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Recent poleward shift of tropical cyclone formation linked to Hadley cell expansion

DOI:10.1038/s41558-018-0227-5
Authors:
Sharmila S. at Bureau of Meteorology
Sharmila S.
Kevin Walsh at University of Melbourne
Kevin Walsh
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Abstract and Figures

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.
Relationship between tropical SST anomaly and atmospheric dry static stability (in the deep tropics) and annual TCG frequency in a recent 35-year period a, Monthly time-series of SST anomaly (ERSST³² 11-month running mean, red curve) averaged over the tropical domain (23.5° N–23.5° S, all longitudes) with respect to the base period 1951–1979. The atmospheric dry static stability (blue curve, the potential temperature θ difference between two vertical levels: θ300–θ900) averaged over the deep tropics (10° S–10° N) based on ERA-Interim reanalysis³⁶. b, annual TCG frequency (two-year running mean, solid curves) over the western Pacific (WP; blue), eastern Pacific (EP; pink), north Atlantic (NA; red), south Indian (SI; grey) and south Pacific (SP; orange) ocean basins based on the latest IBTrACS³⁵ data and their corresponding linear trends (dashed lines). *Significant trend at P < 0.05 calculated for period 1980–2014.
Relationship between tropical SST anomaly and atmospheric dry static stability (in the deep tropics) and annual TCG frequency in a recent 35-year period a, Monthly time-series of SST anomaly (ERSST³² 11-month running mean, red curve) averaged over the tropical domain (23.5° N–23.5° S, all longitudes) with respect to the base period 1951–1979. The atmospheric dry static stability (blue curve, the potential temperature θ difference between two vertical levels: θ300–θ900) averaged over the deep tropics (10° S–10° N) based on ERA-Interim reanalysis³⁶. b, annual TCG frequency (two-year running mean, solid curves) over the western Pacific (WP; blue), eastern Pacific (EP; pink), north Atlantic (NA; red), south Indian (SI; grey) and south Pacific (SP; orange) ocean basins based on the latest IBTrACS³⁵ data and their corresponding linear trends (dashed lines). *Significant trend at P < 0.05 calculated for period 1980–2014.
… 
Epochal changes in TCG and large-scale climate conditions as a function of latitude over the five major ocean basins a–e, composite meridional distributions of TCG frequency (per 2° × 2° latitude–longitude grid box per season per year) averaged over the individual ocean basin (western Pacific (WP), eastern Pacific (EP), north Atlantic (NA), south Indian (SI) and south Pacific (SP)) for the pre-1998 (P1; blue line) and post-1998 (P2; red line) period and their epochal changes (∆TCG = P2 – P1; where blue (red) shading corresponds to positive (negative) ∆TCG). f–j, Probability density histograms of TCG latitude preference (in %) relative to the total number of TCG per individual ocean basin. k–o, Relative changes P2 - P1P1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\frac{\mathrm{P2 - P1}}{\mathrm{P1}}} \right)$$\end{document} in vertical wind shear (VWS; blue, left y-axis), maximum potential intensity (MPI; red, right y-axis) along with the VMS (grey shaded, right y-axis).
Epochal changes in TCG and large-scale climate conditions as a function of latitude over the five major ocean basins a–e, composite meridional distributions of TCG frequency (per 2° × 2° latitude–longitude grid box per season per year) averaged over the individual ocean basin (western Pacific (WP), eastern Pacific (EP), north Atlantic (NA), south Indian (SI) and south Pacific (SP)) for the pre-1998 (P1; blue line) and post-1998 (P2; red line) period and their epochal changes (∆TCG = P2 – P1; where blue (red) shading corresponds to positive (negative) ∆TCG). f–j, Probability density histograms of TCG latitude preference (in %) relative to the total number of TCG per individual ocean basin. k–o, Relative changes P2 - P1P1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {\frac{\mathrm{P2 - P1}}{\mathrm{P1}}} \right)$$\end{document} in vertical wind shear (VWS; blue, left y-axis), maximum potential intensity (MPI; red, right y-axis) along with the VMS (grey shaded, right y-axis).
