Tropical Cyclone Genesis Potential Index Research Articles

A Contrast of the Monsoon–Tropical Cyclone Relationship between the Western and Eastern North Pacific

The monsoon and tropical cyclone (TC) are principal components of global climate variability. The relationship between the monsoon intensity and the TC genesis frequency (TCGF) in different major monsoon regions has not been fully studied. Here, we compared the relationship of monsoon intensity and TCGF during the extended boreal summer between the western and eastern North Pacific, results of which revealed different monsoon–TC relationships (with opposite-sign correlations) in these two regions. A significant positive correlation could be found between the western North Pacific summer monsoon (WNPSM) index and the TCGF over the western North Pacific (WNP). In contrast, a significant negative correlation was identified between the North American summer monsoon (NASM) index and the TCGF over the eastern North Pacific (ENP). The observed different monsoon–TC relationships could be explained by the monsoon-associated changes in the environmental factors over the regions where TCs were formed and the influences from sea surface temperature (SST) anomalies across tropical ocean basins. By comparing the environmental factors in the TC genesis potential index (GPI), the mid-level relative humidity (vertical wind shear) was the factor to make the largest contribution to the monsoon-associated TC genesis changes over the WNP (ENP). In strong (weak) WNPSM years, the high (low) atmospheric mid-level relative humidity could promote (inhibit) the TCGF over the WNP, resulting in a significant positive monsoon–TC correlation. In contrast, in strong (weak) NASM years, the strong (weak) vertical wind shear could inhibit (promote) the TCGF over the ENP, thus leading to a significant negative monsoon–TC correlation. In addition, the WNPSM and the TCGF over the WNP could be modulated by the similar tropical Pacific–Atlantic SST anomalies jointly, thus leading to a significant positive correlation between the WNPSM and the WNP TCGF. In contrast, the signs of tropical Pacific–Atlantic SST anomalies influencing the NASM were almost opposite to those affecting the TCGF over the ENP, thus resulting in a significant negative correlation between the NASM and the ENP TCGF. The results obtained herein highlight the differences of the monsoon–TC relationship between the WNP and the ENP, which may provide useful information for the prediction of monsoon intensity and TC formation number over these two regions.

Possible reasons for the migration of tropical cyclone track over the western north pacific: Interdecadal pacific oscillation modulation

Recent studies have proposed a relatively reliable metric, i.e., the annual-mean latitude of tropical cyclone (TC) lifetime maximum intensity, to evaluate the observed trend of poleward migration of TC. The documented trend in the existing records of the metric suggests that anthropogenic factors might make considerable contributions to the poleward migration of TC. However, here we show that there is no significant TC migration trend in the western North Pacific (WNP). Instead, large interdecadal variation is found in the WNP TC track over the past 35 years. Rather than the anthropogenic factors, it is the natural variability, especially the Interdecadal Pacific Oscillation (IPO), that plays a key role in modulating the migration of WNP TC track. The IPO-related TC migration is assumed to cause systematic changes (i.e., increases and decreases) in regional hazard exposure and risk. The impact of IPO on the WNP TC migration is attributed to its influences on the large-scale circulation, TC genesis potential index, and the potential intensity in the TC prevailing region of the WNP.

Climatological and Seasonal Variations of the Tropical Cyclone Genesis Potential Index Based on Oceanic Parameters in the Global Ocean

Climatological and Seasonal Variations of the Tropical Cyclone Genesis Potential Index Based on Oceanic Parameters in the Global Ocean

Modified tropical cyclone genesis potential index over the Bay of Bengal during southwest and post-monsoon seasons

Modified tropical cyclone genesis potential index over the Bay of Bengal during southwest and post-monsoon seasons

Investigate the relationship between Storm Formation and Tropical Cyclone Genesis Potential Index in the Vietnam East Sea

Abstract: In this paper, the relationship between Tropical Cyclone (TC) Genesis Potential Index (GPI) and the number of TC (NTC) associated with ENSO over the Vietnam East Sea (VES) was investigated. Observed TC data of the Regional Specialized Meteorological Center (RSMC) Tokyo Typhoon Center and ERA Interim reanalysis data for the period 1985-2015 were used. The results show a good agreement between GPI and NTC over the VES with the correlation coefficient is 0.84. There were more TCs formed over the VES during La Nina years and less TCs during El Nino years. There were positive anomalies of GPI, environmental factors (relative humidity, sea surface temperature, absolute vorticity, potential intensity)over the region where the highest densityof TCs genesis locatedduring La Nina years while there were negative anomalies found during El Nino years. Relative humidity has the largest contribution to the positive difference GPI between La Nina years and El Nino years, the less contribution comes from the potential intensity, absolute vorticity, and wind shear.
 Keywords: GPI, Tropical Cyclone Genesis, ENSO, Vietnam East Sea.

