Tropical Cyclone Genesis Potential Research Articles

Modeled variations of tropical cyclone genesis potential during Marine Isotope Stage 3

To advance our understanding of the responses of tropical cyclones (TCs) to abrupt climate transitions, we investigate the variation of genesis potential and the underlying mechanisms during Marine Isotope Stage 3 (MIS 3) that is characterized by frequent millennial-scale oscillations, using a set of coupled model simulations. We illustrate that genesis potential mainly increases over the eastern North Pacific and South Pacific in the storm season during MIS 3 interstadial relative to preindustrial, whereas it decreases over the western North Pacific, North Atlantic, South Indian Ocean, and Australian coast. The relative importance of each genesis factor to genesis potential change varies across oceans. During the transition from warm interstadial to cold stadial induced by freshwater injection, TC genesis potential generally decreases in the Northern Hemisphere, while increases in the Southern Hemisphere. These changes are associated with asymmetric variations of genesis factors between Northern and Southern Hemisphere, which are further tied to the adjustment of large-scale environmental factors in response to the weakened Atlantic meridional overturning circulation. Additionally, TC genesis potential exhibits a regional “overshoot” phenomenon when the freshwater injection is terminated, manifested by an increase over the eastern North Pacific and North Atlantic and a decrease over the South Pacific, compared with the initial interstadial state. Our results may help improve our knowledge on past TC behaviors and shed lights on TC response to potential abrupt climate transitions in a warmer future.

Understanding the Variation and Mechanisms of Tropical Cyclone Genesis Potential over the Western North Pacific during the Past 20 000 Years

Abstract To deepen our understanding of the behavior of tropical cyclones (TCs) over the western North Pacific (WNP) and its response to various external forcings, we investigate the variation of TC genesis potential over the WNP during the past 20 ka that experienced several large forcings. Using a set of transient simulations and a genesis potential index, our results indicate that TC genesis potential in the storm season shows an overall increase averaged over the WNP during the Heinrich Stadial 1 (HS1; ∼17–16 ka) relative to the Last Glacial Maximum. Subsequently, there is a sharp decrease in genesis potential during the Bølling–Allerød (BA; ∼13.5 ka) relative to the HS1, which is followed by an increase during the Younger Dryas (YD; ∼12.9–11.7 ka). During the Holocene, TC genesis potential shows a slight decrease during the early–middle Holocene (∼11.7–6 ka) and a significant increasing trend afterward. Further analysis shows that the contribution of each genesis factor to the genesis potential change varies greatly with time, which is in turn tied to changes in external forcings. The increased meltwater fluxes dominate the increased genesis potential during the HS1 and YD, whereas all external forcings (i.e., changes in meltwater, ice sheets, CO2, and insolation) contribute to genesis potential change during the BA. The long-term increasing trend from the mid-Holocene to the present is controlled by orbital insolation. These results may help improve our understanding on past TC activity and shed lights on TC response to various external forcings in a long-term future.

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.

Reversed and comparable climate impacts from historical anthropogenic aerosol and GHG on global-scale tropical cyclone genesis potential

Emissions of anthropogenic aerosol and greenhouse gases (GHG) have significantly altered various aspects of the climate extremes in recent decades, yet, the observed global tropical cyclone frequency (TCF) shows no significant trend. Untangling this puzzle requires a better understanding of the precise contributions of the individual anthropogenic forcing to global TCF changes. Here, we quantify the relative contributions of anthropogenic aerosol and GHG to global TCF, represented by genesis potential index (GPI), using the single anthropogenic forcing experiments from the 14 Coupled Model Intercomparison Project phase 6 (CMIP6) models. We find that the two forcings have comparable but opposite impacts on GPIs due to their influences on the TC environment, leading to an insignificant change in GPIs in the historical period (1850–2014). Notably, the aerosol radiative forcing’s intensity is only about one-third of that of GHG, suggesting a more effective modulation of aerosol forcing on GPIs. The stable global TC frequency during the past decades could be attributable to the similar pace of the two anthropogenic emissions. The results highlight that a reliable global TC projection depends on both the aerosol and GHG emission policies.

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.

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

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.

Orbitally Induced Variation of Tropical Cyclone Genesis Potential Over the Western North Pacific During the Mid‐Piacenzian Warm Period: A Modeling Perspective

AbstractThe mid‐Piacenzian (3.264 to 3.025 Ma) is the most recent warmer‐than‐present interval in the geological past, with clear orbital‐scale variability in climate and environments. However, it remains unclear how the orbitally induced climate change may regulate the behavior of tropical cyclones (TCs). Here we use a fully coupled climate model to examine the response of TC genesis potential to different orbital configurations specific to the mid‐Piacenzian time slab. Under the Pliocene Research, Interpretation, and Synoptic Mapping version 3 boundary conditions that use modern orbital configurations, our results show that environmental conditions in the mid‐Piacenzian are less favorable for storm formation over the western North Pacific than the preindustrial. By performing a suite of sensitivity experiments, we find that orbital configurations significantly influence the magnitude and phase of genesis potential via the modification of the atmospheric thermal structure and associated large‐scale circulations. Regarding the overall favorability, increased and decreased orbitally induced insolation leads to less and more favorable conditions for storm formation over the western North Pacific, respectively. Under several extreme orbital parameters (e.g., when northern hemisphere summer insolation is at a minimum), there is an increasing favorability for genesis in the mid‐Piacenzian relative to the preindustrial. This is attributed to the fact that the orbitally induced increase in TC genesis exceeds the reduction from the Pliocene Research, Interpretation, and Synoptic Mapping version 3 boundary conditions. Our results highlight potential orbital‐scale variability in TC activity during the mid‐Piacenzian, implying possible changes in TC‐induced vertical mixing, which might play a potential role in regulating the orbital‐scale variation of tropical/global climate during the Pliocene.

