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ScienceDirect
Tropical Cyclone Research and Review 12 (2023) 165–181
www.keaipublishing.com/tcrr

Tropical cyclogenesis: Controlling factors and physical mechanisms
V.P.M. Rajasree a,*, Xi Cao b, Hamish Ramsay c, Kelly M. Nú~
nez Ocasio d, Gerard Kilroy e,George R. Alvey III f,g, Minhee Chang h, Chaehyeon Chelsea Nam i, Hironori Fudeyasu j,
Hsu-Feng Teng k, Hui Yu laIndia Meteorological Department, Goa, India
Chinese Academy of Sciences, Beijing, Chinac
CSIRO, Environment, Aspendale, Victoria, AustraliadNational Center for Atmospheric Research, Colorado, USAe
Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft-und, Berlin, Germanyf
Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, USAg
NOAA/Atlantic Oceanographic and Meteorological Laboratory/Hurricane Research Division, Miami, Florida, USAh
Korea Institute of Science and Technology, Seoul, Republic of Koreai
Colorado State University, Colorado, USAj
Yokohama National University, Yokohama, JapankNational Center for Atmospheric Research/Paciﬁc Science Association, Colorado, USAlShanghai Typhoon Institute/China Meteorological Administration, Shanghai, Chinab

Available online 27 September 2023

Abstract
In this review, advances in the understanding of the controlling factors and physical mechanisms of tropical cyclogenesis (TCG) are summarized from recent (2018–2022) research on TCG, as presented in the Tenth International Workshop on Tropical Cyclones (IWTC-10).
Observational, theoretical, and numerical modeling studies published in recent years have advanced our knowledge on the inﬂuence of large-scale
environmental factors on TCG. Furthermore, studies have shown clearly that appropriate convective coupling with tropical equatorial waves
enhances the development chances of TCG. More recently, illuminating research has been carried out on analyzing the mechanisms by which
oscillations and teleconnections (El Niño Southern Oscillation (ENSO) in particular) modulate TCG globally, in association with changes in thesea surface temperature (SST). In addition to this, recent research has diligently addressed different aspects of TCG. Multiple studies have reported
the applicability of uniﬁed theories and physical mechanisms of TCG in different ocean basins. Recently, research has been carried out on TCG
under different ﬂow pattern regimes, dry air intrusion, importance of marsupial pouch, genesis of Medicanes, wind shear, convection and verticalstructure. Furthermore, studies have discussed the possibility of near equatorial TCG provided that there is enough supply of background verticalvorticity and relatively low vertical wind shear. Progress has been made to understand the role of climate change on global and regional TCG.
However, there are still signiﬁcant gaps which need to be addressed in order to better understand TCG prediction.
© 2023 The Shanghai Typhoon Institute of China Meteorological Administration. Publishing services by Elsevier B.V. on behalf of KeAi
Communication Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Tropical cyclogenesis; AEW; ENSO; SST; Marsupial pouch; Dry air intrusion; Moderate shear; Convection; Near equatorial TCG; Climate change* Corresponding author.
E-mail addresses: rajasree.vpm@imd.gov.in, rajasree.vpm@gmail.com(V.P.M. Rajasree).
Peer review under responsibility of Shanghai Typhoon Institute of China
Meteorological Administration.

Production and Hosting by Elsevier on behalf of KeAi

1. Introduction
Tropical cyclones (TCs) are among the most disastrous
weather phenomena over the global ocean basins. Tropical
cyclogenesis (TCG) prediction remains as a challenging task
due to the limited understanding of the mechanisms associatedwith it. There are multiple controlling factors that modulate thehttps://doi.org/10.1016/j.tcrr.2023.09.004
2225-6032/© 2023 The Shanghai Typhoon Institute of China Meteorological Administration. Publishing services by Elsevier B.V. on behalf of KeAi Communication Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

V.P.M. Rajasree, X. Cao, H. Ramsay et al.

Tropical Cyclone Research and Review 12 (2023) 165–181

TCG, and a detailed understanding of these controlling factors
is important to deduce the underlying physical mechanisms
leading to TCG.
Convectively coupled tropical and equatorial waves are often
the seedlings of TCs and can inﬂuence the environment inwhich a TC forms. TCG is modulated by regions where African
easterly wave (AEW) activity dominates; to the north and southof African easterly jet (AEJ) (Reed et al. 1988a; Lau and Lau
1990). Over the North Atlantic (NA), AEWs are the main
precursors of about 60% of TCs (Landsea 1993; Avila et al.
2000; Russell et al. 2017). Recent studies have shown thatappropriate convective coupling with tropical equatorial waves
enhances the chances of TCG (Fink and Reiner 2003; Hopschet al. 2010; Semunegus et al. 2017; Núñez Ocasio et al.
2020b).This coupling between convection and AEWs isstrongly modulated by moisture-convection interactions (Núñez
Ocasio and Rios-Berrios 2023) that are more likely withfavorable West African Monsoon conditions (Núñez Ocasio
et al. 2021). With an AEW climatology of developing (thosethat do become TCs) and non-developing AEWs, Núñez Ocasio
et al. (2020b) found that the convection coupled to developing
AEWs over western Africa and eastern Atlantic is more intense,
more in phase with the wave vortex, and with larger cloud coverareas than convection coupled to non-developing AEWs. These
results agree with Hopsch et al. (2010) that found the developers
had stronger low-level circulations over the eastern Atlantic
related to more convective activity.
Furthermore, recent studies (Yamada et al. 2019; Song et al.
2020; Lu et al. 2022) have investigated the impacts of different
states of El Niño–Southern Oscillation (ENSO) on variation in
TCG frequency in the western North Paciﬁc (WNP); which is
the most active TCG region worldwide. An extremely active
TCG over the WNP in summer 2018 was primarily caused by
the central-Paciﬁc El Niño (Gao et al. 2020). Li et al. (2022)
have indicated that TCG frequency over the WNP are modulated by the Paciﬁc Meridional Mode (PMM) (Amaya, 2019),
Atlantic Multidecadal Oscillation (AMO), and global warming.
The positive phase of the PMM in the tropical North Paciﬁcremotely enhances TCG frequency over the WNP (Liu et al.
2019a), e.g., in summer 2018 (Takaya, 2019). A decrease in
the moisture condition over South China Sea (SCS) and WNP
associated with a long-lasting MJO has been discussed in Jin
et al. (2022a). Over the North Indian Ocean (NIO), Rajasree
et al. (2016a) have discussed the genesis sequence of TC
Madi (2013). The authors have shown that the protective pouch
associated with the westward moving parent disturbance provided a moistening environment for the pre-disturbance. A
recent study by Núñez Ocasio and Rios-Berrios (2023) suggestthat more moisture content in the atmosphere may not signify
more favorable growth driven by moisture, more intense tropicalwaves nor more intense TCG. Many studies have focused on theimpact of dry air (Fritz and Wang 2013; Komaromi 2013;
Rajasree et al. 2021) on TCG. The pouch associated with theparent disturbance plays an important role in protecting the predisturbance from dry air intrusion (Rajasree et al. 2016b).
As far as knowledge of fundamental mechanisms controlling
the response of TCG to climate is concerned, it remains elusive,

