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Recent research indicates that the annual-mean locations of tropical cyclones have migrated toward higher latitudes. Concurrently, an anthropogenically forced tropical expansion has been observed, yet the connection between the two processes remains little-explored. Here, using observational and reanalysis data, we investigate how large-scale dynamical effects, com-bined with coherent changes in the regional Hadley circulation, explain recent changes in regional tropical cyclone genesis over 1980–2014. We show that the recent anomalous upper-level weakening of the rising branch of the Hadley circulation in the deep tropics, possibly induced by the increased vertical stability, has likely suppressed the low-latitude tropical cyclone genesis in most ocean basins via anomalous large-scale subsidence. Regional Hadley circulation variations have also favoured a poleward displacement of tropical-cyclone-favourable climate conditions through poleward shift of the Hadley circulation’s meridional extent. With projections indicating continued tropical expansion, these results indicate that tropical cyclone genesis will also continue to shift poleward, potentially increasing tropical-cyclone-related hazards in higher-latitude regions.
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Articles
https://doi.org/10.1038/s41558-018-0227-5
1School of Earth Sciences, University of Melbourne, Parkville, Australia. 2Faculty of Science and Technology, Federation University, Mount Helen, Australia.
*e-mail: sharmila.climate@gmail.com
Tropical cyclones (TCs) are among the most catastrophic of
high-impact weather events and have triggered substantial
mortality and huge economic damage over populated tropi-
cal coastal regions in recent decades1. Because the life-cycles of TCs
are critically sensitive to large-scale tropical climate conditions2,3,
the observed increases in the sea surface temperatures (SST) over
tropical ocean basins in recent decades4,5 attributed to anthropo-
genic global warming6 are expected to complicate the status and fate
of TC genesis (TCG) around the globe through gradual modifica-
tion of TC-favourable tropical environments7,8. Despite increasing
research efforts, the issue of how regional-scale TCG has changed or
will change in the context of global warming remains controversial
and challenging6,9,10. An improved understanding of the underlying
warming-induced climatic factors and relevant physical mecha-
nisms that have regulated the regional-scale TCG in recent decades
is therefore crucial to benchmark future TC estimates.
Recent research11 indicates that the observed annual-mean loca-
tions where TCs reach their lifetime-maximum intensity (LMI)
have migrated poleward in most regions in the past 30 years, con-
current with marked changes in the TC-favourable large-scale
climate conditions. A follow-up modelling study12 projected con-
tinuing poleward shift of LMI over the western north Pacific in a
warmer climate. Many recent modelling studies13,14 argue that global
warming could lead to shifts in TC pathways, although the regional-
scale projections remain largely ambiguous4,15. Several recent stud-
ies1619 have noted a significant poleward shift of annual-mean LMI,
particularly over the western north Pacific, but the possible reasons
remain challenging to identify. With some degree of uncertainty,
a recent study20 identified a comparable displacement of annual-
mean TCG locations, linked with sub-tropical displacement of
TC-favourable conditions as measured by potential genesis indi-
ces21,22 (an empirical tool based on a suite of large-scale climate con-
ditions used to identify the potential TC activity) over the Pacific
basins. These results demonstrate potentially increased threats to
locations at higher latitudes that have not been historically prone
to TC-related hazards. However, the physical processes that have
triggered such a migration remain unclear. Although various modes
of natural climate variability at interannual to decadal timescales,
including El Niño/Southern Oscillation (ENSO) and the Pacific
Decadal Oscillations (PDO) influence the regional TCG, no direct
association could be established with the poleward migration of
TCG11,12,20, suggesting that part of the migration could be indepen-
dent of known dominant climate variability12. Previous studies11,12
anticipated that such a poleward migration could possibly be linked
with the independent expansion of the tropical belt observed since
1980, caused by anthropogenic warming23. However, the possible
key mechanisms that link the warming-induced observed tropi-
cal expansion with large-scale climate factors known to modulate
TCG and their poleward displacement in recent decades remains
little-explored.
