Journal articles on the topic 'Cyclogenesis indices'
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1
Menkes, Christophe E., Matthieu Lengaigne, Patrick Marchesiello, Nicolas C. Jourdain, Emmanuel M. Vincent, Jérôme Lefèvre, Fabrice Chauvin, and Jean-Francois Royer. "Comparison of tropical cyclogenesis indices on seasonal to interannual timescales." Climate Dynamics 38, no. 1-2 (July 1, 2011): 301–21. http://dx.doi.org/10.1007/s00382-011-1126-x.
2
Ventrice, Michael J., Chris D. Thorncroft, and Paul E. Roundy. "The Madden–Julian Oscillation’s Influence on African Easterly Waves and Downstream Tropical Cyclogenesis." Monthly Weather Review 139, no. 9 (September 2011): 2704–22. http://dx.doi.org/10.1175/mwr-d-10-05028.1.
Abstract:
The influence of the Madden–Julian oscillation (MJO) over tropical Africa and Atlantic is explored during the Northern Hemisphere summer months. The MJO is assessed by using real-time multivariate MJO (RMM) indices. These indices divide the active convective signal of the MJO into 8 phases. Convection associated with the MJO is enhanced over tropical Africa during RMM phases 8, 1, and 2. Convection becomes suppressed over tropical Africa during the subsequent RMM phases (phases 3–7). African convective signals are associated with westward-propagating equatorial Rossby waves. The MJO modulates African easterly wave (AEW) activity. AEW activity is locally enhanced during RMM phases 1–3 and suppressed during RMM phases 6–8. Enhanced AEW activity occurs during periods of enhanced convection over tropical Africa, consistent with stronger or more frequent triggering of AEWs as well as more growth associated with latent heat release. Enhanced AEW activity occurs during the low-level westerly wind phase of the MJO, which increases the cyclonic shear on the equatorward side of the AEJ, increasing its instability. Atlantic tropical cyclogenesis frequency varies coherently with the MJO. RMM phases 1–3 show the greatest frequency of tropical cyclogenesis events whereas phases 7 and 8 show the least. RMM phase 2 is also the most likely phase to be associated with a train of three or more tropical cyclones over the tropical Atlantic. This observed evolution of tropical cyclogenesis frequency varies coherently with variations in AEW activity and the large-scale environment.
3
Wijnands, Jasper S., Guoqi Qian, and Yuriy Kuleshov. "Variable Selection for Tropical Cyclogenesis Predictive Modeling." Monthly Weather Review 144, no. 12 (November 7, 2016): 4605–19. http://dx.doi.org/10.1175/mwr-d-16-0166.1.
Abstract:
Abstract Variable selection for short-term forecasting (up to 72 h) of tropical cyclone (TC) genesis has been investigated. IBTrACS data (1979–2014) are used to identify the genesis time and position of over 2500 TCs between 30°N and 30°S. Tracks are extended using a tropical cloud cluster (TCC) dataset, which is also used to identify over 28 000 nondeveloping TCCs. Subsequently, corresponding local environment states at various atmospheric pressure levels are retrieved from ERA-Interim data. An initial selection of potentially favorable variables for TC genesis is made based on mutual information, which forms the set of nodes for graphical model structure learning using the Peter–Clark (PC) algorithm. Structure learning identifies the variables with the strongest influence on TC genesis, while taking into account the interrelationship with other variables. Variables are ranked based on the maximum observed p value in all (conditional) independence tests of the variable with the TC genesis node. The results indicate that potential vorticity (600 hPa), relative vorticity (925 hPa), and (vector) vertical wind shear (200–700 hPa) are the highest ranked variables for forecasting up to 72 h. These are followed by the basin and zonal wind speed (200 hPa), and for very short lead-time divergence (925 hPa), air temperature (300 hPa), and average vertical velocity. Predictive modeling with logistic regression confirms the superior performance of the top-ranked variables. The presented variable ranking (methodology) can be used as a building block for the creation of genesis indices or predictive models in the future.
4
Slade, Stephanie A., and Eric D. Maloney. "An Intraseasonal Prediction Model of Atlantic and East Pacific Tropical Cyclone Genesis." Monthly Weather Review 141, no. 6 (June 1, 2013): 1925–42. http://dx.doi.org/10.1175/mwr-d-12-00268.1.