and there is no extant theory for the global rate of TCG. TC
frequency changes at global and regional scales remain one of
the more challenging and uncertain aspects of the projected
impacts of climate change on TCs (Knutson et al. 2020). Many
studies suggest that the number of TCs will decline in a warmer
world (Knutson et al. 2020), especially in the Southern Hemisphere (Roberts et al. 2020), but some dynamical and statisticaldynamical models predict increased TC frequency with warming (Bhatia et al. 2018; Emanuel 2021a; Lee et al. 2020). The
Tenth International Workshop on Tropical Cyclones (IWTC-10)
has discussed various aspects of TCs based on the recent
research (2018–2022). This paper reviews the latest advances in
research on TCG, with a special emphasis on controlling factors
and physical mechanisms. Multi-scale controls of TCG are
discussed in section 2, and thermodynamic conditions inﬂuencing TCG are discussed in section 3. Impacts of shear on
TCG and near equatorial TCG are discussed in section 4 andsection 5, respectively. Section 6 discusses the mesoscale controls and section 7 describes the impacts of climate change on
TCG. Section 8 presents the summary and conclusions.
2. Multi-scale controls of tropical cyclogenesis
2.1. Tropical equatorial waves
The prominent wave types associated with TCG in different
ocean basins include Mixed Rossby Gravity waves, tropicaldepression (TD) disturbances, Equatorial Rossby (ER) waves,
Kelvin waves, and Madden-Julian Oscillation (MJO)
2.1.1. Western North Paciﬁc and North Indian Ocean
Wu and Takahashi (2018) studied the relationship between
different wave types and TCG in the WNP. They found that thesynoptic low-level vorticity and convection associated witheach type of wave contribute to TCG events. It is, however, in
the region where both the cyclonic vorticity and active convection overlap where the majority (83.2%) of TCs form, andnot in the region where there is only low-level vorticity or
active convection. Wu and Takahashi (2018) found that twothirds of TCG events are credited to one or two coexisting
wave types and Kelvin waves account for the smallest percentage of tropical wave-related TCG.
Using empirical orthogonal function (EOF) analyses, Zhao
et al. (2019a) showed that it is the combined role of theintra-seasonal MJO, quasi-biweekly oscillation (QBWO), ER
waves and synoptic-scale waves that modulate the majority of
TCG events in the WNP. Moreover, Lai et al. (2020) showedthat 84% of TCG occurs within the active phase of convectively coupled equatorial waves similar to Wu andTakahashi (2018) ﬁndings. In the active phase of multiple
tropical cyclone (MTC) events where consecutive TC eventsoccur in a period of three days or less, ER waves provide theprimary favorable moist and convective environmental conditions for subsequent TCG (Lai et al. 2020). During the inactive
phase of MTC events when consecutive TCG events occur ninedays or more from each other it is TD-type waves that are thelargest contributor to TCG.
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Tropical Cyclone Research and Review 12 (2023) 165–181(NDAEWs). Unlike the MCSs associated with NDAEWs
which are likely to move south of the AEW trough and faster,
MCSs of DAEWs tend to move with the DAEW, and thus, inphase with the AEW trough (Fig. 1, adapted from Núñez
Ocasio et al. 2020b).These differences between DAEWs and
NDAEWs become important for the intensiﬁcation of the AEW
vortex and subsequent TCG as this slower-moving convection,i.e. moving at the same speed of the AEW trough, supplies
moisture and latent heat to the AEW vortex supporting its
further growth. Núñez Ocasio et al. (2020b) also found that
DAEWs have more squall line type of MCSs associated withthem over western Africa than NDAEWs. In a similar wavefollowing framework, Lawton et al. (2022) found that convectively coupled Kelvin waves (CCKWs) can inﬂuence thelife cycles of NDAEWs that have qualitatively similar characteristics to DAEWs. They found that convective coveragearound AEWs changed in phase with CCKW crests, as also
shown in Núñez Ocasio et al. (2020b). This is relevant to TCG
as the strong convective area is a signiﬁcant contributor to thelatent heat rate associated with DAEWs.
In the Eastern Paciﬁc, the initiation and evolution of easterly
waves (EWs) can be inﬂuenced by anomalous low-level
westerly MJO and Caribbean low-level jet (CLLJ) periods
which together provide favorable conditions for TCG(Whitaker and Maloney, 2018). Whitaker and Maloney (2020)
further showed that the EW that became Hurricane Carlotta(2012) originated from an intense MCS over the Panama Bight.

Similar to the WNP, over the Bay of Bengal (BoB), TCG isfavored when more than one type of wave is present or active(Landu et al. 2020). The dynamically favorable conditions
provided by ER waves (increase in relative vorticity) in combination with the favorable thermodynamic conditions provided by the MJO (increase in RH) are associated with a higher
number of TCG events over the BoB. Landu et al. (2020) also
showed that Kelvin waves contributed the least to cyclogenesis
with relatively low cyclonic vorticity. An example of a TCG
event from a wave, speciﬁcally a convectively coupled equatorial easterly wave, over the BoB was ‘Ockhi’ in November of2017 (Sanap et al. 2020).
2.1.2. Atlantic Ocean and eastern Paciﬁc Ocean
Núñez Ocasio et al. (2021) found that developing AEWs
(DAEWs), and thus, those that undergo TCG, are more likely
southern-track AEWs, where southern-track AEWs develop
along the southern ﬂank of the AEJ. These southern-track
DAEWs are more prone to interact with mesoscale convective systems (MCSs) from the Inter Tropical Convergence
Zone (ITCZ) and West African Monsoon (WAM), as shown in
Núñez Ocasio et al. (2021) and in Núñez Ocasio et al. (2020a)(their Fig. 4) even with origins over eastern Africa, which
eventually leads to subsequent TCG. Previous studies havenoted that the merger of the northern and southern tracks canlead to TCG but recently, Duvel (2021) showed that the merger
does not necessarily lead to TCG, and subsequent deepening of
the vortices and favorable environmental conditions must bepresent.
Núñez Ocasio et al. (2021) investigated the origins of TC
precursors (and thus, DAEWs) and found that AEWs that
initiate over eastern Africa, in the vicinity of the Ethiopian
Highlands, are more probable to undergo TCG than those that
initiate over central or western Africa. Moreover, they found a
relationship between the large-scale environment over east
Africa and the likelihood of an AEW subsequently undergoing
TCG. Núñez Ocasio et al. (2021) proposed the following key
predictors of TCG over eastern Africa.
1. Stronger northerly and WAM ﬂow;
2. More intense zonal Somali jet and stronger convergenceover the Marrah Mountains (region of AEW forcing);
3. Larger WAM moisture and convective signature concentrated west of the Marrah Mountains.
Over western Africa, TC precursors include.
1. Stronger WAM;
2. More intense and extended AEJ off the west coast of Africa
and offshore waters.

Fig. 1. Conceptual model describing how the chances of TCG increase withappropriate convective coupling. For DAEW–MCS systems, TCG is favoredwhen convective cloud clusters (CCCs; denoted by the cloud) move at the same
speed as the AEW trough (direction and magnitude of propagation are denotedby solid black arrows) and are latitudinally in phase with the trough. Maintaining the same speed insures the same phase relationship. Note that convection of NDAEWs is positioned south of the trough and propagates fasterthan the trough; thus, phase locking and further intensiﬁcation are less likely.
Adapted from Núñez Ocasio et al. (2020b).

