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Is the poleward migration of tropical cyclone maximum intensity associated with a poleward migration of tropical cyclone genesis?

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A recent study showed that the global average latitude where tropical cyclones achieve their lifetime-maximum intensity has been migrating poleward at a rate of about one-half degree of latitude per decade over the last 30 years in each hemisphere. However, it does not answer a critical question: is the poleward migration of tropical cyclone lifetime-maximum intensity associated with a poleward migration of tropical cyclone genesis? In this study we will examine this question. First we analyze changes in the environmental variables associated with tropical cyclone genesis, namely entropy deficit, potential intensity, vertical wind shear, vorticity, skin temperature and specific humidity at 500 hPa in reanalysis datasets between 1980 and 2013. Then, a selection of these variables is combined into two tropical cyclone genesis indices that empirically relate tropical cyclone genesis to large-scale variables. We find a shift toward greater (smaller) average potential number of genesis at higher (lower) latitudes over most regions of the Pacific Ocean, which is consistent with a migration of tropical cyclone genesis towards higher latitudes. We then examine the global best track archive and find coherent and significant poleward shifts in mean genesis position over the Pacific Ocean basins.
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Clim Dyn (2018) 50:705–715
DOI 10.1007/s00382-017-3636-7
Is thepoleward migration oftropical cyclone maximum intensity
associated withapoleward migration oftropical cyclone genesis?
AnneSophieDaloz1 · SuzanaJ.Camargo2
Received: 29 November 2016 / Accepted: 16 March 2017 / Published online: 3 April 2017
© Springer-Verlag Berlin Heidelberg 2017
1 Introduction
Kossin et al. (2014) showed that in the past few dec-
ades (1982–2009) the large-scale environment of tropi-
cal cyclones has evolved over the tropics and subtropics.
Indeed, favorable conditions for the development of tropical
cyclones have migrated towards higher latitudes (vertical
wind shear and potential intensity), moving from the trop-
ics closer to the subtropics. With a globally homogenized
record of intensity (Kossin etal. 2013) and a global best-
track archive (Knapp et al. 2010), they also demonstrated
that the location where observed tropical cyclones reach
their maximum intensity has been migrating towards higher
latitudes. More recently, Kossin etal. (2016) used observa-
tions and simulations to examine the changes in lifetime-
maximum intensity and tropical cyclone exposure for the
present and future climates over the western North Pacific
Ocean. The projections of tropical cyclones were simu-
lated by, and downscaled from, an ensemble of numerical
Coupled Model Intercomparison Project Phase 5 (CMIP5)
models (Taylor et al. 2012). They showed a poleward
migration of lifetime-maximum intensity (LMI) latitude in
the present century and continuing into the future using one
of the representative concentration pathways (RCP8.5). A
possible mechanism responsible for these global and local
changes is the expansion of the tropics (Lucas etal. 2014),
however this link has not been proved yet.
The current study expands on the findings by Kossin
etal. (2014, 2016) by analyzing the possible origin of the
poleward migration of the LMI latitude. More precisely, we
would like to answer the following question: Is the pole-
ward migration of tropical cyclones’ LMI location related
to a poleward migration in tropical cyclone genesis loca-
tion? Tropical cyclones are very sensitive to the large-scale
environment both during their genesis and development
Abstract A recent study showed that the global average
latitude where tropical cyclones achieve their lifetime-
maximum intensity has been migrating poleward at a rate
of about one-half degree of latitude per decade over the last
30years in each hemisphere. However, it does not answer
a critical question: is the poleward migration of tropical
cyclone lifetime-maximum intensity associated with a pole-
ward migration of tropical cyclone genesis? In this study
we will examine this question. First we analyze changes
in the environmental variables associated with tropical
cyclone genesis, namely entropy deficit, potential intensity,
vertical wind shear, vorticity, skin temperature and specific
humidity at 500 hPa in reanalysis datasets between 1980
and 2013. Then, a selection of these variables is combined
into two tropical cyclone genesis indices that empirically
relate tropical cyclone genesis to large-scale variables. We
find a shift toward greater (smaller) average potential num-
ber of genesis at higher (lower) latitudes over most regions
of the Pacific Ocean, which is consistent with a migration
of tropical cyclone genesis towards higher latitudes. We
then examine the global best track archive and find coher-
ent and significant poleward shifts in mean genesis position
over the Pacific Ocean basins.