… 
Observed streamfunction (ψ) of the zonal-mean meridional overturning circulation a–c, Pre-1998 (P1), post-1998 (P2) and recent change (P2 – P1) for the boreal summer (JAS). d–f, As with a–c but during boreal winter (JFM). The positive (negative) value of ψ represents clockwise (anticlockwise) circulation in the latitude–pressure coordinate. Values significant above a 95% confidence level are marked with black dots. Solid contours indicate ψ values of zero. The corresponding zonal-mean divergent meridional overturning index (MOI, the difference between 200 hPa and 850 hPa) values are also shown at the bottom of each panel.
Observed streamfunction (ψ) of the zonal-mean meridional overturning circulation a–c, Pre-1998 (P1), post-1998 (P2) and recent change (P2 – P1) for the boreal summer (JAS). d–f, As with a–c but during boreal winter (JFM). The positive (negative) value of ψ represents clockwise (anticlockwise) circulation in the latitude–pressure coordinate. Values significant above a 95% confidence level are marked with black dots. Solid contours indicate ψ values of zero. The corresponding zonal-mean divergent meridional overturning index (MOI, the difference between 200 hPa and 850 hPa) values are also shown at the bottom of each panel.
… 
Meridional-vertical structure of the regional Hadley circulation during peak TC seasons over individual basins Meridional-vertical distribution of the longitudinally averaged divergent meridional wind and vertical pressure velocity (shown in vector) based on ERA-Interim³² datasets. a–e, Pre-1998 (P1). f–j, Post-1998 (P2). k–o, Epochal changes (P2 – P1). The corresponding vertical pressure velocity (omega, × 10–2 hPa s–1) averaged over the same domain is shown in colour shading. Black dots denote values statistically significant at the 95% confidence level. The meridional distribution of the low-level (surface to 700 hPa) integrated moisture flux convergence (MFC) are also shown at the bottom of each panel (blue shading indicates a positive MFC, red shading indicates a negative MFC) for the respective periods.
Meridional-vertical structure of the regional Hadley circulation during peak TC seasons over individual basins Meridional-vertical distribution of the longitudinally averaged divergent meridional wind and vertical pressure velocity (shown in vector) based on ERA-Interim³² datasets. a–e, Pre-1998 (P1). f–j, Post-1998 (P2). k–o, Epochal changes (P2 – P1). The corresponding vertical pressure velocity (omega, × 10–2 hPa s–1) averaged over the same domain is shown in colour shading. Black dots denote values statistically significant at the 95% confidence level. The meridional distribution of the low-level (surface to 700 hPa) integrated moisture flux convergence (MFC) are also shown at the bottom of each panel (blue shading indicates a positive MFC, red shading indicates a negative MFC) for the respective periods.
… 
Schematic of a possible linkage between observed tropical expansion and recent changes in TCG.
Schematic of a possible linkage between observed tropical expansion and recent changes in TCG.
… 
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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-
ies16–19 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
730
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... 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 the period 1980-2014 has been linked to an expansion of the tropics as reflected by changes in regional Hadley circulations (Sharmila and Walsh 2018). Murakami et al. (2020) showed that 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 coupled general circulation model forced with increasing CO2, found decreasing 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 the observational results of Sharmila and Walsh (2018). The decline in TC numbers over the WNP basin since about 1998 has also been linked to multidecadal variabilityspecifically the negative phase of the Interdecadal Pacific Oscillation and associated La Niña like state have likely acted to suppress TCG in parts of the WNP (Zhao et al. 2018;Zhao et al. 2020;Chan and Liu 2022). ...
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... These future changes in TC characteristics (iE intensification, frequency reduction) will increase as sea-surface temperature (SST) becomes warmer (Ying et al. 2012;Walsh et al. 2016;Wehner et al. 2018; Chen et Kawase et al. 2021;Chand et al. 2022). Other changes such as poleward migration, storm duration, and translational speed are also projected for the most intense future TCs (Sharmila and Walsh 2018;Yamaguchi et al. 2020;Studholme et al. 2022). Still, model uncertainties remain a major limitation in projections of future TC impacts (Gallo et al. 2019). ...