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Abstract A real-time statistical model based on the work of Leroy and Wheeler is developed via multiple logistic regression to predict weekly tropical cyclone activity over the Atlantic and east Pacific basins. The predictors used in the model include a climatology of tropical cyclone genesis for each ocean basin, an El Niño–Southern Oscillation (ENSO) index, and two indices representing the propagating Madden–Julian oscillation (MJO). The Atlantic model also includes a predictor representing the variability of sea surface temperature (SST) in the Main Development Region (MDR). These predictors are suggested as useful for the prediction of tropical cyclogenesis based on previous work in the literature and are further confirmed in this study using basic statistics. Univariate logistic regression models are generated for each predictor in each region to ensure the choice of prediction scheme. Using all predictors, cross-validated hindcasts are developed out to a seven-week forecast lead. A formal stepwise predictor selection procedure is implemented to select the predictors used in each region at each forecast lead. Brier skill scores and reliability diagrams are used to assess the skill and dependability of the models. Results show an increase in model skill over the time-varying climatology at predicting tropical cyclogenesis by the inclusion of the MJO out to a three-week forecast lead for the east Pacific and a two-week forecast lead for the Atlantic. The importance of ENSO and MDR SST for Atlantic genesis prediction is highlighted, and the uncertain effects of ENSO on east Pacific tropical cyclogenesis are revisited.
5
Ducić, V., J. Luković, D. Burić, G. Stanojević, and S. Mustafić. "Precipitation extremes in the wettest Mediterranean region (Krivošije) and associated atmospheric circulation types." Natural Hazards and Earth System Sciences 12, no. 3 (March 22, 2012): 687–97. http://dx.doi.org/10.5194/nhess-12-687-2012.
Abstract:
Abstract. The aim of this paper is to analyse indices of extreme precipitation in Krivošije, Montenegro, the wettest Mediterranean region, from the period 1951–2007 and their relationships with atmospheric circulation using "SynopVis Grosswetterlagen" (SVG) series. Data from two stations were analysed, namely Crkvice (42°34'N and 18°39'E) and Herceg Novi (42°27'N and 18°31'E). Four indices of precipitation extremes (SDII, R75p, R95p, R95pTOT) were assessed including number of dry days. The results suggest that the number of days with precipitation decreased. To analyse the relationship between extreme precipitation events and circulation types we have used an efficiency coefficient (Ec). Regarding relation to atmospheric circulation, westerly, southwesterly and northwesterly circulation types with anticyclonic features over Central Europe are more frequent for dry days (days with R<1.0 mm) and northerly, easterly and southerly types for wet and very wet days (R75p and R95p indices). The types with cyclonic condition over Central Europe show a large proportion of wet and very wet days. Also, activity of Genoa cyclogenesis and orographic influence over a small area are the main reasons for the high precipitation amounts recorded in the Krivošije region (Crkvice).
6
Tang, Brian, and Kerry Emanuel. "A Ventilation Index for Tropical Cyclones." Bulletin of the American Meteorological Society 93, no. 12 (December 1, 2012): 1901–12. http://dx.doi.org/10.1175/bams-d-11-00165.1.
Abstract:
An important environmental control of both tropical cyclone intensity and genesis is vertical wind shear. One hypothesized pathway by which vertical shear affects tropical cyclones is midlevel ventilation—or the flux of low-entropy air into the center of the tropical cyclone. Based on a theoretical framework, a ventilation index is introduced that is equal to the environmental vertical wind shear multiplied by the nondimensional midlevel entropy deficit divided by the potential intensity. The ventilation index has a strong influence on tropical cyclone climatology. Tropical cyclogenesis preferentially occurs when and where the ventilation index is anomalously low. Both the ventilation index and the tropical cyclone's normalized intensity, or the intensity divided by the potential intensity, constrain the distribution of tropical cyclone intensification. The most rapidly intensifying storms are characterized by low ventilation indices and intermediate normalized intensities, while the most rapidly weakening storms are characterized by high ventilation indices and high normalized intensities. Since the ventilation index can be derived from large-scale fields, it can serve as a simple and useful metric for operational forecasts of tropical cyclones and diagnosis of model errors.
7
Cattiaux, Julien, Fabrice Chauvin, Olivier Bousquet, Sylvie Malardel, and Chia-Lun Tsai. "Projected Changes in the Southern Indian Ocean Cyclone Activity Assessed from High-Resolution Experiments and CMIP5 Models." Journal of Climate 33, no. 12 (June 15, 2020): 4975–91. http://dx.doi.org/10.1175/jcli-d-19-0591.1.
