activity and tropical cyclone intensity over the northwest Pacific

AbstractLightning data from the World Wide Lightning Location Network along with tropical cyclone (TC) track and intensity data from the China Meteorological Administration are used to study lightning activity in TCs over the northwest Pacific from 2005 to 2009 and to investigate the relationship between inner core lightning and TC intensity changes. Lightning in TCs over the northwest Pacific is more likely to occur in weak storms at tropical depression (10.8–17.1 m s−1) and tropical storm (17.2–24.4 m s−1) intensity levels, in agreement with past studies of Atlantic hurricanes. The greatest lightning density (LD) in the inner core appears in storms undergoing an intensity change of 15–25 m s−1 during the next 24 h. Lightning is observed in all storm intensity change categories: rapid intensification (RI), average intensity change (AIC), and rapid weakening (RW). The differences in LD between RI and RW are largest in the inner core, and the LD for RI cases is larger than for RW cases in the inner core (0–100 km). Lightning activity there, rather than in the outer rainbands, may be a better indicator for RI prediction in northwest Pacific storms. There was a marked increase in the lightning density of inner core during the RI stage for Super Typhoon Rammasun (2008). Satellite data for this storm show that the RI stage had the highest cloud top height and coldest cloud top temperatures, with all the minimum black body temperature values being below 200 K in the inner core.

A statistical analysis of the association between tropical cyclone intensity change and tornado frequency

Tropical cyclones often produce tornadoes that have the potential to compound the injury and fatality counts and the economic losses associated with tropical cyclones. These tornadoes do not occur uniformly through time or across space. Multiple statistical methods were used in this study to analyze the association between tropical cyclone intensity change and tornado frequency. Results indicate that there is an association between the two and that tropical cyclones tend to produce more tornadoes when they are weakening, but the association is weak. Tropical cyclones can also produce a substantial number of tornadoes when they are relatively stable or strengthening.

Effects of Vertical Wind Shear on Inner-Core Thermodynamics of an Idealized Simulated Tropical Cyclone

Abstract A suite of idealized simulations of tropical cyclones (TCs) with weak to strong vertical wind shear (VWS) imposed during the mature stage was employed to examine the effects of VWS on the inner-core thermodynamics and intensity change of TCs using a three-dimensional full-physics numerical model as well as a budget analysis of moist entropy. For sheared TCs with shear-induced convective asymmetries, VWS tends to reduce moist entropy within the midlevel eyewall and the boundary layer (BL) but supply moist entropy outside the eyewall above the BL. Such changes in moist entropy reduce the radial gradient of moist entropy across the eyewall, resulting in weakening of the TC. Budget analysis showed that the intense eddy fluxes are mainly responsible for the reduction and/or increase in entropy in the sheared TCs. The entropy reduction within the midlevel eyewall is a result of both the radial eddy flux and the vertical eddy flux. These eddy fluxes are effective at introducing low-entropy air into the midlevel eyewall. Accompanying the flushing of midlevel low-entropy air into the BL, there is an increase in moist entropy outside the eyewall above the BL due to the upward transport of moisture from the BL by shear-induced convection. This represents a new potential pathway to further restrain the radial gradient of moist entropy across the eyewall and hence TC intensity in the sheared environmental flow.

Satellite derived upper ocean thermal structure and its application to tropical cyclone intensity forecasting in the Indian Ocean

Upper ocean heat content is a factor critical to the intensity change of tropical cyclones (TCs). Because of the inhomogeneity of in situ observations in the North Indian Ocean, gridded temperature/salinity (T/S) profiles were derived from satellite data for 1993–2012 using a linear regression method. The satellite derived T/S dataset covered the region of 10°S–32°N, 25°–100°E with daily temporal resolution, 0.25°×0.25° spatial resolution, and 26 vertical layers from the sea surface to a depth of 1 000 m at standard layers. Independent Global Temperature Salinity Profile Project data were used to validate the satellite derived T/S fields. The analysis confirmed that the satellite derived temperature field represented the characteristics and vertical structure of the temperature field well. The results demonstrated that the vertically averaged root mean square error of the temperature was 0.83 in the upper 1 000 m and the corresponding correlation coefficient was 0.87, which accounted for 76% of the observed variance. After verification of the satellite derived T/S dataset, the TC heat potential (TCHP) was verified. The results show that the satellite derived values were coherent with observed TCHP data with a correlation coefficient of 0.86 and statistical significance at the 99% confidence level. The intensity change of TC Gonu during a period of rapid intensification was studied using satellite derived TCHP data. A delayed effect of the TCHP was found in relation to the intensity change of Gonu, suggesting a lag feature in the response of the inner core of the TC to the ocean.