Subtropical High Affects Interdecadal Variability of Tropical Cyclone Genesis in the South China Sea

AbstractInterdecadal variability of tropical cyclone (TC) genesis in the South China Sea (SCS) during 1982–2015 is investigated using observations and atmospheric reanalysis data. TC genesis primarily occurs in the northern SCS (north of 13 °N) in July–September (summer), while in the southern SCS (south of 13 °N) in October–December (autumn). The TC genesis location is consistent with the climatological distribution of TC genesis potential index. Noticeably, the TC genesis frequency (TCGF) is relatively low in 1982–1993 and 2003–2015 while relatively high in 1994–2002 in summer in the SCS. In autumn, the TCGF shows an abrupt transition from high to low in the early 2000s in the SCS. It is found that such interdecadal change of TCGF is closely related to the east‐westward movement of the subtropical high (SH). When the SH is close to the SCS, large‐scale air subsidence, low‐level divergence, negative vorticity, and high pressure are prominent and inhabit TC genesis in the SCS. On the contrary, when the SH moves away from the SCS, environmental conditions become more favorable for TC genesis. In addition, the localized atmospheric intraseasonal variability can affect TCGF at interdecadal time scales as well.

Tropical Cyclone Genesis Potential Index for Bay of Bengal During Peak Post-Monsoon (October-November) Season Including Atmosphere-Ocean Parameters

ABSTRACTSeveral studies on tropical cyclone genesis potential index (GPI) mainly using atmospheric parameters (relative/absolute vorticity, relative humidity, vertical wind shear, potential instability, vertical velocity etc.) have been reported earlier. Though the ocean plays a vital role in the genesis and intensification of cyclones, no ocean parameter has been included in most of the studies. In this study, we have made an attempt to develop a new GPI for Bay of Bengal during peak post-monsoon (October-November) season including upper ocean heat content (UOHC) using the data for the period 1995–2015. It is found that the new GPI is better correlated with the total number of depressions, cyclones and severe cyclones (TNDC) compared with the existing GPI which was developed for the north Indian Ocean and presently used by India Meteorological Department (IMD), New Delhi. The correlation has significantly enhanced (r=0.86:significant at >99% level) by using the first differences [year(0) –year(−1)] of the time series data. Since, the new GPI which considers atmosphere and ocean (UOHC) parameters, it appears to be more suitable for Bay of Bengal during the peak post-monsoon season.

Cause of interdecadal change of tropical cyclone controlling parameter in the western North Pacific

The interdecadal change of tropical cyclone (TC) controlling parameter in the western North Pacific (WNP) was investigated using observational data. Through the diagnosis of the relative role of each term of the TC genesis potential index (GPI), it was found that the dominant factor controlling interannual TC genesis frequency was specific humidity in 1950–1976, maximum potential intensity or sea surface temperature (SST) in 1977–1998, and relative vorticity in 1999–2014. The change of environmental specific humidity during 1950–1976 was primarily attributed to the advection of the mean moisture by anomalous low-level wind in ENSO developing summer. The change of SST during 1977–1998 was primarily affected by a V-shape SST pattern in western Pacific in ENSO decaying summer. The change of environment relative vorticity during 1999–2014 was primarily controlled by a strong cyclonic flow anomaly associated with CP-type El Nino. The change of dominant environmental controlling parameter is ultimately caused by the change of ENSO behavior. Compared to the first interdecadal period, a stronger EP-type ENSO variability in 1977–1998 leads to a stronger circulation and SST response in ENSO decaying phase. The occurrence of more frequent CP type El Nino in 1999–2014 was possibly attributed to both suppressed convection and low-level divergence over the central equatorial Pacific and a warming trend in the tropics.