Abstract:
AbstractThe evolution of tropical cyclone activity under climate change remains a crucial scientific issue. Physical theory of cyclogenesis is limited, observational datasets suffer from heterogeneities in space and time, and state-of-the-art climate models used for future projections are still too coarse (~100 km of resolution) to simulate realistic systems. Two approaches can nevertheless be considered: 1) perform dedicated high-resolution (typically <50 km) experiments in which tropical cyclones can be tracked and 2) assess cyclone activity from existing low-resolution multimodel climate projections using large-scale indices as proxies. Here we explore these two approaches with a particular focus on the southern Indian Ocean. We first compute high-resolution experiments using the rotated-stretched configuration of our climate model (CNRM-CM6-1), which is able to simulate realistic tropical cyclones. In a 2-K warmer world, the model projects a 20% decrease in the frequency of tropical cyclones, together with an increase in their maximum lifetime intensity, a slight poleward shift of their trajectories, and a substantial delay (about 1 month) in the cyclone season onset. Large-scale indices applied to these high-resolution experiments fail to capture the overall decrease in cyclone frequency, but are able to partially represent projected changes in the spatiotemporal distribution of cyclone activity. Last, we apply large-scale indices to multimodel CMIP5 projections and find that the seasonal redistribution of cyclone activity is consistent across models.
8
Fink, Andreas H., Jon M. Schrage, and Simone Kotthaus. "On the Potential Causes of the Nonstationary Correlations between West African Precipitation and Atlantic Hurricane Activity." Journal of Climate 23, no. 20 (October 15, 2010): 5437–56. http://dx.doi.org/10.1175/2010jcli3356.1.
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Abstract For years, various indices of seasonal West African precipitation have served as useful predictors of the overall tropical cyclone activity in the Atlantic Ocean. Since the mid-1990s, the correlation unexpectedly deteriorated. In the present study, statistical techniques are developed to describe the nonstationary nature of the correlations between annual measures of Atlantic tropical cyclone activity and three selected West African precipitation indices (namely, western Sahelian precipitation in June–September, central Sahelian precipitation in June–September, and Guinean coastal precipitation in the preceding year’s August–November period). The correlations between these parameters are found to vary over the period from 1921 to 2007 on a range of time scales. Additionally, considerable year-to-year variability in the strength of these correlations is documented by selecting subsamples of years with respect to various meteorological factors. Broadly, in years when the environment in the main development region is generally favorable for enhanced tropical cyclogenesis (e.g., when sea surface temperatures are high, when there is relatively little wind shear through the depth of the troposphere, or when the relative vorticity in the midtroposphere is anomalously high), the correlations between indices of West African monsoon precipitation and Atlantic tropical cyclone activity are considerably weaker than in years when the overall conditions in the region are less conducive. Other more remote climate parameters, such as the phase of the Southern Oscillation, are less effective at modulating the nature of these interactions.
9
Ikram, Farah, Kalim Ullah, and Deliang Chen. "Evaluation of Three Genesis Potential Indices for Tropical Cyclogenesis in the Arabian Sea: Two Case Studies Using WRF and ERA5." Monthly Weather Review 150, no. 12 (December 2022): 3275–303. http://dx.doi.org/10.1175/mwr-d-22-0048.1.
