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The poleward migration of the location of tropical cyclone maximum intensity James P. Kossin, Kerry A. Emanuel & Gabriel A. Vecchi // Nature 509, 349–352 (15 May 2014)
Temporally
inconsistent and potentially unreliable global historical data hinder
the detection of trends in tropical cyclone activity1, 2, 3. This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment4, 5.
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 period6. The global migration remains evident and statistically significant under a formal data homogenization procedure3,
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 contributions6.
Inconsistencies in the historical global ‘best-track’ data
can introduce substantial uncertainty into global-mean measures of
tropical cyclone activity. Since the introduction of geostationary
weather satellites in the mid to late 1970s, measures of tropical
cyclone frequency are generally considered to be accurate, and there is
no observed trend in global frequency since that time7, 8. Comparatively, measures of tropical cyclone intensity are considered to be highly uncertain in the global data3, 9.
Consequently, storm duration is also uncertain because identifying the
moment when a cyclone forms (cyclogenesis) requires accuracy in
intensity estimates, as the definition of cyclogenesis is entirely
dependent on a nascent storm’s intensity reaching a formally specified
threshold. Similar uncertainty exists in identifying a cyclone’s demise
(cyclolysis). These uncertainties can project onto metrics such as power
dissipation10 and accumulated cyclone energy11, which are amalgamations of frequency, duration and intensity. But
measurements of a storm’s position taken around the time that it
reaches its lifetime-maximum intensity (LMI) are much less uncertain. By
this time in a storm’s evolution, it is more likely to have been
detected and to be under close observation. Measurements of storm
position at the time of LMI are also less sensitive to inaccuracy in
measurements of intensity, as well as to known interregional differences
in wind-averaging techniques9,
because determining the absolute LMI is not critical—it is necessary
only to know that the intensity has peaked. This also makes measurements
of storm position at the time of LMI comparatively insensitive to
temporal heterogeneity in the historical best-track intensity record3.
It is this heterogeneity that has presented substantial challenges to
trend detection in tropical cyclone metrics that require absolute
measures of intensity5. Here
we consider the 31-yr period 1982–2012. In this period, the global
best-track data are considered most complete and at their highest
quality in each basin9,
and storm position is well monitored globally by geostationary
satellites. This period also encompasses a recent satellite-based global
tropical cyclone intensity reanalysis3, and is the interval over which the atmospheric reanalysis products12, 13, 14 that provide information on the environmental changes that affect tropical cyclones are most reliable. When
the annual-mean latitude of LMI is calculated from the best-track data
in the Northern and Southern hemispheres over this period (Fig. 1a, b, red lines), there are clear and statistically significant poleward trends in both hemispheres of 53 and 62km per decade, respectively (Table 1). The positive contribution to these hemispheric trends from each ocean basin except that of the North Indian Ocean (Table 1 and Extended Data Fig. 1)
suggests that the migration away from the tropics is a global
phenomenon, although there are large regional differences in the trend
amplitudes and their statistical power. These differences are probably
due, in part, to regional differences in interannual to multidecadal
variability15.
The largest contribution to the Northern Hemisphere trend is from the
western North Pacific Ocean, which is also the most active basin in
terms of annual tropical cyclone frequency. By contrast, the North
Indian Ocean has the lowest mean annual frequency, and the small
equatorward trend there has a much lesser effect on the hemispheric
trend. The North Atlantic Ocean and eastern North Pacific exhibit small
poleward trends and also contribute little to the hemispheric trend. In
the Southern Hemisphere, both the South Pacific and the South Indian
Ocean regions contribute substantially to the poleward trend.
Figure 1: Poleward migration of the latitude of LMI away from the tropics.
a, b, Time series of
annual-mean latitude of tropical cyclone LMI calculated from the
best-track historical data (red) and the ADT-HURSAT reanalysis (blue) in
the Northern (a) and Southern (b) hemispheres. c, The annual-mean difference between a and b
shows the global migration of the latitude of LMI away from the
tropics. Linear trend lines are shown with their 95% two-sided
confidence intervals (shaded). Note that the yaxis in b increases downwards.
Within the period 1982–2009, the latitude of LMI can be reanalysed using a globally homogenized record of intensity (ADT-HURSAT3). When this is done, the annual-mean time series exhibit similar variability and trends (Fig. 1a, b,
blue lines), although the ADT-HURSAT-based trend has a greater
amplitude than the best-track-based trend in the Northern Hemisphere and
a lesser amplitude than the best-track-based trend in the Southern
Hemisphere (Table 1),
where the trend is no longer significant with 95% confidence. However,
when both hemispheres are considered together they depict a global
migration away from the deep tropics, and the best-track and ADT-HURSAT
data exhibit similar poleward trends of 115 and 118km per decade, respectively (Table 1). In this global view, the trends in the best-track and ADT-HURSAT data are consistent and both are statistically significant. As
found with the best-track data, the ADT-HURSAT-based time series
exhibit large differences in trend amplitudes and statistical power when
separated by ocean basin (Table 1 and Extended Data Fig. 2).
