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.

Diagnosis of the ENSO modulation of tropical cyclogenesis over the southern South China Sea using a genesis potential index

The modulation of tropical cyclogenesis over the southern South China Sea （SSCS） by the El Nin o- Southern Oscillation （ENSO） is examined in October–December （OND）, when tropical cyclone （TC） activities are most active in this region. The results reveal that there were more TCs formed over the SSCS during La Nin a years and less TCs during El Nin o years. How different environmental factors （including low-level vorticity, mid-level relative humidity, vertical wind shear, and potential intensity） contribute to this influence is investigated, using a genesis potential （GP） index developed by Emanuel and Nolan. Composite anomalies of the GP index are produced for El Nin o and La Nin a years separately, which could account for the changes of TC frequency over the SSCS in different ENSO phases. The degree of contribution by each factor is determined quantitatively by producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology. The results show that the mid-level relative humidity makes the largest contribution to the ENSO modulation of tropical cyclogenesis over the SSCS. Although warmer sea surface temperatures （SSTs） and larger amount of evaporation from the ocean surface were observed over the SSCS during El Nin o years, anomalous descending motions due to the anomalous Walker circulations inhibited the upward transports of water vapor and led to less moisture contents in the middle troposphere, which suppressed TC formations.

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 multi- model 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.

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

This study investigates the global performance of the tropical cyclone(TC)genesis potential index based on oceanic parameters(GPI_(ocean))proposed by Zhang et al.(2016).In six major TC formation basins,GPI_(ocean)can represent the seasonal variations of TC genesis over most basins,except for the North Indian Ocean(NIO).The monthly climatological GPI_(ocean)shows only a single peak in the NIO,which cannot describe the bimodal pattern of the annual cycle of TC genesis.To determine the cause of the poor performance of GPI_(ocean)in the NIO,the relative contributions of different parameters related to GPI_(ocean)are calculated and compared with those related to the genesis potential index developed by Emanuel and Nolan(2004)(GPI04).Results show that the net longwave radiation on the sea surface is responsible for the single peak of TC genesis in the NIO in boreal summer.Compared with GPI04,vertical wind shear is not involved in GPI_(ocean).Vertical wind shear is the dominant factor inhibiting TC genesis in the NIO in boreal summer.Therefore,the absence of vertical wind shear in GPI_(ocean)results in the failure of the annual cycle of TC genesis in the NIO.

Modulation of Tropical Cyclogenesis in the Western North Pacific by the Quasi-Biweekly Oscillation

The quasi-biweekly oscillation （QBWO） is the second most dominant intraseasonal mode over the westem North Pacific （WNP） during boreal summer. In this study, the modulation of WNP tropical cyclogenesis （TCG） by the QBWO and its association with large-scale patterns are investigated. A strong modulation of WNP TCG events by the QBWO is found. More TCG events occur during the QBWO＇s convectively active phase. Based on the genesis potential index （GPI）, we further evaluate the role of environmental factors in affecting WNP TCG. The positive GPI anomalies associated with the QBWO correspond well with TCG counts and locations. A large positive GPI anomaly is spatially correlated with WNP TCG events during a life cycle of the QBWO. The low-level relative vorticity and mid-level relative humidity appear to be two dominant contributors to the QBWO-composited GPI anomalies during the QBWO＇s active phase, followed by the nonlinear and potential intensity terms. These positive contributions to the GPI anomalies are partly offset by the negative contribution from the vertical wind shear. During the QBWO＇s inactive phase, the mid-level relative humidity appears to be the largest contributor, while weak contributions are also made by the nonlinear and low-level relative vorticity terms. Meanwhile, these positive contributions are partly cancelled out by the negative contribution from the potential intensity. The contributions of these environmental factors to the GPI anomalies associated with the QBWO are similar in all five flow patterns--the monsoon shear line, monsoon confluence region, monsoon gyre, easterly wave, and Rossby wave energy dispersion associated with a preexisting TC. Further analyses show that the QBWO strongly modulates the synoptic-scale wave trains （SSWs） over the WNP, with larger amplitude SSWs during the QBWO＇s active phase. This implies a possible enhanced （weakened） relationship between TCG and SSWs during the active （inactive） phase. This study improves our understanding of the modulation of WNP TCG by the QBWO and thus helps with efforts to improve the intraseasonal prediction of WNP TCG.