A downscaling technique to simulate changes in western North Pacific tropical cyclone activity between two types of El Niño events

Changes in tropical cyclone (TC) activity over the western North Pacific (WNP) basin between the Modoki and canonical El Nino years have been simulated and examined in this study based on a downscaling technique. The downscaling technique is used for generating synthetic TCs based on TC formation, TC track, and TC intensity models. Results suggest that the downscaling technique can well simulate the spatial distribution of TC activity during the two types of El Nino years and their differences. It is found that the observed changes in TC tracks during the two types of El Nino years are mainly due to the combined effects of changes in TC formation locations and large-scale steering flows. Further examinations have shown that changes in large-scale steering flows play a more important role than changes in TC formation locations. These results are in accordance with the cyclonic circulation anomaly found during the Modoki El Nino years compared to that during the canonical El Nino years. Numerical simulations further suggest that changes in TC tracks between the two types of El Nino years appear to be the most important factor affecting the TC intensity change. Compared to that during the Modoki El Nino years, TC formation is enhanced in the south quadrant of the WNP basin and more TCs take a northwestward track during the canonical El Nino years, leading to a longer TC lifespan and greater TC intensity.

Impacts of environmental humidity on concentric eyewall structure

AbstractThe tropical cyclone (TC) secondary eyewall (SE) is an important aspect of TC intensity and structure forecast, because it largely affects the horizontal scale of destructive winds and heavy rainfall. Hence, it is important to understand the possible factors that influence the structure and size of SE. Idealized simulations suggest that the environmental humidity is one of the key factors. It is favorable for TC to establish a SE in a moist environment. Furthermore, the SE is likely to be located at a larger radius, thereby leads to a wider moat area. Consequently, the structure of SE would affect TC intensity change during the eyewall replacement cycle. This study strongly motivates further researches with respect to how the environmental conditions influence the storm dynamics and thermodynamics.

Environmental Helicity and Its Effects on Development and Intensification of Tropical Cyclones

Abstract Much attention has been given to the impact of environmental wind shear in the 850–200-hPa layer on tropical cyclones (TCs). However, even with the same magnitude of shear, helicity in this layer can vary significantly. A new parameter is presented, the tropical cyclone–relative environmental helicity (TCREH). Positive TCREH leads to a tilted storm that enhances local storm-scale helicity in regions of convection within the TC. This enhanced local-scale helicity potentially allows for more robust and longer-lasting convection, which is more effective at generating latent heat and subsequent TC intensification. TC vertical tilt is often attributed to wind shear. Different values of helicity modulate this tilt and certain tilt configurations are more favorable for development or intensification than others, suggesting that mean positive environmental helicity is more favorable for development and intensification than mean negative helicity. Idealized modeling simulations demonstrate the impact of environmental helicity on TC development and intensification. Results show that wind profiles with the same 850–200-hPa wind shear but different values of helicity lead to different rates of development. TCREH also is computed from the Interim ECMWF Re-Analysis (1979–2011) and Global Forecast System analyses (2004–11) to determine if a significant signal exists between TCREH and TC intensification. Mean annular helicity is averaged over various time periods and correlated with the TC intensity change during those periods. Results suggest a weak but statistically significant correlation between environmental helicity and TC intensity change with positive helicity being more favorable for intensification.