Keywords Tropical cyclone genesis· Poleward
migration· Tropical cyclone genesis index· Observations
* Anne Sophie Daloz
1 Space andScience Engineering Center, University
ofWisconsin-Madison, 1225 West Dayton Street, Madison,
WI53706, USA
2 Lamont-Doherty Earth Observatory, Columbia University,
Palisades, NY, USA
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... Previous studies found that the basin-wide mean latitude of the lifetime maximum intensity (LMI) of tropical cyclones (TCs) in the western North Pacific (WNP) has shifted northward since the early 1980s (Kossin et al. 2014(Kossin et al. , 2016Moon et al. 2015;Zhan and Wang 2017). This shift in LMI is consistent with a northward migration of the mean TC formation location (Daloz and Camargo 2018;Studholme and Gulev 2018;Sharmila and Walsh 2018). Some studies suggested that the shift in TC formation is the result of the expansion of the tropics and the Hadley circulation in a warmer climate (Lucas et al. 2014;Walsh 2017, 2018;Studholme and Gulev 2018). ...
... with previous studies(Studholme and Gulev 2018;Sharmila and Walsh 2018;Daloz and Camargo 2018), the mean latitude of TC formation locations has shifted northward with a significant linear trend of 0.36° latitude per decade during 1980-2013. This poleward shift becomes weaker during 1980-2017 (0.27° ± 0.17° decade −1 ;Fig. ...
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Consistent with the northward migration of the annual mean latitude of tropical cyclone (TC) lifetime maximum intensity (LMI), the basin-wide mean location of TC formation shifted northward in the western North Pacific (WNP) basin over the past four decades. Whether such a shift was related to the anthropogenic influence is important to understanding the response of TC activity to climate change. Instead of detecting the effects of individual environmental factors on this shift, here we focus on the interdecadal variability of the monsoon trough (MT), within which most TCs in the WNP basin occur, and its roles in the shift of the basin-wide mean location of TC formation using 60-year reanalysis data. Interdecadal variations of the MT exhibit two main modes: one associated with the Pacific decadal oscillation (PDO) and the other associated with the interdecadal Pacific oscillation (IPO). In addition, the north–south shift of the mean latitude of TC formation is accompanied by east–west extension of the tropical upper tropospheric trough (TUTT) and the tropical eastern Pacific cold tongue indicated by the east–west contrast of sea surface temperature (SST) anomalies. The poleward shift of the mean TC formation latitude is closely associated with the IPO mode of the MT. The westward retreat of the northwest-to-southeast-oriented MT and the accompanied westward extension of the TUTT reduced TC formation in the eastern part of the WNP basin when the cold tongue shifted westward. It is indicated that the observed poleward shift of TC formation was mainly attributed to natural variability in recent decades.
... The Western North Pacific basin (WNP; 0-60°N, 100-180°E), accounting for >30% of global TCs, shows the largest poleward migration rate for LMI of all ocean basins. In the WNP, the annual mean TC formation (cyclogenesis) location has also shifted poleward 3 . These migration trends are primarily based on satellite-era TC Best Track observations. ...
... In the Best Track ensemble mean, we find that not only have genesis (40±28 km/decade; ± representing the 95% confidence interval of trend value, with the statistical details in the "Methods" section) and LMI (61±30 km/decade) significantly migrated north over the last four decades, but so too have alltrack-points (78±31 km/decade). Compared to previous studies 1-3 , which used a shorter data record (until 2013), the migration rate of LMI here is similar, but the migration rate of genesis is smaller possibly due to the different definitions of genesis (genesis has been defined with a threshold wind speed of 40 knots in ref. 3 , contrasting with the 34 knots used here). We also find a statistically significant northward trend in the annualmean latitude of lysis (100±59 km/decade). ...
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The average location of observed western North Pacific (WNP) tropical cyclones (TCs) has shifted north over the last several decades, but the cause remains not fully understood. Here we show that, for the annual average, the observed northward migration of WNP TCs is related to changes in TC seasonality, not to a northward migration in all seasons. Normally, peak-season (July–September) TCs form and travel further north than late-season (October–December) TCs. In recent decades, related to less frequent late-season TCs, seasonally higher-latitude TCs contribute relatively more to the annual-average location and seasonally lower-latitude TCs contribute less. We show that the change in TC seasonality is related to the different responses of late-season and peak-season TC occurrence to a stronger Pacific Walker Circulation. Our findings provide a perspective on long-term trends in TC activity, by decomposing the annual-average statistics into seasonal components, which could respond differently to anthropogenic forcing.