Projected Future Changes of Tropical Cyclone Activity over the Western North and South Pacific in a 20-km-Mesh Regional Climate Model

AbstractA high-resolution regional atmospheric model is employed to project the late twenty-first-century changes of tropical cyclone (TC) activity over the western North Pacific (WP) and southwest Pacific (SP). The model realistically reproduces the basic features of the TC climatology in the present-day simulation. Future projections under the representative concentration pathway 4.5 (RCP45) and 8.5 (RCP85) scenarios are investigated. The results show no significant change of TC genesis frequency (TCGF) in the WP by RCP45 due to the cancellation of the reduction over the western part and the increase over the eastern part together with a considerable decrease of TCGF by RCP85 due to the excessive TCGF reduction in the western part. The TCGF over the SP consistently decreases from RCP45 to RCP85. Despite the fact that the simulated maximum surface wind speeds are below 52 m s−1, the change with more strong TCs and fewer weak TCs is robust. The future changes in the TC genesis locations and translational speeds modulate the TC lifetime and frequency of occurrence. The TC genesis potential index (GPI) is used to evaluate the projected TCGF changes. The results show that low-level vorticity and midtropospheric vertical velocity largely contribute to the reduction of GPI in the western part of the WP, while vertical wind shear and midtropospheric vertical velocity mainly contribute to the decrease of GPI over the SP. The weakening of the monsoon trough is found to be responsible for the decreases of GPI and TCGF over the western part of the WP.

Influence of climate regime shift on the interdecadal change in tropical cyclone activity over the Pacific Basin during the middle to late 1990s

In this study, a new interpretation is proposed for the abrupt decrease in tropical cyclone (TC) activity in the western North Pacific (WNP) after the late 1990s. We hypothesize that this abrupt change constitutes a part of the phenomenon of interdecadal change in TC activity in the Pacific Basin, including the WNP, western South Pacific (WSP), and eastern North Pacific. Our analysis revealed that the climate-regime shift (CRS) in the Pacific during the middle to late 1990s resulted in a La Nina-like mean state, which was responsible for the interdecadal change in TC activity in the late 1990s. Analyses of the TC genesis potential index and numerical experiments revealed that the decline in TC activity in both the WNP and WSP was primarily attributable to the increase of vertical wind shear in the central Pacific due to the La Nina-like associated cold sea surface temperature (SST). Conversely, the La Nina-like associated warm SST in the western Pacific produced anomalous vertical transport of water vapor, increasing moisture levels in the mid-troposphere and TC activity in the western WNP. Furthermore, the CRS modified the mean TC genesis position and shifted the steering flow to the west, resulting in the increased frequency of TC landfalls in Taiwan, southeastern China, and northern Australia after the late 1990s.

西北太平洋10~12月台风活动的年代际突变

本文基于日本气象厅区域专业气象中心-东京台风中心(Regional Specialized Meteorological Center-Tokyo Typhoon Center, Japan Meteorological Agency)的热带气旋资料和欧洲中尺度天气预报中心(European Centre for Medium-Range Weather Forecasts)的再分析资料，在前人的研究基础上，对西北太平洋10~12月期间台风活动对大尺度环境因子年代际突变的响应进行了讨论，并且通过台风数量和相关大尺度环境因素的分析，对其影响机制进行了比较详细的阐述。分析西北太平洋1980~2013年10~12月生成的台风，与1980~1994年相比，10~12月的台风数量在1995~2013年期间显著减少。分析发现热带气旋最大潜在强度的年代际变化是导致1995~2013年西北太平洋热带气旋潜在生成指数(Genesis Potential Index, GPI)减少的最主要因素，1995年后西北太平洋SST异常场呈La Nina形态，导致热带气旋最大潜在强度(The Maximum Tropical Cyclone Potential Intensity, MPI)增大，但在热带气旋的发展过程中过早受到西边界的影响使得增强为台风的热带气旋减少。突变后垂直风切变增强造成西北太平洋东南部的动力条件不利于台风生成，这都是造成台风突变减少的原因。分析还发现垂直风切变主要对西北太平洋东南部的台风活动造成影响。 Based on the data sets of Regional Specialized Meteorological Center-Tokyo Typhoon Center, Japan Meteorological Agency and the reanalysis data of European Centre for Medium-Range Weather Forecasts, this paper demonstrated the decadal shift between October to December (OND) typhoon activity over the Western North Pacific (WNP) during 1980 to 2013, and discussed the response of the OND typhoon activity to decadal shift of the large-scale circulation. Meanwhile, by means of the analysis of typhoon activity and its relative large-scale environmental factors, it brought a detail exposition of its impact mechanisms. We analyzed that the WNP typhoon activity during October to December from 1980 to 2013, compared with that from 1980 to 1994, the typhoon activity during October to December among 1995 to 2013 decreased significantly. We discovered the main factor that led to the decrease of Tropical Cyclone Genesis Potential Index was the decadal shift of The Maximum Tropical Cyclone Potential Intensity. The anomalous SST patterns appear to be a La Nina pattern, leading to the increasing of MPI. However, it early affected by the western boundary during tropical cyclones development process so that fewer tropical cyclones enhanced to be typhoon. After 1995, the vertical wind shear and subtropical high to be strengthener, resulting in the southeastern Pacific Northwest dynamic conditions are not conducive to typhoon genesis; all these are the reasons for the decadal shift of typhoon activity. We also found the vertical wind shear mainly affects the eastern part of the Northwest Pacific typhoon activity.