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In November 2020, Typhoon (TY) Vamco (locally named Ulysses) made landfall on the main island of Luzon, the Philippines. It brought intense rainfall resulting in widespread flooding making it the 7 th costliest TY in the Philippines. Its thermodynamic characteristic from radiosonde observations during its closest passage shows that while the convective available potential energy (CAPE) was not abnormally high, the saturated layer from 850-600 hPa height had lapse rates slightly larger than the moist-adiabat. Also, high precipitable water of up to 70.3 mm and high relative humidity (RH) from the surface to 400 hPa likely explain the heavy rainfall associated with TY Vamco. Global warming has exerted profound effects on weather patterns around the globe. Consequently, the impact of TY Vamco was used as an example of climate change in the local popular media. In this study, we investigated the influence of historical warming on the rainfall characteristics of TY Vamco using the Weather Research and Forecasting model. The pseudo-global warming method was applied using a 40-yr regression of sea surface and air temperature, and RH. We then used the modeled rainfall to simulate the river discharges of two rivers in the northern Philippines that experienced extensive flooding. Results show that SST has a major influence on the intensity of TY Vamco. However, other factors such as orography and changes in mid-tropospheric humidity negate the effects of historical warming, which resulted in comparable rainfall between the past and present simulations.
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... To understand the dominant role of scheme uncertainty, we compare the CMIP6 MMM projected trends in the detected TCGF and the GPIs during the period of 1980-2099 under the SSP5-8.5 scenario. Spatially, although all the three TC identification schemes indicate a poleward tendency of tropical cyclogenesis in the WNP basin under global warming, as suggested in previous studies (Sharmila and Walsh 2018, Sun et al 2019, they show different trends over the present-day core genesis area (figures 3(a)-(c)). The different signed responses of the detected TCGF, the DGPI and the EGPI contribute to the large scheme uncertainty. ...
Understanding uncertainties in projections of western North Pacific tropical cyclogenesis
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Reliable projections of tropical cyclone (TC) activities in the western North Pacific (WNP) are crucial for climate policy-making in densely-populated coastal Asia. Existing projections, however, exhibit considerable uncertainties with unclear sources. Here, based on future projections by the latest Coupled Model Intercomparison Project Phase 6 climate models, we identify a new and prevailing source of uncertainty arising from different TC identification schemes. Notable differences in projections of detected TCs and empirical genesis potential indices are found to be caused by inconsistent changes in dynamic and thermodynamic environmental factors affecting TC formations. While model uncertainty holds the secondary importance, we show large potential in reducing it through improved model simulations of present-day TC characteristics. Internal variability noticeably impacts near-term projections of the WNP tropical cyclogenesis, while the relative contribution of scenario uncertainty remains small. Our findings provide valuable insights into model development and TC projections, thereby aiding in adaptation decisions.
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... When explaining the climate impacts on TC activity, another line of active research builds on the concept of the Hadley circulation [19][20][21][22] . The Hadley circulation is a global-scale overturning circulation characterized by equatorial ascent and subtropical descent. ...
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The future risk of tropical cyclones (TCs) strongly depends on changes in TC frequency, but models have persistently produced contrasting projections. A satisfactory explanation of the projected changes also remains elusive. Here we show a warming-induced contraction of tropical convection delays and reduces TC formation. This contraction manifests as stronger equatorial convection and weaker off-equatorial convection. It has been robustly projected by climate models, particularly in the northern hemisphere. This contraction shortens TC seasons by delaying the poleward migration of the intertropical convergence zone. At seasonal peaks of TC activity, the equatorial and off-equatorial components of this contraction are associated with TC-hindering environmental changes. Finally, the convection contraction and associated warming patterns can partly explain the ensemble spread in projecting future TC frequency. This study highlights the role of convection contraction and provides motivation for coordinated research to solidify our confidence in future TC risk projections.