The expansion of the tropics is related to the characteristics of
the thermally driven tropical mean meridional overturning cir-
culation, also known as the Hadley circulation (HC), a crucial
factor for various climatic feedbacks across the globe. Recent evi-
dence23,24 indicates that the HC is expanding and the zones of the
sub-tropical descending branch are progressively shifting poleward
since 1980, although large uncertainty exists on the rate of widen-
ing. Apart from stratospheric ozone depletion25, the HC expansion
is responsive to anthropogenic climate forcing26,27, while increasing
atmospheric stability is predicted to slow down the HC in a warmer
climate28,29. Various modes of natural climate variability, includ-
ing ENSO and PDO at interannual to decadal timescales can also
influence the position of the tropical edge26,30, but interpretation
of such climate trends is difficult and constrained by limited data
records. Because the attribution of any global-warming impact on
regional-scale TCG is complex, and still controversial, a systematic
understanding of the warming-induced behavioural changes in
regional-scale HC in close association with TC-favourable climate
conditions in the recent period will therefore provide a useful per-
spective for future TC estimates. A recent study31 noted that both
TCG and LMI latitudes share trends and magnitudes with shifts
in the hemispheric-mean HC extent and vertically averaged HC
intensity, while associated trends in local HCs remain uncer-
tain. Using a regression approach, the study suggests a potential
Recent poleward shift of tropical cyclone
formation linked to Hadley cell expansion
S. Sharmila 1,2* and K. J. E. Walsh1
Recent research indicates that the annual-mean locations of tropical cyclones have migrated toward higher latitudes.
Concurrently, an anthropogenically forced tropical expansion has been observed, yet the connection between the two processes
remains little-explored. Here, using observational and reanalysis data, we investigate how large-scale dynamical effects, com-
bined with coherent changes in the regional Hadley circulation, explain recent changes in regional tropical cyclone genesis
over 1980–2014. We show that the recent anomalous upper-level weakening of the rising branch of the Hadley circulation in
the deep tropics, possibly induced by the increased vertical stability, has likely suppressed the low-latitude tropical cyclone
genesis in most ocean basins via anomalous large-scale subsidence. Regional Hadley circulation variations have also favoured
a poleward displacement of tropical-cyclone-favourable climate conditions through poleward shift of the Hadley circulation’s
meridional extent. With projections indicating continued tropical expansion, these results indicate that tropical cyclone genesis
will also continue to shift poleward, potentially increasing tropical-cyclone-related hazards in higher-latitude regions.
NATURE CLIMATE CHANGE | VOL 8 | AUGUST 2018 | 730–736 | www.nature.com/natureclimatechange
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... N). These present-day trends appear to be associated with changes in both the ocean (SST patterns; Fig. 4a) and atmospheric thermodynamics (PI; Fig. 4b), and dynamics (vertical shear, large-scale tropospheric winds) 94,96,99 . Additionally, genesis potential has increased during this period (Fig. 4e). ...
... A separate explanation for recent TC poleward migration invokes suppressed genesis in the deep tropics caused by increased dry static stability in the warming tropical atmosphere 99 (Fig. 4a). However, the extent to which changes in the time-mean static stability effect actual TC processes is unclear. ...
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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.
... Moreover, global climate models are not typically run at scales needed to resolve changes in TC characteristics (particularly intensity) at island-scale, substantially limiting our ability to understand the effects of climate change on TCs over small island countries Fig. 5 Schematic representation of the linkages between natural climate variability, human-induced global warming and tropical cyclones. Note that this diagram is not exclusive and does not quantify the changes, but instead demonstrates how different climatic factors may interact to affect TC characteristics and associated impacts over the SWP region ( 1 Vecchi et al. 2006;2 Yeh et al. 2009;3 Power and Kociuba 2011;4 Kim and Yu 2012;5 Sugi et al. 2012;6 Sugi and Yoshimura 2012;7 Tokinaga et al. 2012;8 Church et al. 2013;9 Hartmann et al. 2013;10 Tory et al. 2013;11 Woodruff et al. 2013;12 Cai et al. 2014;13 Kossin et al. 2014;14 Lucas et al. 2014;15 Walsh et al. 2016;16 Chand et al. 2017;17 Taupo and Noy 2017;18 Chand 2018;19 Kossin 2018;20 Sharmila and Walsh 2018;21 Andrew et al. 2019;22 Chand et al. 2020) ▸ Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
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... Our delta approach implicitly captures future TC behavior that has been a topic of discussion in recent literature, such as a poleward shift of TC tracks (31,32,55,56) or a change in TC translational speed (57). A potential poleward extension of TC tracks may follow from the shift in genesis locations in combination with a shift in the first-step changes in longitude and latitude, and the longitudinal and latitudinal residual terms. ...
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