... A similar poleward migration of ϕ LMI was shown in the Coupled Model Intercomparison Project Phase 5 (CMIP5) models for the Representative Concentration Pathway 8.5 scenario (Kossin et al., 2016). Other studies also suggested that more favorable environmental conditions in the WNP subtropics result in relatively higher TC occurrences (i.e., track) over the subtropics than the tropics, which is related to the recent poleward shift of ϕ LMI (Daloz & Camargo, 2018;Kossin et al., 2016;Zhan & Wang, 2017). By decomposing genesis location and latitudinal distance between genesis and LMI locations, Song and Klotzbach (2018) showed that genesis location varies by the decadal time scale, and the latitudinal distance gradually moves poleward. ...
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Plain Language Summary The location where tropical cyclone (TC) attains their lifetime maximum intensity is an important factor to determine the magnitude of its societal impact. Considering the land‐sea distribution in the Atlantic, poleward migration of the location means that the TC can grow longer until it closer to the land. Besides, the location of lifetime maximum intensity is generally better observed than any other development stage. For these reasons, many previous studies investigated its long‐term variation. According to some studies, in the North Atlantic, the location has shifted toward the equator for a recent decade, which appeared to be a part of the multidecadal variability known as the Atlantic Multidecadal Oscillation (AMO). Here, we also found that the location significantly varies by multidecadal time scales, which is well attributable to the AMO. In addition, we revealed that the migration of the location is equally controlled by the track pattern and the in‐track‐pattern location change, all of which are well explained by the large‐scale circulation related with the AMO. Our finding can be helpful to foresee near‐future variation of the location of tropical cyclones lifetime maximum intensity.
... Besides, future studies in these areas should consider a higher temporal resolution to observe the immediate effects of storms on the different PFTs which may respond over different time scales (Painter et al., 2016). On the other hand, the average latitude at which TCs achieve their maximum intensity has been shifting poleward and is projected to continue in the future under a warming climate (Kossin et al., 2014Daloz and Camargo, 2018). Therefore, the assessment of the phytoplankton community response to major hurricanes in subtropical areas is a potential research direction. ...
The open ocean accounts for nearly 70% of Earth’s surface and represents the largest habitat in the biosphere. Phytoplankton, which are drifting microorganisms with the capacity to perform oxygenic photosynthesis, support life in this vast environment. Besides, they are a key component of marine ecosystems as they drive the oceanic biological pump, influence biogeochemical cycles and modulate fishing yields. However, climate change modifies the environmental drivers governing phytoplankton photosynthesis and consequently alters their productivity, diversity and community structure. Understanding the phytoplankton response to environmental stressors is mandatory to ascertain the implications of current and future climate changes on marine ecosystems in general. Important tools in this respect are remote sensing satellite observations and mathematical models. The former provide high-resolution spatial-temporal observations of key ocean variables, while the latter allow to extrapolate knowledge from the laboratory and sparse field observations to global and regional scales. Hence, the main focus of this thesis is the assessment of the marine phytoplankton response to environmental stressors associated with climate change on the basis of multi-platform datasets, i.e. satellite observations and outputs of mathematical models. More specifically, the response of dominant phytoplanktonic cyanobacteria genera on Earth (Prochlorococcus and Synechococcus) to ultraviolet (UV) radiation is investigated as well as the perturbations induced by hurricanes (strongest tropical cyclones (TCs)) on phytoplankton assemblages in several areas of the western North Atlantic Basin in the period 1998–2019. On the basis of biological weighting function (BWF)/photosynthesis-irradiance (P-E) models, we found that UV accounts for roughly two-thirds of the potential photosynthetic inhibition of Prochlorococcus and Synechococcus in the oceanic photoactive layer in the latitudinal band 40º N/S. Prochlorococcus showed a higher inhibition and integrated photosynthetic potential throughout the water column than Synechococcus, since the former is more vulnerable to UV damage at the surface and more successful at greater depths compared to the latter. On the other hand, we demonstrated that hurricanes trigger vertical and horizontal transport of phytoplankton and nutrients leading to an increased satellite chlorophyll (Chl) a concentration (a proxy for phytoplankton biomass) in the waters surrounding Cuba, the eastern Gulf of Mexico and the western Sargasso Sea. Besides, we illustrated that hurricanes drive connectivity of phytoplankton assemblages between coastal and oceanic environments. Climatological analyses showed that the strongest TC-induced Chl a increases in the western Sargasso Sea have been mainly associated with consecutive TCs as they superimpose effects on the upper-ocean response. Finally, data of phytoplankton functional types (PFTs) derived from a biogeochemical ocean general circulation model revealed that Prochlorococcus and Synechococcus respond modestly to post-storm nutrient enrichment in the tropical Sargasso Sea as compared to coccolithophores, diatoms, diazotrophs, mixotrophic dinoflagellates, and picoeukaryotes, whose concentrations increase significantly after a hurricane passage. Besides, a significant post-storm increase of the Shannon diversity index values was also observed indicating that a moderate post-storm nutrient increase in this oligotrophic area positively impacts phytoplankton diversity limiting exacerbated productivity of opportunistic species. Overall, this thesis provides a baseline against which future phytoplankton responses to environmental stressors can be evaluated. Its findings fit the needs of future studies on climate change, ecological variability, environmental protection and fisheries oceanography.