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... There has been a significant increase in the magnitude and intensity of tropical cyclones and their impacts, particularly on southern Africa's livelihoods, economies, and the environment Patricola and Wehner 2018;Kossin 2018). Many authors (e.g., Pillay and Fitchett 2019;Sharmila and Walsh 2018;Walsh et al. 2016;Fitchett and Grab 2014) have noted that changes in the climatic factors have also influenced the pattern of the occurrence of tropical cyclones in the Southwestern Indian Ocean (SWIO). According to Fitchett (2018), SWIO has seen an increase in the intensity of the tropical cyclones, which have also become larger and affect a wider region. ...
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  • Jun 2017
The poleward migration of the annual mean location of tropical cyclone (TC) lifetime maximum intensity (LMI) has been identified in the major TC basins of the globe over the past 30 years, which is particularly robust over the western North Pacific (WNP). This study has revealed that this poleward migration consists mainly of weak TCs (with maximum sustained surface wind speed less than 33 m s-1) over the WNP. Results show that the location of LMI of weak TCs has migrated about 1° latitude poleward per decade since 1980, while such a trend is considerably smaller for intense TCs. This is found to be linked to a significant decreasing trend of TC genesis in the southern WNP and a significant increasing trend in the northwestern WNP over the past 30 years. It is shown that the greater sea surface temperature (SST) warming at higher latitudes associated with global warming and its associated changes in the large-scale circulation favor more TCs to form in the northern WNP and fewer but stronger TCs to form in the southern WNP.
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Tropospheric Warming Over The Past Two Decades
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  • May 2017
Satellite temperature measurements do not support the recent claim of a “leveling off of warming” over the past two decades. Tropospheric warming trends over recent 20-year periods are always significantly larger (at the 10% level or better) than model estimates of 20-year trends arising from natural internal variability. Over the full 38-year period of the satellite record, the separation between observed warming and internal variability estimates is even clearer. In two out of three recent satellite datasets, the tropospheric warming from 1979 to 2016 is unprecedented relative to internally generated temperature trends on the 38-year timescale.
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The Role of Natural Climate Variability in Recent Tropical Expansion
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  • May 2017
Observations show the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Coupled climate models also simulate tropical belt widening, but less so than observed. Reasons for this discrepancy, and the mechanisms driving the expansion remain uncertain. Here, we show the role of unforced, natural climate variability--particularly natural sea surface temperature (SST) variability--in recent tropical widening. Compared to coupled ocean atmosphere models, atmosphere only simulations driven by observed SSTs consistently lead to larger rates of tropical widening, especially in the Northern Hemisphere (NH), highlighting the importance of recent SST evolution. Assuming the ensemble mean SSTs from historical simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Targeted simulations with the Community Atmosphere Model version 5 (CAM5) show that natural SST variability accounts for nearly all of the widening associated with recent SST evolution. This is consistent with the similarity of the unforced SSTs to the observed SSTs, both of which resemble a cold El Nino Southern Oscillation/Pacific Decadal Oscillation (ENSO/PDO)-like SST pattern, which is associated with a wider tropical belt. Moreover, CAM5 coupled simulations with observed central to eastern tropical Pacific SSTs yield more than double the rate of widening compared to analogous simulations without prescribed tropical Pacific SSTs, and reproduce the magnitude of tropical widening in atmosphere only simulations. Our results suggest that the bulk of recent tropical widening--particularly in the NH--is due to unforced, natural SST variability, primarily related to recent ENSO/PDO variability.
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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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Variability of the extent of the Hadley circulation in the southern hemisphere: a regional perspective
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In order to understand the regional impacts of variations in the extent of the Hadley circulation in the Southern Hemisphere, regional Hadley circulations are defined in three sectors centered on the main tropical heat sources over Africa, Asia-Pacific (Maritime Continent) and the Americas. These regional circulations are defined by computing a streamfunction from the divergent component of the meridional wind. A major finding from this study is that year-to-year variability in the extent of the hemispheric Hadley circulation in the Southern Hemisphere is primarily governed by variations of the extent of the Hadley circulation in the Asia-Pacific sector, especially during austral spring and summer when there is little co-variability with the African sector, and the American sector exhibits an out of phase behavior. An expanded Hadley circulation in the Southern Hemisphere (both hemispherically and in the Asia-Pacific sector) is associated with La Niña conditions and a poleward expansion of the tropical wet zone in the Asia-Pacific sector. While La Niña also promotes expansion in the American and African sectors during austral winter, these tropical conditions tend to promote contraction in the two sectors during austral summer as a result of compensating convergence over the Americas and Africa sectors: a process driven by variations in the Walker circulation and Rossby wave trains emanating from the tropical Indian Ocean.
