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The Joint Typhoon Warning Center Tropical Cyclone Best-Tracks, 1945–2000

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... The decay of the tropical cyclone is often associated with landfall. The landfall results in increased surface friction and cutoff in cyclone favorable moisture supply from the ocean. of the eye using satellite observations and tracking algorithms (Chu et al., 2002;Fyfe, 2010;Wong et al., 2008). Here the JTWC derived track of NISHA cyclone is provided in Fig. 2. The tracking algorithm of JTWC can be found from Chu et al. (2002). ...
... The landfall results in increased surface friction and cutoff in cyclone favorable moisture supply from the ocean. of the eye using satellite observations and tracking algorithms (Chu et al., 2002;Fyfe, 2010;Wong et al., 2008). Here the JTWC derived track of NISHA cyclone is provided in Fig. 2. The tracking algorithm of JTWC can be found from Chu et al. (2002). The cyclonic storm Nisha formed north of Sri Lanka on 24th November 2008 as a low-pressure system, by 26th November it was reached to a stage of the cyclonic storm (named as 'Nisha') and made landfall on 27th November 2008. ...
... The cyclone propagation can be best estimated by identifying the eye of the cyclone using tracking algorithms (Chu et al., 2002;Wong et al., 2008). The ocean has a prominent role in determining the genesis, intensification, and track of tropical cyclones (Mc Phaden et al., 2009;Sreenivas et al., 2012aSreenivas et al., , 2012bSreenivas and Gnanaseelan, 2014). ...
Article
Abstract In-situ observations have significant importance for calibration and validation of satellite data, processes studies etc, however in-situ observations are often available with less spatio-temporal coverage due to practical limitations in making observations, as well as cost effectiveness. Albeit satellite observations have better spatial and temporal coverage, they have certain limitations like obstacles of cloud coverage, retrieval algorithms etc. Hence worthy inferences on the processes (that was intended to study) can be obtained only through skillful utilization of both in-situ observations, satellite and/or reanalysis products. We have illustrated a method for deriving synthetic parallel tracks to in-situ track/survey. Using this method, the satellite and/or reanalysis products can be extracted over these synthetic parallel tracks to derive worthy understanding/conclusions on the processes. The points on the original track are reproduced in terms of slope and distance, which can be utilized to construct a new synthetic parallel track initiating at a predetermined distance to the starting point of original track. As an example, we have demonstrated variations in oceanic physical properties along synthetic parallel tracks to the track of the Khai-muk cyclone. It is easy to understood using this technique that the variations in sub-surface heat content (as evidenced by variations in sea level along Khai-muk cyclone track) have a relatively major role in controlling the intensity of the system, compared to the changes in surface temperature. The method illustrated here is applicable to any geophysical track/survey with varying spatial and/or temporal extension.
... Three best track datasets from the Shanghai Typhoon Institute of China Meteorological Administration (CMA, Ying et al., 2014), the Regional Specialized Meteorological Center of Japan Meteorological Agency (JMA, RSMC Tokyo, 2000), and the Joint Typhoon Warming Center (JTWC, Chu et al., 2002) were used to examine the robustness of the relationship between PMM and the WNP TC genesis frequency. TCs are defined as the records in the data set whose maximum wind speed reaches tropical storm intensity (17.2 m s −1 ). ...
... Tropical cyclone data of CMA (Ying et al., 2014), JMA (RSMC Tokyo, 2000) and JTWC (Chu et al., 2002) can be requested at https://tcdata.typhoon.org.cn/en/zjljsjj_zlhq.html [Dataset], https://www.jma.go.jp/jma/jma-eng/ jma-center/rsmc-hp-pub-eg/besttrack.html [Dataset] and https://www.metoc.navy.mil/jtwc/jtwc.html?western-pacific [Dataset], respectively. ...
Article
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Pacific Meridional Mode (PMM) has been found to be significantly correlated with tropical cyclone (TC) genesis in the western North Pacific (WNP), while the seasonality in their relationship remains unknown. Here we found that their relationship experiences remarkable seasonality, with a significant positive correlation in January–April and August–December but an insignificant relationship in May–July. This seasonality stems from the diverse responses of large‐scale conditions to the different magnitude of the PMM‐related sea surface temperature (SST) warming that is dependent on the strength of trade wind in the three seasons. In January–July, strong trade wind facilitates great PMM SST warming that stimulates large‐scale ascending motion in the eastern WNP but compensated descending motion in the western WNP, favoring TC genesis in January–April because its main genesis region is in the eastern WNP. However, the main genesis region extends to the whole WNP in May–July, the inconsistent large‐scale condition anomalies across the western and eastern WNP thus lead to the insignificant relationship in May–July. In August–December, the PMM‐related SST warming and the associated ascending motion fade away due to the much‐weakened trade wind, and the favorable large‐scale ascending motion over the whole WNP is linked to the PMM‐related SST cooling induced descending motion through a modification of the Walker circulation. The results highlight the season‐dependent mechanism of PMM influencing TC genesis and have some insights for improving seasonal forecasting.
... TC data from 1979 to 2020 are obtained from the Joint Typhoon Warning Center (JTWC) best track dataset (Chu et al., 2002). This dataset includes information on TC location and maximum sustained wind at 6-h intervals. ...
Article
This study highlights a significant inter-decadal modulation of the South Asian high (SAH) on the meridional distribution of western North Pacific (WNP) tropical cyclone genesis frequency (TCGF). There is an obvious inter-decadal change in the first leading SAH mode, with an abrupt weakening since the early 1990s. During 1979–1993, when there was a stronger SAH, TCs were suppressed over the northern WNP and enhanced over the southern WNP in response to inter-annual SAH changes. The northern WNP TCGF had a significant negative correlation with the SAH (r = −0.69), while the southern WNP TCGF had a significant positive correlation with the SAH (r = 0.78). This north-south seesaw pattern driven by the SAH weakened during 1994–2019. This inter-decadal modulation of the inter-annual SAH-WNP TC genesis relationship can be explained by changes in large-scale environmental parameters. There is anomalous ascending (descending) motion and negative (positive) vorticity centered at 850-hPa as well as less (more) moisture at 600-hPa in the northern WNP (southern WNP) during 1979–1993. The maintenance of a strong low-level anomalous anticyclone accompanied by the concurrent influence of the SAH and East Asian upper-tropospheric jet stream decreases northern WNP TCGF. Linking to an El Niño-like SST pattern, the cyclonic circulation forced by a Gill-type forcing and enhanced by equatorial low-level westerly anomalies increases southern WNP TCGF. In contrast, we find a relatively weak inter-annual relationship of large-scale factors to the SAH since 1994. This weakening relationship is largely due to the inter-decadal weakening of the SAH and East Asian upper-tropospheric jet stream. Inter-decadal changes of the East Asian summer monsoon and the phase shift of the Atlantic Multi-decadal Oscillation appear to be two important factors modulating the inter-annual relationship between the SAH and the meridional distribution of WNP TCGF.
... The COAWST model starts at the same time as the WRF model in LC for each TC, which is 60 hours prior to its landfall in RegCM. The results produced in this study are compared with (a) the results from LC, referred as the "WRF-only" model, and (b) the Joint Typhoon Warning Center (JTWC) Best Track dataset (Chu et al., 2002). Because the new model has a finer horizontal resolution (6 km) than that in LC, the wind fields from both model systems are interpolated to the same 9 km horizontal grid to determine TC intensity for evaluation. ...
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An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones (TCs) along the South China coast is upgraded by including ocean and wave models. A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea. Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size. In particular, interaction with the ocean does not necessarily imply a weakening of the TC, with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC. These results suggest that to simulate the evolution of TC structure more accurately, it is essential to use an air-sea coupled model instead of an atmosphere-only model.