... Subtle but robust poleward trends of 53 ± 43 and 62 ± 48 km per decade 94 in TC seasonal-mean lifetime maximum intensity (LMI) latitudes are detectable in observations of the Northern and Southern hemispheres, respectively (1982 to 2012). Although these estimates are largely drawn from analyses of the International Best Track Archive for Climate Stewardship (IBTrACS) archive, which aggregates multiple records, such a poleward migration is found across different datasets and also for genesis latitudes [94][95][96] . The magnitudes of these trends depend on the period and TC intensity considered 94 . ...
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Tropical cyclones (TCs, also known as hurricanes and typhoons) generally form at low latitudes with access to the warm waters of the tropical oceans, but far enough off the equator to allow planetary rotation to cause aggregating convection to spin up into coherent vortices. Yet, current prognostic frameworks for TC latitudes make contradictory predictions for climate change. Simulations of past warm climates, such as the Eocene and Pliocene, show that TCs can form and intensify at higher latitudes than of those during pre-industrial conditions. Observations and model projections for the twenty-first century indicate that TCs may again migrate poleward in response to anthropogenic greenhouse gas emissions, which poses profound risks to the planet’s most populous regions. Previous studies largely neglected the complex processes that occur at temporal and spatial scales of individual storms as these are poorly resolved in numerical models. Here we review this mesoscale physics in the context of responses to climate warming of the Hadley circulation, jet streams and Intertropical Convergence Zone. We conclude that twenty-first century TCs will most probably occupy a broader range of latitudes than those of the past 3 million years as low-latitude genesis will be supplemented with increasing mid-latitude TC favourability, although precise estimates for future migration remain beyond current methodologies. Hurricanes and typhoons are tracking further poleward due to the effects of climate change, according to a synthesis of numerical modelling results, observations and palaeoclimate records.
... Emanuel and Nolan (2004) developed a genesis potential index (referred to as GPI04 hereafter) by considering four parameters related to TC genesis, that is, potential intensity (PI), relative humidity, low-troposphere wind vorticity, and vertical wind shear. GPI04 is widely applied to analyze the variations of TC activity on multiple timescales for further use in conducting reanalysis and deriving model outputs (Camargo et al., 2007a(Camargo et al., , 2007bNolan et al., 2007;Vecchi and Soden, 2007;Camargo et al., 2009;Daloz and Camargo, 2018;Zhang et al., 2018). For example, Camargo et al. (2007a) used the index to diagnose the effects of the El Niño-Southern Oscillation (ENSO) on cyclone genesis and found that GPI04 could reproduce the variations of the observed frequency and location of TC genesis in the global ocean during El Niño and La Niña. ...
This study investigates the global performance of the tropical cyclone (TC) genesis potential index based on oceanic parameters (GPIocean) proposed by Zhang et al. (2016). In six major TC formation basins, GPIocean can represent the seasonal variations of TC genesis over most basins, except for the North Indian Ocean (NIO). The monthly climatological GPIocean 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 GPIocean in the NIO, the relative contributions of different parameters related to GPIocean are calculated and compared with those related to the genesis potential index developed by Emanuel and Nolan (2004) (GPI04). Results show that the net long-wave 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 GPIocean. Vertical wind shear is the dominant factor inhibiting TC genesis in the NIO in boreal summer. Therefore, the absence of vertical wind shear in GPIocean results in the failure of the annual cycle of TC genesis in the NIO.