The interactions between AEWs and MCSs have beenshown to be signiﬁcant for TCG in the Atlantic. Núñez Ocasio
et al. (2020b) evaluated the propagation of MCSs relative to the
AEW they are associated with (i.e., wave-relative framework)and they found statistically signiﬁcant differences between
MCSs associated with DAEWs and non-developing AEWs
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Tropical Cyclone Research and Review 12 (2023) 165–181

The coupled MCS-EW system was strongly modulated by the
Choco and Papagayo jets (that are an extension of the CLLJ).
2.2. Oscillations and teleconnections
It is of great importance to elucidate the mechanisms
responsible for changes in TCG to improve accurate forecasts
over the major TC development regions, such as the WNP,
eastern North Paciﬁc (ENP) and the tropical North Atlantic
Ocean (TNA). TCG variation from the interdecadal to theintraseasonal time scales has been actively discussed in theliterature during the last ﬁve years (2018–2022).
On the interdecadal time scale, Zhao et al. (2019b) found thatthe covariability of TCG latitude and longitude is increasing
since 1998, which is found to be closely linked to shifting
ENSO conditions and tropical Paciﬁc climate regime shift. In
addition, Wang et al. (2022) identiﬁed an anti-phase decadal
variation in TCG between the WNP and TNA. The trans-basin
TC connection results from a subtropical east-west “relay” teleconnection triggered by AMO, involving a chain atmosphereocean interaction in the North Paciﬁc. Both studies (Liu andChen, 2018; Cao et al. 2018a) have detected an intensiﬁed
impact of the ENSO Modoki on TCG frequency over the WNP
since early 1990s, but with different mechanisms.
On the interannual time scale, Chen et al. (2018a) found that
TCG is closely related to the ENSO, which is a dominant signalof the atmosphere and ocean. Other than SST spatial pattern,
Liang et al. (2022a) found that the onset time difference of El
Niño events has various inﬂuences on TCG. The equatorial
Paciﬁc Ocean warming can be separated into spring(April–June; SP) and summer (July–October; SU) modes basedon the onset time of El Niño events. In the SP type, the locationof TCG over the WNP shifts signiﬁcantly eastward and equatorward, whereas in the SU type, only an eastward shift occurs.
In addition to the SST anomalies in the central-eastern Paciﬁc,
Pu et al. (2019) demonstrated that the spring Victoria mode, thesecond EOF (EOF2) of SST anomalies in the North Paciﬁcnorth of 20◦ N, exhibits a positive correlation with the following
summer TC frequency over the eastern WNP and negative
correlation over the western WNP. A combination mode (Cmode), formed by nonlinear interactions between the western
Paciﬁc warm pool annual cycle and ENSO variability (Stuecker
et al. 2013), has a signiﬁcant negative relationship with WNP
TCG frequency on a monthly timescale from 1970 to 2019
(Fig. 2, adapted from Song et al. 2021). The related atmosphericconditions are linked to an anomalous large-scale anticyclone
over the WNP induced by the C-mode. Zhang et al. (2020)
found a dominant role of the SST variation in the central Paciﬁc in the TCG over the WNP compared to the simultaneous
impact of the PMM. However, Wu et al. (2021) revisited theinterannual impact of the PMM on TCG over the WNP andfound the important effect of the PMM on WNP TC activity,
especially during neutral ENSO years. Liu et al. (2019a) foundthat the statistical relationship between PMM and WNP TCG isdominated by their co-variability on the decadal time scales.
Thus, the role of the PMM deserves to be further studied.

Fig. 2. Composite monthly evolution of the ﬁrst PC (green solid line), thesecond PC (green dashed line), and the WNP TC frequency anomaly percentage relative to its corresponding climatological mean (bar) for(a) 7 El Niñoand (b) 9 La Niña winters during 1970–2019. The abscissa indicates a 19month period from March of year 0 to September of year 1. Filled bars indicate composited values signiﬁcant at the 0.05 level based on a two-sample
Student's t-test, compared to 36 neutral winters. Adapted from Song et al.
(2021).

On the other hand, atmospheric variability also plays a rolein the interannual variation of TCG over the WNP. Cao et al.
(2021) found that TCG frequency has a signiﬁcant positive
correlation with the intensity of intraseasonal oscillation (ISO),but a weak correlation with the intensity of synoptic variationof environmental factors during the peak season over the WNP.
Zhan et al. (2022) found a signiﬁcant positive correlation between TCG frequency over the WNP and the jet intensity in theentrance region of the tropical easterly jet over the tropicalwestern Paciﬁc, which is associated with strong ageostrophic
northerly winds in the entrance region.
On the intraseasonal scale, the 20-70-day oscillation mainly
modulates the frequency of TCG, while the 10-20-day QBWO
usually inﬂuences the genesis location over the WNP and SCS
region (You et al. 2019). Through affecting the strength of
monsoon trough and the location of the western Paciﬁc subtropics high, the ISO combinations modulate the thermodynamic
conditions and lead to changes in the frequency and location ofTCG. A comparison research through observational analysis andnumerical simulation indicated that RH associated with the MJO
plays the most important role in modulating TCG over the
WNP, the Gulf of Mexico and the western Caribbean Sea, while
VWS has the most signiﬁcant impact on TC activity over the168

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Tropical Cyclone Research and Review 12 (2023) 165–181

convection contrasts between inner and outer pouch region(neither convection intensity nor the area) is the key feature.
The Marsupial paradigm has been examined in TCs indifferent ocean basins. Rajasree et al. (2021) examined adeveloping and a non-developing disturbance in the NIO basedon the marsupial theory using an operational numerical model.
The evidence of a synoptic scale parent disturbance, its protection of the vortex from hostile environmental factors andpositive vorticity tendency at low-level are clearly observed,
not only during the developing disturbance but also during thenon-developing one. However, the eventual failure of development was due to the interaction of the vortex with strong
deep layer (850-200 hPa) environmental wind shear and astrong ridge developed over the central Indian land mass andcolder sea water. Yoshida and Fudeyasu (2020) examined thehorizontal distribution of the genesis environment in different
ﬂow pattern regimes. In terms of geographical location, TCG
most frequently occur over the eastern edge of the area presenting high genesis potential index, high RH and weak VWS.
In terms of temporal evolution, such favorable conditions arefound to gradually organize three days before genesis in the
monsoon shear line and easterly wave regimes, while they
organize 13 days before genesis in monsoon conﬂuence regions. Raavi and Walsh (2020) conducted a basinwide statistical analysis of factors limiting tropical storm formation by
adopting the Okubo-Weiss Zeta Parameter (OWZP) introducedby Tory et al. (2013) to trace developing and non-developing
depressions. The contrast between developing and nondeveloping depressions is pronounced as strong positive(negative) OWZP was found in the core (surrounding) region
for developing cases but a weaker shear sheath in the northwest/west side of the depression was found in non-developing
cases.