... The best track data set provided by the Joint Typhoon Warning Center from 1979 to 2017 at a six-hour interval over the WNP basin is considered for the analysis (Chu et al., 2002). Only those systems that are categorized as a tropical storm or higher category during June-September are considered for the analysis. ...
Article
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Plain Language Summary The monsoon low‐pressure systems (LPSs) are cyclonic vortices of diameter 1,000–2,000 km that are predominantly present over the Bay of Bengal (BoB) during the summer monsoon. These systems are responsible for more than half of the summer monsoon rainfall over the highly populated central India and Gangetic plains. The genesis mechanisms of monsoon LPSs are not fully understood but can be broadly classified into two types ‐ local processes (in situ) and remote forcing by the westward propagating atmospheric disturbances from the Pacific (downstream amplification). In this study, we show that the Rossby waves from four clusters of Western North Pacific (WNP) tropical cyclones might be responsible for triggering most of the downstream genesis of the synoptic‐scale storms over BoB during the summer monsoon season. Our results suggest that the downstream storm development over the BoB can be reliably predicted, possibly using deep learning models, by considering predictors from WNP.
... Maximum 10-m wind speed was also added to the tracking where 10-m wind speed maxima should reach at least 17.5 m s −1 , following the tropical storm intensity threshold value for 1-min maximum sustained wind speed (Manganello et al. 2012). Best track data from Joint Typhoon Warning Center (JTWC; Chu et al. 2002) were used to validate model results. TC intensities were classified using the Saffir-Simpson hurricane wind scale (SSHWS; Simpson and Riehl 1981) where it is defined based on the maximum sustained wind Analyses were focused on the impact of PBL schemes on the intensity and structure of TCs. ...
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The influence of planetary boundary layer (PBL) parameterization on tropical cyclones (TCs) over the Philippine region is examined using the Holtslag and the University of Washington (UW) schemes in RegCM4.7 at 25-km resolution. Comparisons made between the model-simulated TCs and best track data indicate that more TCs were reproduced in both the RegCM4.7 simulations than the ERA-Interim reanalysis. It is further revealed that while only small biases were obtained in the number of TCs detected from the two RegCM4.7 simulations, only the UW scheme was able to simulate strong (Category 4–Category 5) TCs. A composite analysis on the radial cross section of azimuthally averaged wind fields shows that the UW scheme generates stronger wind velocities with narrower and elevated maximum tangential wind, and enhances low-level momentum convergence, compared with the Holtslag scheme. In addition, the radial positioning of the strong diabatic heating of UW simulation within the radius of maximum wind supported the needed conditions for warmer core formation and contributed to the enhancement of the secondary circulation by vertical advection of high absolute angular momentum, hence the higher intensity of TCs. The resulting higher surface maximum winds with increased diabatic heating within the eyewall and stronger warm-core structures in the UW simulation suggest that the induced convergence has offset the dissipative effects of momentum loss more than that in the Holtslag run.
... After comparing the datasets, the JTWC dataset was then selected as the only data source to further examine seasonal, interannual variations of TC landfalling and post-landfall behaviors in Southeast Asia by region, sub region and country due to the following reasons: (a) the JTWC area of responsibility has included the Bay of Bengal since 1971 (Chu et al., 2002) and so this dataset observes TCs further to the west and south of the East Asia mainland where most Southeast Asia countries are located ( Figure 1b). JTWC also tracks TCs from the early tropical depression stage resulting in the largest number of data points (Table 1), (b) TOKYO does not track TCs moving west of the WNP west border (100°E) and only record TCs after they reach TS intensity ( Figure 1d) while HKO rarely tracks TCs west of 100°E longitude ( Figure 1e). ...
Article
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In this study, the landfalling tropical cyclone (TC) exposure in Southeast Asia for a 50‐year period from 1970 to 2019 is investigated relative to the total western North Pacific (WNP) climatology taking disparities in historical records into account. Long‐term trends in landfalling TCs are analyzed and intercompared among the Regional Specialized Meteorological Center of Tokyo (TOKYO), China Meteorological Administration (CMA), Hong Kong Observatory (HKO), and Joint Typhoon Warning Center (JTWC) best track datasets. Interannual and intra‐seasonal variations are further examined by sub‐region and nation using JTWC records. Approximately half of the WNP TCs make landfall in Southeast Asia representing over 75% of the total WNP landfalls in all datasets. Over the study period, there is a slight upward trend in landfalling TC frequency in both the WNP and Southeast Asia in the JTWC dataset, while the number of landfall events has decreased in the CMA and TOKYO datasets. A consistent northward shift in landfalling locations over the 50‐year period is found in all datasets such that landfalls have decreased in the Philippines, Vietnam, but increased in some South China areas. The TOKYO dataset alone suggests that landfalling TCs in South China have slowed down over the study period, which would increase rainfall and wind risks in their path if substantiated. Less TC landfalls occur in El Niño years with landfalling locations shifting northwestward over the Asian mainland, while landfalls are higher and more distributed in La Niña, and highest in Neutral years.
... The maximum winds provided by IBTrACS is time-averaged values, where the period of the average is different depending on the agencies which furnished their best-track datasets. Thus, we use only the datasets provided by the National Hurricane Center (North Atlantic and eastern North Pacific; Jarvinen et al. 1984) and the Joint Typhoon Warning Center (western North Pacific, north Indian Ocean and Southern Hemisphere; Chu et al. 2002) among those provided by IBTrACS, because both centers use 1 min for the period to average the maximum winds. ...
Article
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Nonhydrostatic Icosahedral Atmospheric Model (NICAM) coupled with a slab ocean model was applied to a paleoclimate research for the first time. The model was run at a horizontal resolution of 56 km with and without a convective parameterization, given the orbital parameters of the last interglacial (127,000 years before present). The simulated climatological mean-states are qualitatively similar to those in previous studies reinforcing their robustness, however, the resolution of this model enables to represent the narrow precipitation band along the southern edge of the Tibetan Plateau. A particular focus was given to convectively coupled disturbances in our analysis. The simulated results show a greater signal of the Madden–Julian Oscillation and weakening of the moist Kelvin waves. Although the model's representation of the boreal summer intraseasonal oscillation in the present-day simulations is not satisfactory, a significant enhancement of its signal is found in the counterpart of the last interglacial. The density of the tropical cyclones decreases over the western north Pacific, north Atlantic and increases over the south Indian Ocean and south Atlantic. The model's performance is generally better when the convective parameterization is used, but the tropical cyclones are better represented without the convective parameterization. Additional simulations using the low-resolution topography reveals that the better representation of the Tibetan Plateau enhances the boreal summer Asian monsoon and its impact is similar and comparable to that of the orbital parameters over the south Asia and the Indian Ocean.
... This study uses the South Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) database for best track TC data that are mostly available at 6-hour intervals for the entire TC lifecycle (Diamond et al., 2012). The SPEArTC database is built using several existing TC databases including the International Best Tracks for Climate Stewardship (IBTrACS; Knapp et al., 2010), Joint Typhoon Warning Centre (JTWC; Chu et al., 2002), Southern Hemisphere Tropical Cyclone Data Portal (Australian Bureau of Meteorology, 2018), Regional Specialised Meteorological Centre Nadi, Tropical Cyclone Warning Centre Wellington, TC tracking maps from Fiji, New Caledonia, New Zealand, Tonga, Solomon Islands and Vanuatu, and old historical records such as Visher (1922) (Diamond et al., 2012;Magee et al., 2016). ...
