Western North Pacific Tropical Cyclone Research Articles

Recent shortening of the mature tropical cyclone stage over the western North Pacific

AbstractThis study investigates long‐term trends related to the mature stage duration of tropical cyclones (TCs) over the western North Pacific (WNP) during June–November between 1982 and 2021. There is a significant shortening in the mean duration of the WNP mature TC stage, which is defined as the period when a TC is within 5 kt of its lifetime maximum intensity. This shortening is induced by a significant (weak) poleward migration in the starting (ending) location of the WNP mature TC stage, which can be further explained by changes in environmental conditions. From 1982 to 2021, there have been significant increases in maximum potential intensity and 700–500‐hPa relative humidity over most of the WNP, which has broadly expanded the TC‐favorable area poleward. Consequently, WNP TCs can start their mature stages and reach their lifetime maximum intensities at higher latitudes. By contrast, there are only weak changes in 850‐hPa relative vorticity and 850–200‐hPa vertical wind shear (VWS). Given the dominant role of VWS in modulating the TC weakening rate, the TC‐suppressive area over the subtropical WNP has shown lesser changes, thus leading to insignificant changes in the ending location of the mature TC stage.

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

El Niño–Southern Oscillation-Independent Regulation of Western North Pacific Tropical Cyclone Genesis

Western North Pacific tropical cyclone activity modulated by phytoplankton feedback under global warming

Western North Pacific tropical cyclone activity modulated by phytoplankton feedback under global warming

Uptrend of the Western North Pacific Tropical Cyclone Outflow Height during 1959–2021

Uptrend of the Western North Pacific Tropical Cyclone Outflow Height during 1959–2021

Trends in the ocean mixed‐layer depth in the western North Pacific tropical cyclone basin

AbstractThe upper ocean mixed‐layer depth (MLD) has been recognized as a crucial factor in understanding the impact of climate change on tropical cyclone (TC) intensity, while the specific MLD variations in the western North Pacific (WNP) basin have received comparatively little attention. Using two ocean reanalysis datasets and the output of 28 CMIP6 models, the climatological mean MLD and its trend in the TC active region are examined. Compared to the ORAS4 and EN4 datasets, the spatial pattern of the climatological mean MLD can be well simulated in the multi‐model ensemble (MME). The mean MLD decreases with increasing latitude and maintains the maximum in the equatorial region. The spatial patterns of MLD long‐term (1958–2017) and short‐term (1979–2017) trends are deepening in the TC active region, with the relatively large positive trends occurring to the southeastern WNP primarily between 5°N and 15°N. However, the simulated trend in the MME is dominant with shoaling MLD, and the shoaling of the MLD is also projected under the SSP5‐8.5 scenario. It is suggested that the bias on the simulated shoaling of the MLD in CMIP6 models may result from the overestimated enhancement of the upper‐ocean stratification. Thus, there is a discrepancy between the observed and simulated MLD trends in the WNP basin, which may have implications for TC intensity change under global warming scenarios.

Impact of Extratropical Circulation in East Asia on Western North Pacific Tropical Cyclone Frequency and Intensity

Abstract This study examines the East Asia and western North Pacific (WNP) monsoon circulation patterns for strong and weak WNP tropical cyclone (TC) numbers in summer. It suggested that years with more intense TCs are coupled with the tropical monsoon circulations, including the northward cross-equatorial airflow and the extending tropical monsoon trough toward the central-eastern WNP. However, a higher frequency of weak TCs can be largely attributed to the mutual interactions among the tropical monsoon trough west of 140°E, the westward South Asia high, and the high pressure anomaly in Northeast Asia (NEA). Then the potential influence of the NEA extratropical system is focused on. The resultant local negative potential vorticity (PV) anomaly is carried southeastward by the prevailing flow. It stimulates a descending flow around 30°N, which favors the westward retreat of the South Asian high and the decreased zonal vertical wind shear around 20°N. The associated lower-level outflow converges in the tropical WNP and reinforces the ascending motion around 10°–20°N. Meanwhile, the warm air column in NEA also contributes to anomalous easterlies in a band around 30°N, intensifying the lower-level cyclonic vorticity in the northwestern WNP. Consequently, the ascending motion, cyclonic vorticity, and the weakened zonal vertical wind shear in northwestern WNP promote the WTC formation. A set of physically based empirical models is developed using various physically based predictors to reconstruct the number of intense and weak TCs. Cross-validated hindcasts suggest that the NEA extratropical circulation can serve as an additional source of predictability for the weak TC variability. Significance Statement Tropical cyclones (TCs) are a highly destructive type of natural disaster that have garnered widespread attention. By comparison with intense TCs (ITCs), weak TCs (WTCs) are much more numerous and often form closer to the coastal regions of East Asia, whose mechanism has not been fully understood. In this study, we suggest that more ITCs are controlled by tropical monsoon circulations, while the WTC variability is closely coupled with both tropical and extratropical monsoon systems. In addition to the tropical monsoon trough west of 140°E and the westward South Asian high, the Northeast Asian circulation can regulate the WTC number by changing the lower-level vorticity, vertical motion, and vertical wind shear in the WTC genesis-prone region, which can be applied to improve the seasonal prediction skill of WTCs.