eastern Atlantic (Zhao and Li 2019). In addition to the WNP andTNA, the study of Bhardwaj et al. (2019) speciﬁcally focused on
the MJO's impacts on TCs in the BoB during the two peak TC
periods, April–June and October–December. TC activity issigniﬁcantly enhanced (suppressed) over the BoB when theconvectively active MJO phase is positioned over the eastern
Indian Ocean and the Maritime Continent (the western hemisphere and Africa). Chen et al. (2018a) synthetically examined
the inﬂuence of intraseasonal-to-interannual oscillations on TCG
by measuring the productivity of TCG from the developing andnon-developing precursory tropical disturbances. They foundthat the percentage value of precursory tropical disturbances
evolving into TCs increases (decreases) during the phases of
positive-vorticity (negative-vorticity) ER wave, the active(inactive) MJO and El Niño (La Niña) years. Relative vorticity
acts as the most important factor to modulate the percentage
value of precursory tropical disturbances evolving into TC
compared with VWS and mid-level RH. Previous studies aremostly concerned with the environmental conditions of the TCG
on a large spatial scale averaged in a certain period. Cao et al.
(2018b, 2019) examined the state at a locality of TCG to ﬁnd
out the relative contributions of different time scales (interannual, intraseasonal and synoptic variations) to the TCG over the
WNP and TNA. They found that the synoptic variation is themost important component over the TNA, while the leading
component is the intraseasonal variation over the WNP.
2.3. Marsupial paradigm
There have been signiﬁcant advances in studies related to
the Marsupial paradigm, particularly focusing on the evolutionof the mid-level pouch circulation and its accompanied inﬂuence on TCG. Similar analyses were conducted for the case of
Typhoon Nepartak (2016) in the WNP (Wu and Fang 2019)and Hurricane Karl (2010) in the NA (Bell and Montgomery2019). From both case studies, alternative enhancement oflow- and midlevel circulations (MLCs) is observed during each
TCG event and both emphasized the thermodynamic role of
MLC aiding low-level spin up. From the vorticity budget
analysis, Wu and Fang (2019) revealed that the MLC isenhanced during deep convection due to vertical advection andtilting of vorticity from low-levels and is further intensiﬁed due
to accompanying midlevel convergence during stratiform precipitation. According to Bell and Montgomery (2019), such
midlevel convergence of vorticity may increase the depth ofprotective pouch and the cooling and moistening in the stratiform precipitation region may promote convective bursts andrecur the enhancement of low-level vortex. However, one
should be cautious that the dynamic impact of the stratiform
divergence at low-levels would weaken the near-surface circulation and lead to eventual low-level spin-down. That is,
stratiform precipitation and the midlevel vortex play a supporting role in the genesis process, whereas the deep convection is a primary element in the genesis. Among various
components of convection, a key feature leading up to TCG isinvestigated by Wang (2018). By investigating more than 150
named Atlantic storms, the author conﬁrmed that the3. Thermodynamic conditions inﬂuencing tropical
cyclogenesis
3.1. Impacts of sea surface temperature
TCG is strongly forced by conditions in the atmosphere andocean, which provide energy for storm development. Because
large-scale atmospheric environments are modulated by SSTs,
an understanding of the relationship between SST variability
and TCG is important for predicting TCG frequency. TCG
frequency on an interannual time scale is modulated by SSTs inassociation with climate mode such as the ENSO (Amaya,2019).
A potential role of SST warming in PMM effects on TCG
frequency was suggested by Ishiyama et al. (2022) in relation
to the 2015 super-El Niño. Thus, remote effects such as SST
anomalies outside of the WNP appear to affect TCG frequency.
Recent studies (Wu et al. 2018; Wu et al. 2019; Wu et al. 2020;
Gao et al. 2018; Zhang et al. 2018; Zhan et al. 2019) haveidentiﬁed various impacts of SST anomalies in NA Ocean and/or Indian Ocean (IO) on TCG locations and frequencies over
the WNP, demonstrating the effects of large-scale environmental modulation. For example, Liu et al. (2019b) estimated169V.P.M. Rajasree, X. Cao, H. Ramsay et al.

Tropical Cyclone Research and Review 12 (2023) 165–181

environmental conditions forced by subtropical NA SST
warming (Wang et al. 2019b). Future research should continue
to investigate the inﬂuence of SST spatial patterns in different
basins and different background ﬂows on TCG over different
scales. Moreover, investigating the inﬂuence of SST anomalies
on TCG would lead to accurate predictions of TCG frequency.

the impacts of SST anomalies in the Indo-Paciﬁc region on
TCG frequency over the WNP. The combined impacts of the
ENSO and Indian Ocean Dipole (IOD) enhanced an anomalous
cyclonic circulation in the lower troposphere, leading to higher
TCG frequency in the southeastern WNP, while an anomalous
anticyclonic circulation led to lower TCG frequency in thenorthwestern WNP. A signiﬁcant correlation was detected
between the IOD and TCG frequency over the SCS (Wanget al. 2019a). According to Shi and Fang (2022), high SSTs
in the TNA in June and July 2020 triggered cyclonic circulations over the tropical ENP, resulting in strong upward motion
and subsidence in the tropical central-Paciﬁc, which caused thewestern Paciﬁc subtropical high to extend westward andstrengthened easterly winds on its south side, in turn weakening the monsoon trough and producing unfavorable conditions for TCG and “the no-typhoon (no-TCG over the WNP)”
phenomena in July 2020. Furthermore, the categorization of
TCG environments over the WNP according to shear line,
monsoon gyre, and Rossby wave energy dispersion from preexisting cyclones showed that TCG is associated with higher
SSTs in the WNP (Fudeyasu et al. 2020). Jin et al. (2022b) has
discussed the possible role of a local monsoon trough favoring
weak TC formation over SCS and WNP.
Climate modes that inﬂuence TCG in the NA include the
ENSO, AMO, the Atlantic meridional mode, and global
warming, while in the ENP they include the ENSO and AMO(Li et al. 2022). The ENSO was found to be the main factor
inﬂuencing TCG frequency in the ENP and southwest Paciﬁcdue to the effects of local SSTs. The PMM also exerts an inﬂuence on TCG frequency over the ENP, as demonstrated by
the strongly positive SST component of the PMM, which led tofavorable TCG conditions in 2018 (Wood et al. 2019). Magee
and Verdon-Kidd (2018) found a relationship between SSTvariability in the IO and TCG over the southwest Paciﬁc when
the ENSO was in an inactive phase. Warmer (cooler) SSTs in
the IO caused statistically signiﬁcant northeastward (southwestward) migration of TCG over the southwest Paciﬁc.
Ensemble simulations have shown that due to interannual
variability forced by observed SSTs, TCG over the NA ismodulated primarily by potential intensity and VWS (Mei et al.
2019). The role of ocean heat content (OHC) including SSTs in
TCG over the BoB, a semi-marginal sea in the NIO, wasinvestigated in Albert and Bhaskaran (2020) that detecteddifferences in positive (negative) OHC anomalies to the south
during the pre- (post-) monsoon season, leading to the higher
TCG over the BoB.
Recent studies (e.g., Takaya, 2019; Wang et al. 2019b;
Wood et al. 2019; Gao et al. 2020) have investigated theextremely active TCG that occurred over the Northern Hemisphere in summer 2018. The active TCG in the North Paciﬁc
was found to have been caused primarily by warming in thesubtropical Paciﬁc and secondarily by warming in the tropicalPaciﬁc (Qian et al. 2019). The impact of midlatitude SST
anomalies has been found to affect TCG at lower latitudes in
the WNP due to large-scale atmospheric responses and intraseasonal oscillation (Nasuno et al. 2022). Enhanced TCG
over the NA was caused by favorable large-scale