Tropical cyclone genesis potential index over the western North Pacific simulated by CMIP5 models

Tropical cyclone (TC) genesis over the western North Pacific (WNP) is analyzed using 23 CMIP5 (Coupled Model Intercomparison Project Phase 5) models and reanalysis datasets. The models are evaluated according to TC genesis potential index (GPI). The spatial and temporal variations of the GPI are first calculated using three atmospheric reanalysis datasets (ERA-Interim, NCEP/NCAR Reanalysis-1, and NCEP/DOE Reanalysis-2). Spatial distributions of July–October-mean TC frequency based on the GPI from ERA-interim are more consistent with observed ones derived from IBTrACS global TC data. So, the ERA-interim reanalysis dataset is used to examine the CMIP5 models in terms of reproducing GPI during the period 1982–2005. Although most models possess deficiencies in reproducing the spatial distribution of the GPI, their multimodel ensemble (MME) mean shows a reasonable climatological GPI pattern characterized by a high GPI zone along 20°N in the WNP. There was an upward trend of TC genesis frequency during 1982 to 1998, followed by a downward trend. Both MME results and reanalysis data can represent a robust increasing trend during 1982–1998, but the models cannot simulate the downward trend after 2000. Analysis based on future projection experiments shows that the GPI exhibits no significant change in the first half of the 21st century, and then starts to decrease at the end of the 21st century under the representative concentration pathway (RCP) 2.6 scenario. Under the RCP8.5 scenario, the GPI shows an increasing trend in the vicinity of 20°N, indicating more TCs could possibly be expected over the WNP under future global warming.

Chinese contribution to CMIP5: An overview of five Chinese models’ performances

An overview of Chinese contribution to Coupled Model Intercomparison Project-Phase 5 (CMIP5) is presented. The performances of five Chinese Climate/Earth System Models that participated in the CMIP5 project are assessed in the context of climate mean states, seasonal cycle, intraseasonal oscillation, interannual variability, interdecadal variability, global monsoon, Asian-Australian monsoon, 20th-century historical climate simulation, climate change projection, and climate sensitivity. Both the strengths and weaknesses of the models are evaluated. The models generally show reasonable performances in simulating sea surface temperature (SST) mean state, seasonal cycle, spatial patterns of Madden-Julian oscillation (MJO) amplitude and tropical cyclone Genesis Potential Index (GPI), global monsoon precipitation pattern, El Nino-Southern Oscillation (ENSO), and Pacific Decadal Oscillation (PDO) related SST anomalies. However, the performances of the models in simulating the time periods, amplitude, and phase locking of ENSO, PDO time periods, GPI magnitude, MJO propagation, magnitude of SST seasonal cycle, northwestern Pacific monsoon and North American monsoon domains, as well as the skill of large-scale Asian monsoon precipitation need to be improved. The model performances in simulating the time evolution and spatial pattern of the 20th-century global warming and the future change under representative concentration pathways projection are compared to the multimodel ensemble of CMIP5 models. The model discrepancies in terms of climate sensitivity are also discussed.

Will typhoon over the western North Pacific be more frequent in the Blue Arctic conditions?

How would typhoon activity over the western North Pacific change for various scenarios of future global warming? Using the model projections of the Coupled Model Intercomparison Project phase 3 (CMIP 3) under the SRES A1B scenario, we generated summer (September) ice-free Arctic conditions, also referred to as Blue Arctic conditions, and then used the corresponding monthly sea surface temperature (SST) and a set of CO2 concentrations to drive an AGCM model to simulate the resulting changes in background conditions affecting typhoon activity over the western North Pacific. Our results show that, during typhoon season (June to October), atmospheric and ocean circulations over the western North Pacific would be significantly different from the present circulations. Changes in the vertical shear of zonal wind and outgoing longwave radiation (OLR) in the western North Pacific are favorable for westward and northward shift, respectively, of the location of typhoon genesis. Moreover, changes in the above fields over the key area may be conducive to less frequent typhoons. In addition, the tropical cyclone genesis potential index (GPI) over the western North Pacific would decrease (increase) east (west) of 150°E (140°E).