Article
Extreme rainfall events often lead to excessive river flows and severe flooding for Pacific Island nations. Fiji, in particular, is often exposed to extreme rainfall events and associated flooding, with significant impacts on properties, infrastructure, agriculture, and the tourism sector. While these occurrences are often associated with tropical cyclones (TCs), the specific characteristics of TCs that produce extreme rainfall are not well understood. In particular, TC intensity does not appear to be a useful guide in predicting rainfall, since weaker TCs are capable of producing large rainfall compared to more intense systems. Therefore, other TC characteristics, in particular TC track morphology and background climate conditions, may provide more useful insights into what drives TC related extreme rainfall. This study aimed to address this problem by developing a decision tree to identify the most important predictors of TC related extreme rainfall (i.e., 95th percentile) for Fiji. TC attributes considered include; TC duration, the average moving speed of TCs, the minimum distance of TCs from land, seasonality, intensity (wind speed) and the geometry of TCs (i.e., geographical location, shape and length via cluster and sinuosity analyses of TC tracks). In addition, potential predictors based on the phases of Indo-Pacific climate modes were input to the decision tree to represent large scale background conditions. It was found that a TC's minimum distance from land was the most important influence on extreme rainfall, followed by TC cluster grouping, seasonality and duration. The application of this model could result in improved TC risk evaluations and could be used by forecasters and decision-makers on mitigating the TC impacts over the Fiji Islands.
... We obtained six-hourly TC data during 1982-2018 from the Joint Typhoon Warning Center (accessible at https:// www.metoc.navy.mil/jtwc/jtwc.html?western-pacific), including the TC center location (latitude and longitude) and maximum sustained wind speed (Chu et al., 2002). We also used the PDO index data set from the University of Washington's Joint Institute for the Study of the Atmosphere and Ocean (available at http://research.jisao. ...
Article
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Plain Language Summary Tropical cyclones (TCs) are destructive natural hazards in Far East Asia due to strong winds, storm surges, and heavy precipitation, which could lead to considerable casualties and property damage. The relationship between TCs and Pacific Decadal Oscillation (PDO) was examined over the western North Pacific (WNP) during 1982–2018. We focused on an analysis area from 30–45°N and 120–150°E, which included the Korean Peninsula and Japan. The frequency of TCs passing through the analysis area (TCm) and PDO had a negative correlation in September. In the negative PDO phase, large‐scale environments, including sea‐surface temperature, vertical wind shear, and steering winds, were favorable for TC activity in the mid‐latitudes. Large‐scale environments in the positive phase showed the opposite trend. However, TCm and PDO were not related in August because the difference in WNP subtropical high was insignificant. Furthermore, the anomalies of large‐scale environments (e.g., vertical wind shear and steering wind) over the mid‐latitudes were not significant in August compared to those in September.
... The study period extends between 1987 and 2020. TC data pre-1987 was not considered due to a lack of in situ aircraft reconnaissance before this time 56 , resulting in inconsistent intensity estimates. This analysis does not account for interbasin differences in defining the intensity of TY and STY events and uses the definitions are outlined above. ...
Article
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With an average of 26 tropical cyclones (TCs) per year, the western North Pacific (WNP) is the most active TC basin in the world. Considerable exposure lies in the coastal regions of the WNP, which extends from Japan in the north to the Philippines in the south, amplifying TC related impacts, including loss of life and damage to property, infrastructure and environment. This study presents a new location-specific typhoon (TY) and super typhoon (STY) outlook for the WNP basin and subregions, including China, Hong Kong, Japan, Korea, Philippines, Thailand, and Vietnam. Using multivariate Poisson regression and considering up to nine modes of ocean-atmospheric variability and teleconnection patterns that influence WNP TC behaviour, thousands of possible predictor model combinations are compared using an automated variable selection procedure. For each location, skillful TY and STY outlooks are generated up to 6 months before the start of the typhoon season, with rolling monthly updates enabling refinement of predicted TY and STY frequency. This unparalleled lead time allows end-users to make more informed decisions before and during the typhoon season.
... tressed an already ailing sugar industry and resulted in massive production losses which ultimately caused exports to fall by over 25% (Brown, 1989;Fraenkel, 2001;Premdas, 1993). This was exacerbated by the predominant land tenure and lease issues. A further reason could be the Increased periods of extreme weather conditions (Vincent et. al., 2011;Chu et. al. 2002). Particularly, the long droughts in the 1981-1983 cane growing season. Also, both periods were also marked by dramatic increases in the frequency of tropical storms with the 1982 season being listed as one of the most active in the Pacific region (Hastings, 1990;Vincent et. al., 2011). ...
Research
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In this study, we examine the effect of agriculture on economic growth in Fiji over the period 1975 to 2015 which amounts to 41 annual observations. Agricultural output has been consistently declining overtime and the current study would provide new evidence to boost support for the sector. We apply the autoregressive distributed lags bounds approach to estimate the long run and short run parameters, identify structural breaks using the Bai-Perron break test, and examine causality using Toda and Yamamoto's augmented VAR model. Real GDP per capita is regressed on per-capita agricultural output, with the per-capita capital stock and per-capita visitor arrivals as control variables. Our results indicate that in the long run, the contribution of agriculture exceeds that of tourism whereas in the short run, the opposite is observed. We find a unidirectional causality from tourism and agriculture to output. The findings have significant implications for agricultural and developmental policy in Fiji.
... TC wind radii have been estimated with varying degrees of accuracy for decades (see Chu et al., 2002;Landsea & Franklin, 2013;Sampson et al., 2017). Such information is used in practical applications such as wind speed probability forecasts, wave and storm surge forecasting, and evacuation planning Powell & Reinhold, 2007;Sampson et al., 2010Sampson et al., , 2012. ...
Article
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The sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance. Existing studies have explored important influences on SST of TC translation speed, maximum surface winds, ocean thermal condition, and ocean stratification. But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked. In this study, we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014–2018 seasons. Results show a 6%–20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35%–40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model. Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds (R64) due to its effects on the integrated kinetic energy of the TC and subsequent mixing of the ocean surface layer. It is also found that the correlation between SST cooling induced by the TC and its size is 0.49, which is the highest among all the parameters tested. This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.
... There are three different datasets, which maintain the TCs information over the AS, namely, Indian Meteorological Department (IMD 2017), Joint Typhoon Warning Center (JTWC) (Chu et al. 2002), and International Best Track Archive for Climate Stewardship (IBTrACS) (Knapp et al. 2010). IMD provides an extensive data summary of TCs in the NIO from 1891, such as monthly and annual frequency, direction, and speed. ...
Article
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Three recent Tropical Cyclones (TCs) over the Arabian Sea, i.e., the super cyclone Gonu, the low intense cyclone Ashobaa, and cyclone Phet (with an unusual track), are studied using a high-resolution atmosphere-wave model and observations. The reliable performances of both atmospheric and wave models were established using the available field measurements, in which the modeling improvement in comparison with the previous studies is met. The effects of input parameterizations on the wave and wind estimations in the presence of TCs were examined. TCs track and intensity were more sensitive to the microphysics and cumulus schemes. Concerning the multi and single peaked wave spectra, three steps of wave generation, growth, and dissipation were addressed. Bimodal seas and swells exist in both generation and dissipation stages, whereas the seas are dominant in the growth stage. The performance of Westhuysen was better in modeling the combination of seas and swells.
... For comparison, we use the seasonal climatology of the TCF, which is defined as the number of TC tracks (segment of the TC trajectory) that fall within 5° × 5° bins in a season, computed from observations made available by the Joint Typhoon Warning Center (JTWC, Chu et al., 2002; data can be found at https://www. metoc.navy.mil/jtwc/jtwc.html?best-tracks). ...