... However, even if this was to be true, a shift to the more intense storm mechanisms associated with higher temperatures is not inconsistent with climate change (Berg et al., 2013). For example, the poleward shifts of the tropics is likely to bring with it changing weather types Seidel et al., 2008;Staten et al., 2018) and increased tropical cyclone activity (Daloz and Camargo, 2018). ...
Increases in the magnitude of storm and flood related catastrophes due to climate change are predicted to increase associated economic losses. There exists, however, conflicting evidence for greater economic losses despite well acknowledged increases in the severity of observed extreme events in recent decades. Here, using a worldwide catastrophe insurance database from 1970 to 2015, we link the catastrophe economic loss from extreme storms and floods to local temperature. Although no statistically significant temporal trend is detected in standardised global catastrophe economic losses, we find a statistically significant positive association between economic losses expressed as a proportion of GDP and local temperature. The association between economic losses and temperature is greater as the event becomes more extreme, with the signal muted for flooding as compared to storms. Although local associations of economic loss with temperature cannot be directly linked to rising global temperatures as a result of climate change, the positive economic loss-temperature associations are consistent with observed extreme precipitation-temperature associations, and hence pertinent to the advancement of understanding future natural catastrophes.
... The increase in TCHP at lower latitudes particularly over the south-central ARB is responsible for genesis at lower latitudes along with other favorable atmospheric conditions. This shift in the genesis location equatorward is in agreement with Daloz and Camargo (2018) which show that during the period 1980-2013, an average equatorward shift of genesis location for the north Indian Ocean TCs is 78 km decade −1 . ...
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Climatologically, the frequency of tropical cyclones (TCs) in the Bay of Bengal (BoB) is higher relative to that over the Arabian Sea (ARB). However, recent years exhibit a greater number of TCs forming in the ARB than in the BoB. During the study period (1982–2019), a significant increasing trend in the intensity, frequency, and duration of cyclonic storms (CS) and very severe CS (VSCS) is observed over the ARB. There is a 52% increase in the frequency of CS during the recent epoch (2001–2019) in the ARB, while there is a decrease of 8% in the BoB. Over the ARB, increment in CS duration is 80% and VSCS is almost threefold in the recent epoch as compared to the past epoch (1982–2000). Also, lifetime maximum intensity and accumulated cyclone energy have increased over the ARB implying an increase in the strength of TCs. The increase in TC duration over the ARB is prominent during May, June, and October and a decrease over the BoB is noted during November. The increase in the duration of TCs in the ARB is associated with an increase in mid-level relative humidity and column averaged (950-150 hPa) moist static energy, which is significantly correlated to an increase in sea surface temperatures and tropical cyclone heat potential in the basin. In the recent epoch, TC genesis is observed at lower latitudes (< 8° N), which is another factor contributing to longer durations of TCs. This increases the probability of TC intensification with the support from other favourable environmental parameters. Significant changes in TC tracks are also noted in May, June, and October due to changes in steering currents.
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Recent studies have noted a poleward shift in tropical cyclone (TC) lifetime maximum intensity (LMI) location. Whether this observed shift is due to global warming, natural variability or a combination of both factors remains inconclusive. The western North Pacific (WNP) has been shown in prior research to be the most robust contributor to the observed poleward LMI migration. This study explores inter-annual and inter-decadal climate drivers of WNP LMI location from 1979–2018. On inter-annual time scales, there are more northward-moving TC tracks during El Niño years compared with La Niña years. However, there is substantially smaller variance in the latitudinal distance from TC genesis latitude to LMI latitude than the variance in the TC genesis latitude. Thus, TC genesis El Nino years tend to reach their LMI at a lower latitude given the increased likelihood that they undergo genesis at a lower latitude. On decadal or longer timescales, global warming has contributed to the recent poleward shift of LMI location by causing more northwestward/northward-moving TC tracks, while the PDO also significantly modulates decadal variability in TC genesis latitude, thus also contributing to LMI latitude changes. Further analyses of the extended period from 1960–2018 suggests that the trends in TC LMI latitude and TC tracks are dominated by global warming, and the PDO phase change likely reinforces this trend during 1979–2018. These three leading modes of climate variability (e.g., ENSO, PDO and global warming) offer a more complete picture of the meridional migration of WNP LMI location on various timescales.