3.2. Impacts of moist/dry air
Hypotheses explaining the physical mechanisms related tomoisture entrainment and dry air intrusion have been tested indifferent ocean basins in the recent years. Most of these studies
discuss the underlying physical mechanisms by which the intrusions of moist/dry air into the core of tropical disturbances
lead to/inhibit TCG. A moisture pooling effect is discussed by
Liang et al. (2022b) in WNP TCG case studies. Ramakrishna
et al. (2019) show the presence of cyclonic circulation andmoisture convergence six days ahead of the development of
NIO TC Thane (2011) and the cyclone is strengthened bymoisture advection from the SCS. In the genesis environmentof Paciﬁc Ocean tropical disturbance pre-Faxai (2019), thedevelopment of a Kelvin cat's eye is found to be one of thereasons for the persistence of vortices (Fudeyasu et al. 2022).
Positive feedback between near surface wind and latent heatﬂux increases the boundary layer speciﬁc humidity and leads to
TCG under favorable conditions (Gao et al. 2019). Teng et al.
(2021) addressed the sensitivity to moisture patterns in thecases of TCG related to monsoon environment compared tothat of easterly environment. The importance of mid-level
moisture is higher in the case of WNP TCG associated witheasterly environments and more sensitive to moisture patterns
compared to that of monsoon environments. In the developing
cases, TCG is favored by the generation of stronger potential
vorticity produced by diabatic heating as a result of increased
mid-level moisture. Moist Static Energy can be used as a proxyfor understanding moistening processes in the lower to middle
troposphere (Chen et al. 2019a; Wing et al. 2019; Yu et al.
2019; Fu et al. 2021; Carstens 2022). Signiﬁcant advances
have been reported in the case of Medicanes (Mediterranean
tropical-like cyclones) in the last ﬁve years; particularly inunderstanding the role of moist and dry air advection on thegenesis of Medicanes. The increase in humidity induces an
early onset of genesis in Medicanes by producing stronger andlonger persisting vortices (Miglietta et al. 2021).
Dry air intrusion reduces both the thickness of the moist
absolutely unstable layer and maximum intensity of convective
updrafts (Saito et al. 2022) and longer residence time of dry air
causes the disruption of convection (Alland et al. 2021b). A drylower troposphere may limit diffusive exchange (Sun et al.
2022), however, smallness in size of the core as well as
weak VWS can offset the adverse effects of dry air and ocean
cooling (Shimada, 2022). Once the convection is generated
then the instability index is more important than saturation
fraction (SF, Column RH) in the inner ring of the core whereas
in the outer ring, the SF is more important than instability
indices (Raymond and Kilroy, 2019). In a study carried out by
Akter (2022), on TCG connected to dryline over BoB, the170

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The focus of recent research in this ﬁeld has been on
moderate VWS, because the uncertainty of TCG forecasts inmoderate shear is the largest (Nam and Bell 2021; Rios-Berrios
et al. 2018; Rogers et al. 2020). Weak VWS provides a
conducive environment and strong VWS is generally unfavorable for TCG, but under moderate shear, genesis scenarios
can swing a lot depending on environmental and internal factors. TCs that successfully underwent genesis in moderate
shear share the characteristics of continuous and strong convection (Rogers et al. 2020; Nam and Bell 2021). Deep convection near the vorticity center is a necessary factor for vortex
reformation and eventually successful TCG despite shear (e.g.,
Rios-Berrios et al. 2018). Schecter and Melanou (2020)
showed that the vortex advection and misalignment can be
simulated with an adiabatic dry model, but vortex realignment
and intensiﬁcation in a sheared environment can be only
modeled with moist convection included. In addition to deep
convection, recent studies have highlighted the roles ofdifferent kinds of convection in vortex reformation and precession – the role of convective congestus in tilting horizontal
vorticity to vertical vorticity (Nam and Bell, 2021), stratiform
precipitation in intensifying the mid-level vortex and preventing dry air ventilation (Bell and Montgomery 2019), and coldpools with radiative effects (Rios-Berrios 2020).
Various large or synoptic scale weather systems such as
upper-level troughs serve as sources of VWS (Fig. 4 adaptedfrom Nam and Bell 2021; Yoshida and Fudeyasu 2020). Thenthe impact of shear reduces to convective meso scales, andconvection in turn affects the tropical disturbance's resilienceagainst VWS through vorticity generation of vortex stretching
and tilting (Fig. 4b, d). The idealized modeling studies havemade progress on bridging the gap between the model world
and the complex real atmosphere – adding radiative effects(Rios-Berrios 2020), diversifying the timing of shear introduction (Finocchio and Rios-Berrios 2021), and simulating the
combined effects of shear and dry air (Alland et al. 2021a, b).
Nevertheless, more modeling research is needed to analyze themulti-dimensional variable space of internal and environmental

author showed that large values of convective inhibition, due to
hot and dry air, support the stable environment over thenorthern and Northwestern BoB. As shown in Helms andBosart (2021), the core of persistent convection collapses
when an Atlantic Ocean tropical disturbance, pre-Gabrielle(2013) interacts with mid-level dry air ﬂow layer. The authors suggested that the pre- Gabrielle disturbance might havebeneﬁted if a protective pouch was present in the genesis
environment. Similar case studies of dry air intrusion have beenreported from different ocean basins in the last ﬁve years. In a
comparative study of developing and non-developing TCs,
Rajasree et al. (2021) found that the non-developing system
weakened because of the intrusion of dry air from the north ofthe core and the trajectories were traced back to the northwestern part of India as a possible source as shown in Fig. 3,
adapted from Rajasree et al. (2021). Future research should
continue research on understanding how different aerosol types
interact with the TCG, using available chemical models.
4. Impacts of shear on tropical cyclogenesis
Strong VWS often delays or completely thwarts TCG due tovortex tilt, precipitation asymmetry, and dry air ventilation.
These dynamic and thermodynamic processes constitute a
positive feedback cycle. TCG follows the vertical alignment of
the vortex tower, and strong VWS advects the vortex indifferent vertical layers to opposite directions, in other words,
tilts the vortex tower (e.g., Rios-Berrios et al. 2018). Then atilted vortex tower is more susceptible to dry air ventilation asshown by Alland et al. (2021a, b) who investigated the combined effect of dry air and VWS on TC intensiﬁcation. VWS
also moves the convection down-shear side, and convection
farther from the vorticity center is not efﬁcient in amplifying
the vortex (e.g., Schecter and Melanou 2020). Well-established
TCs may withstand moderate to even the strong intensity of
VWS, but pre-genesis TCs are rather vulnerable to the negative
impacts of strong VWS (Finocchio and Rios-Berrios 2021;
Fischer et al. 2022).