The western North Pacific is the largest contributor to the trend in
the Northern Hemisphere, and the eastern North Pacific also contributes
significantly, unlike the best-track data from that region. The
equatorward trend in the North Indian Ocean best-track data is not found
in the ADT-HURSAT data, which shows essentially no trend in that region
(the lack of any poleward trend in the North Indian Ocean might be
expected given the confines of the basin and the close proximity of land
to the north). In the North Atlantic, the best-track and ADT-HURSAT
data sets both show essentially no trend. There are similar poleward
trends in the best-track and ADT-HURSAT data from the South Pacific, but
the ADT-HURSAT data in the South Indian Ocean exhibits a smaller,
statistically insignificant trend. Although regional differences are evident, the migration of the
mean latitude of LMI away from the deep tropics is observed in both
hemispheres, which indicates that this is a global phenomenon. The
genesis and subsequent intensification period of tropical cyclones,
which precedes LMI and controls when and where LMI occurs, is strongly
modulated by the environment that the storms move through in this
period. Known major factors controlling tropical cyclone evolution are
the environmental vertical wind shear and the potential intensity16, 17, 18.
Potential intensity describes the thermodynamically based maximum
tropical cyclone intensity that the environment will support, all other
factors being optimal. Vertical wind shear is one of the key factors
that inhibit a storm from achieving this maximum. Greater shear and
lesser potential intensity each inhibits genesis and intensification,
and vice versa, and increased shear in the deep tropics, decreased shear
at higher latitudes, or both, can thus be plausibly linked to a
poleward migration of the latitude of LMI. Decreased potential intensity
in the deep tropics, increased potential intensity at higher latitudes,
or both, could be expected to result in a similar migration. Here we
explore these environmental factors using three different atmospheric
reanalysis products, NCEP/NCAR12, ERA-Interim13 and MERRA14. All three products exhibit broad regions of increased shear in the deep tropics and decreased shear in the subtropics (Fig. 2),
which is consistent with the observed changes in the tropical cyclones.
The changes in mean potential intensity are not as consistent among the
different reanalysis products, particularly in the tropics, which is
probably a result of spurious differences in upper tropospheric
temperatures19, 20.
However, the meridional structure of potential intensity change is
generally consistent in showing greater increases at higher latitudes,
and the MERRA data, in particular, also show a broad reduction of
potential intensity in the deep tropics.
Figure 2: Observed changes in the mean environment where tropical cyclones form and track.
Percentage changes from 1980–1994 to 1995–2010 in mean vertical wind shear (a, c, e) and potential intensity (b, d, f).
Annual means are taken over the peak tropical cyclone seasons in each
hemisphere (August–October in the north and January–March in the south)
from three different reanalysis products: MERRA (a, b), ERA-Interim (c, d) and NCEP/NCAR (e, f).
The observed changes in shear and potential intensity provide
evidence that the global migration of tropical cyclones away from the
tropics is being modulated by systematic environmental changes. Shifts
in tropical cyclone tracks in most regions have also been linked to
phase changes in El Niño/Southern Oscillation21, 22, 23, 24
(ENSO), which can potentially contribute to the poleward trends in LMI
identified here. To test this, we decrease the contribution of ENSO by
regressing the LMI latitude time series onto an index of ENSO
variability. When this is done (Fig. 3),
the amplitude of the interhemispheric migration rates is found to
decrease only slightly in both the best-track and the ADT-HURSAT data,
and the statistical power of the trends in fact increases. This makes it
unlikely that natural ENSO variability has a role in the observed
multidecadal poleward migration of LMI, although it plays a substantial
part in its interannual variability.
Figure 3: Global trends of the latitude of LMI with ENSO variability reduced.
Time series of the latitude of LMI calculated from the best-track historical data (red; trend, 9959km per decade) and the ADT-HURSAT reanalysis (blue; trend, 10753km
per decade) with ENSO variability reduced. The values are calculated
from residuals of the regression of LMI latitude onto an index of ENSO
variability. Shading represents the 95% two-sided confidence interval of
the trend.
The potential for contributions from natural variability occurring
on decadal or longer timescales still exists, but quantifying this is
difficult using relatively short observation records. We propose that
there is a linkage between the poleward migration of LMI and the
observed expansion of the tropics. The rate of expansion since 1979
varies considerably among existing studies6,
but the rate of LMI migration identified here falls well within this
range. This potential linkage between tropical cyclones and the
expansion of the tropics further heightens interest in establishing the
forcing mechanisms of the expansion, which are at present uncertain but
are generally thought to have anthropogenic contributions6.
The expansion of the tropics, as measured by the meridional extent of
the tropical Hadley circulation, exhibits a step change in the late
1990s6.
Formal change-point analysis applied to the global time series of LMI
latitude reveals a significant change point in 1996, providing further
support for a linkage between the two independently observed phenomena. Observed changes in vertical wind shear and potential intensity over the past 30yr
seem to have resulted in a poleward shift, in both hemispheres, of the
regions most favourable for tropical cyclone development (Fig. 2), and an associated migration of tropical cyclone activity away from the tropics (Fig. 1).
If these environmental changes continue, a concomitant continued
poleward migration of the latitude where tropical cyclones achieve their
LMI would have potentially profound consequences for life and property.