Tropical cyclone genesis potential index over the western North Pacific simulated by LASG/IAP AGCM

Tropical cyclone genesis potential index (GPI) is a useful metric for gauging the performance of global climate models in the simulation of tropical cyclone (TC) genesis. The performance of LASG/IAP AGCM GAMIL2.0 in the simulation of GPI over the western North Pacific (WNP) is assessed in this paper. Since GPI depends on large scale environmental factors including low-level vorticity at 850 hPa, relative humidity at 700 hPa, vertical wind shear between 850 and 200 hPa, maximum potential intensity (MPI), and vertical velocity, the bias of GPI simulation is discussed from the perspective of thermal and dynamical factors. The results are compared with the ECMWF reanalysis data (ERA40). The analyses show that both the climatological spatial pattern and seasonal cycle of GPI over the WNP are reasonably simulated by GAMIL2.0, but due to the overestimation of relative humidity, the simulated GPI extends to 170°E, about 10° east to that in the reanalysis data. It is demonstrated that the bias in the simulation of monsoon trough, which is about 5° north to the reanalysis, leads to an overestimation of GPI during May–June and September–October, but an underestimation during July–August. Over the WNP, the response of GPI to ENSO is well captured by GAMIL2.0, including the eastward (westward) shift of TC genesis location during El Nino (La Nina) years. However, the anomalous convective center associated with El Nino shifts westward about 20° in comparison to ERA40, which leads to the biases in both vertical velocity and relative humidity. These eventually result in the westward deflection of the boundary between the positive and negative GPI centers along 20°-30°N. The results from this study provide useful clues for the future improvement of GAMIL2.0.

Resolution dependence of tropical cyclone formation in CMIP3 and finer resolution models

Detection of tropical lows is performed in a suite of climate model simulations using objectively-determined detection thresholds that are resolution-dependent. It is found that there is some relationship between model resolution and tropical cyclone formation rate even after the resolution-dependent tropical cyclone detection threshold is applied. The relationship is investigated between model-simulated tropical cyclone formation and a climate-based tropical cyclone Genesis Potential Index (GPI). It is found that coarser-resolution models simulate the GPI better than they simulate formation of tropical cyclones directly. As a result, there appears to be little relationship from model to model between model GPI and the directly-simulated cyclone formation rate. Statistical analysis of the results shows that the main advantage of increasing model resolution is to give a considerably better pattern of cyclone formation. Finer resolution models also simulate a slightly better pattern of GPI, and for these models there is some relationship between the pattern of GPI simulated by each model and that model’s pattern of simulated tropical cyclone formation.

Changes in the tropical cyclone genesis potential index over the western north pacific in the SRES A2 scenario

The Tropical Cyclone Genesis Potential Index (GPI) was employed to investigate possible impacts of global warming on tropical cyclone genesis over the western North Pacific (WNP). The outputs of 20th century climate simulation by eighteen GCMs were used to evaluate the models’ ability to reproduce tropical cyclone genesis via the GPI. The GCMs were found in general to reasonably reproduce the observed spatial distribution of genesis. Some of the models also showed ability in capturing observed temporal variation. Based on the evaluation, the models (CGCM3.1-T47 and IPSL-CM4) found to perform best when reproducing both spatial and temporal features were chosen to project future GPI. Results show that both of these models project an upward trend of the GPI under the SRES A2 scenario, however the rate of increase differs between them.

Tropical cyclone genesis potential index in climate models

Tropical cyclone genesis potential index in climate models

Tropical cyclone genesis potential index in climate models

The potential for tropical cyclogenesis in a given ocean basin during its active season has been represented by genesis potential indices, empirically determined functions of large-scale environmental variables which influence tropical cyclone (TC) genesis. Here we examine the ability of some of today’s atmospheric climate models, forced with historical observedSSTover a multidecadal hindcast period, to reproduce observed values and patterns of one such genesis potential index (GP), as well as whether the GP in a given model is a good predictor of the number of TCs generated by that model. The effect of the horizontal resolution of a climate model on its GP is explored.The five analysed models are capable of reproducing the observed seasonal phasing of GP in a given region, but most of them them have a higher GP than observed. Each model has its own unique relationship between climatological GP and climatological TC number; a larger climatological GP in one model compared to others does not imply that that model has a larger climatological number of TCs. The differences among the models in the climatology of TC number thus appear to be related primarily to differences in the dynamics of the simulated storms themselves, rather than to differences in the simulated large-scale environment for genesis. The correlation of interannual anomalies in GP and number of TCs in a given basin also differs significantly from one model to the next.Experiments using the ECHAM5 model at different horizontal resolutions indicate that as resolution increases, model GP also tends to increase. Most of this increase is realized between T42 and T63.