This study analyzes landfall locations of tropical cyclones (TCs) over the western North Pacific during 1979–2018. Results demonstrate that the landfall locations of TCs over this region have shifted northward during the last four decades, primarily due to the shift of landfalling TC tracks, with the decreasing/increasing proportion of westward/northward TC tracks. In particular, the northward shift of the landfalling TCs was not related to their formation locations, which have not markedly changed, whereas “no-landed” TCs have significantly shifted northward. TC movement was significantly and positively correlated to the zonal component of the steering flow, while the correlation between TC movement and the meridional component of the steering flow was relatively unobvious. The westward steering flow in the tropical central Pacific that occurred around the formation and early development of the westward TCs was significantly weakened, which was unfavorable for their westward movement, thereby, causing the higher proportions of northward moving tracks. This weakened westward flow was related to the northward shift of the subtropical high ridge, which was caused by significant weakening of the southern part of the subtropical high. The vertical wind shear, sea surface temperature, and convective available potential energy also showed that the northern region of the western North Pacific became more favorable for TC development, whereas the upper divergence, low-layer relative vorticity, and accumulated water vapor content were not obviously related to the northward shift of TCs.
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In the first part of this chapter, we give a review of the relationship of climate and tropical cyclones on various time scales, from intra-seasonal to decadal. The response of tropical cyclone activity to natural modes of variability, such as El Niño-Southern Oscillation and the Madden Julian Oscillation in various regions of the world are discussed. Genesis location, track types and intensity of tropical cyclones are influenced by these modes of variability. In the second part, a review of the state of the art of seasonal tropical cyclone forecasting is discussed. The two main techniques currently used to produce tropical cyclone seasonal forecasts (statistical and dynamical) are discussed, with a focus on operational forecasts.
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The ventilation index serves as a theoretically-based metric to assess possible changes in the statistics of tropical cyclones to combined changes in vertical wind shear, midlevel entropy deficit, and potential intensity in climate models. Model output from eight Coupled Model Intercomparison Project 5 models is used to calculate the ventilation index. The ventilation index and its relationship to tropical cyclone activity between two twenty-year periods are compared: the historical experiment from 1981 to 2000 and the RCP8.5 experiment from 2081 to 2100. The general tendency is for an increase in the seasonal ventilation index in the majority of the tropical cyclone basins, with exception of the North Indian basin. All the models project an increase in the midlevel entropy deficit in the tropics, although the effects of this increase on the ventilation index itself are tempered by a compensating increase in the potential intensity and a decrease in the vertical wind shear in most tropical cyclone basins. The nonlinear combination of the terms in the ventilation index results in large regional and intermodel variability. Basin changes in the ventilation index are well correlated with changes in the frequency of tropical cyclone formation and rapid intensification in the climate models. However, there is large uncertainty in the projections of the ventilation index and the corresponding effects on changes in the statistics of tropical cyclone activity.
The average latitude where tropical cyclones (TCs) reach their peak intensity has been observed to be shifting poleward in some regions over the past 30 years, apparently in concert with the independently observed expansion of the tropical belt. This poleward migration is particularly well observed and robust in the western North Pacific Ocean (WNP). Such a migration is expected to cause systematic changes, both increases and decreases, in regional hazard exposure and risk, particularly if it persists through the present century. Here, it is shown that the past poleward migration in the WNP has coincided with decreased TC exposure in the region of the Philippine and South China Seas, including the Marianas, the Philippines, Vietnam, and southern China, and increased exposure in the region of the East China Sea, including Japan and its Ryukyu Islands, the Korea Peninsula, and parts of eastern China.Additionally, it is shown that projections ofWNP TCs simulated by, and downscaled from, an ensemble of numerical models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) demonstrate a continuing poleward migration into the present century following the emissions projections of the representative concentration pathway 8.5 (RCP8.5). The projected migration causes a shift in regional TC exposure that is very similar in pattern and relative amplitude to the past observed shift. In terms of regional differences in vulnerability and resilience based on past TC exposure, the potential ramifications of these future changes are significant. Questions of attribution for the changes are discussed in terms of tropical belt expansion and Pacific decadal sea surface temperature variability.