Orbitally-induced variation of tropical cyclone genesis potential over the western North Pacific during the mid-Piacenzian warm period: A modeling perspective

Orbitally-induced variation of tropical cyclone genesis potential over the western North Pacific during the mid-Piacenzian warm period: A modeling perspective

Dominating Roles of Ice Sheets and Insolation in Variation of Tropical Cyclone Genesis Potential Over the North Atlantic During the Last 21,000 Years

AbstractTo advance our knowledge on tropical cyclone (TC)'s sensitivity to various external forcings, we investigate evolution of TC genesis potential over the North Atlantic during the last 21,000 years that experienced more varied forcings than present‐day using transient simulations. Abrupt large amplitude of fluctuations in genesis potential during the studied period was dominated by meltwater discharge, whereas insolation controlled the long‐term trend. Increased (decreased) meltwater discharge from Northern Hemisphere ice sheets leads to detrimental (conducive) conditions for genesis via altering the Atlantic Meridional Overturning Circulation and hence three‐dimensional temperature structure and Hadley circulation. This effect is larger than the orographic effect of retreated ice sheets that favors TC formation. Decreasing (increasing) insolation broadly results in more (less) favorable conditions for genesis arising from larger (smaller) local vertical temperature contrast and stronger (weaker) meridional circulation. Our study highlights potential changes of ice sheets and insolation in affecting future long‐term TC activity.

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.

An Anomalous Genesis Potential Index for MJO Modulation of Tropical Cyclones

Abstract Modulation of tropical cyclone (TC) genesis by the Madden–Julian oscillation (MJO) has been quantitatively diagnosed by using a climatological genesis potential index (GPI). Analysis of TC genesis during November–April of 1979–2014 indicates the most effective factors controlling intraseasonal TC genesis are 850-hPa relative vorticity weighted by the Coriolis parameter fζr850 and 500-hPa vertical motion ω500. The total vertical wind shear and maximum potential intensity are unimportant, and the role of 600-hPa relative humidity is greatly represented by ω500. The MJO modulates TC genesis primarily through changing low-level vorticity induced by its Rossby wave gyres and meridional shears of equatorial zonal winds. A new intraseasonal GPI (ISGPI) is proposed to quantify the MJO’s modulation of TC genesis. The ISGPI significantly improves representation of intraseasonal variation of TC genesis in the tropics and in each subregion of the southern Indian Ocean, Australian monsoon, and South Pacific. In the hot spots of the Southern Hemisphere TC genesis zone, the probability of TC genesis can differ by a factor of 5–19 as a result of MJO modulation. The results suggest that the large-scale factors controlling TC genesis may vary with different time scales, and the climatological GPI may not be quite applicable for diagnoses of climate variability and future change of TC genesis potential. To simulate realistic impacts of the MJO on TC genesis, general circulation models must reproduce not only realistic eastward propagation but also the MJO low-level circulation structure. Application of the new ISGPI may have a large potential to improve dynamical subseasonal prediction of TC genesis.

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.

Tropical cyclone genesis potential across palaeoclimates

Abstract. The favourability of the mid-Pliocene, Last Glacial Maximum (LGM) and mid-Holocene for tropical cyclone formation is investigated in five climate models. This is measured by a genesis potential index, derived from large-scale atmospheric properties known to be related to storm formation. The mid-Pliocene and Last Glacial Maximum (LGM) were periods where carbon dioxide levels were higher and lower than preindustrial levels respectively, while the mid-Holocene differed primarily in its orbital configuration. The cumulative global genesis potential is found to be fairly invariant across the palaeoclimates in the multi-model mean. Despite this all ensemble members agree on coherent responses in the spatial patterns of genesis potential change. During the mid-Pliocene and LGM, changes in carbon dioxide led to sea surface temperature changes throughout the tropics, yet the potential intensity (a measure associated with maximum tropical cyclone strength) is calculated to be relatively insensitive to these changes. Changes in tropical cyclone genesis potential during the mid-Holocene are found to be asymmetric about the Equator: being reduced in the Northern Hemisphere but enhanced in the Southern Hemisphere. This is clearly driven by the altered seasonal insolation. Nonetheless, the enhanced seasonality drove localised changes in genesis potential, by altering the strength of monsoons and shifting the intertropical convergence zone. Trends in future tropical cyclone genesis potential are consistent neither between the five models studied nor with the palaeoclimate results. It is not clear why this should be the case.

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.

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.

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.