Article
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The potential predictability determined by sea surface temperature (SST) of the frequency distribution of tropical cyclones (TCs) is studied using ensemble simulations with a 50‐km resolution atmospheric general circulation model (AGCM). In this experiment, the interannual variability of the TC frequency over the western North Pacific (WNP) is primarily determined by SST, as the simulated ensemble mean signal is dominant over the ensemble spread and effectively follows the observed TC frequency in June‐July‐August (JJA). In contrast, the correlation between the observed and simulated TC frequency variability is less significant in September‐October‐November (SON). To explore the TC frequency‐SST linkage, the relationship between the TC frequency variability over the WNP and climate modes determined by SST indices is investigated. Through a correlation analysis, we found that El Niño Modoki is the major driver of the TC frequency variability over the WNP in JJA. Partial correlation analysis further reveals that internal atmospheric dynamics are important for the TC frequency variability over the WNP in SON. These results suggest that the tropical climate mode is responsible for the seasonal predictability of the TC frequency variability over the WNP and that the prediction skill is seasonally dependent with a high (low) skill in JJA (SON).
... In our analysis, we choose TCs of which a 1-min maximum sustained wind speed exceeds 17.5 ms −1 in the IBTrACS. The wind speed was based on data reported by the national Hurricane Center (Jarvinen et al. 1984) and the Joint Typhoon Warning Center (Chu et al. 2002). We used the IBTrACS between 1980 and 2014. ...
Article
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Previous projections of the frequency of tropical cyclone genesis due to global warming, even in terms of sign of the change, depends on the chosen model simulation. Here, we systematically examine projected changes in tropical cyclones using six global atmospheric models with medium-to-high horizontal resolutions included in the sixth phase of the Coupled Model Intercomparison Project/High-Resolution Model Intercomparison Project. Changes in the frequency of tropical cyclone genesis could be broken down into the contributions from (i) the tropical cyclone seed, a depression having a closed contour of sea level pressure with a warm core and (ii) the survival rate, the ratio of the frequency of tropical cyclone genesis to that of tropical cyclone seeds. The multi-model ensemble mean indicates that tropical cyclone genesis frequencies are significantly decreased during the period 1990–2049, which is attributable to changes in tropical cyclone seeds. Analysis of the individual models shows that although most models project a more or less decreasing trend in tropical cyclone genesis frequencies and seeds, the survival rate also contributes to the result in some models. The present study indicates the usefulness of decomposition into the frequency of the tropical cyclone seeds and the survival rate to understand the cause of uncertainty in projected frequencies of tropical cyclone genesis.
... Best track data from the Joint Typhoon Warming Center (JTWC) were used to obtain TC information (Chu et al. 2002). TCs in this study were defined as tropical storms whose maximum wind speed reach or exceeds 17.2 m s −1 . ...
Article
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As a semi-permanent circulation system at the upper troposphere, the South Asian high (SAH) profoundly influences the Asian climate. However, its impact on the underlying tropical cyclone (TC) genesis in the South China Sea (SCS) remains unknown. Here we show that the leading mode of eastern-central SAH and TC genesis (TCG) number in the SCS during July–September are significantly correlated with a correlation coefficient of − 0.71 during 1979–2017. During the strong SAH years, the SAH-related upper-level convergence favors descending motion and low-level divergence in the SCS, decreasing mid-level humidity and low-level vorticity and thus suppressing TCG in the SCS. We find that the variations in the leading mode of eastern-central SAH are coupled to the meridional gradient in surface temperature over the western North Pacific. An enhanced meridional surface temperature gradient can increase the meridional gradient in 200 hPa geopotential height and thus intensify the SAH. Meanwhile, the intensified SAH and the related descending motion, in turn, re-enforce the meridional surface temperature gradient through positive SST-cloud-shortwave radiation feedback. The positive feedback between the SAH and the WNP meridional temperature gradients provides persisting large-scale circulation anomalies that influence the TCG in the SCS from July through September. These results highlight the importance of the SAH and its interaction with the WNP meridional temperature gradients for regulating the SCS TCG and suggest that the precursors that are known to be linked to the SAH intensity can be used to the seasonal prediction of TCG in the SCS.
... In our analysis, we choose TCs of which a 1-minute maximum sustained wind speed exceeds 17.5 ms -1 in the IBTrACS. The wind speed was based on data reported by the national Hurricane Center (Jarvinen et al., 1984) and the Joint Typhoon Warning Center (Chu et al., 2002). We used the IBTrACS between 1980 and 2014. ...
Preprint
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Previous projections of the frequency of tropical cyclone genesis due to global warming, even in terms of sign of the change, depends on the chosen model simulation. Here, we systematically examine projected changes in tropical cyclones using six global atmospheric models with medium-to-high horizontal resolutions included in the sixth phase of the Coupled Model Intercomparison Project/High-Resolution Model Intercomparison Project. Changes in the frequency of tropical cyclone genesis could be broken down into the contributions from (i) the tropical cyclone seed, a depression having a closed contour of sea level pressure with a warm core and (ii) the survival rate, the ratio of the frequency of tropical cyclone genesis to that of tropical cyclone seeds. The multi-model ensemble mean indicates that tropical cyclone genesis frequencies are significantly decreased during the period 1990–2049, which is attributable to changes in tropical cyclone seeds. Analysis of the individual models shows that although most models project a more or less decreasing trend in tropical cyclone genesis frequencies and seeds, the survival rate also contributes to the result in some models. The present study indicates the usefulness of decomposition into the frequency of the tropical cyclone seeds and the survival rate to understand the cause of uncertainty in projected frequencies of tropical cyclone genesis.
... Halong, Vongfong, and Nuri are three super typhoons over the Western Pacific, and they are isolated from other typhoons temporally, making it easy to study S wave microseisms from individual storms. Here we use center locations and intensities of typhoons from Joint Typhoon Warning Center best-track data set (Chu et al., 2002) (http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/best_tracks/). The seismic waveform data are from WTA in Northwest China (Figure 1), which consist of 150 three-component broadband (Guralp CMG-3T and CMG-3ESPC) seismometers with sampling rate of 100 Hz. ...
Article
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Plain Language Summary Besides causing property damages and life losses, ocean storms can also generate seismic waves named as microseisms, which could also be used as signals for studying interior of the Earth. Microseisms mostly propagate as surface waves and P waves. Yet S waves from storms are only detected in the North Atlantic Ocean shown by previous studies. However, there are no reports of S wave microseism by storms in the Pacific Ocean, though the typhoons there are strong and frequent. In this study, we observe and locate S wave microseisms generated by three super typhoons in Western Pacific by processing continuous ambient seismic waveforms recorded in China. The source regions of P and S waves are located very close to each other in the deep ocean near typhoons. Our results show that observability of S wave microseisms depends on P wave energy, which is controlled by strength of typhoons. It is demonstrated that S wave microseisms not only can be generated in the regions with steep topography but also can be excited in the flat seafloor regions. This work is helpful for understanding the excitation mechanism of the S waves.
... Another interesting thing is that the correlation coefficient between WNP TC average intensity and the AMO index reaches 0.86 (p < 0.05), further confirming the strong connection between them as is inferred from Figure 2. We can then conclude that the linkage between AMO and WNP TC intensity is robust at decadal time scales, while other TC metrics such as ACE, frequency and TC days show weaker correlations with the AMO. Previous studies have suggested that the TC intensity records may be less reliable before the mid-1970s (Dvorak 1975;Chu et al., 2002;Ackerman et al., 2018). We repeat the above analysis but for a more recent period 1975-2018. ...