Independent contributions of tropical sea surface temperature modes to the interannual variability of western North Pacific tropical cyclone frequency

AbstractThis study investigates the independent contributions of three tropical sea surface temperature anomaly (SSTA) modes to interannual changes in summertime western North Pacific (WNP) tropical cyclone (TC) frequency from 1982 to 2022. We primarily analyse partial regressions of TC metrics and environmental variables onto the El Niño–Southern Oscillation Modoki (ENM), North Indian Ocean (NIO) SSTA and tropical North Atlantic (TNA) SSTA indices. We find that although basinwide WNP TC frequency significantly correlates with both the ENM and NIO indices, the regions with the most significant changes in TC formation induced by these two indices are well separated. During El Niño Modoki, significant increases in TC formation concentrate over the southeastern part of the WNP, while during a warm NIO, significant decreases in TC formation concentrate over the northwestern part of the WNP. By contrast, insignificant changes in TC genesis over the WNP are solely induced by TNA SSTAs. We show that the two pathways of TNA SSTA's influence on WNP TC activity through modulation of Pacific and NIO SSTAs are of comparable importance. During El Niño Modoki, significantly enhanced TC genesis over the southeastern WNP can be explained by more favourable environmental conditions, which is linked to an anomalous low‐level cyclonic circulation over the WNP. During a warm NIO, TC genesis is significantly suppressed over the northwestern WNP, due primarily to an anomalous low‐level anticyclonic circulation‐induced decrease in relative vorticity. By contrast, during a warm TNA, no notable changes in environmental variables are found over most of the WNP, while significant flow anomalies are limited to the central‐to‐eastern Pacific and the western Atlantic. These results highlight that the TNA SSTA by itself has a limited impact on WNP TC genesis and the associated large‐scale environment.

Emergent Constraint on Projection of the North Pacific Monsoon Trough and Its Implications for Typhoon Activity Using CMIP6 Models

AbstractThe North Pacific monsoon trough (NPMT) is an imperative large‐scale circulation pattern influencing tropical cyclone (TC) activity in the western North Pacific (WNP), thus its future change has great implications for the WNP TC activity. Here future change in the NPMT and its uncertainty are examined by using 35 climate models from Phase six of Coupled Model Intercomparison Project (CMIP6). Due to the El Niño‐like sea surface temperature (SST) pattern, the multi‐model ensemble (MME) mean projects an eastward extension of the NPMT in the latter half of the 21st century. However, considerable inter‐model uncertainty exists in the projection, which is associated with the diversity in the zonal SST gradient across the tropical Pacific. This diversity in the projected SST is largely rooted in the simulated precipitation biases in the tropical Pacific in the historical experiments of CMIP6 models, which can modulate future Pacific SST distribution through the shortwave‐SST feedback. This linkage between biases in historical climate and future climate allows us to constrain the projection by using observational data sets. Emergent constraint using observational precipitation data set reduces the projection uncertainty by 48% and projects an eastward extension of the north Pacific NPMT by 6.2°. This eastward extension of the NPMT indicates an eastward migration of TC genesis location, and elongated TC track and thus strengthened TC intensity, suggesting an increased TC‐related disaster for residents near the WNP.

Increase in western North Pacific tropical cyclone intensification rates and their northwestward shifts

The tropical cyclone intensification rates (TCIRs) over the western North Pacific (WNP) are investigated on the basis of 6-hourly atmospheric and oceanic reanalyses and TC best track data from 1982 to 2020. Results indicate that 24-h-TCIRs over the WNP have increased remarkably by ∼17.2% in the past four decades. A remarkable difference before and after the changepoint 2004 is detected in 24-h TCIRs over the WNP basin. The correlation between the TCIR and sea surface temperature (SST) during 2005–2020 shows a significant positive correlation over the western Pacific. Composite analyses of atmospheric and oceanic environments between P1 (1982–2004) and P2 (2005–2020) suggest greater contributions of the underlying SST and relative vorticity (VORT) than those of vertical wind shear (VWS) and mid-tropospheric relative humidity (RH) to TCIR increase. The maximum potential intensity (MPI) has increased since the 1980s, with a significant change from P1 to P2, which further demonstrates that the environmental fields since 2004 are more favorable for TC intensification. At the same time, TCIR geolocation shows a northwestward shift over the TC-active region bounded by 10° ∼ 25°N, 125° ∼ 150°E since 2004, which is possibly modulated by the interdecadal difference of steering flow and ocean state. This study may provide inspirations to clarify the mechanisms of WNP-TCIR interdecadal increase for general TC intensity forecast.