Fig. 3. Backward trajectories of speciﬁc humidity (g kg−1) obtained from HYSPLIT model at deep depression (DD) stages of (a) TC Ockhi and (b) depression valid
at 00 UTC of 30 November and 08 December 2017. Red * mark represents the center of the storms. Adapted from Rajasree et al. (2021).
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Fig. 4. Schematic of the multiscale interactions in Hagupit (2008)'s cyclogenesis in a shear environment: (a) Synoptic-scale active features are the upper-level troughand easterly wave that carries the marsupial wave pouch, denoted as red circulation. (b) In the meso-α-scale wave pouch, the low-level cyclonic streamlines areshown with the circulation center marked with a black cross and the midlevel center marked with a yellow cross. Overlaid on the clouds to the southwest of the lowlevel center is vorticity (positive: red, negative: blue), illustrating vorticity dipoles from tilting, enhanced vorticity from stretching, and a midlevel MCV over the
stratiform area. (c) In the meso-β scale, the local wind shear proﬁle that inﬂuences the convective organization comprises winds from cyclonic circulation inside thepouch, easterly wave propagation, and the trough. (d) Meso-γ-scale convective cells produce vorticity that serves as building blocks for the wave-pouch intensiﬁcation through tilting of horizontal vorticity associated with low-level wind shear (S cell) and stretching of the local vertical vorticity (N cell). Adapted from Namand Bell (2021).

(2016) there was a monsoon-like shear line close to the equator. In all cases, TC development occurs when deep convection
consistently ﬂares close to the center of these large-scale circulations. The authors conclude that cyclone development near
the equator appears to be dynamically the same as in otherhigher latitude storms.
Another study by Li et al. (2019), using best track data fromthe Joint Typhoon Warning Center and ECMWF interim
reanalysis, showed that TCs form close to the equator in the
WNP in boreal winter as the large-scale ﬂow changes, when
the north-easterly trade winds turn anticlockwise near theequator. This turning leads to the occurrence of an increased
region of low-level absolute vorticity within 5◦ of the Equator.
Other changes in the large-scale environment, such as RH, SST
or mean atmospheric temperature ﬁelds did not seem to beundergoing drastic seasonal changes, and were deemed unimportant (Deng and Li, 2020).
Lu et al. (2021) further analyzed the controlling parametersin the formation of TCs near the equator (in the WNP and NA
basins). They found that no cases of near-equatorial development occurred in the NA. As in the studies by Li et al. (2019)and Deng and Li (2020), it is found that low-level relative
vertical vorticity is likely a dominant factor. They also showedthat VWS is more favorable in the WNP compared to the NA.
In an analysis of Advanced Research Weather Research andForecasting (WRF-ARW) model output, Deng and Li (2020)highlighted the importance of small-scale vortical hot towers(VHTs) in converging the large-scale background vorticity into
vortex-scale monopole. This convergence of vorticity is notdifferent from that which occurs at higher latitudes. Using ahighly-idealized numerical model setup, Kilroy et al. (2020)
showed that TCs occurring at the equator can also undergo
rapid intensiﬁcation (RI), however, they decay rather rapidly

factors for TCG in sheared environments. For observational
analyses, one of the challenges is that high-resolution observational data of developing disturbances under moderate tostrong shear are scarce. High-resolution data are essential toshow the upscale cascade of energy from the convective scale
to larger scales in the vortex realignment process (Nam andBell 2021; Rogers et al. 2020), and to verify the ﬁndings
from idealized modeling studies. We recommend that future
research should focus on understanding the complicated multiscale interactions in TCG under moderate vertical wind shear
which would enable better understanding of vortex resilience,
precession, or reformation in sheared environments.
5. Near equatorial tropical cyclogenesis
It is typically stated in classical meteorological textbooks
that TCs do not form within 5◦ of the equator, due to thediminishing effects of the Coriolis force closer to the equator.
In other words, according to these textbooks, the formation ofTCs depends on there being adequate background planetaryrotation present. A series of papers in the recent years haveshown that, while relatively rare, TCs can occur close to theequator provided that there is a large enough supply of background relative vertical vorticity.
A study by Steenkamp et al. (2019) investigated several realcases of near equatorial TCs using European Centre for
Medium-Range Weather Forecasts (ECMWF) operational analyses, with an emphasis on understanding the origins of theseregions of enhanced vertical vorticity. For Typhoon Vamei(2001), the initial enhanced background rotation was provided
by the Borneo vortex, while for Super Typhoon Dolphin(2015) the rotation was associated with unstable roll-up ofvorticity in a shear line marking an ITCZ. For Hurricane Pali
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through which low-level convectively induced vorticity
anomalies (LCVA) initiate TCG. At the edges of cold pools,
vorticity is concentrated by convection and LCVA forms above
the regions of increased vorticity. LCVA has maximum
vorticity near the surface, outlives the convection itself andoccasionally get strengthened by subsequent convection. Thesemechanisms stretch the ambient vorticity into a coherent vortex. LCVA reaching the center of the parent disturbance becomes the low-level core of the TC, forming a small vortex thatgenerates in the center of the parent disturbance. LCVA undergoes merger similar to VHTs (Hendricks et al. 2004). Future
research should continue to explore the convection physics,
and diabatic heating to improve the forecast skill of TCG
prediction. Furthermore, research should continue to understand the top-down and bottom-up mechanisms of TCG in
detail, and further quantify the stochastic processes of
convective development and TCG using ensemble probabilistic
forecast. Further research should focus on understanding theroles of water vapor and convection and how these interact atdifferent scales in inﬂuencing TCG prediction.

also. After a few days of development, the storms deplete theirinitial cyclonic vorticity source and begin to ingest the negative
vertical vorticity in the surrounding environment as air parcels
large distances from the center are drawn inward. The ingestingof negative vertical vorticity leads to the development of regions that are inertially unstable close to the cyclone center,
which consequently leads to a rapid vortex breakdown. At
higher latitudes, away from the equator, planetary vorticity
would play an important role at this stage of development.
6. Meso-scale controls
6.1. Role of convection/heating
Generally, the location of TCG is related to environmental
vorticity and moisture, while the probability and timing of TCG
mainly depend on convective characteristics (Narenpitak et al.
2020). Convection is a precursor, trigger, and energy sourceof TCG at the initial stage. It provides heating, vertical ascent,and moisture around the disturbance center and leads to midlevel vortex development and vertical mass transport during
the TCG process. It is also highly stochastic in nature, leading to
uncertainty in model forecasts. Therefore, understanding therole of convection and heating in the TCG environment isimportant to interpret the physical mechanisms that differentiate
the developing systems from non-developing systems. Cumulus
congestus and deep convection respectively moisten the lower
to middle troposphere and upper troposphere (Wang, 2014),
convergence enhances around the center through ice microphysical processes and diabatic forcing, and then strengthens
the mid-level circulation/vortex due to conservation of angularmomentum (Nicholls et al. 2018; Bell and Montgomery 2019;
Wang et al. 2019c; Carstens and Wing 2020).
The mid-level circulation/vortex protects the disturbance fromthe dry air intrusion and further enhances the convection andvertical mass transport, favoring low-level spin up and TCG.
Because convection-related characteristics affect the TCG
process, they may become important indicators for TCG. The
total rain area, rain volume, and coverage area of deep convection near the circulation center have notable differences
between developing and non-developing disturbances. They
are useful predictors for TCG in observations (Zawislak 2020).
Moreover, in the inner core, the structure of vortical convection
substantially changes before the initial disturbance intensiﬁes
rapidly into a TC, i.e., an increase in cyclonic vertical vorticity.
This is a potential indicator to characterize the period of TCG(Kilroy 2021). Conversely, boundary layer physics of the large
scale vortex, producing an Ekman pumping, is a key mechanism for convection maintenance and intensiﬁcation.
Convection-induced convergence in the boundary layer andmass ﬂux governed by thermodynamic conditions highly
impact inner-core vortex expansion and the subsequent TCG
process (Raymond and Kilroy 2019; Smith et al. 2021). These
characteristics can be further applied to improve convective
parameterizations for models and TC formation forecasts.
Many studies have investigated the pathways of TCG.
Smith and Nicholls (2019), described different mechanisms