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Sea surface temperature (SST) over the western North Pacific (WNP) exhibits strong decadal to multidecadal variability and in this region, warm waters fuel the tropical cyclones (TCs). Observational records show pronounced decadal variations in WNP TC metrics during 1950–2018. Statistical analysis of the various TC metrics suggests that the annual average intensity of WNP TCs is closely linked to the AMO (r = 0.86 at decadal timescales, p < 0.05). Observations and coupled atmosphere-ocean simulations show that the decadal WNP SST variations regarded as the primary driver of TC intensity, are remotely controlled by the AMO. Corresponding to the WNP SST warming, the local SLP gets lower and the tropospheric air becomes warmer and moister, enhancing atmospheric instability and the generation of convective available potential energy. These favorable changes in the background environment provide more “fuel” to the development of deep convection and intensify the WNP TCs. The footprints of AMO in WNP SST and atmospheric states through trans-basin interaction eventually exert a significant impact on the TC intensity over the WNP region.
... western-pacific). The study period of this paper is 1979-2016, since the record of the WNP TCs is more reliable after 1970 with the introduction of more new technologies (Chu et al., 2002). Following the previous studies (e.g., Yu et al., 2016;Gao et al., 2018), only TCs reaching the intensity level of tropical storm or above (i.e., maximum sustained wind speed ≥ 34 kt) were included. ...
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Based on the observational evidences that the tropical cyclones (TCs) number in the western North Pacific (WNP) in autumn is more than that in summer, this study mainly compares the disturbance of some conditions that associated with the formation of TCs between the two seasons. Results show that the most of the distributions of the disturbance field of atmospheric dynamic factor are similar, while the unique feature of the upper level geopotential height/vortex anomaly over east Asia/western Pacific sector varies greatly and may play the important role in generating more TCs in autumn. Further analyses indicate that the high-pressure/anticyclonic anomaly in autumn is originated from the north tropical Atlantic Ocean. When a cold sea surface temperature (SST) anomaly appears in north tropical Atlantic, a Rossby wave train over Eurasia continent could be induced which then leads a favorable condition for TC genesis over east Asia/western Pacific sector at upper troposphere, resulting in more WNP TCs in autumn.
... For instance, Fraedrich et al. (2003) predicted cyclone tracks over the Australian basin by using an analog ensemble forecast model and historical storm tracks (Fraedrich et al. 2003). The Joint Typhoon Warning Center (JTWC) used climatology mean forecasting method to produce storm tracks through un-weighted averaging of previous storm motions over the Western North Pacific (Chu et al. 2002). Climatology and persistence (CLIPER) method was developed by Neumann and Hope (1972) which used current storm locations, storm motions, maximum sustained wind speeds, and prior storm motion records as predictors to derive a linear regression equation to produce storm tracks for up to 3 days (Neumann and Hope 1972). ...
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An ensemble machine learning model for tropical cyclone (TC) track forecasts over the Western North Pacific was developed and evaluated in this study. First, we investigated predictors including TC climatology and persistence factors which were extracted from TC best-track dataset and storm’s surrounding atmospheric conditions which were extracted from ERA-Interim reanalysis. Then, we built a Gradient Boosting Decision Tree (GBDT) nonlinear model for TC track forecasts, in which 30-year data was used. Finally, using tenfold cross-validation method, the GBDT model was compared with a frequently used technique: climatology and persistence (CLIPER) model. The experimental results show that the GBDT model performs well in three forecast times (24 h, 48 h, and 72 h) with relatively small forecast error of 138, 264, and 363.5 km, respectively. The model obtains excellent TC moving direction aspects. However, the model is still insufficient to produce aspects of storm acceleration and deceleration, with mean moving velocity sensitivities all less than 60%. Nevertheless, the model obtains much more robust and accurate TC tracks relative to CLIPER model, where the forecast skills are 17.5%, 26.3%, and 32.1% at three forecast times, respectively. The presented study demonstrates that the GBDT model could provide reliable evidence and guidance for operational TC track forecasts.
... In our analysis, we choose TCs of which a 1-minute maximum sustained wind speed exceeds 17.5 ms -1 in the IBTrACS. The wind speed was based on data reported by the national Hurricane Center (Jarvinen et al., 1984) and the Joint Typhoon Warning Center (Chu et al., 2002). We used the IBTrACS between 1980 and 2014. ...
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Previous projections of the frequency of tropical cyclone genesis due to global warming, even in terms of sign of the change, depends on the chosen model simulation. Here, we systematically examine projected changes in tropical cyclones using six global atmospheric models with medium-to-high horizontal resolutions included in the sixth phase of the Coupled Model Intercomparison Project/High-Resolution Model Intercomparison Project. Changes in the frequency of tropical cyclone genesis could be broken down into the contributions from (i) the tropical cyclone seed, a depression having a closed contour of sea level pressure with a warm core and (ii) the survival rate, the ratio of the frequency of tropical cyclone genesis to that of tropical cyclone seeds. The multi-model ensemble mean indicates that tropical cyclone genesis frequencies are significantly decreased during the period 1990–2049, which is attributable to changes in tropical cyclone seeds. Analysis of the individual models shows that although most models project a more or less decreasing trend in tropical cyclone genesis frequencies and seeds, the survival rate also contributes to the result in some models. The present study indicates the usefulness of decomposition into the frequency of the tropical cyclone seeds and the survival rate to understand the cause of uncertainty in projected frequencies of tropical cyclone genesis.
... The TC data from 1979 to 2012 is obtained from the besttrack dataset of the Joint Typhoon Warning Center (JTWC) (Chu et al. 2002). This data includes TC latitude and longitude, maximum sustained surface winds and minimum sea level pressure (since 2001 for most TCs) at a 6-h interval. ...
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This study examines extended boreal summer (May–October) tropical cyclogenesis events (TCGEs) associated with large-scale flow patterns (LFPs) over the western North Pacific (WNP) between 1979–1997 and 1998–2012. WNP TCGEs are objectively identified to be associated with five LFPs [e.g., monsoon shear line (SL), monsoon confluence region (CR), monsoon gyre (GY), Pacific easterly wave (EW) and preexisting tropical cyclone (PTC)]. Results show that an apparent decrease in TCGEs from 1998–2012 was due to the significant decrease in TCGEs associated with the PTC pattern and to a somewhat lesser degree, TCGEs associated with the GY pattern. In contrast, TCGEs associated with the SL pattern show a small increase, which seems to contradict the weakened monsoon circulation since 1998 but corresponds well to cyclonic anomalies over the Philippines region. Decreased TCGEs associated with the GY pattern and increased TCGEs associated with the EW pattern are closely related to the strengthening of Pacific easterly waves in response to the Mega La Niña-like pattern that predominated during 1998–2012. Weakened easterly shear over the eastern WNP is not conducive to the development and propagation into the southeastern WNP of Rossby wave trains induced by preexisting TC energy dispersion. Consequently, there is a significant reduction of TCGEs associated with the PTC pattern and a weakening in the contribution of TCGEs associated with the PTC pattern to TCGEs associated with the EW pattern. An increased correlation between TCs associated with the SL/GY/EW patterns and central Pacific (CP)-type ENSO during 1998–2012 is observed. A stable and robust association between TCGEs associated with the CR pattern and tropical North Atlantic sea surface temperature is observed regardless of decadal climate regime shifts. However, there is no significant link between TCGEs associated with the PTC pattern and more CP ENSO events during 1998–2012, but there is a strong association between the Pacific meridional mode and TCGEs associated with the PTC pattern during 1979–1997. More observational analyses and numerical simulations are needed to further investigate the underlying physical mechanism.