A new index for hurricane development is introduced which indicates regions of the tropical ocean where tropical cyclones are likely to be generated and regions where existing cyclones would be spun-down. The index physical basis, which can be computed from the fields of sea surface temperature (SST) and evaporation, is briefly discussed. The principal result is that regions favorable to the development of tropical cyclone development are characterized by strong negative spatial gradients of evaporation with respect to SST, whereas cyclones are likely to be spun-down in regions of strong positive gradients. The application to the Caribbean region indicates a corridor of favorable conditions in the central Caribbean, possibly promoted by the presence of easterly waves, and a southern unfavorable zone that shelters the Venezuelan coast from hurricane impact. The results show in detail the reasons for the variability in hurricane seasons, using 1983 and 2005 as examples. On the large scale, the sign and strength of the SST dipole anomaly between the Pacific and the Atlantic oceans appears to be the controlling influence, a positive anomaly in the Atlantic Ocean leading to hurricane formation. The monthly standard deviation of the hurricane index simulates reasonably well the historical fluctuations in hurricane occurrence in 1979-2005, and using the results of a coupled climate model, it predicts that the hurricane season in 2051-2080 will be lengthened to include an early season maximum in June and another in September-October, in contrast to the current single maximum in September.
Temperatures in the upper troposphere of the atmosphere, near the tropopause, play a key role in the evolution of tropical cyclones (TC) by controlling their potential intensity (PI), which describes the thermodynamically based maximum TC intensity that the environment will support. Accurately identifying past trends in PI is critical for understanding the causes of observed changes in TC intensity, but calculations of PI trends using different atmospheric reanalysis products can give very different results, largely due to differences in their representation of upper-tropospheric temperatures. Without a means to verify the fidelity of the upper-tropospheric temperatures, PI trends calculated from these products are very uncertain. Here, a method is introduced to validate the upper-tropospheric temperatures in the reanalysis products by using the TCs themselves as thermometers. Using a 30-yr global dataset of TC cloud-top temperatures and three widely utilized atmospheric reanalysis products—Modern-Era Retrospective Analysis for Research and Applications (MERRA), ECMWF interim reanalysis (ERA-Interim), and NCEP–NCAR Global Reanalysis 1—it is shown that storm-local upper-level temperatures in the MERRA and ERA-Interim data vary similarly to the TC cloud-top temperatures on both interannual and decadal time scales, but the NCEP–NCAR data have substantial biases that introduce an increasing trend in storm-local PI not found in the other two products. The lack of global storm-local PI trends is due to a balance between temporal increases in the mean state and the poleward migration of TCs into lower climatological PI, and it has significant implications for the detection and attribution of mean TC intensity trends.
Tropical cyclone genesis indices (TCGIs) are functions of the large-scale environment that are designed to be proxies for the probability of tropical cyclone (TC) genesis. While the performance of TCGIs in the current climate can be assessed by direct comparison to TC observations, their ability to represent future TC activity based on projections of the large-scale environment cannot. Here the authors examine the performance of TCGIs in high-resolution atmospheric model simulations forced with sea surface temperatures (SST) of future, warmer climate scenarios. They investigate whether the TCGIs derived for the present climate can, when computed from large-scale fields taken from future climate simulations, capture the simulated global mean decreases in TC frequency. The TCGIs differ in their choice of environmental predictors, and several choices of predictors perform well in the present climate. However, some TCGIs that perform well in the present climate do not accurately reproduce the simulated future decrease in TC frequency. This decrease is captured when the humidity predictor is the column saturation deficit rather than relative humidity. Using saturation deficit with relative SST as the other thermodynamic predictor overpredicts the TC frequency decrease, while using potential intensity in place of relative SST as the other thermodynamic predictor gives a good prediction of the decrease's magnitude. These positive results appear to depend on the spatial and seasonal patterns in the imposed SST changes; none of the indices capture correctly the frequency decrease in simulations with spatially uniform climate forcings, whether a globally uniform increase in SST of 2K, or a doubling of CO2 with no change in SST.