6.2. Vertical structure
Pre-genesis and weak TCs often develop a top-heavy potential vorticity structure (Raymond and Session 2007; Murthy
and Boos 2019) via stratiform precipitation. Although stratiform precipitation commonly cools the lower troposphere, its
latent heating maximizes in the middle troposphere and provides a key component for the initial spin-up and maintenanceof a mid-level circulation. The persistence of convection, on
the other hand, is also critical to achieve early stage of TC
development, as it provides a more bottom-heavy mass ﬂux tospin up vorticity in the lower troposphere. A recent study from
Levina (2018) hypothesized that a criterion summarizing the
combined effects of favorable helicity and instability (e.g.,convective available potential energy) may be beneﬁcial towards identifying sufﬁcient convective development in TCs
that results in TCG. Persistent vortical convection can generatevertical vorticity by tilting of horizontal vortex ﬁlaments within
updrafts that are also ampliﬁed by stretching, which can help
spin up the low-level vortex.
VWS of the horizontal wind can detrimentally cause
misalignment between developing low-level and mid-levelvortices, particularly in undeveloped or weak TCs, wherein
they have increased susceptibility to environmental inﬂuences(Jones 1995; Frank and Ritchie 2001). Previous studies havehighlighted the importance of achieving a vertically aligned
conﬁguration of the vortex (e.g., Zhang and Tao 2013; RiosBerrios et al. 2016; Munsell et al. 2017; Schecter, 2022) tofavorably promote a more symmetric precipitation structure,
which is favorable for RI and may also increase resiliency fromsurrounding environmental dry air and VWS (Tang andEmanuel 2010, 2012; Davis and Ahijevych 2012; Alland
et al. 2021a, b; Chen et al. 2019b; Alvey et al. 2020). Multiple pathways towards achieving an aligned vortex have been
identiﬁed in the literature: precession (Jones 1995; Reasor et al.
2004), downshear reformation (Nguyen and Molinari 2015;173

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Tropical Cyclone Research and Review 12 (2023) 165–181

reanalysis datasets suggest a long-term decline in global TC
frequency since about 1900 (Chand et al. 2022). Fig. 5 shows the
hypothesized link between anthropogenic-induced global
warming and associated reduction in annual TC numbers,
adapted from Chand et al. (2022). Declining trends in thenormalized composite index derived from the environmental
vertical wind shear, upward mass ﬂux and saturation deﬁcit
suggest a similar declining trend in TC numbers for all the TC
basins around the globe. The authors have attributed thedecreasing upward mass ﬂux and increasing saturation deﬁcit andenvironmental vertical wind shear to the warming of the climate
over the twentieth century. Further, the reduction in the upward
mass ﬂux has been linked to the weakening of the Walker andHadley circulations during twentieth century compared to thepre-industrial period, which is further increasing the likelihood of
dry air entrainment and thereby reducing chances of TCG.
Although the annual global number of TCs has remained
steady since the early 1980s, there have been notable regional

Chen et al. 2018b; Rogers et al. 2020; Schecter 2020; Alvey
et al. 2022), and (diabatically induced) and advection(Schecter and Menelaou 2020). More common in weaker,
disorganized storms, a new low-level circulation can reform
either beneath or near the MLC via persistent convection. A
recent study by Zhang et al. (2022) have indicated that distribution of deep convection is mainly affected by wind shear.
Furthermore, sensitivities to the environment like increasing or
decreasing the SST and changes in the VWS (Finocchio andRios-Berrios 2021) can affect the distribution/intensity of
convection, which can alter the relationship between vortex tilt
magnitude and subsequent TC intensity change.
7. Role of climate change
There has been no discernible trend in the annual global
number of TCs over the last four decades (Schreck et al. 2014;
Ramsay 2017; Klotzbach et al. 2022), however some global

Fig. 5. Schematic showing the hypothesized link between anthropogenic-induced global warming and associated reduction in annual TC numbers. a, Linear trends in
SSTs since the mid-nineteenth century, overlaid with the mean trade wind directions (red arrows). b, Linear trends in sea-level pressure (SLP) (dashed blue rectangles
are used to deﬁne the Indo–Paciﬁc SLP gradient, ΔSLP, which serves as a proxy of changes in mean intensity of the Paciﬁc Walker circulation. The three Walker
cells are represented in orange. Sinking dry air is represented by green arrows and moist rising air is represented by grey arrows). c, Changes in the Indo–Paciﬁc SLP
gradient since the mid-nineteenth century. d, An illustration of the Hadley circulation. e, Observed Hadley circulation pattern, represented by mass stream function of
the zonal-mean meridional winds during the period 1900–2012 (red shadings indicate regions where the summertime mean intensity of the circulation has weakened
signiﬁcantly relative to the pre-industrial counterpart). f, Environmental conditions represented as the normalized composite index. Adapted from Chand et al.
(2022).
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trends in annual TC frequency (Murakami et al. 2020),
including a sharp increase in the NA basin and decreases in theAustralian region (Chand et al. 2019) and in the WNP basin(Liu and Chan 2013; Zhang et al. 2018). The signiﬁcant upward
trend in the NA is associated with a regional TC drought during
the 1970s and 1980s (Vecchi et al. 2021), when thermodynamic
conditions over the main development region were unfavorable(Emanuel 2021b) due to both enhanced anthropogenic aerosols
and an increase in Saharan dust (Rousseau-Rizzi and Emanuel2022). The warming of the TNA has resulted in several
notable active hurricane seasons: Murakami et al. (2018)attributed the active major hurricane season of 2017 to therelative warming of the NA (i.e., warming relative to the tropical mean SST), rather than La Niña conditions, and theextremely active hurricane seasons of 2005 and 2020 have been
also attributed to rising regional SSTs (Pﬂeiderer et al. 2022).
TC frequency trends in basins outside the NA have received
less attention and are arguably not as well understood. An
observed poleward shift in the latitude of TCG during theperiod 1980–2014 has been linked to an expansion of thetropics as reﬂected by changes in regional Hadley circulations(Sharmila and Walsh 2018). Murakami et al. (2020) showedthat decreasing TC numbers in the South Indian, Coral Sea and
WNP basins since 1980 are generally consistent with a forced
response from monotonically increasing greenhouse gases.
Similarly, Chu et al. (2020), using a high-resolution coupledgeneral circulation model forced with increasing CO2, founddecreasing TC numbers in all TC basins except for the NA. The
forced TCG response in their experiments was linked to a
weakening of the summer Hadley cells, consistent with theobservational results of Sharmila and Walsh (2018). The
decline in TC numbers over the WNP basin since about 1998
has also been linked to multidecadal variability – speciﬁcallythe negative phase of the Interdecadal Paciﬁc Oscillation andassociated La Niña like state have likely acted to suppress TCG
in parts of the WNP (Zhao et al. 2018; Zhao et al. 2020; Chanand Liu 2022). In the NIO basin, the number of intense TCs(Lifetime maximum intensity≥90 kts) has increased over theperiod 1970–2020 together with increasing potential intensity
and OHC (Swapna et al. 2022).
Idealized models have served as a valuable testbed for understanding climatic controls on TCG (Merlis and Held 2019),
allowing for a more systematic exploration of the parameter
space. Federov et al. (2019) investigated the effect of a reduced
equator-to-pole temperature gradient on TCG using a cloudsystem resolving model and found an increase in both thetotal number of TCs and the number of category 4-5 storms for
sufﬁciently reduced SST gradients. In a series of aquaplanet
experiments with uniform thermal forcing, Chavas and Reed(2019) showed that TCG rate increased almost linearly with
Coriolis parameter f up to a critical value, and also that theminimum genesis distance from the equator scaled with theequatorial Rhines scale.
Using a similar approach, Walsh et al. (2020) found that
increased static stability associated with surface warming wasstrongly related to a reduction in TCG. The global number ofTCs has also been shown to be sensitive to the latitude of the