Interannual variation of the Philippines affecting tropical cyclone intensity and its possible causes

This study analyzed changes in tropical cyclone (TC) intensity for the past 62 years (1951–2012) by calculating annual average value of central pressure (CP) of TCs that affected the Philippines from July to September. Although TC intensity slightly weakened for the last 62 years, it was not statistically significant. In order to examine the causes of changes in intensity of TCs that influenced the Philippines, nine low CP years and nine high CP years among the 62 years were selected to analyze differences between the two groups. TCs largely occurred in the southeastern quadrant of tropical and subtropical western North Pacific during low CP years and tended to move to the Philippines from the far sea in the southeast of the nation. In differences in deep-layer-mean wind between the two groups, western North Pacific subtropical high (WNPSH) did not develop toward the middle latitudes of East Asia but toward the low latitudes of the region during low CP years. Therefore, TCs occurred in the southeastern quadrant of tropical and subtropical western North Pacific, triggering its movement of long distance westward toward the Philippines. As positive anomalies in precipitable water and at 600 hPa relative humidity, 850 hPa air temperature, and sea surface temperature continued to the Philippines from the southeastern quadrant, a favorable environment where intensity of TCs can be strengthened while they move to the Philippines during low CP years has been formed. In the end, because WNPSH developed toward the low latitudes during low CP years, anomalous easterlies that strengthened in the southern regions of WNPSH blew to the Philippines, and SST in the sea near the nation was heightened. Therefore, because TCs were able to obtain sufficient energy from the relatively warm sea while TCs moved a long distance to the Philippines, TC intensity was able to be strengthened.

Future Changes in the Western North Pacific Tropical Cyclone Activity Projected by a Multidecadal Simulation with a 16-km Global Atmospheric GCM

Abstract How tropical cyclone (TC) activity in the northwestern Pacific might change in a future climate is assessed using multidecadal Atmospheric Model Intercomparison Project (AMIP)-style and time-slice simulations with the ECMWF Integrated Forecast System (IFS) at 16-km and 125-km global resolution. Both models reproduce many aspects of the present-day TC climatology and variability well, although the 16-km IFS is far more skillful in simulating the full intensity distribution and genesis locations, including their changes in response to El Niño–Southern Oscillation. Both IFS models project a small change in TC frequency at the end of the twenty-first century related to distinct shifts in genesis locations. In the 16-km IFS, this shift is southward and is likely driven by the southeastward penetration of the monsoon trough/subtropical high circulation system and the southward shift in activity of the synoptic-scale tropical disturbances in response to the strengthening of deep convective activity over the central equatorial Pacific in a future climate. The 16-km IFS also projects about a 50% increase in the power dissipation index, mainly due to significant increases in the frequency of the more intense storms, which is comparable to the natural variability in the model. Based on composite analysis of large samples of supertyphoons, both the development rate and the peak intensities of these storms increase in a future climate, which is consistent with their tendency to develop more to the south, within an environment that is thermodynamically more favorable for faster development and higher intensities. Coherent changes in the vertical structure of supertyphoon composites show system-scale amplification of the primary and secondary circulations with signs of contraction, a deeper warm core, and an upward shift in the outflow layer and the frequency of the most intense updrafts. Considering the large differences in the projections of TC intensity change between the 16-km and 125-km IFS, this study further emphasizes the need for high-resolution modeling in assessing potential changes in TC activity.

The Tropical Cyclone Diurnal Cycle of Mature Hurricanes

Abstract The diurnal cycle of tropical convection and the tropical cyclone (TC) cirrus canopy has been described extensively in previous studies. However, a complete understanding of the TC diurnal cycle remains elusive and is an area of ongoing research. This work describes a new technique that uses infrared satellite image differencing to examine the evolution of the TC diurnal cycle for all North Atlantic major hurricanes from 2001 to 2010. The imagery reveals cyclical pulses in the infrared cloud field that regularly propagate radially outward from the storm. These diurnal pulses begin forming in the storm’s inner core near the time of sunset each day and continue to move away from the storm overnight, reaching areas several hundreds of kilometers from the circulation center by the following afternoon. A marked warming of the cloud tops occurs behind this propagating feature and there can be pronounced structural changes to a storm as it moves away from the inner core. This suggests that the TC diurnal cycle may be an important element of TC dynamics and may have relevance to TC structure and intensity change. Evidence is also presented showing the existence of statistically significant diurnal signals in TC wind radii and objective Dvorak satellite-based intensity estimates for the 10-yr hurricane dataset that was examined. Findings indicate that TC diurnal pulses are a distinguishing characteristic of the TC diurnal cycle and the repeatability of TC diurnal pulsing in time and space suggests that it may be an unrealized, yet fundamental TC process.