... In this research, we would like to explore this recent trend of increasing Uh over the WNP and neighboring seas, the associated TC-ocean interaction, and the possible impact on TC intensity. The reason for focusing on this most recent period (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) is also due to the data quality, since the data quality from the earlier period is not as reliable [19][20][21][22]. Also, 1998 is a meaningful year in the WNP, as PDO (Pacific decadal oscillation) also changed from the warm phase to the cold phase [23][24][25]. ...
Article
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Tropical cyclone (TC) translation speed is an important parameter. In the context of TC-ocean interaction, faster translation speed can contribute to less TC-induced ocean cooling and thus enables more air-sea enthalpy flux supply to favor TC intensification. In 2018, Kossin published an interesting paper in Nature, reporting a global slowdown of TC translation speed since the 1950s. However, upon close inspection, in the last two decades, TC translation speed actually increased over the western North Pacific (WNP) and neighboring seas. Thus, we are interested to see which sub-region in the WNP and neighboring seas had the largest increase during the last two decades, and whether such increases contribute to TC intensification. Our results found statistically significant translation speed increases (~0.8 ms −1 per decade) over the South China Sea. Ruling out other possible factors that may influence TC intensity (i.e., changes in atmospheric vertical wind shear, pre-TC sea surface temperature or subsurface thermal condition), we suggest, in this research, the possible contribution of TC translation speed increases to the observed TC intensity increases over the South China Sea in the last two decades (1998-2017).
... Three TCs, namely Hato, Pakhar, and Mawar, influenced our wave glider in the summer of 2017 successively ( Figure 1a). The TC best-track datasets are from the Joint Typhoon Warning Center (JTWC [48]; www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/best_tracks), the China Meteorological Administration (CMA; http://tcdata.typhoon.org.cn/zjljsjj_zlhq.html; [49]), and the Japan Meteorological Agency (JMA; http://www.jma.go.jp/jma/jma-eng/jma-center/rsmc-hp-pubeg/besttrack.html; ...
Article
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Surface waves induced by tropical cyclones (TCs) play an important role in the air–sea interaction, yet are seldom observed. In the 2017 summer, a wave glider in the northern South China Sea successfully acquired the surface wave parameters when three TCs (Hato, Pakhar, and Mawar) passed though successively. During the three TCs, surface wave period increased from 4–6 s to ~8–10 s and surface wave height increased from 0–1 m to 3–8 m. The number of wave crests observed in a time interval of 1024 s decreased from 100–150 to 60–75. The sea surface roughness, a key factor in determining the momentum transfer between air and sea, increased rapidly during Hato, Pakhar, and Mawar. Surface waves rotated clockwise (anti-clockwise) on the right (left) side of the TC track, and generally propagated to the right side of the local cyclonic tangential direction relative to the TC center. The azimuthal dependence of the wave propagation direction is close to sinusoidal in a region within 50–600 km. The intersection angle between surface wave direction and the local cyclonic tangential direction is generally smallest in the right-rear quadrant of the TC and tends to be largest in the left-rear quadrant. This new set of glider wave observational data proves to be useful for assessing wave forecast products and for improvements in corresponding parameterization schemes.
... In this research, we would like to explore this recent trend of increasing Uh over the WNP and neighboring seas, the associated TCocean interaction, and the possible impact on TC intensity. The reason for focusing on this most recent period (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) is also due to the data quality, since the data quality from the earlier period is not as reliable [19][20][21][22]. Also, 1998 is a meaningful year in the WNP, as PDO (Pacific decadal oscillation) also changed from the warm phase to the cold phase [23][24][25]. ...
Article
Full-text available
Tropical cyclone (TC) translation speed is an important parameter. In the context of TC–ocean interaction, faster translation speed can contribute to less TC-induced ocean cooling and thus enables more air–sea enthalpy flux supply to favor TC intensification. In 2018, Kossin published an interesting paper in Nature, reporting a global slow-down of TC translation speed since the 1950s. However, upon close inspection, in the last two decades, TC translation speed actually increased over the western North Pacific (WNP) and neighboring seas. Thus, we are interested to see which sub-region in the WNP and neighboring seas had the largest increase during the last two decades, and whether such increases contribute to TC intensification. Our results found statistically significant translation speed increases (~ 0.8 ms-1 per decade) over the South China Sea. Ruling out other possible factors that may influence TC intensity (i.e., changes in atmospheric vertical wind shear, pre-TC sea surface temperature or subsurface thermal condition), we suggest, in this research, the possible contribution of TC translation speed increases to the observed TC intensity increases over the South China Sea in the last two decades (1998–2017).
... In this study, the best track dataset from the Joint Typhoon Warning Center (available at https://www.usno.navy.mil/ NOOC/nmfc-ph/RSS/jtwc/best_tracks/wpindex.php) was used to analyze the trends in TCs over the WNP (Chu et al., 2002). To analyze track density, all categories of TCs (i.e., tropical depression, tropical storm, severe tropical storm, and typhoon) were included, and the genesis of a TC was defined from the moment that a TC changed from a tropical depression to a tropical storm. ...
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This study investigates long‐term trends in tropical cyclones (TCs) over the extratropical western North Pacific (WNP) over a period of 35 years (1982–2016). The area analyzed extended across 30–45°N and 120–150°E, including the regions of Korea and Japan that were seriously affected by TCs. The northward migration of TCs over the WNP to the mid‐latitudes showed a sharp increase in early fall. In addition, the duration of TCs over the WNP that migrated northwards showed an increase, specifically in early to mid‐September. Therefore, more recently, TC tracks have been observed to significantly extend into the mid‐latitudes. The recent northward extension of TC tracks over the WNP in early fall was observed to be associated with changes in environmental conditions that were favorable for TC activities, including an increase in sea surface temperature (SST), decrease in vertical wind shear, expansion of subtropical highs, strong easterly steering winds, and an increase in relative vorticity. In contrast, northward migrations of TCs to Korea and Japan showed a decline in late August, because of the presence of unfavorable environmental conditions for TC activities. These changes in environmental conditions, such as SST and vertical wind shear, can be partially associated with the Pacific decadal oscillation. The northward migrations of tropical cyclones over the western North Pacific to the mid‐latitudes significantly increased in early fall. The changes in environmental conditions can be related to increasing migration of tropical cyclones to the mid‐latitudes in early to mid‐September.
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This study assesses the relative impacts of model resolutions, tropical cyclone (TC) trackers, and ocean coupling on simulating TC climatology over the western North Pacific (WNP) based on six Coupled Model Intercomparison Project phase 6 (CMIP6) High-Resolution Model Intercomparison Project (HighResMIP) models from 1979 to 2014. The HighResMIP multimodel ensemble (MME) analysis shows that the high resolution has a higher Taylor skill score II (S 2) in both temporal and spatial patterns of TC genesis frequency and accumulated cyclone energy (ACE) than the low resolution. In contrast, the TempestExtremes tracker (coupled run) results in a higher S 2 in temporal patterns but a lower S 2 in spatial patterns than the TRACK tracker (uncoupled run). Among the three factors, increased resolution leads to the greatest improvement in S 2 in both temporal and spatial patterns. Furthermore, this study investigates the projections of future TC activity over the WNP by HighResMIP under the SSP5-8.5 scenario. Overall, HighResMIP MMEs project a decrease in the genesis frequency, track density, and ACE of all TCs, with the high-resolution, TRACK tracker, and un-coupled run showing greater magnitude. The high-resolution MMEs, using both trackers, project an increase in the genesis frequency and ACE of intense TCs in the coupled run. Moreover, TC track density and ACE show a larger poleward migration in the coupled run than in the uncoupled run, consistent with the significant surface warming in the northern WNP.