Tropical cyclone genesis over the western North Pacific simulated by Coupled Model Intercomparison Project Phase 6 models

Threatening millions of people and causing billions of dollars in losses,tropical cyclones(TCs)are among the most severe natural hazards in the world,especially over the western North Pacific.However,the response of TCs to a warming or changing climate has been the subject of considerable research,often with conflicting results.In this study,the abilities of Coupled Model Intercomparison Project(CMIP)Phase 6(CMIP6)models to simulate TC genesis are assessed through historical simulations.The results indicate that a systematic humidity bias persists in most CMIP6 models from corresponding CMIP Phase 5 models,which leads to an overestimation of climatological TC genesis.However,the annual cycle of TC genesis is well captured by CMIP6 models.The abilities of 25 models to simulate the geographical patterns of TC genesis vary significantly.In addition,seven models are identified as well simulated models,but seven models are identified as poorly simulated ones.A comparison of the environmental variables for TC genesis in the well-simulated group and the poorly simulated group identifies moisture in the mid-troposphere as a key factor in the realistic simulation of El Niño-Southern Oscillation(ENSO)impacts on TC genesis.In contrast with the observations,the poorly simulated group does not reproduce the suppressing effect of negative moisture anomalies on TC genesis in the northwestern region(20°–30°N,120°–145°E)during El Niño years.Given the interaction between TC and ENSO,these results provide a guidance for future TC projections under climate change by CMIP6 models.

What Controls Early or Late Onset of Tropical North Atlantic Hurricane Season？

The occurrence of first hurricane in early summer signifies the onset of an active Atlantic hurricane season.The interannual variation of this hurricane onset date is examined for the period 1979-2013.It is found that the onset date has a marked interannual variation.The standard deviation of the interannual variation of the onset day is 17.5 days,with the climatological mean onset happening on July 23.A diagnosis of tropical cyclone（TC） genesis potential index（GPI） indicates that the major difference between an early and a late onset group lies in the maximum potential intensity（MPI）.A further diagnosis of the MPI shows that it is primarily controlled by the local SST anomaly（SSTA）.Besides the SSTA,vertical shear and mid-tropospheric relative humidity anomalies also contribute significantly to the GPI difference between the early and late onset groups.It is found that the anomalous warm（cold） SST over the tropical Atlantic,while uncorrected with the Nino3 index,persists from the preceding winter to concurrent summer in the early（late） onset group.The net surface heat flux anomaly always tends to damp the SSTA,which suggests that ocean dynamics may play a role in maintaining the SSTA in the tropical Atlantic.The SSTA pattern with a maximum center in northeastern tropical Atlantic appears responsible for generating the observed wind and moisture anomalies over the main TC development region.A further study is needed to understand the initiation mechanism of the SSTA in the Atlantic.

Comparison of Controlling Parameters for Near-Equatorial Tropical Cyclone Formation between Western North Pacific and North Atlantic

In this study,the differences in spatial distribution and controlling parameters for the formation of near-equatorial tropical cyclones(NETCs)between the western North Pacific(WNP)and the North Atlantic(NA)are investigated.NETCs exhibit distinctive spatial variabilities in different basins.Over the past few decades,the majority of NETCs took place in WNP while none was observed in NA.The mechanism behind such a distinguishing spatial distribution difference is analyzed by using statistical methods.It is noted that the dynamical variables such as low-level relative vorticity and vertical wind shear(VWS)are likely the primary controlling parameters.Compared with NA,larger low-level vorticity and smaller VWS appear over WNP.The increase of vorticity attributes a lot to the turning of northeast trade wind.NETCs in WNP tend to occur in the areas with VWS less than 9 m s^(-1),while the VWS in NA generally exceeds 10 m s^(-1).On the other hand,the sea surface temperature in the near-equatorial region of both of the two oceans exceeds 26.5℃and the difference of mid-level moisture is not significant;thus,thermal factors have little contribution to the distinction of NETC activities between WNP and NA.Intraseasonal oscillation(ISO)and synoptic-scale disturbances in WNP are also shown to be more favorable for NETC genesis.More NETCs were generated in ISO active phase.Synoptic-scale disturbances in WNP obtain more energy from the mean flows through the barotropic energy conversion process.The overall unfavorable thermal and dynamic conditions lead to the absence of NETCs in NA.