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Concurrent Changes to Hadley Circulation and the Meridional Distribution of Tropical Cyclones
Article
  • Mar 2018
Poleward trends in seasonal-mean latitudes of tropical cyclones (TCs) have been identified in direct observations from 1980 to the present. Paleoclimate reconstructions also indicate poleward–equatorward migrations over centennial–millennial time scales. Hadley circulation (HC) is often both implicitly and explicitly invoked to provide dynamical linkages to these shifts, although no direct analysis of concurrent changes in the recent period has been presented. Here, the observational TC record (1981–2016) and ERA-Interim, JRA-55, and MERRA-2 are studied to examine potential relationships between the two. A zonally asymmetric HC is defined by employing Helmholtz theory for vector decomposition, and this permits the derivation of novel HC diagnostics local to TC basins. Coherent variations in both long-term linear trends and detrended interannual variability are found. TC genesis and lifetime maximum intensity latitudes share trend sign and magnitude with shifts in local HC extent, with rates being approximately 0.25° ± 0.1° lat decade−1. Both these life cycle stages in hemispheric means and all Pacific TC basins, as well as poleward-extreme North Atlantic lysis latitudes, shared approximately 35% of their interannual variability with HC extent. Local HC intensity is linked only to eastern North Pacific TC latitudes, where strong local overturning corresponds to equatorward TC shifts. Examination of potential dynamical linkages implicates La Niña–like sea surface temperature gradients to poleward HC termini. This corresponds to increased tropical and reduced subtropical vertical wind shear everywhere except in the North Atlantic and western North Pacific, where the opposite is true. These results quantify a long-hypothesized link between TCs and the large-scale oceanic–atmospheric state.
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Long-term trends of typhoon-induced rainfall over Taiwan: In situ evidence of poleward shift of typhoons in western North Pacific in recent decades: Trends of Typhoon-Induced Precipitation
Article
  • Feb 2017
A significant poleward shift of tropical cyclones (TCs or typhoons) [Kossin et al. 2014; 2016] and TC-induced storm surge [Oey and Chou 2016] in the western North Pacific has occurred in recent decades. Here we use 64-year rainfall observations around Taiwan to provide an independent evidence of the shift. We show that due to the island's unique location relative to typhoon tracks [Sampe and Xie 2007], TC-induced rainfall trends are significantly rising west and north of the island, but are insignificant east and southeast, caused by a preference in recent decades for TCs to veer more poleward. Analyses of large-scale fields indicate that the TCs’ poleward shift is caused by the weakening of the steering flow and western North Pacific subtropical high, which in turn is due to tropic-subtropical Indo-Pacific warming and a weakened monsoon, consistent with the expansion of the tropics due to climate change [Fu et al. 2006; Loarie et al. 2009].
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Observed warming trend in sea surface temperature at tropical cyclone genesis
Article
  • Jan 2017
Tropical cyclone (TC) activity is influenced by environmental factors, and it is expected to respond to anthropogenic climate change. However, there is observational uncertainty in historical changes in TC activity, and attributing observed TC changes to anthropogenic forcing is challenging in the presence of internal climate variability. The sea surface temperature (SST) is a well-observed environmental factor that affects TC intensity and rainfall. Here we show that the SST at the time of TC genesis has a significant warming trend over the three decades of the satellite era. Though TCs are extreme events, the warming trend at TC genesis is comparable to the trend in SST during other tropical deep convection events and the trend in SST in the TC main development regions throughout the TC season. This newly documented, observed signature of climate change on TC activity is also present in high-resolution global atmospheric model simulations that explicitly simulate TCs.
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