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The variability and predictability of tropical cyclone genesis frequency (TCGF) during 1973–2010 at both basinwide and sub-basin scales in the northwest Pacific are investigated using a 100-member ensemble of 60-km-resolution atmospheric simulations that are forced with observed sea surface temperatures (SSTs). The sub-basin regions include the South China Sea (SCS) and the four quadrants of the open ocean. The ensemble-mean results well reproduce the observed interannual-to-decadal variability of TCGF in the southeast (SE), northeast (NE), and northwest (NW) quadrants, but show limited skill in the SCS and the southwest (SW) quadrant. The skill in the SE and NE quadrants is responsible for the model’s ability to replicate the observed variability in basinwide TCGF. Above-normal TCGF is tied to enhanced relative SST (i.e., local SST minus tropical-mean SST) either locally or to the southeast of the corresponding regions in both the observations and ensemble mean for the SE, NE, and NW quadrants, but only in the ensemble mean for the SCS and the SW quadrant. These results demonstrate the strong SST control of TCGF in the SE, NE, and NW quadrants; both empirical and theoretical analyses suggest that ensembles of ∼10, 20, 35, and 15 members can capture the SST-forced TCGF variability in these three sub-basin regions and the entire basin, respectively. In the SW quadrant and the SCS, TCGF contains excessive noise, particularly in the observations, and thus shows low predictability. The variability and predictability of the large-scale atmospheric environment and synoptic-scale disturbances and their contributions to those of TCGF are also discussed.
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This study investigates the climate drivers of western North Pacific (WNP) tropical cyclone season onset date (TCSOD). We find that the inter‐annual relationship between WNP TCSOD and ENSO varies considerably on inter‐decadal time scales. The association between TCSOD and ENSO, as measured by the Niño‐3.4 index, is significant during the first sub‐period from 1979–1998 (r=0.51) but becomes insignificant (r=0.01) during the second sub‐period from 1999–2019. This relationship degradation is mainly due to shifting ENSO conditions with more frequent occurrences of tropical central Pacific ENSO events since the late 1990s. Tropical eastern Pacific ENSO events cause changes in the low‐level large‐scale circulation over the tropical western WNP, where early season WNP TC genesis typically occurs, via a Gill‐type Rossby wave response. Tropical central Pacific ENSO events have a limited impact over this region. These relationships are confirmed by a stable and strong association between tropical eastern Pacific ENSO and TCSOD for the whole period (r=0.63) and for the two sub‐periods (r=0.72/r=0.56), while the correlation between central Pacific ENSO events and TCSOD is weak for the whole period (r=‐0.01) and for the two sub‐periods (r=‐0.03/r=‐0.13). We further find that the Pacific meridional mode is a robust indicator of TCSOD over the WNP regardless of shifting ENSO conditions. Results of this study may provide useful information for decadal climate prediction of WNP TCSOD. This article is protected by copyright. All rights reserved.
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This paper reviews the history of tropical cyclone research from the late nineteenth century until the present with an emphasis on observational studies. The topics include tropical cyclone theory, climatology, structure, genesis indices and intraseasonal-to-centennial variability and trends. Possible future directions are also suggested.
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The understanding and prediction of tropical cyclones (TCs) in a changing climate have been active research topics. Over the past few decades, studies have found important relationships between TC activity and climate variability. However, the temporal heterogeneity of TC best track data due to changes in observational and analytical technology makes long-term analysis of the relationship difficult. In particular, it was difficult to quantify when TC data were reliable, which led to the use of different start years for the analysis and eventually hindered a consistent study. This study developed a new method to quantitatively evaluate the reliability of the data, specifically TC genesis frequency in the western North Pacific (WNP), using the relationship between TCs and the El Niño Southern Oscillation (ENSO). This is based on the fact that TC genesis extends east of 140°E in the WNP during El Niño but is limited to within the west of 140°E during La Niña. Using the relationship, this study proposes that the period with the most reliable TC data began in 1985. The analysis revealed that the unreliability during earlier periods was related primarily to the technical difficulties in detecting and classifying weak TCs, such as tropical depressions and tropical storms. These results will reduce the uncertainty in the TC data and will facilitate the detection of a more solid relationship between TCs and climate variability in future studies.
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There is a distinct gap between tropical cyclone (TC) prediction skill and the societal demand for accurate predictions, especially in the western Pacific (WP) and North Atlantic (NA) basins, where densely populated areas are frequently affected by intense TC events. In this study, seasonal prediction skill for TC activity in the WP and NA of the fully coupled FGOALS-f2 V1.0 dynamical prediction system is evaluated. In total, 36 years of monthly hindcasts from 1981 to 2016 were completed with 24 ensemble members. The FGOALS-f2 V1.0 system has been used for real-time predictions since June 2017 with 35 ensemble members, and has been operationally used in the two operational prediction centers of China. Our evaluation indicates that FGOALS-f2 V1.0 can reasonably reproduce the density of TC genesis locations and tracks in the WP and NA. The model shows significant skill in terms of the TC number correlation in the WP (0.60) and the NA (0.61) from 1981 to 2015; however, the model underestimates accumulated cyclone energy. When the number of ensemble members was increased from 2 to 24, the correlation coefficients clearly increased (from 0.21 to 0.60 in the WP, and from 0.18 to 0.61 in the NA). FGOALS-f2 V1.0 also successfully reproduces the genesis potential index pattern and the relationship between El Niño–Southern Oscillation and TC activity, which is one of the dominant contributors to TC seasonal prediction skill. However, the biases in large-scale factors are barriers to the improvement of the seasonal prediction skill, e.g., larger wind shear, higher relative humidity, and weaker potential intensity of TCs. For real-time predictions in the WP, FGOALS-f2 V1.0 demonstrates a skillful prediction for track density in terms of landfalling TCs, and the model successfully forecasts the correct sign of seasonal anomalies of landfalling TCs for various regions in China.
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This study quantifies the contributions of tropical sea surface temperature (SST) variations during the boreal warm season to the interannual-to-decadal variability in tropical cyclone genesis frequency (TCGF) over the Northern Hemisphere ocean basins. The first seven leading modes of tropical SST variability are found to affect basin-wide TCGF in one or more basins, and are related to canonical El Niño–Southern Oscillation (ENSO), global warming (GW), the Pacific Meridional Mode (PMM), Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO) and Atlantic Meridional Mode (AMM). These modes account for approximately 58%, 50% and 56% of the variance in basin-wide TCGF during 1969–2018 over the North Atlantic (NA), Northeast Pacific (NEP) and Northwest Pacific (NWP), respectively. The SST effect is weak on TCGF variability in the North Indian Ocean. The dominant SST modes differ among the basins: ENSO, the AMO, AMM and GW for the NA; ENSO and the AMO for the NEP; and the PMM, interannual AMO and GW for the NWP. A specific mode may have opposite effects on TCGF in different basins, particularly between the NA and NEP. Sliding-window multiple linear regression analyses show that the SST effects on basin-wide TCGF are stable in time in the NA and NWP, but strengthen after the mid-1970s in the NEP. The SST effects on local TC genesis and occurrence frequency are also explored, and the underlying physical mechanisms are examined by diagnosing a genesis potential index and its components.