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Abstract Tropical cyclones (TCs) generated over the Arabian Sea can cause significant damage to infrastructure, human lives, landfall, and property near inshore and maritime trade route areas. A key to successful prediction of TCs is a skillful prediction of potential cyclogenesis locations. This study focuses on evaluating three genesis potential indices (GPIs) derived from a global reanalysis (ERA5) and dynamically downscaling using a regional model (WRF) for two TC cases: Gonu in 2007 and Kyarr in 2019, selected by analyzing the accumulated cyclone energy trend from the International Best Track Archive for Climate Stewardship (IBTrACS) dataset over the period of 1981–2019. The two TCs belong to category 4 and above on the Saffir–Simpson scale. To test the sensitivity of downscaling to cumulus parameterizations, two WRF experiments were conducted using the Kain–Fritsch and New Tiedke cumulus schemes, respectively. The calculated genesis locations with help of the three GPIs were compared with IBTrACS. The results show that 1) all indices have reasonable skills in reproducing genesis locations, although their performances differ somewhat; 2) the dynamic downscaling with two WRF experiments added value to the study by comparing two numerical schemes for estimating genesis locations; and 3) WRF with the New Tiedke and Kain–Fritsch schemes showed good skill in reproducing the spatial distribution of the most relevant dynamical parameters. The pattern correlations are well correlated with environmental parameters of untransformed GPI and higher correlations with binary logarithmic transformed GPI. The applicability to other cyclones is also tested (e.g., TC Nilofar in 2014) with encouraging results. This study demonstrates the usefulness of GPIs for forecasting TC genesis in the region. Significance Statement The trend analysis of accumulated cyclone energy (ACE) over the Arabian Sea (AS) shows an increase over the period of 1981–2019 with the highest ACE values for 2019. The genesis potential indices (GPI) show strong ability for use as a forecasting tool for tropical cyclone genesis, and hence, are helpful for providing a reference for future studies. WRF experiments were able to reproduce the GPI with slight differences from the observations and ERA5. WRF schemes show good performance in reproducing key meteorological fields. The analysis of the GPI and WRF schemes shows the potential to be implemented for maritime forecasts of the tropical cyclones in the region. This study will be helpful scientifically and strategically with a significant impact on socioeconomic activities in the region.
10
Mailier, Pascal J., David B. Stephenson, Christopher A. T. Ferro, and Kevin I. Hodges. "Serial Clustering of Extratropical Cyclones." Monthly Weather Review 134, no. 8 (August 1, 2006): 2224–40. http://dx.doi.org/10.1175/mwr3160.1.
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Abstract The clustering in time (seriality) of extratropical cyclones is responsible for large cumulative insured losses in western Europe, though surprisingly little scientific attention has been given to this important property. This study investigates and quantifies the seriality of extratropical cyclones in the Northern Hemisphere using a point-process approach. A possible mechanism for serial clustering is the time-varying effect of the large-scale flow on individual cyclone tracks. Another mechanism is the generation by one “parent” cyclone of one or more “offspring” through secondary cyclogenesis. A long cyclone-track database was constructed for extended October–March winters from 1950 to 2003 using 6-h analyses of 850-mb relative vorticity derived from the NCEP–NCAR reanalysis. A dispersion statistic based on the variance-to-mean ratio of monthly cyclone counts was used as a measure of clustering. It reveals extensive regions of statistically significant clustering in the European exit region of the North Atlantic storm track and over the central North Pacific. Monthly cyclone counts were regressed on time-varying teleconnection indices with a log-linear Poisson model. Five independent teleconnection patterns were found to be significant factors over Europe: the North Atlantic Oscillation (NAO), the east Atlantic pattern, the Scandinavian pattern, the east Atlantic–western Russian pattern, and the polar–Eurasian pattern. The NAO alone is not sufficient for explaining the variability of cyclone counts in the North Atlantic region and western Europe. Rate dependence on time-varying teleconnection indices accounts for the variability in monthly cyclone counts, and a cluster process did not need to be invoked.
11
Berdahl, Mira, Asa Rennermalm, Arno Hammann, John Mioduszweski, Sultan Hameed, Marco Tedesco, Julienne Stroeve, Thomas Mote, Tomoko Koyama, and Joseph R. McConnell. "Southeast Greenland Winter Precipitation Strongly Linked to the Icelandic Low Position." Journal of Climate 31, no. 11 (May 17, 2018): 4483–500. http://dx.doi.org/10.1175/jcli-d-17-0622.1.
Abstract:
Abstract Greenland’s largest precipitation flux occurs in its southeast (SE) region during the winter, controlled primarily by easterly winds and frequent cyclogenesis in the North Atlantic. Several studies have attempted to link SE Greenland precipitation to the North Atlantic Oscillation (NAO) but results are inconsistent. This work uses reanalysis, automatic weather station data, and regional climate model output to show that the east–west position of the Icelandic low is a better predictor of SE Greenland precipitation (average correlation of r = −0.48 in DJF) than climate indices such as the NAO (r = −0.06 in DJF). In years when the Icelandic low is positioned extremely west, moisture transport increases up to ~40% (or up to 40 kg m−1 s−1) off the SE Greenland coast compared to when the low is in an extreme east position. Furthermore, in years when the Icelandic low is positioned extremely west, storm track density and intensity increase just off the SE coast of Greenland. Thus, the Icelandic low’s longitudinal position dominates SE Greenland ice sheet’s wintertime precipitation, a positive term in the ice sheet mass balance. Given SE Greenland’s importance in the overall ice sheet mass balance, the position of the Icelandic low is therefore important for making projections of future sea level.