The historical global “best track” records of tropical cyclones extend back to the mid-nineteenth century in some regions, but formal analysis of these records is encumbered by temporal heterogeneities in the data. This is particularly problematic when attempting to detect trends in tropical cyclone metrics that may be attributable to climate change. Here the authors apply a state-of-the-art automated algorithm to a globally homogenized satellite data record to create a more temporally consistent record of tropical cyclone intensity within the period 1982–2009, and utilize this record to investigate the robustness of trends found in the best-track data. In particular, the lifetime maximum intensity (LMI) achieved by each reported storm is calculated and the frequency distribution of LMI is tested for changes over this period. To address the unique issues in regions around the Indian Ocean, which result from a discontinuity introduced into the satellite data in 1998, a direct homogenization procedure is applied in which post-1998 data are degraded to pre-1998 standards. This additional homogenization step is found to measurably reduce LMI trends, but the global trends in the LMI of the strongest storms remain positive, with amplitudes of around +1 m s−1 decade−1 and p value = 0.1. Regional trends, in m s−1 decade−1, vary from −2 (p = 0.03) in the western North Pacific, +1.7 (p = 0.06) in the south Indian Ocean, +2.5 (p = 0.09) in the South Pacific, to +8 (p < 0.001) in the North Atlantic.
The authors examine the change in tropical cyclone (TC) tracks that results from projected changes in the large-scale steering flow and genesis location from increasing greenhouse gases. Tracks are first simulated using a Beta and Advection Model (BAM) and NCEP-NCAR reanalysis winds for all TCs that formed in the North Atlantic Ocean's Main Development Region (MDR) for the period 1950-2010. Changes in genesis location and large-scale steering flow are then estimated from an ensemble mean of 17 models from phase 3 of the Coupled Model Intercomparison Project (CMIP3) for the A1b emissions scenario. The BAM simulations are then repeated with these changes to estimate how the TC tracks would respond to increased greenhouse gases. As the climate warms, the models project a weakening of the subtropical easterlies as well as an eastward shift in genesis location. This results in a statistically significant decrease in straight-moving (westward) storm tracks of similar to 5.5% and an increase in recurving (open ocean) tracks of similar to 5.5%. These track changes decrease TC counts over the southern Gulf of Mexico and Caribbean by 1-1.5 decade(-1) and increase counts over the central Atlantic by 1-1.5 decade(-1). Changes in the large-scale steering flow account for a vast majority of the projected changes in TC trajectories.
A Poisson regression between the observed climatology of tropical cyclogenesis (TCG) and large-scale climate variables is used to construct a TCG index. The regression methodology is objective and provides a framework for the selection of the climate variables in the index. Broadly following earlier work, four climate variables appear in the index: low-level absolute vorticity, relative humidity, relative sea surface temperature (SST), and vertical shear. Several variants in the choice of predictors are explored, including relative SST versus potential intensity and satellite-based column-integrated relative humidity versus reanalysis relative humidity at a single level; these choices lead to modest differences in the performance of the index. The feature of the new index that leads to the greatest improvement is a functional dependence on low-level absolute vorticity that causes the index response to absolute vorticity to saturate when absolute vorticity exceeds a threshold. This feature reduces some biases of the index and improves the fidelity of its spatial distribution. Physically, this result suggests that once low-level environmental vorticity reaches a sufficiently large value, other factors become rate limiting so that further increases in vorticity (at least on a monthly mean basis) do not increase the probability of genesis. Although the index is fit to climatological data, it reproduces some aspects of interannual variability when applied to interannually varying data. Overall, the new index compares positively to the genesis potential index (GPI), whose derivation, computation, and analysis is more complex in part because of its dependence on potential intensity.
Temporally inconsistent and potentially unreliable global historical data hinder the detection of trends in tropical cyclone activity. This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment. Here we mitigate this difficulty by focusing on a metric that is comparatively insensitive to past data uncertainty, and identify a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. The poleward trends are evident in the global historical data in both the Northern and the Southern hemispheres, with rates of 53 and 62 kilometres per decade, respectively, and are statistically significant. When considered together, the trends in each hemisphere depict a global-average migration of tropical cyclone activity away from the tropics at a rate of about one degree of latitude per decade, which lies within the range of estimates of the observed expansion of the tropics over the same period. The global migration remains evident and statistically significant under a formal data homogenization procedure, and is unlikely to be a data artefact. The migration away from the tropics is apparently linked to marked changes in the mean meridional structure of environmental vertical wind shear and potential intensity, and can plausibly be linked to tropical expansion, which is thought to have anthropogenic contributions.