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A recurrent neural network model for predicting the trajectories and intensities of tropical cyclones (TCs) in the northwestern Pacific Ocean basin is described in the present study. By training an echo state network with a meteorological dataset, the recurrent neural network model, named Reservoir Computing for Tropical Cyclone Prediction (RCTCP) was developed to predict four attributes of TCs, i.e., latitude, longitude, maximum sustained wind speed, and minimum sea level pressure. To comprise the dataset, best track data of the TCs occurred in the basin from 1945 to 2017 were obtained from the U.S. Joint Typhoon Warning Center. The CERA-20C and ERA-Interim reanalysis datasets were used along with dynamic time warping to compensate for the unreported pressure information in the best track data. The data for each TC were then attached end-to-end based on the order of occurrence to form a single sequential dataset. The dataset was interspersed with artificial dummy data to strengthen its regularity. After training, the model was able to yield every six hours the likely position and intensity of a TC 6 to 24 h into the future. Validation of the model’s 6-h forecasting produced mean absolute errors for distance, wind speed, and pressure of 32.73 km, 3.84 kn, and 3.12 mbar, respectively. The accuracy of the 6 to 24-h forecasts by the model was comparable or better than climatology and persistence (CLIPER), statistical typhoon intensity forecast (STIFOR), and existing AI-based approaches. The RCTCP thus represents an attractive option to aid in prompt decision making in relevant fields due to its computational efficiency.
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Tropical cyclones do not form easily near the equator but can intensify rapidly, leaving little time for preparation. We investigated the number of near-equatorial (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon seasons (October to December) over the past 60 years. A marked 43% decline in the number of such cyclones was detected in recent decades (1981-2010) compared to earlier (1951-1980). This decline in tropical cyclone frequency is primarily due to the weakened low-level vorticity modulated by the Pacific Decadal Oscillation (PDO). In the presence of basin-wide warming at low latitudes, and a favorable phase of the PDO, both the intensity and frequency of such cyclones is expected to increase. Such dramatic and unique changes in tropical cyclonic activity due to the interplay between natural variability and climate change call for appropriate planning and mitigation strategies.
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This study analyzes decadal modulation of trans-basin variability (TBV) on extended boreal summer (May-October) tropical cyclone frequency (TCF) over the western North Pacific (WNP), central-eastern North Pacific (CENP) and North Atlantic (NATL) basins. There are distinct decadal regimes (P1:1979-1997, P2:1998-2008, and P3:2009-2019) with changes in the interannual relationship between TBV and TCF over these three basins. During P1 and P3, there is a significant inter-annual TBV-TCF relationship over the CENP and NATL, but these relationships become insignificant during P2. Changes in the interannual TBV-TCF relationship over the WNP are opposite to those over the CENP and NATL basins, with significant relationship during P2 but insignificant relationship during P1 and P3. Changes in all three basins coincide with decadal changes in large-scale parameters associated with TBV. Consistent basin-wide changes in lower-tropospheric vorticity (vertical wind shear) associated with TBV appear to be largely responsible for changes in total TCF over the NATL (CENP) during P1 and P3. In contrast, a dipole pattern in lower-tropospheric vorticity and vertical wind shear anomalies associated with TBV over the NATL and CENP basins occurs during P2, leading to an insignificant interannual TBV-TCF relationship over the NATL and CENP basins. Over the WNP, a basin-wide consistent distribution of lower-tropospheric vorticity associated with TBV is consistent with changes in total TCF during P2, while a dipole correlation pattern between TBV-associated factors and TCF during P1 and P3 leads to a weak correlation between TBV and WNP TCF. These three distinct observed decadal regimes may be associated with interactions between ENSO and the Pacific Decadal Oscillation on decadal timescales.
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This study analyzes the impacts of tropical cyclones (TCs) that form 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 article is protected by copyright. All rights reserved.
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The coupling between the ocean activity driven by winds and the solid Earth generates seismic signals recorded by seismometers worldwide. The 2–10 s period band, known as secondary microseism, represents the largest background seismic wavefield. While moving over the ocean, tropical cyclones generate particularly strong and localized sources of secondary microseisms that are detected remotely by seismic arrays. We assess and compare the seismic sources of P, SV, and SH waves associated with typhoon Ioke (2006) during its extra-tropical transition. To understand their generation mechanisms, we compare the observed multi-phase sources with theoretical sources computed with a numerical ocean wave model, and we assess the influence of the ocean resonance (or ocean site effect) and coastal reflection of ocean waves. We show how the location and lateral extent of the associated seismic source is period- and phase-dependent. This information is crucial for the use of body waves for ambient noise imaging and gives insights about the sea state, complementary to satellite data. The authors locate the maximum seismic energy imprint and lateral extent of the seismic sources generated by Typhoon Ioke. Based on this data set, they present a new tool to shed light on the generation mechanism of secondary microseisms body waves.
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The traditional view holds that a weakened upwelling has often been observed off the coast of southeast Vietnam during the post-El Niño summer. This study investigated a strong upwelling and concurrent phytoplankton bloom off the coast of southeast Vietnam in August 2016 by comparing it with another case in 1998. Analyses of the upwelling structure and formation mechanisms indicated that the abnormal strong upwelling in August 2016 was attributable to strong wind-driven offshore Ekman transport and Ekman pumping, which were caused by the accompanying southwesterly anomalies south of the anomalous cyclone (AC) over the western North Pacific (WNP), and vice versa in August 1998. This anomalous southwesterly wind associated with the AC over the WNP could not be explained by La Niña, the negative Indian ocean dipole, or the positive Pacific meridional mode events. Further analyses showed that the Madden-Julian oscillation (MJO)-induced westerly winds could have contributed more than 75% of the original zonal winds. Nine tropical cyclones generated over the WNP were favorable for excessive precipitation. The opposite configurations of precipitation patterns over the WNP and the Maritime Continent could have further strengthened the AC via a Gill response.
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This study explores the variability of tropical cyclone (TC) intensification rates (IRs) in the postmonsoon Bay of Bengal (BoB) for the satellite period of 1980–2015. It is found that both number of rapid intensification (RI) events and magnitude of IRs show a robust increase, with a northeastward shift of intensification events. Analyses show that the temporal variability of sea surface temperature dominated the IR variability during 1980–1997. However, the thick barrier layer in the northern BoB was considerably responsible for IR variability during 1998–2015, which significantly contributed to the IR increase. Due to more intensification events occurring over the northeastern region in two recent decades, the thick barrier layer with strong salinity stratification in the northern BoB limits TC-induced sea surface cooling and in turn favors TC intensification. This study has an important implication that air–sea coupled climate model need to realistically simulate upper ocean salinity variability on projecting TC intensity change over the BoB.
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Rapid intensification (RI) refers to a significant increase in tropical cyclone (TC) intensity over a short period of time. A TC can also undergo multiple RI events during its lifetime, and these RI events pose a significant challenge for operational forecasting. The long-term tendency in RI magnitude of TCs over the western North Pacific is investigated in this study. During 1979–2018, a significant increasing trend is found in RI magnitude, which primarily results from the significant increasing number of strong RI events, defined as 24 h intensity increases of at least 50 kt. Furthermore, there are significantly more (slightly fewer) strong RI occurrences west (east) of 155°E in 1999–2018 than in 1979–1998. Significant increases in strong RI occurrences are located over the region bounded by 10°∼20°N, 120°∼150°E. These changes are likely induced by the warming ocean but appear uncorrelated with changes in the atmospheric environment. By contrast, there are slight decreases in strong RI occurrences over the region bounded by 12.5°∼22.5°N, 155°∼170°E, likely due to the offset between RI-favorable influences of the warming ocean and RI-unfavorable influences of increasing vertical wind shear (VWS).
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