The combined influence of west Pacific subtropical high and tropical cyclones over the northwest Pacific on Indian summer monsoon rainfall

AbstractWe examined the interrelationship between the Indian summer monsoon rainfall (ISMR), the west Pacific subtropical high (WPSH), and tropical cyclone (TC) activity over the western north Pacific (WNP) Ocean during the peak monsoon season in 1951–2019. When WNP TCs are inactive (active), there is a noticeable 10° westward (eastward) shift observed along with intensification (weakening) of the WPSH. The WPSH and WNP TC activities are divided into four categories—an eastward shift of WPSH with active TCs (EA) and inactive TCs (EI), and a westward shift of WPSH with active TCs (WA) and inactive TCs (WI)—in order to investigate the combined influence of the WPSH and WNP TCs on ISMR during non‐El Niño–Southern Oscillation (non‐ENSO), El Niño, and La Niña phases. In the EA non‐ENSO cases, a surplus (deficit) rainfall was noticed over central (southern peninsular) India as a result of moisture convergence (low convective available potential energy, abnormal moisture divergence and downdraft); however, the rainfall is above normal over the Indian subcontinent. During the EI non‐ENSO case, an excess (deficit) regional rainfall activity was observed over east‐central India and parts of north India (southern peninsular India) through regional changes in atmospheric circulations and midtropospheric vertical velocity. In contrast, in WI (WA) non‐ENSO cases, the majority of India (southern peninsular and northeast India) experiences excessive precipitation while east‐central India (east and west central India) undergoes a deficit rainfall. This pattern is consistent with the spatial distribution of moisture flux convergence, convective available potential energy, and midtropospheric vertical velocity. In the case of non‐ENSO, the westward shift of the WPSH associated with inactive TCs is one of the favourable conditions for ISMR. In general, the effect of an eastward (westward) shift of the WPSH along with more (less) TCs during the non‐ENSO case is favourable to ISMR. On the other hand, rainfall patterns during ENSO are significantly associated with the circulation changes teleconnected to El Niño and La Niña conditions.

Predicting Short-Term Intensity Change in Tropical Cyclones Using a Convolutional Neural Network

Abstract This study details a two-method, machine learning approach to predict current and short-term intensity change in global tropical cyclones (TCs), “D-MINT” and “D-PRINT.” D-MINT and D-PRINT use infrared imagery and environmental scalar predictors, while D-MINT also employs microwave imagery. Results show that current TC intensity estimates from D-MINT and D-PRINT are more skillful than three established intensity estimation methods routinely used by operational forecasters for North Atlantic and eastern and western North Pacific TCs. Short-term intensity predictions are validated against five operational deterministic guidances at 6-, 12-, 18-, and 24-h lead times. D-MINT and D-PRINT are less skillful than NHC and consensus TC intensity predictions in North Atlantic and eastern North Pacific TCs, but are more skillful than the other guidances for at least half of the lead times. In western North Pacific, north Indian Ocean, and Southern Hemisphere TCs, D-MINT is more skillful than the JTWC and other individual TC intensity forecasts for over half of the lead times. When probabilistically predicting TC rapid intensification (RI), D-MINT is more skillful in North Atlantic and western North Pacific TCs than three operationally used RI guidances, but less skillful for yes–no RI forecasts. In addition, this work demonstrates the importance of microwave imagery, as D-MINT is more skillful than D-PRINT. Since D-MINT and D-PRINT are convolutional neural network models interrogating two-dimensional structures within TC satellite imagery, this study also demonstrates that those features can yield better short-term predictions than existing scalar statistics of satellite imagery in operational models. Finally, a diagnostics tool is revealed to aid the attribution of the D-MINT/D-PRINT intensity predictions. Significance Statement This study develops a method to predict current and short-term forecasts of tropical cyclone (TC) intensity using artificial intelligence. The resultant models use a convolutional neural network (CNN) that can identify two-dimensional features in satellite imagery that are indicative of TC intensity and future intensity change. The performance results indicate that in several TC basins, the CNN approach is generally more skillful than alternative satellite-based estimates of TC intensity as well as operational short-term forecasts of deterministic intensity change and of similar skill to probabilistic rapid intensification forecasts.