ITCZ in aquaplanets, with increasing TC frequency associatedwith a poleward migration of the ITCZ up to ~25◦ N (Burnett
et al. 2021), consistent with previous work. Using a tropicalchannel aquaplanet model, Vu et al. (2021) showed that the
global number of TCs reached a similar upper bound despite
differences in the frequency of synoptic-scale precursors. In
addition, midlevel moisture and 850 hPa absolute vorticity
were shown to be key factors associated with episodic global
TCG.
At cloud-permitting resolutions (~3 km), TCs can form
spontaneously (i.e., without an imposed pre-existing disturbance) over uniform SST on an f-plane (Bretherton et al. 2005).
This approach has been used to explore the sensitivity of TCG
to a range of climates and feedback processes, including
radiative feedbacks (Wing et al. 2016; Muller and Romps2018), surface warming (Ramsay et al. 2020), surface moisture (Cronin and Chavas 2019), and planetary vorticity(Carstens and Wing 2020). The absence of convective
parameterization as well as the ability to resolve important
dynamical features of TCs (e.g., eyes) and their precursors(e.g., mid-level vortices) has allowed for a more detailed,process-based understanding of TCG in response to different
climate parameters. A drawback of this approach is that typically only a single TC is simulated (at least for Earth-like f )due to domain size constraints, which makes it difﬁcult to test
the dependence of TC frequency on climate with ﬁdelity.
Finally, there is ongoing debate about the role of TC precursor
disturbances (“seeds”) in determining the rate of TCG in bothobservations and climate model projections. Speciﬁcally, there isdebate as to whether precursor disturbances themselves are sensitive to climate change or should be regarded as backgroundweather noise that is largely independent from TC climatology.
Vecchi et al. (2019) argued that TC frequency response to
global warming can be thought of as the product of the change
in seed frequency and the change in likelihood that a seed
ampliﬁes to a TC. The probability of seeds and the seed-to-TC
transition have been linked to different environmental constraints (Hsieh et al. 2020). This framework has also been
applied successfully to explain the annual cycle of TCs invarious basins (Yang et al. 2021). Sugi et al. (2020) andYamada et al. (2021) showed that changes in TC seed frequency played an important role in determining the overall TC
frequency response to warming in climate change simulations,
including inter-model differences. On the other hand, several
studies have shown that global and regional TC climatologies
are largely insensitive to the frequency of low-amplitude synoptic disturbances (Patricola et al. 2018; Vu et al. 2021;
Emanuel 2022). There are a number of avenues for future
research that are likely to advance our understanding of TCG
and climate change. These include regional climate change andTCG, including the inﬂuence of ENSO and other natural modesof variability, additional idealized simulations, including
aquaplanet experiments, aimed at exploring and testing theparameter space in relation to TCG. In parallel, future researchshould address the role of seed disturbances in determining
climatological aspects of TCG in recent historical observations
and in future climate projections.
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ER
Equatorial Rossby
EWs
Easterly waves
HYSPLIT Hybrid Single-Particle Lagrangian Integrated
Trajectory
IO
Indian Ocean
IOD
Indian Ocean Dipole
ISO
Intraseasonal oscillation
ITCZ Inter Tropical Convergence ZoneLCVA Low level convectively induced vorticity anomalies
MCSs Meso-scale Convective Systems
MCV Meso-scale Convective Vortex
MJO
Madden-Julian Oscillation
MLC Midlevel circulation
MTC Multiple tropical cyclone
NA
North Atlantic
NDAEWs Non-developing AEWs
NIO
North Indian Ocean
OHC
Ocean Heat Content
OWZP Okubo-Weiss Zeta parameter
PMM Paciﬁc Meridional Mode
QBWO Quasi-Biweekly Oscillation
RH
Relative Humidity
RI
Rapid Intensiﬁcation
SCS
South China Sea
SF
Saturation FractionSP
Spring (April–June)
SST
Sea Surface Temperature
SU
Summer (July–October)TC
Tropical Cyclone
TCG
Tropical Cyclogenesis
TD
Tropical Depression
TNA
Tropical North Atlantic Ocean
VHTs Vortical Hot Towers
VWS Vertical Wind Shear
WAM West African Monsoon
WNP Western North Paciﬁc
WRF-ARW Advanced Research Weather Research andForecasting

8. Summary and conclusions
The results of the recent studies demonstrate the importanceof understanding the potential effects of regional- tohemispheric-scale variation in TCG. During the last ﬁve years,
multi scale inﬂuences on TCG have been investigated indifferent environments and background ﬂows with TC representations from global ocean basins. Also, alternative pathways of TCG have been proposed. The development of TCs
near the equator is possible, provided that there is a reasonably
large enough source of background vertical vorticity, andrelatively weak vertical shear. Conditions are often morefavorable for near equatorial TCG over the WNP in boreal
winter, while development over the NA basin has not been
well documented. Storms that occur close to the equator
should not survive for more than a few days if they do nottrack poleward, as ingestion of negative relative vorticity mustoccur once the initial cyclonic supply is depleted. Recent
studies have focused on convection, vortex alignment, developing and non-developing systems. However, some of thesestudies require validation with real case studies. A few of thestudies represent important validations and continuation ofearlier studies. Understanding convective processes andimproving model forecasts of TCG would lead to the ability tomake better decisions at the operational level. Our understanding of the impact of climate change on TCG has beenremarkably improved.
Acknowledgments
We would like to thank the WMO for organizing IWTC-10
and providing this opportunity to publish the topic summary
report. We are grateful to IWTC-10 co-chairs Joe Courtney andRobert Rogers for their guidance and support. We would also
like to thank the topic lead Andrew Burton for the insightful
discussions.
H.R. acknowledges funding support from the Climate Systems Hub of the Australian Government's National Environmental Science Program (NESP).

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