Lightning activity within tropical cyclones in the South West Indian Ocean

Lightning activity is investigated within tropical convective events of the South-West Indian Ocean (SWIO). The World Wide Lightning Location Network (WWLLN) provides global lightning data since 2005 and is therefore a well suited network for the study of systems like tropical cyclones as they remain over ocean most of their life. Firstly, a 7-year lightning climatology over the South West Indian Ocean has been performed using the WWLLN data from 2005 to 2011. Results show the same main features presented in other studies: the hot spots in this region are found in Madagascar and over the Great Lakes in East Africa (> 10-20 fl. km 2 yr 1 ). Lightning flashes within tropical cyclones represent 50% to 100% of the total lightning activity in some oceanic areas (between 10 S and 20 S). Moreover, lightning activity is more intense during the November-to-April period which corresponds to the wet/cyclonic season. Then, lightning activity in tropical storms of the basin was studied using both WWLLN data and best- tracks from the Regional Specialized Meteorological Center La R´ eunion from January 2005 to May 2013. The South-West Indian Ocean is divided into three distinct regions (open ocean, Mozambique channel, and the oceanic region 400 km offshore the eastern coast of Madagascar) to account for the impact of land on the tropical storm structure. The tropical storms are divided into annular rings based on a radius of maximum wind criteria. We show that lightning activity is mainly found in the eyewall region whatever the system stage in the open ocean. The possible link between lightning activity and tropical cyclone intensity change is investigated using different thresholds for the intensification rate. We show that, whatever the intensity stage is, the proportion of 6-h period with lightning activity is higher during rapid intensifications. For systems rapidly intensifying, 20 to 50% of the 6-h periods are associated to lightning flashes.

Environmental influences on the intensity changes of tropical cyclones over the western North Pacific

Abstract. The influence of environmental conditions on the intensity changes of tropical cyclones (TCs) over the western North Pacific (WNP) is investigated through examination of 37 TCs during 2000–2011 that interacted directly with the western North Pacific subtropical high (WNPSH). Comprehensive composite analysis of the environmental conditions is performed for two stages of storms: one is categorized as intensifying events (maximum wind speed increases by 15 kn over 48 h) and the other is categorized as weakening events (maximum wind speed decreases by 15 kn over 48 h). Comparison of the composite analysis of these two cases show that environmental conditions associated with the WNPSH play important roles in the intensity changes of TCs over the WNP. When a TC moves along the southern periphery of the WNPSH, the relatively weaker easterly environmental vertical wind shear helps bring warm moist air from the south and southeast to its southeast quadrant within 500 km, which is favorable for the TC to intensify. However, when a TC moves along the western edge of the WNPSH, under the combined influences of the WNPSH and an upper-level westerly trough, a strong westerly vertical shear promotes the intrusion of dry environmental air associated with the WNPSH from the north and northwest, which may lead to the inhibition of moisture supply and convection over the western half of the TC and thus its weakening. These composite results are consistent with those with additional geographic restrictions, suggesting that the dry air intrusion and the vertical wind shear (VWS) associated with the WNPSH, indeed affect the intensity changes of TCs over the WNP beyond the difference related solely to variations in geographical locations. The average sea surface temperature (SST) of 27.6 °C for the weakening events is also lower than an average of 28.9 °C for the strengthening events, but remains above the critical value of 27 °C for TC intensification, suggesting that the SST may be regarded as a less positive factor for the weakening events.