12
Werner, Angelika, and Neil J. Holbrook. "A Bayesian Forecast Model of Australian Region Tropical Cyclone Formation." Journal of Climate 24, no. 23 (December 1, 2011): 6114–31. http://dx.doi.org/10.1175/2011jcli4231.1.
Abstract:
Abstract A new and potentially skillful seasonal forecast model of tropical cyclone formation [tropical cyclogenesis (TCG)] is developed for the Australian region. The model is based on Poisson regression using the Bayesian approach. Predictor combinations are chosen using a step-by-step predictor selection. The three-predictor model based on derived indices of June–August average convective available potential energy, May–July average meridional winds at 850 hPa (V850), and July–September geopotential height at 500 hPa produces the smallest standard error (se = 0.36) and root-mean-squared error (RMSE = 5.20) for the leave-one-out cross-validated TCG hindcasts over the 40-yr record between 1968/69–2007/08. The corresponding correlation coefficient between observed annual TCG totals and cross-validated model hindcasts is r = 0.73. Using fourfold cross validation, model hindcast skill is robust, with 85% of the observed seasonal TCG totals hindcast within the model standard deviations. Seasonal TCG totals during ENSO events are typically well captured with RMSE = 5.14 during El Niño, and RMSE = 6.04 during La Niña years. The model is shown to be valuable in hindcasting seasonal TCG totals in the eastern Australian subregion (r = 0.73) and also provides some skill for the western Australian region (r = 0.42), while it not useful for the northern region. In summary, the authors find that the three-predictor Bayesian model provides substantial improvement over existing statistical TCG forecast models, with remarkably skillful hindcasts (forecasts) of Australian region and subregional seasonal TCG totals provided one month ahead of the TC season.
13
Sobel, Adam H., and Suzana J. Camargo. "Influence of Western North Pacific Tropical Cyclones on Their Large-Scale Environment." Journal of the Atmospheric Sciences 62, no. 9 (September 1, 2005): 3396–407. http://dx.doi.org/10.1175/jas3539.1.
Abstract:
Abstract The authors investigate the influence of western North Pacific (WNP) tropical cyclones (TCs) on their large-scale environment by lag regressing various large-scale climate variables [atmospheric temperature, winds, relative vorticity, outgoing longwave radiation (OLR), column water vapor, and sea surface temperature (SST)] on an index of TC activity [accumulated cyclone energy (ACE)] on a weekly time scale. At all leads and lags out to several months, persistent, slowly evolving signals indicative of the El Niño–Southern Oscillation (ENSO) phenomenon are seen in all the variables, reflecting the known seasonal relationship of TCs in the WNP to ENSO. Superimposed on this are more rapidly evolving signals, at leads and lags of one or two weeks, directly associated with the TCs themselves. These include anomalies of positive low-level vorticity, negative OLR, and high column water vapor associated with anomalously positive ACE, found in the region where TCs most commonly form and develop. In the same region, lagging ACE by a week or two and so presumably reflecting the influence of TCs on the local environment, signals are found that might be expected to negatively influence the environment for later cyclogenesis. These signals include an SST reduction in the primary region of TC activity, and a reduction in column water vapor and increase in OLR that may or may not be a result of the SST reduction. On the same short time scale, an increase in equatorial SST near and east of the date line is seen, presumably associated with equatorial surface westerly anomalies that are also found. This, combined with the correlation between ACE and ENSO indices on the seasonal time scale, suggests the possibility that TCs may play an active role in ENSO dynamics.
14
Huang, Xin, Tianjun Zhou, Johnny C. L. CHAN, Ruifen ZHAN, Ziming CHEN, and Jiuwei ZHAO. "Understanding uncertainties in projections of western north pacific tropical cyclogenesis." Environmental Research Letters, October 12, 2023. http://dx.doi.org/10.1088/1748-9326/ad02ad.
Abstract:
Abstract Reliable projections of tropical cyclone (TC) activities in the western North Pacific (WNP) are crucial for climate policy-making in densely-populated coastal Asia. Existing projections, however, exhibit considerable uncertainties with unclear sources. Here, based on future projections by the latest Coupled Model Intercomparison Project Phase 6 climate models, we identify a new and prevailing source of uncertainty arising from different TC identification schemes. Notable differences in projections of detected TCs and empirical genesis potential indices (GPIs) are found to be caused by inconsistent changes in dynamic and thermodynamic environmental factors affecting TC formations. While model uncertainty holds the secondary importance, we show large potential in reducing it through improved model simulations of present-day TC characteristics. Internal variability noticeably impacts near-term projections of the WNP tropical cyclogenesis, while the relative contribution of scenario uncertainty remains small. Our findings provide valuable insights into model development and TC projections, thereby aiding in adaptation decisions.