Any related changes in positions where storms make landfall will have
obvious effects on coastal residents and infrastructure. Increasing
hazard exposure and mortality risk from tropical cyclones25
may be compounded in coastal cities outside the tropics, while being
offset at lower latitudes. Tropical cyclones also have an important role
in maintaining regional water resources26, 27,
and a poleward migration of storm tracks could threaten potable water
supplies in some regions while increasing flooding events in others.
Given these motivating factors, further study of the poleward migration
of tropical cyclone LMI identified here, and its potential link to the
observed expansion of the tropics, is warranted.
Best-track data were taken from the International Best Track Archive for Climate Stewardship (IBTrACS) v03r05 (ref. 28) and are available at http://www.ncdc.noaa.gov/ibtracs/. Following ref. 3,
when a storm has overlapping data from multiple sources, we used the
source with the greatest reported LMI. In storms that achieve their LMI
more than once, the latitude of LMI is taken at the first occurrence.
The homogenized intensity data were taken from the Advanced Dvorak
Technique Hurricane Satellite (ADT-HURSAT) data set3.
The data reflect the additional homogenization procedure addressing the
discontinuity in satellite coverage that occurred in 1997. The global
distribution of the ADT-HURSAT LMI is known to be spuriously leptokurtic3,
which is the likely cause of the consistent equatorward bias in the
mean latitude of LMI when compared to the best-track data, but there is
no expectation that this bias has any time dependence and it is not
expected to affect the trends. ENSO variability was removed from
the LMI latitude time series by regressing the individual series from
the Northern and Southern hemispheres onto the Niño-3.4 index30
averaged over the most active periods of tropical cyclone activity
(August–October in the north and January–March in the south), and
analysing the residuals. The index is available at http://www.cpc.ncep.noaa.gov/data/indices/. None
of the time series explored in this paper exhibits autocorrelation
after detrending, as determined with the Durbin–Watson test statistic,
and no corrections were necessary when calculating the confidence
intervals. In addition to linear trend analysis, the time series were
explored for change points with models based on batch detection using
both the Studentt and Mann–Whitney statistics to test for significance at 95% confidence or greater using the ‘cpm’ package31 in the software environment R. The
global trends in the annual mean latitude of LMI are a result of both
intrabasin and interbasin changes. The climatological mean latitude of
LMI varies by basin (see, for example, Extended Data Fig. 1)
such that, in addition to meridional shifts within each basin, changes
in the relative annual frequency of storms from each basin can also
contribute to the global trends in the latitude of LMI. To quantify this
contribution, the LMI latitude of every storm was normalized by the
respective basin-mean LMI latitude, and the analysis of Fig. 1c was repeated. When this was performed, the trend in the best-track data decreased from 115±70 to 78±66km per decade and the trend in the ADT-HURSAT data decreased from 118±70 to 92±65km per decade. Thus, both factors contribute, but the intrabasin poleward migration of LMI dominates the trends. Monthly-mean vertical wind shear and potential intensity were calculated over water in the region spanning latitudes 35°S–35°N. In the Southern Hemisphere, the longitude was confined to 30°–240°E, which excludes the region where storms are not observed to form or track. Vertical wind shear was calculated as32 where u250, u850, v250 and v850 are the zonal (u) and meridional (v) winds at the 250- and 850-hPa pressure levels; u′250, u′850, v′250 and v′850
are departures of the corresponding daily means from their monthly
means; and overbars represent monthly-mean quantities. Potential
intensity was calculated following ref. 29.
In the Northern Hemisphere the shear and potential intensity were
averaged over August–October, and in the Southern Hemisphere they were
averaged over January–March.
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NOAA National Climatic Data Center, University of Wisconsin/CIMSS, 1225 West Dayton Street, Madison, Wisconsin 53706, USA
James P. Kossin
Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
Kerry A. Emanuel
NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08540, USA
Gabriel A. Vecchi
Contributions
J.P.K. had the idea for, and designed, the study, and performed the
analyses with input from K.A.E. and G.A.V. J.P.K., K.A.E. and G.A.V.
provided data and participated in interpretation of the results and the
writing of the manuscript.
Competing financial interests
The authors declare no competing financial interests.
The basins are those in the North Atlantic (a), the western North Pacific (b), the eastern North Pacific (c), the Northern Indian Ocean (d), the Southern Indian Ocean (e) and the South Pacific (f). Linear trend lines are shown with their 95% two-sided confidence intervals (shaded). Note that the yaxes in e and f increase downwards.
The basins are those in the North Atlantic (a), the western North Pacific (b), the eastern North Pacific (c), the Northern Indian Ocean (d), the Southern Indian Ocean (e) and the South Pacific (f). Linear trend lines are shown with their 95% two-sided confidence intervals (shaded). Note that the yaxes in e and f increase downwards.
59 km per decade) and the ADT-HURSAT reanalysis (blue; trend, 107
53 km
per decade) with ENSO variability reduced. The values are calculated
from residuals of the regression of LMI latitude onto an index of ENSO
variability. Shading represents the 95% two-sided confidence interval of
the trend.
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