Revisiting Koba’s Relationship to Improve Minimum Sea-Level Pressure Estimates of Western North Pacific Tropical Cyclones

Revisiting Koba’s Relationship to Improve Minimum Sea-Level Pressure Estimates of Western North Pacific Tropical Cyclones

Shifted relationship between the Pacific decadal oscillation and western North Pacific tropical cyclogenesis since the 1990s

The Pacific Decadal Oscillation (PDO) and Pacific Meridional Mode (PMM) are prominent climate modes in the North Pacific with well-established impacts on tropical cyclone (TC) genesis in the western North Pacific (WNP) basin. While previous research has primarily focused on the roles of the PDO and PMM in regulating TC genesis through the modification of large-scale environmental factors, this study investigates the evolving influence of the PDO on WNP TC genesis since the 1950s. Remarkably, our analysis reveals a shift in the PDO-TC genesis relationship, transitioning from a significant negative correlation to a significant positive correlation since the 1990s. This shift is attributed to variations in the specific large-scale factors through which the PDO affects TC genesis. Furthermore, this study suggests that these changes appear to be linked to the PMM strengthening on the interdecadal timescale in recent decades. The linkage of the PMM strengthening to the PDO-related atmospheric circulation is further confirmed by the results of a 500 year pre-industrial numerical experiment, suggesting that the PMM strengthening may result from natural internal variability. The results underscore the non-stationary relationship between PDO and WNP TC genesis, with the PMM intensity probably influencing their relationship.

Intense Tropical Cyclones in the Western North Pacific Under Global Warming: A Dynamical Downscaling Approach

AbstractThis study aims to assess the global warming's impact on intense tropical cyclones (TCs) over the western North Pacific (WNP) through dynamical downscaling. 379 and 179 TCs reaching Category 1 in the High‐Resolution Atmospheric Model (HiRAM) are downscaled for use in the Weather Research and Forecasting (WRF) model at 5‐km horizontal resolution in the current climate (1979–2015) and for use in the Representative Concentration Pathways 8.5 (RCP8.5) in the future climate (2074–2100) scenarios, respectively. Inclusion of the downscaling simulations by WRF helps better reproduce the probability distribution of the TC's lifetime maximum intensity (LMI). In the warmer climate, the LMI of very intense TCs in WNP are projected to be stronger. Such an increase in intensity is statistically significant, and can be primarily explained by enhanced intensification rate. Meanwhile, TCs among the top 5% in LMI can reach higher intensities which cannot be attained in the current climate. After downscaling, the probability of WNP TCs reaching Category 4–5 increases by 6.5 percentage points in the late 21st century. This increase is 1.7 percentage points higher than the increase projected exclusively by HiRAM. Moreover, for TCs among the top 5% in LMI, a 233‐km and 300‐km westward shift of LMI locations is identified in the late 21st century for simulations with and without the downscaling approach, respectively. Both results suggest that very intense TCs would pose a higher threat to the WNP lands under global warming, as they become substantially stronger, and as their LMI locations migrate toward the coast.

Mutating ENSO Impact on Northwest Pacific Tropical Cyclones Under Global Warming

AbstractA prominent feature of the western North Pacific tropical cyclone genesis frequency (TCGF) anomaly in response to El Niño‐Southern Oscillation (ENSO) is a distinct west‐east dipole structure in the present‐day (PD) climate. Here, large ensemble high‐resolution simulations show that the current dipole may transform into a monopole under global warming (GW). In the PD climate, the ENSO‐induced dipole effect on TCGF is mainly observed in late autumn. However, GW accelerates this effect into the summer by amplifying atmospheric circulation anomalies. Consequently, this leads to a significant increase in the coastal TCGF during the development phase of El Niño under GW. Meanwhile, it weakens the anomalous anticyclonic circulation during late autumn, leading to an increase in coastal TCGF during the mature phase of El Niño, and vice versa. Therefore, the combined effect suggests a potential trend toward spatial homogenization during ENSO phases under GW.

Opposite Skills of ENGPI and DGPI in Depicting Decadal Variability of Tropical Cyclone Genesis over the Western North Pacific

Abstract The genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel–Nolan GPI (ENGPI) and the dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979–2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of −0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI. On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and midlevel humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis. Significance Statement Tropical cyclones (TCs) account for one-third of the deaths and economic losses from weather-, climate-, and water-related disasters. Understanding variations in TC activity from the perspective of large-scale conditions is of great importance to seasonal forecasting and disaster mitigation. Here we find that two genesis potential indexes (GPIs), the Emanuel–Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skill in quantifying decadal variability of TC genesis in the western North Pacific (WNP). The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis. The result suggests a critical role of large-scale dynamic control in the decadal variability of TC genesis in the WNP.