15
Meng, Lingwei, and Stephen T. Garner. "Non-local Controls on Tropical Cyclogenesis: A Trajectory-based Genesis Potential Index." Journal of the Atmospheric Sciences, October 31, 2023. http://dx.doi.org/10.1175/jas-d-23-0025.1.
Abstract:
Abstract Tropical cyclone (TC) genesis is initiated by convective precursors or “seeds” and influenced by environmental conditions along the seed-to-TC trajectories. Genesis Potential Indices (GPIs) provide a simple way to evaluate TC genesis likelihood from environmental conditions, but have two limitations that may introduce bias. First, the globally fixed GPIs fail to represent inter-basin differences in the relationship between environments and genesis. Second, existing GPIs are only functions of local environmental conditions, whereas non-local factors may have a significant impact. We address the first limitation by constructing basin- and timescale-specific GPIs (local-GPIs) over the Eastern North Pacific (ENP) and North Atlantic (NA) using Poisson regression. A sequential feature selection algorithm (SFS) identifies vertical wind shear and a heating condition as leading factors controlling TC genesis in the ENP and the NA, respectively. However, only a slight improvement in performance is achieved, motivating us to tackle the second limitation with a novel trajectory-based GPI (traj-GPI). We merge adjacent non-local environments into each grid point based on observed seed trajectory densities. The seed activity, driven mainly by upward motion, and the transition to TCs, controlled primarily by vertical wind shear or heating conditions, are captured simultaneously in the traj-GPI, yielding a better performance than the original GPIs. This study illustrates the importance of seed activity in modeling TC genesis and identifies key environmental factors that influence the process of TC genesis at different stages.
16
Sagar, Aswin, R. Krishnan, and T. P. Sabin. "Anthropogenically-forced weakening of the Indian summer monsoon and enhancement of the western North Pacific tropical cyclogenesis." Frontiers in Earth Science 11 (March 1, 2023). http://dx.doi.org/10.3389/feart.2023.1149344.
Abstract:
This study addresses the role of human-induced climate change on the interactions of convective activities between the Indian summer monsoon (ISM) and western North Pacific (WNP) regions - an important scientific issue which has been hitherto overlooked. We have examined this problem using two numerical experiments of a high-resolution climate model, with and without anthropogenic forcing (i.e., HIST and HISTNAT) for the historical period 1951–2005, supplemented by innovative diagnostics like causal network analysis. Our findings suggest that an anthropogenically-forced weakening of the ISM circulation tends to significantly enhance the genesis potential index (GPI) of the WNP tropical cyclones by 13.5% and associated convective activities, by reorienting the large-scale flow over the tropical Indo-Pacific in a manner that is conducive for enhancement of the WNP tropical cyclogenesis. Additionally, it is found that the probability of extremely low sea-level pressure (SLP) (<995.5 hPa) around Taiwan and Chinese mainland is significantly higher by 10.3% in the anthropogenically-forced simulation as compared to the natural run. Using the model outputs from HIST and HISTNAT, we also performed a causal effect network (CEN) analysis to understand the causal connections among the three indices involved in the ISM-WNP interactions (a) Indian monsoon circulation index (IMI) which is the difference in the area-averaged 850 hPa zonal winds between the boxes (40oE-80oE, 5oN-15oN) and (70oE-90oE, 20oN-30oN) (b) WNP tropical cyclone activity expressed as the genesis potential index (GPI) averaged over the region (120oE-180oE, 5oN-30oN) and (c) Tropical Indo-Pacific wind index (IPWND) which is based on the 850 hPa zonal winds averaged over the domain (100oE-130oE, 5oN-20oN). Results from the CEN analysis indicate that an anthropogenically-forced weakening of the IMI can lead to possible strengthening of GPI and IPWND with time-lags of 5 and 7 days, respectively. In general, it is noted that the causal relationships among IMI, GPI and IPWND are associated with shorter time-lags (∼4–9 days) in HIST and longer time-lags (∼19–28 days) in HISTNAT.