What Controls the Interannual Variation in Rapid Intensification Magnitude of Western North Pacific Tropical Cyclones?

AbstractThis study investigates the interannual variability of rapid intensification (RI) magnitude of western North Pacific (WNP) tropical cyclones. There is a significant correlation between basin‐averaged RI magnitude during July–November and the simultaneous Western Pacific (WP) teleconnection index from 1982 to 2021. RI magnitude is, on average, larger (smaller) in positive (negative) WP phases. During a positive WP, RI magnitude changes exhibit a southwest‐northeast dipolar pattern, with significant increases over the southwestern quadrant of the WNP and weak decreases over the northern part of the WNP. The WP teleconnection relates to RI magnitude primarily through modulation of 850–200‐hPa vertical wind shear, with less influence from 850‐hPa relative vorticity. The changes in these two dynamic conditions can be linked to WP‐induced circulation anomalies at lower and upper levels. Our results highlight that different contributors may be responsible for changes in WNP RI occurrence and RI magnitude.

Influence of Qinghai-Xizang Plateau snow cover on interannual variability of Western North Pacific tropical cyclone tracks

Influence of Qinghai-Xizang Plateau snow cover on interannual variability of Western North Pacific tropical cyclone tracks

The Pacific Meridional Mode Influences ENSO by Inducing Springtime Near-Equatorial TCs in the Western North Pacific

Abstract Previous research has shown that tropical cyclones in the near-equatorial western North Pacific (NE-WNP TCs) in boreal spring are associated with the onset of El Niño–Southern Oscillation (ENSO). However, the cause of these TCs is unclear. Here, we find that the interannual activity of the springtime NE-WNP TCs is insensitive to the simultaneous ENSO intensity but is closely related to the springtime Pacific meridional mode (PMM). The lead–lag correlation between PMM and NE-WNP TCs is high from January to April, and the evolution of the SST anomaly associated with the NE-WNP TCs strongly manifests the seasonal footprint of the PMM. Analysis using AGCM experiments confirms that the positive PMM tends to reduce the vertical shear of zonal wind and sea level pressure and to increase vertical velocity in the middle atmosphere and surface humidity in the WNP. All of these conditions are favorable for NE-WNP TC genesis. We conducted CGCM experiments to validate that the NE-WNP TCs significantly influence ENSO development. A statistical regression model with respect to the impact of PMM on NE-WNP TCs and further on ENSO intensity was also conducted. The results imply that modulating the springtime NE-WNP TCs is another effective way in which the PMM influences ENSO. Significance Statement Some studies have found that springtime near-equatorial western North Pacific (NE-WNP) tropical cyclones (TCs) heavily impact ENSO development. Using observations and AGCM experiments, we showed that the genesis of these TCs is closely associated with the Pacific meridional mode (PMM), presumably because the PMM provides favorable conditions for TC genesis. We further showed that including the PMM-induced NE-WNP TCs in a CGCM leads to an El Niño–like response. The linkage between the PMM and NE-WNP TCs not only supplements the existing theories about tropical–subtropical interactions but also provides a new way to improve ENSO simulation and prediction.

Asymmetric influence of the Pacific meridional mode on tropical cyclone formation over the western North Pacific

AbstractThis study investigates the asymmetric response of western North Pacific (WNP) tropical cyclone (TC) formation during August–November to the Pacific meridional mode (PMM) from 1961 to 2021. Basinwide WNP TC frequency significantly increases (slightly decreases) during positive (negative) PMM years, implying a nonlinear PMM–TC frequency relationship. Only small spatial changes in TC formation occur during negative PMM years, while there is nearly a basinwide enhancement of TC formation during positive PMM years, particularly over the region of 5°–30°N and 135°–155°E. This region is characterized by significantly enhanced low‐level vorticity, mid‐level updrafts and upper‐level divergence during positive PMM years, all favouring TC development. By contrast, environmental anomalies are of a smaller magnitude and mostly insignificant over the WNP during negative PMM years. These distinct environmental changes during different PMM phases can be explained by differences in PMM strength. Positive PMM events are of a greater strength and are associated with a stronger wind–evaporation–sea surface temperature feedback than negative PMM events, leading to a notable anomalous large‐scale low‐level cyclonic circulation over the WNP in positive PMM events.